Architectures and Technologies for Wavelength Division Multiplexed Access Networks

Transcription

1 Architectures and Technologies for Wavelength Division Multiplexed Access Networks Nishaanthan Nadarajah B. E (Hons.) A thesis submitted to the University of Melbourne in total fulfilment of the requirements of the degree of Doctor of Philosophy January 2006 Department of Electrical and Electronic Engineering The University of Melbourne Victoria 3010 Australia Produced on archival quality paper

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3 To My Parents

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5 Abstract Optical fibre communication is very much preferred for the communication of signals over bandwidth of a gigabits per second over distances more than hundreds of kilometres. For a long period of time optical fibre communication has been about how to provide higher bandwidths with reduced cost per bit transmitted. However, this trend has changed from optical transmission to optical networking. By exploiting the wavelength division multiplexing (WDM) technology, optical networks have expanded from backbone networks to metropolitan and access networks to deliver high bandwidth services to the users in a seamless fashion with reduced cost. The ultimate evolution of the optical access network involves fibre-to-the-home (FTTH) technologies, which can potentially offer every kind of information and communication related services. Out of all FTTH technologies, the passive optical network (PON) can potentially offer the most cost-effective solution as the optical network is shared between a number of end users. PONs have significant advantage over competing access technologies as the fibre infrastructure can be effectively future-proofed for upgrades. A number of demonstrations have been carried out for the cost effective deployment of the PONs. However, as these networks evolve, advanced functionalities have to be added over the existing end to end transmissions between the service providers and end users. This thesis addresses several advanced functionalities that are required for a next generation optical access network infrastructure. These advanced functionalities include local area network (LAN) emulation over the existing PON infrastructure, protection capabilities against feeder and distribution fibre breaks, multiple virtual private networking (VPN) within the PON, and an efficient control packet signalling mechanism for a future-packet based access network. Novel schemes are developed and experiments are carried out for successful demonstration of these functionalities. To date, user networking has become an important part of the access networks. Emulating a LAN over the existing PON for this purpose is cost effective. In this thesis, two separate schemes are developed for the optical layer LAN emulation within a PON. Both these - i -

6 schemes use RF subcarrier multiplexing for the transport of the traffic amongst the users within the PON. These schemes are experimentally demonstrated to verify the feasibility of the schemes. A detailed comparison study is also carried out to identify the benefits and drawbacks of each scheme. As more and more users become connected to PON environment, protection of services delivered to the users will become paramount. In this thesis, a number of architectures are proposed and experimentally demonstrated for the protection against feeder and distribution fibre breaks. Feeder fibre protection architecture proposes an overlay the affected transmissions on another similar network. One of the distribution fibre protection schemes uses dual fibres for resilient protection switching. The other scheme uses interconnection amongst the users for rerouting the signals. The distribution fibre protection schemes show that the users perform intelligent monitoring of their own fibres and perform independent protection switching. Electronic code division multiple access (E-CDMA) has the potential to offer several features for a PON. In this thesis, an experimental demonstration and a theoretical analysis are performed to show the capabilities of E-CDMA as an upstream access scheme. E-CDMA has also been used to provide physical layer security of the transmitted signals amongst the users. Moreover, a scheme for multiple and secure VPN capability is also experimentally demonstrated. A detailed theoretical analysis is then carried out to study the performance and scalability limitations of this scheme. As the optical access network infrastructure grows with increasing number of users and increasing demand for more bandwidth, the transport of signals will be required to very efficient. Therefore, future access networks will be based on packet centric architectures. In these packet based access networks, an efficient signalling mechanism will be required to coordinate the transport of packets to obtain low packet latency and transparent switching. In this thesis, E-CDMA based signalling mechanism is proposed and experimentally demonstrated. Several possible architectures that could employ this signalling technique are described in detail. A detailed theoretical study is also carried out to understand the limitations of the technique in the presence of several noise contributions. - ii -

7 Acknowledgements I would like to begin by thanking a number of people who have assisted me in my studies during my candidature. I would like to express my gratitude and sincere thanks to my supervisors Assoc. Prof. Ampalavanapillai Nirmalathas and Dr. Elaine Wong. I am very grateful to Assoc. Prof. Thas. A. Nirmalathas for his constant support and guidance throughout my candidature. Since my first day of the candidature, his words of encouragements in many aspects in the research field have certainly helped me many ways. I owe a huge debt of gratitude for making me a better person and a professional. I am extremely grateful to Dr. Elaine Wong for her constant guidance throughout my candidature. It has been a pleasure to work along side her and learn many skills especially the art of writing. The technical and non-technical advices she provided have helped me tremendously in my development. I am very lucky to have had two wonderful supervisors and thank them much. I am very thankful to Dr. Manik Attygalle for his constant support, encouragement and help. He certainly was a mentor throughout my candidature and his encouragements during the difficult phase of the candidature have helped me stay focused on the work. I am very grateful to Dr. Thomas Chae for his guidance, help and discussions especially in early stages of my candidature. I also would like to thank Dr. Christina Lim, Dr. Alan Lee, Dr. An Vu Tran for the discussions and help. I am very grateful to all other research staff in the Photonics Research Laboratory (PRL), for the technical discussions and help. I wish to thank fellow students of PRL for their help and wonderful environment. Many thanks go to Masud, who I have had a good relationship since our first meeting, Teddy, Bipin, Prasanna, Milan, Thisara, Goutam, Kate, Xingwen and Leigh for the entertaining and non-technical discussions as well as proof reading many parts of my thesis. On a personal level, I am forever indebted to my parents for their understanding and encouragement all my years of study. The hardships they had to take to make me a better person is immeasurable; I cannot thank them enough. I am extremely thankful to my wonderful twin sister, a very special brother and an extremely kind brother-in-law for their love and support. I thank you all very much. - iii -

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9 Declaration This thesis is the result of my own work and, except where acknowledged, includes no material previously published by any other person. I declare that none of the work presented in this thesis has been submitted for any other degree or diploma at any University and that this thesis is less than 100,000 words in length, excluding figures, tables, bibliographies, appendices and footnotes. Nishaanthan Nadarajah - v -

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11 Table of Contents Abstract Acknowledgements Declaration i iii v 1 Introduction 1.1 Optical access networks Passive optical networks Emerging PS-PON technologies Thesis outline Original contributions Publications arising from the work completed in this thesis References 14 2 Literature Review 2.1 Introduction LAN Emulation in PONs LAN emulation using additional transceiver and a FBG LAN emulation using fibre loopback LAN emulation using a common regenerator LAN emulation using optical switches at ONU LAN emulation using dual distribution fibres Survivable optical access network architectures Ring based protection schemes Redundant tree protection schemes Protection in PS-PONs Protection in WDM-PONs Protection for WDM-PON using dual fibres Protection for WDM-PON using interconnections between ONUs 40 - vii -

12 Centrally controlled protection switching in WDM-PON Feeder fibre protection scheme for WDM-PON Direct sequence spread spectrum Performance monitoring using DS-SS ONU authentication in PON Upstream access in PS-PONs Optical beat interference Signalling for packet based access networks Conclusions References 55 3 Local Area Network Emulation in Passive Optical Networks 3.1 Introduction Higher layer LAN emulation Optical layer LAN emulation schemes LAN emulation using RF subcarrier multiplexing LAN emulation using RF subcarrier multiplexing and narrowband FBG Transmission protocol in the upstream direction Optical combination of baseband and RF subcarrier multiplexed data Experimental demonstration using a single notch FBG Optical spectra BER results Experimental demonstration of LAN emulation using double notch FBG Optical spectra BER results LAN emulation using separate distribution fibre loopback Transmission protocol in the upstream direction Experimental demonstration Optical spectra BER results 95 - viii -

13 3.5 Comparison of the proposed LAN emulation schemes Bandwidth requirements Dispersion tolerance Optical source stability Power budget Downstream power budget Upstream power budget RF LAN data power budget Conclusions References Protection and Restoration in Passive Optical Networks 4.1 Introduction Protection against feeder fibre breaks Feeder fibre protection scheme using CWDM separation Procedure for the fast protection switching Experimental demonstration Optical spectra BER results Protection against distribution fibre breaks Protection using two distribution fibres to each ONU Experimental demonstration Optical spectra BER results Protection and LAN emulation using dual distribution fibres Experimental demonstration Optical spectra BER results Protection using the interconnections amongst the ONUs Experimental demonstration Optical spectra BER results ix -

14 4.3.4 Protection using interconnections amongst the ONUs with LAN emulation Protocol for upstream transmissions and ranging Protection with LAN emulation using narrowband FBG Experimental demonstration Optical spectra BER results Protection with LAN emulation using fibre loop back Experimental demonstration Optical spectra BER results Scalability of the protection architectures Switching time Conclusions References Applications of Electronic CDMA in Passive Optical Networks 5.1 Introduction Upstream access in PS-PONs TDMA enabled upstream access in PS-PONs SCMA enabled upstream access in PS-PONs E-CDMA enabled upstream access in PS-PONs Experimental demonstration Optical and RF spectra BER results Requirement for power control Scalability in the presence of optical beat interference Secure LAN emulation using E-CDMA with fibre loopback Experimental demonstration Optical spectra BER results Power budget x -

15 5.5 Secure LAN emulation using E-CDMA with a FBG and an additional optical transceiver Experimental demonstration Optical spectra BER results Multiple and secure virtual private networking using E-CDMA Experimental demonstration Oscilloscope traces Optical spectra BER results Theoretical analysis of scalability Conclusions References Signalling Mechanism using E-CDMA for Packet-Based Access Networks 6.1 Introduction Control packet signalling schemes Packet signalling using electronic code division multiple access Optical network architectures incorporating E-CDMA control packet signalling Passive star architectures incorporating E-CDMA signalling WDM ring architectures incorporating E-CDMA signalling E-CDMA access control interface CSMA/CA protocol for WDM packet ring networks using E-CDMA signalling Simulation Experimental demonstration BER results Theoretical scalability analysis of WDM channels E-CDMA control packet analysis Optimum power budget analysis for E-CDMA control signalling for WDM packet- based access networks xi -

16 6.9 Conclusions References Conclusions and Future Work 7.1 Thesis overview Directions for future work Conclusions References 288 Appendices A Acronyms 291 B Publications xii -

17 Chapter 1 Introduction 1 Introduction 1.1 Optical access networks Optical fibre communication is ubiquitous in the telecommunication infrastructure. Over the last twenty years, optical fibre has become the preferred medium for high capacity transmissions over longer distances while successfully continuing to reduce the cost per bit transmitted due to its low loss and enormous bandwidth over a single fibre [1-4]. The history of optical communication has been mostly about providing transmissions capabilities with reduced cost per bit transmitted. However, the perspective of optical transmission has changed from transmission to networking, whereby the focus is now shifting towards reducing the cost per connected bit transmitted [4]. Optical fibre communication technology has kept up the speed with the increasing demand of traffic transport using wavelength division multiplexing (WDM), with much of the capacity growth being in the point-to-point long distance backbone networks [5, 6]. While the growth of the backbone network capacity has been tremendous, end-user access to this capacity is limited and expensive due to existing access network infrastructures. The rapid increase in demand for truly broadband telecommunication services makes the cost-effective realisation of an access infrastructure highly challenging. Different access technologies based on wired infrastructure such as Asymmetric Digital Subscriber Loop (ADSL), Hybrid Fiber Coax (HFC), and Fibre In The Loop (FITL) are being deployed in the access networks currently [7-13]. ADSL or its high speed implementations (VDSL, HDSL) running along the conventional copper based telephony infrastructure, cable modem (usually for analog video and data services) based on HFC technologies may not be able to keep up with the customer demand for bandwidth and may not facilitate the development of a scaleable access infrastructure in the long term. Optical fibre is an attractive transmission medium capable of delivering large bandwidth for longer distances [14-19]. By taking this high bandwidth and low loss - 1 -

18 Chapter 1 Introduction advantage of optical fibres, fibre-optic distribution network can be used to realise a broadband access network infrastructure and is an attractive solution for the first mile problem [20-23]. In addition to the large bandwidth offered with the use of fibre in the access networks, this transport medium also offers improved reliability and security. Consequently, fibre based access networks have the potential to make true broadband connection to home and small medium enterprises a reality. Compared to point-to-point fibre networks between the head end and customer terminals, and active optical networks (AON), passive optical networks (PONs) have been described as a solution for future-proof technology for the optical access networks [24-28]. There are advantages in using PON as it offers the potential of providing transparent network with a capability to offer a range of broadcast, data and multi-media services Passive Optical Networks Central Office OLT Feeder fibre Passive splitter SC Feeder fibre Passive splitter SC Figure 1.1: Schematic of a passive optical network (PON) architecture for customer access network applications Figure 1.1 shows a schematic of a PON infrastructure where a central office (CO) with optical line terminals (OLTs) provide a network interface to a large number of optical network units (ONUs) which in turn provide the network access to the customer base. In terms of the size of the network, the distance between a CO and ONU can be longer than 20 km with links operating at very high bandwidth in the order of 1 Gb/s. PONs are point-to-multipoint - 2 -

19 Chapter 2 Literature Review 2 Literature Review 2.1 Introduction Fibre based access networks have the capability to provide the greatest bandwidth for both downstream traffic from the central office (CO) to the optical network units (ONUs) that are located at the customer premises and the upstream traffic from the ONUs to the CO [1-5]. Fibre based access networks not only provide huge capacity for the end users, but also provide high degree of configurability enabling efficient sharing of resources and easier network upgrades compared to conventional synchronous optical network (SONET) based networks. Moreover, wavelength division multiplexed (WDM) access networks provide a degree of transparency, where services are carried independent of the data rate and format [5-7]. There have been a number of proposals for the connectivity between CO and the ONUs such as point-to-point fibre links, active optical networks (AONs), and passive optical networks (PONs). In these schemes, PONs are preferred as the next generation optical access network architecture [8-13]. PONs are typically based on double star architectures whereby a collector or distributor device is placed at the remote node (RN). An arrayed waveguide grating (AWG) is used as this branching device for a WDM-PON [14-23], while a star coupler (SC) is used for a power splitting (PS) PON [24-29]. These architectures have been extensively investigated in providing higher and higher bandwidth in both upstream and downstream directions. As the customers and the traffic demand by the customers increase, PONs should be capable of providing the additional capacity on demand. Moreover, this infrastructure should be able to support different types of traffic between the customers to obtain a seamless connectivity between the customers. Next generation optical access network infrastructure can be built on a PON supporting variety of services on demand by the customers with added functionalities. As discussed in Chapter 1, these optical access networks should be capable of providing several advanced features such as virtual private networking (VPN) capabilities between the

20 Chapter 2 Literature Review customers, protection against fibre breaks and terminal equipment failures, dynamic bandwidth allocation (DBA) schemes, and efficient signalling mechanism. Physical architecture of the access network should be developed such that the network is capable of providing intelligent network functionalities at the optical layer as well as allowing the optical and electrical switching layers to operate in synergy [30]. As the access network evolves, the customer units gradually take control and manage the network such that the operations are distributed from the conventionally centralised network operation [31]. In this chapter, some of the previously demonstrated advanced functionalities of the next generation optical access network architecture are discussed. Section 2.2 discusses a number of previously proposed optical layer local area network (LAN) emulation schemes. The advantages and disadvantages of each scheme are analysed. Survivable optical access network architectures that were developed for the protection against feeder fibre and distribution fibre breaks as well as the equipment failures are described in detail in Section 2.3. Section presents several protection architectures proposed for WDM PON. In Section 2.4, direct sequence spread spectrum (DS-SS) is discussed. The applications of the DS-SS CDMA in the optical access networks are also presented. 2.2 LAN Emulation in PONs As the access network grows with increasing number of customers and demand for more bandwidth, added services also need to be delivered in an efficient way. For example, customers within a PON environment may require private communication links between themselves for various computer applications and telecommunication services, such as distributed data processing, broadcast information systems, teleconferencing, and interactive video games. To serve this purpose, two solutions can be found. The first one is to deploy another optical network interconnecting all customers within the PON to facilitate the customer networking. The second solution is to use the existing PON infrastructure more intelligently to provide the additional services. Deploying a separate network for internetworking amongst the customers is extremely complex especially in a densely populated area. Moreover, some other requirements such as connectivity requirements, geographic layout, inter-network topology, inter-network capacity, and network management

21 Chapter 2 Literature Review should be considered [32]. Furthermore, deploying a separate fibre network is also very costly. Therefore, using the existing PON infrastructure to provide inter-networking amongst the customers is a cost effective solution. There have been a number of demonstrations on emulating a LAN over the existing PON. A number of higher layer or router based LAN emulation schemes have been proposed to the Ethernet in the First Mile (EFM) alliance IEEE 802.3ah [33, 34]. Three solutions have been considered [35]. One of these solutions considered using higher layers protocols to deal with the LAN traffic, which is carried among the customers within a PON. The second solution proposed the use of PON tags below the media access control (MAC) layer to emulate bundle of point-to-point links. The third solution proposed the use of PON tags and a trivial reflector function at the CO to emulate a shared medium and therefore reflection all upstream traffic back to the ONUs. In the point-to-point emulation scheme, each optical line terminal (OLT)-to-ONU frame contains a logical link identifier (LLID) identifying the ONU. At CO, a logical MAC with standard MAC interface is used for each ONU to emulate a bundle of point-to-point links. Even though this scheme is compatible with higher layer, to enable multicasting to several ONUs, the frame transmission should be carried multiple times, which wastes bandwidth. In the trivial shared LAN emulation scheme, every downstream frame is received by every ONU in the PON. Every upstream frame is reflected by the CO, tagged with the originating ONU s LLID. It is received by every ONU except the originating ONU. This emulates a single shared medium. This scheme is also compatible with higher layer functions; however, any transmitted upstream frame is reflected back to the ONUs, which wastes bandwidth. In the advanced upper-layer shared LAN emulation (ULSLE) scheme, the LAN traffic is separated using the bridges and/or routers at the CO. Using the MAC address of the packet, intelligent decisions are made and the LAN traffic is rerouted back to the appropriate ONUs [35]. In all ULSLE schemes, LAN traffic is required to be transmitted to the CO on the upstream wavelength channel along with the upstream traffic to the CO and then retransmitted back to the ONUs on the downstream wavelength channel along with the downstream traffic. These schemes waste bandwidth in both wavelength channels. Moreover, the processing complexities are increased at the CO incurring additional cost. An optical layer LAN emulation can be provided whereby the LAN traffic physically redirected back to the ONUs and therefore obtaining higher bandwidth utilisation for the upstream and downstream

22 Chapter 2 Literature Review wavelength channels. Some of the previously proposed optical layer LAN emulation schemes are discussed below LAN emulation using additional transceiver and a FBG Central Office λ up FBG λ LAN Passive Coupler λ LAN λ LAN ONU ONU ONU λ LAN λ up Figure 2.1: Optical layer LAN emulation scheme using a FBG placed in the feeder fibre close to the SC with the use of an additional optical transceiver at each ONU. Figure 2.1 shows an architecture used for optical layer LAN emulation technique [36]. In this technique, two optical transceivers are used at each ONU, whereby one is used for the transmission and reception of signals between the ONU and OLT, while the other optical transceiver is used for the transmission and reception of the customer traffic that is carried amongst the ONUs within a PON. A fibre Bragg grating (FBG) device is placed in the feeder fibre close to the 1 N SC. The Bragg wavelength of this FBG device matches the wavelength of the optical transmitter used at each ONU for the transmission of the LAN traffic. Therefore, the transmitted LAN traffic on the LAN wavelength channel λ LAN that is reflected from the FBG is broadcast to all ONUs. This way, one ONU can communicate with any other ONU in the PON. As each ONU uses the same wavelength channel for the customer traffic, a MAC protocol is required to coordinate the transmissions of customer traffic. As the upstream traffic is carried on a separate wavelength ( λ U ), the transmission of customer traffic does not affect the transmissions of the upstream traffic. However, this technique requires a separate optical transmitter at each ONU to perform optical layer LAN emulation technique. Moreover, optical components such as a coarse wavelength division multiplexing (CWDM) device are also used at each ONU to separate and combine these wavelength channels. This adds to the cost of the ONU

23 Chapter 2 Literature Review LAN emulation using fibre loopback 1.5 um LAN Receiver Downstream Transmitter Feeder Fibre λ 1 λ 1 Downstream Receiver Upstream Receiver 1.5µm/ 1.3µm CWDM N x N Star Coupler λ um LED LAN Transmitter Upstream Transmitter Figure 2.2: LAN emulation scheme using broadband optical source with loopback. In the optical layer LAN emulation scheme discussed in Section 2.2.1, a distributed feedback laser (DFB) is required at each ONU for the transmission of LAN traffic. Moreover, as the LAN traffic goes through the SC twice, the wavelength channel that carries the LAN traffic experiences high loss [37]. As the required bandwidth of the LAN traffic increases, this scheme may not be able to support high bandwidth LAN traffic due to inadequate power budget. Figure 2.2 shows another proposed scheme for optical layer LAN emulation, whereby better power budget could possibly obtained by looping the LAN wavelength channel [38, 39]. In this scheme, an N N SC is used at the RN. For the transmission of LAN traffic, a light emitting diode (LED) operating in the 1.5 µm wavelength window is used. LAN wavelength channel and the upstream wavelength channel, which is in the 1.3 µm wavelength window are transmitted in the upstream direction and the LAN wavelength channel is separated from the upstream wavelength channel using a 1.3 µm/1.5 µm coarse wavelength division multiplexing (CWDM) coupler placed at the SC. Several loop-backs are used to interconnect the ports of the SC facing the CO. Using the loop backs, the LAN wavelength channel is distributed to all ONUs in the PON. As the number of loop backs increases, the received power at an ONU also increases. Therefore, higher bandwidth LAN traffic can be supported. Moreover, the use of a thermally stable FBG in the passive plant is avoided. However, this scheme also requires an additional optical source at each ONU. As LAN traffic

24 Chapter 2 Literature Review signals reach an ONU through multiple loop backs, the detected LAN traffic experiences penalty due to beat interference at the photodetector (PD) LAN emulation using a common regenerator Common Regenerator λ R Central Office Feeder Fibre 2 x N Star Coupler WDM Coupler ONU N ONU 1 λ R + λ d Receiver λ u Transmitter Figure 2.3: Optical layer LAN emulation scheme using a common regenerator and a common receiver at the ONU for the reception of both downstream data and customer traffic. Figure 2.3 shows an architecture for an optical layer LAN emulation scheme that uses a common regenerator in addition to the conventional PON [40, 41]. This regenerator enables the services to be distributed all the way to the users over the PON after necessary signal processing, which includes amplification, reformatting and appropriate encoding. The upstream signals that consist of traffic to the CO and the customer traffic to the other ONUs are transmitted in the upstream wavelength channel. At the common regenerator, the customer traffic is remodulated on to a RF subcarrier signal and optically modulated onto a wavelength channel in the 1.5 µm wavelength window. As the transmission distances from the common regenerator to the ONUs are relatively shorter, low cost optical sources such as Fabry-Perot laser diodes (FP-LDs) can be used. The customer traffic on wavelength channel λ R and downstream traffic from the CO on wavelength channel λ d are received at the ONU using the same optical receiver. The RF subcarrier frequency that is chosen to carry the customer traffic is larger than the bandwidth of the downstream traffic. This scheme requires cheaper optical sources at each ONU for the transmission of upstream and customer traffic. However, a

25 Chapter 2 Literature Review common regenerator is required close the SC. The common regenerator increases the cost of the network for installation and maintenance. Moreover, higher bandwidth optical receiver is required for the reception of both signals and upgrading the transmission bit rates in the network may requires the optical components being replaced at the ONUs and the common regenerator LAN emulation using optical switches at ONU CO Upstream λ u Receiver WDM Downstream Transmitter λ d Feeder Fibre N+1 x N+1 Star Coupler Distribution Fibres WDM λ u λ d 4 3 λ u 2 1 OSW Burst mode Receiver 2 1 OSW 2 Upstream Transmitter ONU 1 Figure 2.4: LAN emulation using dual distribution fibres and optical switches placed at each ONU. Figure 2.4 shows the LAN emulation architecture using dual distribution fibres and optical switches placed at each ONU [42]. A N N SC is placed at the RN with an isolator at each input port except the one that is connected to the feeder fibre. Each ONU has two optical switches for switching the mode of operation. In the normal mode of operation, PON system has the OSW 1 set in the cross state and OSW 2 in the bar state. The CO sends downstream signals on wavelength channel λ d and the signals are split into N+1 portions and broadcast to all N ONUs. Since OSW 2 is in the bar state, the common receiver at each ONU receives the downstream data. The upstream signal on wavelength channel λ u goes through OSW 1 in the cross state and received at the CO. Optical isolators at the SC block the upstream signal from coming back to the ONUs. When OSW 1 is set in the bar state and OSW 2 is set in the cross state, the PON system is considered to be in the local mode. In this mode of operation, the optical signal on λ u is passed to the SC and redirected to each ONU. The downstream signal on λ d also reaches OSW 2, but is routed to the other port, which is anti-reflection treated. So the common receiver at the ONU receives signals from all ONUs including its own ONU, but

26 Chapter 2 Literature Review not from OLT. In this LAN emulation scheme, LAN traffic and the downstream traffic cannot be received at the same time. This makes the used MAC protocol for the transmission of both signals very complex LAN emulation using dual distribution fibres Redirection of upstream signals Upstream Receiver Downstream Transmitter λ u Feeder Fibre CO WDM 1.3/1.5 µm λ d Distribution λ u Fibres N+1 x N+1 Star Coupler Terminated unused port LAN data Receiver Downstream Receiver WDM Upstream 1.3/1.5 µm Transmitter ONU 1 ONU N Figure 2.5: LAN emulation architecture using dual distribution fibres. Figure 2.5 illustrates the proposed LAN emulation architecture [43, 44]. Each ONU is connected to the SC using two distribution fibres such that the transmitted upstream signals from an ONU are redirected to each ONU through the second distribution fibre. PON uses the carrier-sense multiple access with collision detection (CSMA/CD) protocol as its upstream multiple access scheme [25]. Using PON tags assigned to each ONU, differentiated services are allocated to the ONUs. As the upstream wavelength channel is used for the transmission of both upstream traffic and LAN traffic, higher bandwidth utilisation is not obtained. Moreover, complex filtering mechanisms are required to separate the LAN traffic from the upstream traffic at each ONU. So far, a number of PON architectures incorporating optical layer LAN emulation are discussed. The advantages and disadvantages of each scheme are also explained. 2.3 Survivable Optical Access Network Architectures Several architectures and technologies are developed for optical access networks not only for obtaining higher transmission rates, but also at provisioning services effectively and reliably

27 Chapter 2 Literature Review to the customers. Next generation customer access networks must be able to deliver services based on customer demand and with guaranteed service requirements as agreed with the service provider. The use of optical networking in high data rate LANs such as storage area networks (SANs), high-end enterprise networks, application service providers, and web hosting sites have continually increased. Therefore, reliability of these optical access networks has become paramount important for the provision of such new value added services. In optical access networks, aggregated high speed traffic is transmitted between the customers and the CO and therefore a failure in network elements can cause serious problems. The reliability and the robustness of the access networks depend on the architectures and the topologies that were used in the design and implementation of the network. Providing protection against fibre network failures at any level is expensive due to the high cost associated with equipments and components. Survivable architectures in the optical backbone networks have been extensively studied [45-48]. Protection architectures are usually categorised in terms of their path availability, response time and complexity. Faster and simpler methods such as N+1 link protection methods, whereby additional dedicated link is used to protect N links in a shared manner, were usually used since these protection systems reduces the unavailability of the services with minimal cost [49]. In any protection scheme chosen for the deployment, it must be able to provide the requirements of the specific network with a capability to migrate to higher traffic demand with more flexibility. However, these architectures and techniques applied to the backbone networks are neither directly applicable nor equally effective for cost-sensitive customer access networks. The design and implementations of an optical access network with reduced protection costs while maintaining acceptable level of protection have become a challenge. There have been several survivable architectures explored over the years. These include automatic protection switching with diverse protection (APS/DP), self healing rings, and dynamically path rearrangeable mesh architectures [50]. The survivable network architectures can be divided into two categories and they are dedicated facility restoration and dynamic facility restoration. Dedicated facility restoration uses the dedicated facility for the service restorations while the dynamic facility restoration uses the spare capacity within the working facilities for service restoration. APS and rings fall within the dedicated facility restoration category while the dynamic path rearrangeable mesh architectures and dual homing fall within the dynamic facility restoration category. There are tradeoffs between the flexibility, system complexity and the additional spare capacity required for each restoration category [51, 52]

28 Chapter 2 Literature Review Ring based protection schemes Failure Failure (a) (b) Figure 2.6: Protection in a ring network whereby (a) shows unidirectional path switched ring and (b) shows bidirectional line switched ring. Survivable ring networks are rapidly filling up the gap for the fibre network survivability from the backbone network to the access network. Many of the metropolitan area networks (MANs) and LANs have used ring topology as the building block for reliable communications. Ring networks can recover from the failure using path protection or link/node recovery [50]. Figure 2.6 illustrates both path protection and link/node recovery protection in a ring network. Figure 2.6(a) shows the path protection technique. As a node fails, the connection in the clockwise direction is disrupted. Therefore, a connection in the anticlockwise direction is formed to provide connectivity to the affected nodes. The protected operation is similar to the unidirectional path switched rings (UPSRs) in synchronous optical network (SONET). Figure 2.6(b) shows the link/node rerouting technique, whereby the clockwise connection is redirected to the anticlockwise direction by the upstream node after the failure. The protection can also be made for the fibre cable failures as well. This type of 1:1 diverse protection architecture is totally automatic and provides restoration capability for equipment failures and fibre cable breaks. These types of ring networks can be overlaid on a physical mesh network providing service protection and restoration. However, in a physical network, a ring overlay may not exist. Even if one exists, it may be too large leading to several issues such as larger propagation delay, increased jitter etc. Mesh architecture is not usually preferred for the deployment of the optical access networks. Therefore, the use of

29 Chapter 2 Literature Review overlay ring networks for the protection and restoration of optical access networks is not widely used as they require costly over-building of the network Redundant tree protection schemes The vast majority of the optical access networks uses star and tree architectures for deployment, as these topologies provide low propagation delays between the CO and the customer network units [53]. Star architectures present many weaknesses in terms of reliability. A network element failure in the CO could potentially affect the operation of the entire network. Breaks in both in feeder or distribution fibre cables could lead to one or more customers being denied of network access. If an amplifier is placed in the links, the failure of these devices could disrupt the operation of the network. For the star networks, providing recovery through 1+1 can be made, whereby the entire system can be replicated. Moreover, several scenarios can be followed to provide only the fibre cables or the equipments at the end terminals. Dual homing approaches can be adopted, whereby each ONU is connected to the CO using dual fibre access. The redundancy of the tree based PON can also be obtained by overlapping two independent diverse routed trees [52]. In this scheme, an optical switch is placed at each customer premise to permit the customer to choose one of the two diverse routed paths. The internal nodes of the networks are assumed to be very reliable while the links are subject to failure. Node Figure 2.7: Redundant tree architecture

30 Chapter 2 Literature Review Figure 2.7 shows a fault tolerant tree architecture that can be applicable for the access networks. In normal operation, the head end transmits on one or both inputs and a station could receive on either output. The outputs can be identical although the different propagation delays through each path may cause different delays. If a customer terminal fails to receive signal from one output, it switches to the other output. This failure is also reported to the network control centre. If neither output is operational, then the customer terminal is cutoff from the network and therefore waits until links are repaired. The probability of both paths to a customer terminal is cut is low; however this topology introduces additional 3 db loss in the optical coupler in each level. Moreover, one path to a customer terminal may be longer and therefore resulting in higher losses in the transmission path. So, this redundant scheme suffers from inadequate power budget if the customer terminals are connected in many levels Protection in PS-PONs PONs are usually based on double star architectures and therefore are vulnerable to fibre link cuts. Full services access network (FSAN) group considers protection of the APON (ATM PON) systems in its proposals. In ITU T G.983.1, four protection schemes have been defined [54-58]. These architectures propose solutions against feeder and distribution fibre link cuts and equipment failures at the optical line terminal (OLT) and ONUs. Most of the solutions proposed consider duplicating the systems. Depending on the level of protections, the architectures and therefore the incurring cost are varied. OLT PON 2 x N SC Feeder Fibre Optical Switch ONU 1 ONU N Figure 2.8: Protection architecture 1, whereby feeder fibre is protected against breaks

31 Chapter 2 Literature Review Figure 2.8 shows a proposed protection architecture against feeder fibre breaks [54-58]. In this protection scheme, a spare fibre is equipped between the OLT and the SC. The PON interface at the OLT is capable of detecting the fibre cut and then can switch the connection to the spare fibre. During the protection switching, signal loss is inevitable. As the upstream transmission is based on the time division multiple access (TDMA) protocol, to support the multipoint to point communications, re-ranging of all connected ONUs are required as the distances from the OLT to the ONUs are changed through the spare protection fibre. However, this scheme requires no redundant equipment used in the OLT and the ONUs. To implement this protection scheme, an 1 2 optical switch at the OLT along with a 2 N SC is used. Splitter OLT PON (P) Feeder Fibre ONU 1 PON (W) ONU N Figure 2.9: Protection architecture 2, whereby feeder fibre and interfaces at the OLT are protected by duplicating the systems. Figure 2.9 shows the protection architecture 2 proposed by FSAN group. This scheme requires a redundant PON protection interface (PON-P) in cold-standby mode in the OLT in addition to the PON working interface (PON-W). To perform switching between the interfaces, a selector is required at the OLT to switch between the working and protection interface modules. However no redundant parts are used at the ONUs. The failure in the working PON-W interface at the OLT and a cut in the feeder fibre between the OLT and the splitter cause the activation of tree protection switching. As before, the signal or packet frame loss is inevitable during the protection switching. Compared to protection architecture 1, this architecture provides additional protection against the failure of the PON-W at the OLT

32 Chapter 2 Literature Review Splitter OLT PON (W) PON (P) Feeder Fibre PON (W) PON (P) ONU... PON (W) PON (P) ONU Figure 2.10: Protection architecture 3, whereby the failure against feeder and distribution fibre, splitter and interfaces at the OLT and ONUs are protected. The protection architecture 3 proposed in FSAN is illustrated in Figure In this scheme, OLT and ONUs are equipped with redundant modules. Hot standby circuits are placed at the OLT and the ONUs making hitless switching possible in an event of a failure in the equipment and fibre cuts. To perform hitless switching, constant synchronisation is carried out for each ONU. Tree switching is performed at the OLT in an event of a failure in the interface at the OLT and a distribution fibre cut. This mechanism is quite simple in terms of operation. However, there might be ONUs that do not have a redundant interface for the associated OLT. Therefore, in tree switching, as one distribution fibre is cut, the entire operation of the PON both at the OLT and ONUs are switched to the protected mode. Therefore, the ONUs that do not have a backup link cannot communicate with the OLT even though it has no link failure. Individual PON interface failures at the ONU can be recovered by single branch switching. Branch switching mechanism can potentially solve the problems associated with the tree switching mechanism. In this mechanism, only the ONU with the failed link switches to the backup link rather than entire network as in tree switching. Therefore, this protection mechanism can be independent of the operation of other ONUs. Operation, administration, and maintenance (OAM) message sequences are used between the OLT and ONUs for the setup and operation [59]

33 Chapter 2 Literature Review Splitter OLT PON (W) SW PON (P) Protection Path Recovery Path PON (W) PON (P) ONU... Working Path PON ONU Figure 2.11: Protection architecture that is extended version of architecture 3 to support various protection levels. The protection architecture 3 can be modified to provide various protection levels to each customer by providing separate links for working, protection and recovery. An optical switch is placed at the OLT to the feeder part of the protection path and another feeder fibre for the recovery path to the protection path as shown in Figure This scheme is capable of supporting suitable network protection for an ONU with a special protection demand. This configuration as shown above enables each ONU being protected from the failure of the OLT interface, feeder and distribution (branch) fibres. It is also shown that this architecture can be less expensive than the protection architecture 3. Figure 2.12 illustrates the protection architecture 4. This scheme is an extension of the architecture 3. A redundant optical distribution network (ODN) is implemented on the existing protection scheme at the OLT and the ONUs. As the ODN is protected, multiple point failure can be protected. As the redundant modules in the network are increased, the reliability is also increased; however the complexity and the management of the network are also increased with a requirement of re-ranging of all ONUs from the OLT

34 Chapter 2 Literature Review 1:2 Splitter Feeder Fibre 2:N Splitter PON (W) PON (P) ONU OLT PON (W) PON (P)... 1:2 Splitter 2:N Splitter PON (W) PON (P) ONU Figure 2.12: Protection architecture 4. Architecture 1 Architecture 2 Architecture 3 Architecture 4 Required Components Unprotected interfaces at OLT and ONU Protected OLT, Unprotected ONU All protected All protected Duplicated Region Feeder fibre OLT interface and feeder fibre OLT, ONU interfaces, splitter, feeder and branch fibres Architecture 3 + independence redundance of the of the fibres Recovery Region Feeder fibre cut OLT, ONU interface, feeder fibre failure All parts of PON All parts of PON Protection 1:1 1:1 1+1, 1:1 1:1 Fast Switching No No Yes No Flexibility Bad Bad Good Good Cost Low Normal High Very High Table 2.1: Summary of the protection architectures proposed by FSAN

35 Chapter 2 Literature Review Table 2.1 gives a summary of the protection architectures described above showing all the advantages and disadvantages of each protection scheme. FSAN recommends that protection architectures 2 and 3 are most suitable for fibre to the building (FTTB) deployments while protection architectures 1 and 2 are more suitable for fibre to the home (FTTH) deployments. Among all four protection architectures, schemes 3 and 4 require higher cost as all parts of the PON such as the interfaces at the OLT and ONU, feeder and distribution fibres, and splitters are duplicated. Architecture 3 could possibly provide faster switching due to 1+1 protection scenario, whereby hot standby circuits are placed at the OLT and ONU and therefore enabling hitless switching. Other architectures follow 1:1 protection, whereby the fibre link and equipments at the OLT and ONUs are changed using cold standby circuits, which consumes time due to ranging all ONUs from the OLT. 2:N Splitter Switch PON.. ONU OLT PON (W 1 ) PON (W 2 ) PON (W 3 ). PON (P) PON PON.. ONU ONU APS Control Signal PON ONU 2:N Splitter Figure 2.13: Protection architecture 5. Figure 2.13 shows a protection architecture for PON, whereby the PON interfaces are protected with 1: N protection scheme [55, 56]. In this scheme, the PON interfaces are divided into several groups and each group of interfaces are protected with 1: N scheme whereby N depends on the vendor specific implementation and optical switch characteristics. So, one PON interface is assigned for protection mode at the OLT. This module can be installed in any slot in the OLT interfaces. All the PON interface information including PON

36 Chapter 2 Literature Review ID, ONU s passwords, ranging intervals, and current alarm status are stored in a common control card (CCC) to supervise protection switching. If a working PON module fails, its information stored at the CCC will be transferred to the protecting PON module to perform APS. The switching time is restricted such that connections on the affected PON interface module will not be dropped. If more than one PON interface module in a group fails at a time, then the PON interface with more traffic flow or with higher priority will be switched to the protecting interface module. This scheme is more economical compared to protection architecture 3 described before. However, it is difficult to realise protection time of less than 50 ms since re-ranging is required. This scheme is suitable to FTTH systems only. SC ONU 1 SC ONU 2 OLT Figure 2.14: Protection architecture 6. Figure 2.14 shows the protection architecture 6 of a PON that delivers resilient fast protection switching at full bandwidth usage [56]. In normal state, data are transmitted on both east and west paths. Therefore 100% of the available capacity is used. When a branch fibre is broken, OLT detects the failure and switches the data to the protected path. This protection mechanism enables the data transmissions being carried with higher QoS requirement. The fast protection switching is performed at the physical layer and realised by hardware and therefore the switching time is faster. This protection scheme enables higher reliability, while the failures in the branch fibres and ONUs do not affect the operation of the entire network. Moreover, close to 100% of the capacity on the PON can be used to deliver services. Moreover, this scheme enables that the protection scheme functions can be installed on customers demand and therefore making the operation independent of each other

37 Chapter 2 Literature Review OLT 1 Feeder Fibre 2 x N SC ONU 1 OLT 2 FTTH Switch ONU N Figure 2.15: Protection architecture using a switch at CO and redundant fibres. There are similar protection architectures proposed for the protection against feeder fibre breaks using a switch at the CO and redundant fibres between CO and SC as shown in Figure 2.15 [60, 61]. In this scheme, no cold standby equipment is used. Protection is provided by rescheduling time slot. Half of available time slots of ONUs served by working OLT are distributed to ONUs served by failed OLT(s). PONs share the available bandwidth during the failure time. Proposed protection control unit stores interface information like PON ID and ranging interval of ONUs. It also controls the protection optical switch that is installed at CO. CO Fibre Break RN Ring 1 RN Ring 2 RN ONU ONU Figure 2.16: Protection of a star-ring architecture using active optical switches at RN. Figure 2.16 shows a self healing star-bus-ring architecture that is proposed for high capacity RF subcarrier multiplexed broadband subscriber networks [62-65]. In these schemes, the transmissions are performed using RF subcarrier multiplexing and the ONUs are interconnected in a ring topology as shown. RN contains CWDM couplers, optical switches

38 Chapter 2 Literature Review and control circuits to change the operating state of the switch. If a failure of a feeder fibre is detected, then the state of the optical switch is changed such that the signals are carried through next feeder fibre. However, in this scheme, RNs require electrical power to operate the optical switches, making the network active. Moreover, the signals are regenerated at each ONU and the cost and operational complexities at each ONU are increased. This scheme is suitable for low bit rate transmissions from the ONUs only. As the transmission bit rate of the signals increases, then the optoelectronic components of all other ONUs have to be changed. PS-PON tree infrastructure Partial CWDM upgrade PS-PON tree CWDM overlay Full CWDM upgrade CWDM PON DWDM upgrade DWDM PON Installing disjoint paths Installing disjoint paths PS-PON tree Protection for selected nodes DWDM Upgrade WDM-PON Disjoint paths Figure 2.17: Evolution paths for protected PONs. Figure 2.17 describes an evolution path for the PONs. The PS-PON infrastructure can be upgraded to a CWDM PON and then the CWDM PON can be upgraded to a DWDM PON. The tree structured PS-PON architecture can gradually be changed into a ring architecture, whereby the RNs are replaced with CWDM devices and these RNs can be interconnected. Gradually DWDM components can be inserted and a complete DWDM ring architecture protecting all ONUs can be obtained [66-67]. So far, the protection architectures against the equipment failures at the OLT and ONUs and fibre cut in a PS-PON were presented

39 Chapter 2 Literature Review Protection in WDM-PONs Similar to the protection architectures for the PS-PONs, protection against feeder and distribution fibre breaks have also been developed for WDM-PONs. As an AWG is used as a branching device at the RN, and each ONU is allocated a set of wavelength channels for the transmission of signals, these architectures vary from those developed for PS-PONs Protection for WDM-PON using dual fibres CO Downstream Receiver WDM Upstream Transmitter 3 db Coupler 1 x N AWG Working Feeder Fibre Protection Feeder Fibre Distribution Fibres N x N AWG 2 1 Control Electronics Optical Switch ONU 1 Downstream Receiver WDM Upstream Transmitter ONU N-1 Figure 2.18: Protection architecture for a WDM-PON using dual fibres. A protection capability with additional deployment of feeder fibre and distribution fibre for a PON can be implemented for a WDM PON. Figure 2.18 shows the protection architecture for a WDM-PON that uses dual distribution and feeder fibres. A 1 N AWG is placed at the CO for the multiplexing and demultiplexing of the downstream and upstream wavelength channels. An N N AWG is placed at the RN and this AWG has the same free spectral range (FSR) as the AWG placed in the CO. The AWG placed in the RN is connected to the CO using two feeder fibres as shown in Figure 2.17, whereby the working feeder fibre is connected to one of the port of the AWG facing towards the CO, while the protection feeder fibre is connected to another port of AWG facing the ONUs. The proposed network could support a maximum of ( N 1) ONUs. The upstream wavelength channel and downstream wavelength channel for an ONU is spaced by some FSRs of the AWGs, thus one port of the AWG can support the transmission and routing of the two bidirectional wavelength channels. The WDM coupler placed at the CO and the ONU can separate these wavelength channels. At

40 Chapter 2 Literature Review each ONU, a 1 2 optical switch is placed such that once a failure in the working distribution fibre or feeder fibre is detected, the transmissions of signals are automatically changed to the protection path. In this protection architecture, if the working distribution fibre of an ONU is cut, then only that particular ONU performs transmissions of signals in both directions through the protection path, while the other ONUs carry out their operation in normal working path. If the working feeder fibre fails, then the transmissions of all ONUs are switched simultaneously to the protection path Protection for WDM-PON using interconnections between ONUs OLT 1 x N AWG A 1, A 2,. A N ; C 1, C 2,. C N B 1, B 2,. B N ; D 1, D 2,. D N R/B Filter R/B Filter A 1 B 1 D 1 B 1 D 1 C 1 ONU 1 ONU 2 WDM WDM Do wnstream R eceiver Upstream Transmitter Downstream Receiver Upstream Transmitter Figure 2.19: Protection architecture against distribution fibre cuts in a WDM-PON In this PON architecture, protection against distribution fibre cuts can also be carried out by grouping two ONUs and interconnecting them [69-71]. Using the FSR of the AWG placed at the RN, upstream and downstream wavelength channels to a group of ONUs use only one port of the AWG for the transmissions. There have been two different protection architectures that were experimentally demonstrated against the distribution fibre breaks. One architecture demonstrates distributed fast protection switching, whereby the switching of the signals to the protection path are performed at the ONUs using an optical switch [69, 70]. The second

41 Chapter 2 Literature Review architecture demonstrates a centrally controlled protection switching capability, whereby an optical switch placed at the CO performs the switching of signals in an event of a failure in the distribution fibres [71]. Figure 2.19 shows the protection architecture against distribution fibre breaks for 2N ONUs. The RN consists of a 1 N AWG and 3 db couplers to route the wavelength channels to two ONUs. As can be seen, each port of the AWG connects two adjacent ONUs that are interconnected using a fibre. For each ONU, two separate wavelengths are assigned for the upstream and downstream transmissions and these wavelength channels are separated using the FSR of the AWG. FSR FSR FSR FSR Upstream Wavelengths Downstream Wavelengths Upstream Wavelengths Downstream Wavelengths A 1 A 2 A 3 A 4 B 1 B 2 B 3 B 4 C 1 C 2 C 3 C 4 C 1 C 2 C 3 C 4 Blue band Red band Figure 2.20: Wavelength assignment, whereby upstream and downstream wavelengths are separated by FSR of AWG. Figure 2.20 shows wavelength assignment scheme for the ONUs in this protection architecture. The failure of one distribution fibre of an ONU leads to the transmissions to the affected ONU being carried out using the connected adjacent ONU and therefore four wavelength channels are carried through a single port of the AWG. Therefore, all four wavelengths are separated using the FSR of the AWG. In normal operating state, the downstream wavelength channels B and D are delivered to both ONUs in the same group. i i In ONU1, wavelength channel that is in the blue band is filtered by the red/blue (R/B) B i filter, while wavelength channel Di that is in the red band is filtered at ONU2. For the upstream wavelength channels, wavelength channel Ai from ONU1 and wavelength channel Ci from ONU2 pass their R/B filters and their respective links. These wavelength channels are combined at the 3 db coupler at the RN and fed to the same out put port of the AWG. The WDM coupler at each ONU is used to separate and combine the upstream and downstream

42 Chapter 2 Literature Review wavelength channels of an ONU. In normal state, as each ONU is connected to the CO through their respective distribution fibres, interconnection of both ONUs in the same group is not utilised. Assume the distribution fibre that connects ONU 2 is cut. The ONUs are independently capable of detecting the fibre cut using the loss of power. The operating state of the optical switches at ONU 2 is changed. In this protected state, the upstream wavelength channel of Ci of ONU2 is rerouted through the interconnected link. At ONU 1, using the power meter, the presence of upstream signal of ONU 2 is detected and the optical switch at ONU 1 is reconfigured such that the upstream wavelength channel C i of ONU2 is transmitted to the CO through ONU 1. Using this way, the signal transmissions to and from ONU 2 are restored. Similarly, the protection against a break in the distribution fibre of ONU 1 can be supported by ONU 2. Even though the signal transmissions can be restored using the interconnected adjacent ONU, the signals experience additional loss in the protected state. In normal state, the signals to an ONU go through only one optical switch, while in protected state, the signals go through three optical switches resulting in higher loss Centrally controlled protection switching in WDM-PON Tx & Rx Optical Isolators Optical switch Fiber 1 Fiber 2 2 x N AWG ONU 1 1 ONU 2 1 ONU 1 2 ONU 2 2 Figure 2.21: Centrally controlled protection architecture against distribution fibre breaks in a WDM- PON. In the protection architecture described in Figure 2.20, two optical switches are required at

43 Chapter 2 Literature Review each ONU for resilient fast protection switching. This potentially increases the cost of the ONUs. Moreover, in protected state, the signals in both directions go through additional optical switches and optical taps and therefore the required power budget margin may not be satisfied to obtain error-free transmission for high data rate signals. Therefore, to reduce the cost and complexities associated with this protection architecture, a centrally controlled protection architecture is demonstrated that simplifies the ONU design with reduced network resources. Figure 2.21 shows the centrally controlled protection architecture for a WDM-PON against distribution fibre breaks. At the OLT, two optical isolators with opposite directions and a 2 2 optical switch are placed. At the RN, a 2 N AWG is placed and the spectral transmission peaks of the two ports are spaced by half of the FSR of the AWG. Two adjacent ONUs are assigned to a group as shown in the figure and each ONU is connected to a specified port of the AWG through a distribution fibre. The distribution fibres to each ONU in the same group are diversely run. The fibre connection to each ONU is attributed to a proposed wavelength assignment plan. Wavebands A and B are in the blue band of the R/B filter and the wavelengths in these bands are allocated for the downstream and upstream channels respectively for particular ONUs in the groups. Wavebands C and D are in the red band of the R/B filter and the wavelengths in these bands are allocated for the downstream and upstream channels respectively for the other ONUs in the groups. Figure 2.22 illustrates the structure of each group in both normal and protected states. At ONU 1 i, a coupler is used to combine the blue band outputs from ONU 1 i with that from ONU 2 i. A WDM coupler is used at each ONU to separate and combine the upstream and downstream wavelength channels of an ONU. The structure of ONU 2 i is similar to that of ONU 1 i except for the fact that red band outputs of two ONUs in the same group are combined. In normal state, the optical switch is set to bar state such that the downstream and upstream transmissions are carried through fibre 1 and fibre 2 respectively. Due to the cyclic property of the AWG and the proposed wavelength assignment plan, all the downstream wavelength channels are routed to ONU 1 i. The R/B filter at ONU 1 i separates the wavelength channels for the ONU 1 i from that of ONU 2 i. The upstream channels at each ONU are split by the 3 db coupler and delivered to the CO through two different paths. As the isolator is used at the CO, only the upstream wavelength channels passing through the left distribution fibres reach CO while the other path is blocked. It can be said that only half of the distribution fibres and the corresponding interconnecting fibres are used for the transmission of signals. In an event of a

44 Chapter 2 Literature Review failure in the distribution fibre, the operating state of the optical switch is changed to cross state. In this state, fibre 1 becomes the upstream path while fibre 2 becomes the downstream path. Therefore, all signal transmissions are switched from the working fibre to the protection fibre. An additional advantage of this protection scheme is that the interconnecting fibre can also be protected against fibre breaks. This centrally controlled protection scheme uses only one optical switch compared to N optical switches in the distributed protection switching scheme. However, a disadvantage of the scheme is that each ONU receives signals of the adjacent ONU and therefore the security of the signal transmissions in the WDM-PON is reduced. A, C B, D B, D ONU 1 1 B R/B Filter R/B Filter Blue Red Blue Red 3 db coupler 3 db coupler WDM A B C D WDM D ONU 2 1 Downstream Receiver Upstream Transmitter Downstream Receiver Upstream Transmitter (a) A, C B, D ONU 1 1 R/B Filter R/B Filter Blue Red Blue Red 3 db coupler 3 db coupler WDM A B C C WDM D ONU 2 1 Downstream Receiver Upstream Transmitter Downstream Receiver Upstream Transmitter (b) Figure 2.22: Operation of ONUs in (a) normal state, (b) protected state for the centrally controlled protection architecture for WDM-PON. The protection architectures against distribution fibre cuts in a WDM-PON that are discussed above can be expanded into a star-ring architecture as well. Rather than grouping two ONUs

45 Chapter 2 Literature Review to provide protection capabilities against distribution and feeder fibre breaks, the ONUs can be continuoisly interconnected to form a ring architecture on top of the existing star architecture [46] Feeder fibre protection scheme for WDM-PON In the self protecting architecture for WDM-PON, two separate wavebands can be used for the protection against feeder fibre breaks [73]. Using the cyclic properties of an AWG, the upstream and downstream wavelength channels can be allocated within a single waveband. Figure 2.23 shows the architecture of a WDM PON with the protection capability against feeder fibre breaks. The OLTs that serve each WDM-PON are interconnected through the optical switch. The AWGs used at the OLTs and the RNs have similar cyclic properties. The CWDM filters placed at the OLTs and the RNs separate and combine the wavebands that are allocated to each PON. Additionally, each RN consists of a 3 db coupler to complete the transmission path in normal and protected states. Downstream Transmitter OLT Upstream Receiver AWG 1 Feeder Distribution Fiber 1 3 db Control Fiber CWDM CWDM Coupler 1 2 Coupler 1 Coupler Downstream Receiver 1 3 Optical Upstream AWG 3 Switch Normal path for Transmitter ONU transmission for 1 PON 1 Optical Line Terminal 2 AWG 2 Control Optical Switch Feeder Fiber 2 To other PON CWDM CWDM Coupler 3 Coupler 4 Protection path for transmission for PON 1 Figure 2.23: Protection achitecture against feeder fibre breaks in WDM-PON using the CWDM overlay. In nomal state, the transmissions to each PON are carried directly using the respective feeder fibres. For WDM-PON 1, transmissions between OLT 1 and the ONUs are carrried through

46 Chapter 2 Literature Review feeder fibre 1, while for WDM-PON 2, transmissions between OLT 2 and the ONUs are carrried through feeder fibre 2. In an event of a break in the feeder fibre 1, the optical switch at OLT 1 is changed to cross state such that transmissions in both directions for the WDM- PON 1 are carried through feeder fibre 2. the CWDM filters 3 and 4 are used to combine and separate the wavelength channels of WDM-PON 1 from that of WDM-PON 2. The protection scenario can be completely different for the greenfield FTTH networks. The installation of ring based fibre infrastructure enables easier upgrade path towards protected networks [67]. There are variety of optical ring protection schemes that can be utilised [5, 74-78]. Amplified PON technologies have been proposed to support larger number of customers using a single fibre PON infrastructure over longer transmission distance [79-84]. These superpon systems propose solution for redundant strategies for these systems. Most of the devices and the links are duplicated (1+1 protection) to provide protection for the entire superpon system. 2.4 Direct Sequence Spread Spectrum Binary data Pseudo Random Bit Stream XOR Direct Sequence Spread Spectrum Data PRBS 1 0 Power Power frequency DSSS Power frequency frequency Figure 2.24: DS-SS modulation: A high chip rate pseudo-random bit stream is multiplexed with a low bandwidth data to generate a broader bandwidth DS-SS signal. Direct sequence spread spectrum (DS-SS) is one of the most popular transmission techniques within the code division multiple access (CDMA) scheme. In DS-SS, the signal is spread over

47 Chapter 2 Literature Review a bandwidth much wider than the original signal bandwidth. Figure 2.24 shows how the DS- SS signal is generated. The binary data stream (user data) is used to modulate a pseudorandom bit stream (PRBS). The PRBS signal is faster than the user data rate and the bits of the PRBS are called chips. The ratio between the chip rate ( ) called the processing gain equation 2.1. B and the data rate is DS SS B data ( G p ) or the spread ratio of the spread DS-SS signal as shown in G p B DS SS = Equation 2.1 B data As can be seen from Figure 2.24 the bandwidth of the spread signal is the same as that of PRBS. The DS-SS technique allows a method to share the same bandwidth among many users as different chips are used for the coding of the data. At the receiver, the user correlates its received signal with a known PRBS while other signals are filtered out. DS-SS technique results in a signal which is very hard to distinguish from background noise unless the PRBS that is used to generate the signal is known. Therefore, this DS-SS multiplexing scheme provides inherent security for the transmitted signals. The use of DS-SS techniques allows several benefits to the customer access networks with improved performance because the transmission and noise characteristics are much more easily controlled in this environment. Some of the benefits of the DS-SS in a LAN environment are listed below. Contention free concurrent channel access for a given number of users Immediate and unlimited access to the channel Asynchronous transmission requiring no centralised control The ability to support many different types of users in terms of data rates The ability to add new stations easily subject to spreading sequence considerations Noise and interference rejections

48 Chapter 2 Literature Review Performance monitoring using DS-SS Using the multiple access capability and the noise and interference rejection capability of the DS-SS, optical performance monitoring techniques for the transmission systems have been developed [85-91]. λ 1 λ 2 Tunable optical filter 10:90 coupler Demodulation & despreading DS Signal λ 3 DS signal Baseband data Figure 2.25: Using DS-SS for signal monitoring purposes. Figure 2.25 shows a simple setup for monitoring the signal path and the signal-to-noise (SNR) of the signals carried on a particular wavelength channel. DS-SS signal can be digitally generated and electrically or optically combined with the payload data of the wavelength channel. As a unique electronic code is assigned to each signal path, the path of signals can be easily identified. For monitoring the signals, using a low bandwidth PD and all the DS-SS signals are detected. The detected DS-SS signals are then demodulated to recover the data. DS-SS enables simpler detection circuit and smaller amount of required optical power for monitoring the wavelength channels ONU authentication in PON DS-SS signal transmission shows good performance in the presence of high interference and noise. Figure 2.26 shows an ONU authentication technique to identify a malicious ONU in a PS-PON [92, 93]. In a PS-PON, the transmission in the upstream direction from each ONU follows TDMA protocol and therefore each ONU performs transmissions in the allocated time slots. However, a malicious ONU could flood light in the upstream direction in other ONUs transmission period. This process could impact the performance of the PON. Therefore, to identify the malicious ONU, DS-SS signals on a RF carrier was sent to all ONUs and looped

49 Chapter 2 Literature Review back. Using digital signal processing, the malicious ONU could easily be identified. Results show that even with low SNR, low bandwidth DS-SS signals are recovered error free. 1.5 um test signal command ONU OLT 1.3 um loopback signal Star Coupler Disturbing light FPGA WDM Passive coupler ONU Transmitter 3.91 Mb/s x 62.5 MHz subcarrier Receiver Correlator A/D BPF Figure 2.26: ONU diagnostic method using DS in PONs Upstream access in PS-PONs The upstream transmission from an ONU to the CO in a PS-PON can be performed using several multiple access techniques such as TDMA, subcarrier multiple access (SCMA), and CDMA. TDMA is the most commonly used protocol for the upstream access [94-96]. A detailed description of TDMA will be provided in Section of Chapter 5. SCMA is also a popular upstream access scheme in PS-PONs [97-100]. The advantages and disadvantages of the SCMA scheme are also described in Section of Chapter 5. DS-SS CDMA has also been used for the transmission of signals in LANs [ ]. In these schemes, data can be electronically coded. However, external modulators such as lithium niobate (LiNbO 3 ) modulators have also been employed as optical AND gates [104, 105] to impress a code on a high repetition rate pulse train. In these schemes, the repetition rate of the optical source and the switching rate of the modulator must be greater than the data rate from each user due to the length of the spreading codes. Therefore, the use of DS-SS for the transmission of signals in the LAN requires high speed PD at the receiver if electronic

50 Chapter 2 Literature Review processing is used to encode and decode the data. Moreover, the multiple access interference (MAI) and optical beat interference (OBI) limit the number of active users. MAI between the DS-SS signals can be obtained by using both PN & Walsh codes and a symmetric CDMA PON system was achieved using this setup [102]. However, the data rate of the proposed network was very low. Even though OBI limits the capacity and the scalability of the network, DS-SS has shown better performance in the presence of OBI compared to pure SCM systems [109, 110]. To increase the capacity of the incoherent CDMA system, encoding and decoding can be carried out in the optical domain. The processing bandwidth exceeds that of the electronic encoders. This type of coherence multiplexing schemes were also used for the asynchronous transmission of signals in the upstream direction [ ] that uses selective interference to discriminate between desired and undesired signals. Unbalanced Mach-Zehnder interferometers (MZI) are used to coherence multiplex and demultiplex the signals at both ends. Demultiplexing of the desired signal is achieved by matching the differential delay of the receiver MZI, to that of the transmitter MZI. Coherence interference occurs at the outputs of the receiver MZI and the original signal is reconstructed using balanced detection. However, the number of ONUs that can be supported using this technique are limited to common mode rejection ratio of the receiver and OBI caused by mixing of uncorrelated optical fields at the balanced receiver. Moreover, these interferometer delays cannot be easily be reconfigured Optical beat interference In the optical CDMA systems, the lightwaves from several sources are simultaneously incident at the PD for the recovery of data. Since the PD acts as a square law detector to the optical fields, OBI can occur in the bandwidth of the optical receiver [ ]. The OBI resulting from the beating of K number of incoherent Lorenzian lightwaves in a single PD can be written as [118]

51 Chapter 2 Literature Review P beat L, K ( RZ ) ( 1 δ ) 2 K K Bτ C 1 ij Pi Pj 2 i= 1 j= 1 1+ C ij Q = ( πτ f ) 2 Equation 2.2 where f ij = f i f j Here, R, Z, τ, C B are responsivity of the PD, impedance, coherence time of the source, and the noise equivalent of the receiver respectively. Here, Q is a constant and equals 2 for the case of lightwaves are randomly related in polarisation, and it equals 4 for linearly and identically polarised waves. In this calculation, it is assumed that all lightwaves have identical spectrum types and widths. If one of the lightwaves has power P w, and other (K-1) lightwaves have power P l with an assumption that all lightwaves are randomly related in polarisation with the worst case, whereby there is no difference in the operating frequencies, then the OBI can be written as [118] P 2 ( P ) 2 ( RZ ) B ( K 1) P P + ( K 1)( K ) beat = 2 2 L, K C w l l τ Equation 2.3 Using equation 2.3, OBI can be calculated. In Chapter 5, the scalability of the PS-PON for the DS-SS CDMA upstream access scheme and the DS-SS CDMA VPN scheme are calculated. DS-SS has also been used for the direct optical switching of the optical signals enabling alloptical connectivity and simpler detection [ ] Signalling for packet based access networks Optical packet routing involves decision making in the packet router and may not be realised in a fully optically transparent way. Therefore, an efficient signalling mechanism is required to obtain highly efficient packet based access network. There have been a number of proposals for signalling mechanisms in a WDM network [ ]. Out of all these schemes, RF subcarrier based signalling [ ], a separate wavelength channel signalling [139, 140], optical code division multiplexing (OCDM) [141, 142] based signalling are the most popular signalling techniques. In RF subcarrier multiplexed signalling schemes, each

52 Chapter 2 Literature Review wavelength channel is identified by a unique RF carrier frequency. For the transmission of the packet, the header of the packet is multiplexed with the unique RF carrier frequency and the RF upconverted header data is then combined with the payload of the packet, which is carried at baseband. Therefore the header and the payload of the packet are transmitted simultaneously in the same time slot as shown in Figure At each network terminal, by detecting the RF carrier frequency, the wavelength channel of interest can be identified without the use of a WDM demultiplexer. Header Payload λ 1 f 1 f 2 f n Header Payload λ 2 Payload Data.... Figure 2.27: RF subcarrier signalling scheme, whereby payload and header are transported simultaneously in the same timeslot. However, this technique adds complexity in the forms of active microwave components at the modulation and detection. In addition, the modulation depth of the payload data is decreased and hence a power penalty is paid. In a WDM packet network, increasing the RF subcarrier multiplexed signals will lead to nonlinear effects in the RF and optical domain. Payload λ Drop Control λ Drop Control λ Add Coupler MAC Transceiver Tunable Tx Inserted Packet Figure 2.28: Node architecture of a network employing dedicated wavelength signalling scheme. In the dedicated wavelength channel signalling, parallel transmission of payload and header of a packet in separate wavelength channels increases the throughput of the network since most

53 Chapter 2 Literature Review of the wavelength channels are occupied with the payload data of the packet. Figure 2.28 shows an architecture of a node of a network that employs dedicated wavelength signalling scheme. The dedicated wavelength channel that carries the control information of the packets is dropped at each node and the header data is processed. Using the information, the wavelength channel availability can be found. Therefore, a packet can be inserted into a wavelength channel by tuning the tunable wavelength transmitter. The time slot in the dedicated control wavelength channel is modified accordingly and sent back to the network. As a dedicated wavelength channel is used for carrying the header of each packet, the utilisation of the wavelength channels that carry the payload data can be high. However, due to the fibre dispersion, the synchronisation between the packet header and payload may be lost. Therefore, realignment of the payload data and header at each network node is required and this could make the system more complex. OCDM signalling scheme has been demonstrated using bit level coding and header processing. Packet routing and switching functionalities are performed using this signalling technique. Although 80 Gb/s photonic packet routing has been experimentally demonstrated, it is too difficult and costly to perform low-level functions using this technique especially in local area networks. System Issues RF Subcarrier Dedicated Wavelength Optical CDM Electronic CDMA Wavelength walk off Scalability? Payload bit rate Complexity? Table 2.1: A summary of the signalling schemes for WDM packet networks

54 Chapter 2 Literature Review Table 2.2 gives a summary of the signalling schemes that are used in the WDM packet networks. RF subcarrier signalling scheme is limited by scalability. The payload data bit rate is limited by the lower RF carrier frequency, since the chosen RF carrier frequency is placed outside the bandwidth of the payload data. The processing complexity of the RF subcarrier signalling is limited by the bandwidth of the PD. Moreover, the performance of this scheme is limited by the analog processing circuits. Furthermore, dispersion, and OBI also limit the scalability of this signalling scheme. In the dedicated wavelength channel signalling scheme, the complexity of the network node is higher because proper synchronisation is required between the header data in the dedicated wavelength channel and the payload data in other WDM channels. However, higher utilisation of the WDM channels is obtained with the capability to carry high bandwidth payload data. In OCDM signalling techniques, higher capacity payload data rate can be supported. However the processing complexity at each network node limits the performance of the network. In this thesis, a novel signalling scheme using DS-SS CDMA is proposed for the packet based optical access networks. It is shown that this scheme is more scalable than RF subcarrier signalling scheme. It has the capability to support high bandwidth payload data and the processing of the DS-SS CDMA control signals can be digitally performed and therefore higher performance can be obtained. A detailed experimental and theoretical analysis of this scheme is discussed in Chapter Conclusions This chapter presented a literature review on the next generation optical access network architectures. A number of advanced functionalities of a next generation WDM access network are discussed. Optical layer LAN emulation architectures for PS-PONs, protection schemes against distribution fibre breaks and feeder fibre breaks in PS-PONs, a number of applications of the DS-SS CDMA in the optical access networks, and a few signalling mechanisms are described. The advantages and disadvantages of these schemes are identified. Using the knowledge gained from the literature review, a number of novel schemes for the

55 Chapter 2 Literature Review above mentioned advanced functionalities for the next generation optical access architectures were developed. These schemes are presented in the technical chapters 3, 4 5 and 6 of this thesis. 2.6 References [1] T. Nakashima, Y. Hamazumi, N. Tokura, and K. Kikuchi, Photonic access network architecture, in Proc. IEEE Global Telecommunications Conference, vol. 1, pp , [2] G. Gobl, C. Lundquist, B. Hillerich, and M. Perry, Fiber to the residential customer, in Proc. IEEE Global Telecommunications Conference, vol. 1, pp , [3] Y. Mochida, "Technologies for local-access fibering," IEEE Commun. Mag., vol. 32, pp , Feb [4] T. H. Wood, What architectures make sense for fiber access networks in Proc. 11 th Annual Meeting of the IEEE Lasers and Electro-Optics Society (LEOS 98), vol. 2, pp , [5] A. A. M. Saleh, and J. M. Simmons, "Architectural principles of optical regional and metropolitan access networks," IEEE J. Lightw. Technol., vol. 17, pp , Dec [6] T. Miki, "The potential of photonic networks," IEEE Commun. Mag., vol. 32, pp , Dec [7] T. Miki, Toward the service-rich era (optical access networks)," IEEE Commun. Mag., vol. 32, pp , Feb [8] N. J. Frigo, Recent progress in optical access network, in Proc. Optical Fiber Communication Conference (OFC 96), vol. 2, pp , [9] N. J. Frigo, Optical networking for local access, in Proc. IEEE/LEOS Summer Topical Meetings in Advanced Applications of Lasers in Materials Processing, Broadband Optical Networks - Enabling Technologies and Applications, Smart Pixels; Optical MEMS and their Applications, pp , [10] M. Shibutani, H. Ishikawa, H. Tezuka, W. Domon, M. Soda, and K. Emura, A gigabit-to-the-home (GTTH) system for future broadband access infrastructure, IEEE International Conference on Communications (ICC 97), vol. 2, pp ,1997. [11] D. Kettler, H. Kafka, and D. Spears, Driving fiber to the home, IEEE Commun. Mag., vol. 38, pp , Nov [12] Y.-K. M. Lin, and D. R. Spears, Passive optical subscriber loops with multi-access, IEEE J. Lightw. Technol., vol. 7, pp , Nov [13] D. W. Faulkner, D. B. Payne, J. R. Stern, J. W. Ballance, Optical networks for local loop applications, IEEE J. Lightw. Technol., vol. 7, pp , Nov [14] G. Maier, M. Martinelli, A. Pattavina, and E. Salvadori, "Design and cost performance of the multistage WDM-PON access networks," IEEE J. Lightw. Technol., vol. 18, pp , Feb [15] M. Zirngibl, C. H. Joyner, L. W. Stulz, C. Dragone, H. M. Presby, and I. P. Kaminow, "LARnet, a local access router network," IEEE Photon. Technol. Lett., vol. 7, pp , Feb [16] N. J. Frigo, P. P. Iannone, P. D. Magill, T. E. Darcie, M. M. Downs, B. N. Desai, U. Koren, T. L. Koch, C. Dragone, H. M. Presby, and G. E. Bodeep, "A wavelength-division multiplexed passive optical

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57 Chapter 2 Literature Review [32] G. Schatzberg, Internetworking Dissimilar LANs with FDDI, in Proc. 15 th Conference on Local Computer Networks, pp , [33] E. J. Hernandez-Valencia, "Architectures for broadband residential IP services over CATV networks," IEEE Network., vol. 11, no. 1, pp , [34] G. Kramer, B. Mukherjee, and A. Maislos, "Ethernet passive optical networks," in IP over WDM: Building the Next Generation Optical Internet, S. Dixit, Ed. New York: Wiley, ch. 8, pp , [35] Norman Finn, "Compatibility Model for IEEE 802.3ah EPONs," in www. ieee802.org/1/files/public/docs2002. [36] C. -J. Chae, L. Seung-Tak, K. Geun-Young, and P. Heesang, "A PON system suitable for internetworking optical network units using a fiber Bragg grating on the feeder fiber," IEEE Photon. Technol. Lett., vol. 11, pp , Dec [37] D. Podwika, D. Stefanski, J. S. Witkowski, and E. M. Pawlik, "Computer networks based on optical passive couplers," in Proc. 2 nd International Conference on Transparent Optical Networks (ICTON 00), pp , [38] C.-J. Chae, P. Heesang, and E. Jong-Hoon, "An ATM PON system overlaid with a 155-Mb/s optical star network for customer networking and fiber to the premises," IEEE Photon. Technol. Lett., vol. 13, pp , Oct [39] Chang-Joon Chae, Heesang Park, and Jong-Hoon Eom, "A new ATM PON system suitable for local access and local private networking services, in Proc. Optical Fiber Communication Conference (OFC 01), vol. 3, pp. WU4-1 - WU4-3, [40] T. Jayasinghe, C.-J. Chae, and R. S. Tucker, "Rebroadcasting of broadband services over passive optical network in residential community," IEE Electron. Lett., vol. 41, pp , Nov [41] T. Jayasinghe, and C.-J. Chae, "Implementation of Local Re-broadcasting and LAN Services over an Ethernet Passive Optical Network," in Proc. 31 st European Conference on Optical Communications (ECOC 05), vol. 3, pp , [42] A. V. Tran, C.-J Chae, and R. S. Tucker, "PON architecture providing local customer networking capability," in Proc. the 18 th Annual Lasers and Electro Optics Society Meeting (LEOS 05), pp [43] E. Wong and C.-J. Chae, "CSMA/CD-based Ethernet passive optical network with optical internetworking capability among users," IEEE Photon. Technol. Lett., vol. 16, pp , Sep [44] E. Wong and C.-J. Chae, "CSMA/CD-based Ethernet passive optical network with shared LAN capability," in Proc. Optical Fiber Communication Conference (OFC 04), vol. 1, [45] O. Gerstel and R. Ramaswami, "Optical layer survivability: A post-bubble perspective," IEEE Commun. Mag., vol. 41, pp , Sep [46] O. Gerstel and R. Ramaswami, "Optical layer survivability: A services perspective," IEEE Commun. Mag., vol. 38, pp , Mar [47] T.-H. Wu, "Emerging technologies for fiber network survivability," IEEE Commun. Mag., vol. 33, pp , Feb

58 Chapter 2 Literature Review [48] M. M.-K. Liu, "Proton-a concept for highly reliable and low cost optical fiber networks," in Proc. 10 th Annual International Phoenix Conference on Computers and Communications, pp , [49] J. Davidson, I. Hawker, and P. Cochrane, The evolution of service protection in the BT network, in Proc. Global Communications Conference (GLOBECOM 89), vol. 2, pp , [50] M. Medard, and S. Lumetta, "Architectural issues for robust optical access," IEEE Commun. Mag., vol. 39, pp , [51] Tsong-Ho Wu, A novel architecture for optical dual homing survivable fiber networks, in Proc. IEEE International Conference on Communications (ICC 90), vol. 2, pp , [52] M. Gerla, P. Camarda, and G. Chiaretti, Fault tolerant PON topologies, in Proc. 11 th Annual Joint IEEE Conference on Computer and Communications, vol. 1, pp , [53] O. K. Tonguz, and K. A. Falcone, "Fiber-optic interconnection of local area networks: physical limitations of topologies." IEEE J. Lightw. Technol. Vol. 11, pp , May [54] Broadband optical access systems based on passive optical networks (PON), ITU-T, Recommendation G.983.1, [55] D. J. Xu, W. Yen, and E. Ho, Proposal of a new protection mechanism for ATM PON interface, in Proc. IEEE International Conference on Communications (ICC 01), vol.7, pp , [56] C. Xue, S. Shuhe, Z. Xu, L. Dong, and D. Yu, Proposal of a novel protection mechanism for Ethernet PONs, in Proc. IEEE Region 10 Conference on Computers, Communications, Control and Power Engineering, vol. 2, pp , [57] Y. Okumura, S. Aoyagi, and E. Maekawa, Duplex system configuration in passive double star system, in Proc. IEEE Global Telecommunications Conference, vol. 3, pp , [58] Y.-m. Kim, J. Y. Choi, J.-H. Ryou, H.-M. Baek, O.-S. Lee, H.-S. Park, and M. Kang, Cost effective 8protection architecture to provide diverse protection demands in Ethernet passive optical network, in Proc. International Conference on Communication Technology, vol. 1, pp , [59] T. Yokotani, K. Murakami, and T. Yasushi, Simplified PON protected mechanism using L2 control protocols, in Proc. 10 th Optoelectronics and Communications conference (OECC 05), pp , [60] M. K. Abdullah, W. T. P'ng, P. W. Lau and E. R. Tee, "FTTH access network protection using a switch," in Proc. 9 th Asia-Pacific Conference on Communications (APCC 03), pp , [61] W. T. P'ng', M. K. Abdullah, S. Khatun, S. B. Ahmad-Anas and S. Shaari, Protection networking of Ethernet PON FTTH access network, 7 th International Conference on Laser and Fiber-Optical Networks Modeling, pp , [62] Wen-Piao Lin, Ming-Seng Kao, and Sien Chi, "The modified star-ring architecture for high-capacity subcarrier multiplexed passive optical networks", IEEE J. Lightw. Technol., vol. 19, pp , Jan [63] Wen-Piao Lin, Ming-Seng Kao, and Sien Chi, A star-bus-ring architecture for DWDM/SCM passive optical networks, in Proc.4 th Pacific Rim Conference on Lasers and Electro-Optics (PR-CLEO 01), vol. 2, pp , [64] Wen-Piao Lin, Ming-Seng Kao, and Sien Chi, "A DWDM/SCM Self-Healing Architecture for Broad- Band Subscriber Networks", IEEE J. Lightw. Technol., vol. 21, pp , Feb

59 Chapter 2 Literature Review [65] Wen-Piao Lin, Ming-Seng Kao, and Sien Chi, "A high capacity DWDM/SCM Architecture for Broad- Band Subscriber Networks", in Proc. Optical Fiber Communication Conference (OFC 02), pp , [66] F. -T. An, Kyeong Soo Kim, D. Gutierrez, S. Yam, E. Hu, K. Shrikhande, and L. G. Kazovsky, "SUCCESS: a next-generation hybrid WDM/TDM optical access network architecture," IEEE J. Lightw. Technol., vol. 22, no. 11, pp , [67] K.-D. Langer, J. Grubor, and K. Habel, Promising evolution paths for passive optical access networks, 6 th International Conference on Transparent Optical Networks (ICTON 04), vol. 1, pp , [68] S.-B. Park, D. K. Jung, D. J. Shin, H. S. Shin, S. Hwang, Y. J. Oh, and C. S. Shim, Bidirectional Wavelength-Division-Multiplexing Self-Healing Passive Optical Network, in Proc. Optical Fiber Communication Conference (OFC 05), vol., pp., [69] Tsan-Jim Chan, Chun-Kit Chan, Lian-Kuan Chen, and Frank Tong, A Self-Protected Architecture for Wavelength-Division-Multiplexed Passive Optical Networks, IEEE Photon. Technol. Lett., vol. 15, no. 11, pp , [70] T.-J. Chan, Y. C. Yu, C. K. Chan, L. K. Chen, and F. Tong, A novel bidirectional wavelength division multiplexed passive optical network with 1:1 protection, in Proc. Optical Fiber Communication Conference (OFC 03), vol. 15, pp , [71] Z. Wang, X. Sun, C. Lin, C. K. Chan, and L. K. Chen, "A novel centrally controlled protection scheme for traffic restoration in WDM passive optical networks," IEEE Photon. Technol. Lett., vol. 17, pp , Mar [72] Xiaofeng Sun, Zhaoxin Wang, Chun-Kit Chan, and Lian-Kuan Chen, A novel star-ring protection architecture scheme for WDM passive optical access networks, in Proc. Optical Fiber Communications Conference (OFC 04), [73] E. S. Son, K. H. Han, J. H. Lee, and Y. C. Chung, "Survivable Network Architectures for WDM PON", in Proc. Optical Fiber Communications Conference (OFC 05), vol. 5, [74] A. F. Elrefaie, Self healing WDM ring networks with all optical protection path, in Proc. Optical Fiber Communications Conference (OFC 92), pp , [75] O. Gerstel, Opportunities for optical protection and restoration, in Proc. Optical Fiber Communications Conference (OFC 98), pp , [76] P. A. Bonefant, Optical layer survivability: A comprehensive approach, in Proc. Optical Fiber Communications Conference (OFC 98), pp , [77] Z. Wang, C. Lin, C.-K. Chan, Demonstration of a Single-Fiber Self-Healing CWDM Metro Access Ring Network With Unidirectional OADM, IEEE Photon. Technol. Technol., vol. 18, no. 1, pp , Jan [78] Z. Wang, C. Lin, C.-K. Chan, A simple single-fiber CWDM metro/access ring network with unidirectional OADM and automatic protection, in Proc. Optical Fiber Communications Conference (OFC 05), vol. 6,

60 Chapter 2 Literature Review [79] J. M. Senior, A. J. Phillips, and M. O. Van Deventer, Physical layer dimensioning of superpon architectures, in in Proc. SPIE, vol. 3230, pp , [80] M. O. Van Deventer, J. D. Angelopoulos, J. J. M. Binsma, A. J. Boot, P. Crahay, E. Jaunart, P. J. M. Peters, A. J. Phillips, X. Z. Qiu, J. M. Senior, M. Valvo, M. Vandewege, P. J. Vetter, and I. Van de Voorde, Architectures for 100 km 2048 split bidirectional SuperPON s from ACTS-PLANET, in Proc. SPIE, vol. 2919, pp , [81] I. Van de Voorde, M. O. Van Deventer, P. J. M. Peters, P. Crahay, E. Jaunart, A. J. Phillips, J. M. Senior, X. Z. Qiu, M. Vandewege, and P. J. Vetter, Network topologies for SuperPON, in Proc. Optical Fiber Communications Conference (OFC 97), pp , [82] M. O. Van Deventer, Y. M. van Dam, P. J. M. Peters, F. Vermaerke, and A. J. Phillips, Evolution phases to an ultra-broadband access network: Results from ACTS-PLANET, IEEE Commun. Mag., vol. 35, no. 12, pp 72 77, Dec [83] A. J. Phillips, J. M. Senior, R. Mercinelli, M. Valvo, P. J. Vetter, C. M. Martin, M. O. Van Deventer, P. Vaes and X. Z. Qiu, "Redundancy strategies for a high splitting optically amplified passive optical network," IEEE J. Lightw. Technol., vol. 19, pp , Feb [84] A. J. Phillips, J. M. Senior, P. J. Vetter, M. O. Van Deventer, M. Valvo, and R. Mercinelli "Reliability of Supoer PON systems," in Proc. 6 th IEE Conference on Telecommunications, pp , [85] L. G. Giehmann, A. Gladisch, J. Rudolph, "Field trial of OAM-signal transport capabilities with a 10 Mchip/s LED-direct sequence spread spectrum system suited for OAM-signal-transport in transparent optical WDM-networks," in Proc. Optical Fiber Communication Conference and the International Conference on Integrated Optics and Optical Fiber Communication (OFC/IOOC '99), [86] T. Welsch, and K. Jobmann, Measurement of linear optical crosstalk for bit error rate estimation by using spread spectrum measurement symbols, in Proc. IASTED International Conference on Wireless and Optical Communications, pp , [87] T. Welsch, and K. Jobmann, Reliable quality of transport monitoring in optical transparent paths of all optical DWDM networks, in Proc. Optical Fiber Communication Conference (OFC 01), vol. 1, pp. MH6-1 MH6-3, [88] D. Enguang, W. Deming, and X. Anshi, Application of spread spectrum technology at 64 Mcps and 2 Mbps as OAm signals in transparent optical networks, in Proc. Optical Fiber Communication Conference (OFC 01), paper TuV1-1, [89] V. A. Vaishanpayan, and M. D. Feuer, An overlay architecture for managing light paths in optically routed networks, IEEE Trans. Commun., vol. 53, no. 10, pp , Oct [90] M. D. Feuer, and V. A. Vaishanpayan, Demonstration of an In-band auxiliary channel for path trace in photonic networks, in Proc. Optical Fiber Communication Conference (OFC 05), paper OWB6, [91] M. D. Feuer, and V. A. Vaishanpayan, In band management channel for light paths in photonic networks, in Proc. European Conference on Optical Communications (ECOC 04), paper Tu 3.6, [92] Y. Horiuchi, K. Ohara, H. Tanaka and M. Suzuki, "ONU Diagnostic Methodology of Passive Optical Network in Disturbance Environment for Physical Layer Security," in Proc. 29 th European Conference on Optical Communication and 14 th International Conference on Integrated Optics and Optical Fiber Communication (ECOC-IOOC 03), paper Th2.4.7,

61 Chapter 2 Literature Review [93] Y. Horiuchi, and N. Edagawa, ONU authentication technique using loopback modulation within a PON disturbance environment, in Proc. Optical Fiber Communication Conference (OFC 05), paper OF13, [94] S. Topliss, D. Beeler, and L. Altwegg, Synchronization for passive optical networks, IEEE J. Lightw. Technol., vol. 13, pp , May [95] S. Culverhouse, R. A. Lobbett, and P. J. Smith, Optically amplified TDMA distributive switch network with Gb/s capacity offering interconnection to over 1000 customers at 2 Mb/s, IEE Electron. Lett., vol. 28, pp , [96] B. Miah, and L. Cuthbert, An economic ATM passive optical network, IEEE Commun. Mag., pp , [97] T. E. Darcie, "Subcarrier multiplexing for multiple-access lightwave networks," IEEE J. Lightw. Technol., vol. LT-5, pp , [98] W.-P. Lin, M. S. Kao, S. Chi, The modified star-ring architecture for high-capacity subcarrier multiplexed passive optical networks, IEEE J. Lightw. Technol., vol. 19, pp , Jan [99] C. R. Giles, R. D. Feldman, T. H. Wood, M. Zirngibl, G. Raybon, T. Strasser, L. Stulz, A. McCormick, C. H. Joyner, and C. R. Doerr, "Access PON using downstream 1550-nm WDM routing and upstream 1300-nm SCMA combining through a fiber-grating router," IEEE Photon. Technol. Lett., vol. 8, pp , Nov [100] S. L. Woodward, G. E. Bodeep, T. E. Darcie, G. Huang, Z. Wang, and J. J. Werner, "A passive-optical network employing upconverted 16-CAP signals," IEEE Photon. Technol. Lett., vol. 8, pp , Sep [101] A. J. Mendez, Code division multiple access enhancement of wavelength division multiplexing schemes, in Proc. IEEE International Conference on Communications (ICC 95), vol. 1, pp , [102] B. Ahn and Y. Park, "A symmetric-structure CDMA-PON system and its implementation," IEEE Photon. Technol. Lett., vol. 14, no. 9, pp , [103] M. J. Parham, C. Smythe, and B. L. Weiss, Code division multiple access techniques for use in opticalfiber local-area networks, J. Electronics and Communication Engineering, vol. 4, no. 4, pp , Aug [104] M. Kavehrad, F. Khaleghi, and G. Bodeep, An experiment on a CDM subcarrier multiplexed optical fiber local area network, IEEE Photon. Technol. Lett., vol. 4, pp , Jul [105] G. J. Foschini, and G. Vannucci, Using spread spectrum in a high capacity Fiber optic local network, IEEE J. Lightw. Technol., vol. 6, pp , Mar [106] P. M. Lam, and K. Sripimanwat, Synchronous optical fiber code division multiple access networks using Walsh codes, in Proc. 7 th IEEE International Symposium on Spread Spectrum and Applications, vol. 2, pp , [107] D. Enguang, W. Deming, X. Anshi, A novel WDM-CDMA optical fiber communication system with asynchronous despreading technology, IEEE Ultrasonics Symposium, pp ,

62 Chapter 2 Literature Review [108] F. Ayadi, P. Fortier, E. Hamelin, B. Ruchet, L. A. Rusch, and M. Tetu, Network architecture for a high bandwidth WDM/CDMA Local Area Network, IEEE Pacific Rim Conference on Communications, Computers and Signal Processing, vol. 2, pp , [109] C.C. W. Hsiao, B. H. Wang, and W. I. Way, "Multiple access in the presence of optical-beat and gochannel interference using Walsh-code-based synchronized CDMA technique," IEEE Photon. Technol. Lett., vol. 9, pp , Aug [110] F. Yamamoto, and T. Sugie, "Reduction of optical beat interference in passive optical networks using CDMA technique," IEEE Photon. Technol. Lett., vol. 12, pp , Dec [111] M. J. L. Cahill, G. J. Pendock, and D. D. Sampson, "Hybrid coherence multiplexing/coarse wavelengthdivision multiplexing passive optical network for customer access," IEEE Photon. Technol. Lett., vol. 9, pp , [112] G. J. Pendock, and D. D. Sampson, Capacity of coherence-multiplexed CDMA networks, Optical Communications, vol. 143, pp , [113] G. J. Pendock, M. J. L. Cahill, and D. D. Sampson, Multi-gigabit per second demonstration of photonic code division multiplexing, IEE Electron. Lett., vol. 31, pp , [114] G. J. Pendock, and D. D. Sampson, Transmission performance of high bit rate spectrum-sliced WDM systems, IEEE J. Lightw. Technol., vol. 14, pp , [115] P. R. Prucnal, M. A. Santaro, and T. R. Fan, Spread spectrum fiber optic local area network using optical processing,, IEEE J. Lightw. Technol., vol. LT-4, pp , May [116] G. J. Pendock, and D. D. Sampson. Increasing the transmission capacity of coherence multiplexed communication systems by using differential detection, IEEE Photon. Technol. Lett., vol. 7, pp , Dec [117] G. C. Gupta, N. Karafolas, and D. Uttamchandani, Low coherence optical CDMA for LAN, in Proc. Annual Meeting of the IEEE Lasers and Electro-Optics Society (LEOS 94), vol. 2, pp , [118] T. Demeechai, and A. B. Sharma, Beat noise in a non-coherent optical CDMA system, in Proc. 8 th International Conference on Communication Systems, vol. 2, pp , [119] C. Desem, Measurement of optical interference due to multiple optical carriers in subcarrier multiplexing, IEEE Photon. Technol. Lett., vol. 3, pp , Apr [120] T. H. Wood, and N. K. Shankaranarayanan, Operation of a passive optical network with subcarrier multiplexing in the presence of optical beat interference, IEEE J. Lightw. Technol., vol. 11, pp , Oct [121] C. Desem, Optical interference in subcarrier multiplexed system with multiple optical carriers, IEEE J. Sel. Areas Commun., vol. 8, pp , [122] T. Higashino, K. Tsukamoto, and S. Komaki, Experimental study of the received signal performance in direct optical switching CDMA ROF system, in Proc. International Topical Meeting on Microwave Photonics, pp , [123] K. Kumamoto, K. Tsukamoto, and S. Komaki, Theoretical consideration on transferring transparency for RF bandwidth on direct optical switching CDMA Radio-on-Fiber Networks, in Proc. International Topical Meeting on Microwave Photonics, pp ,

63 Chapter 2 Literature Review [124] Y. Takushima, and K. Kikuchi, Photonic switching using spread spectrum technique, IEE Electron. Lett., vol. 30, no. 5, pp , Mar [125] A. Narasimha, and E. Yablonovitch, IEE Electron. Lett., vol. 39, no. 7, pp , Apr [126] A. Narasimha, and E. Yablonovitch, Code-selective RF photonic mixing for use in optical CDMA demultiplexers, in Proc. Annual Meeting of the IEEE Lasers and Electro-Optics Society (LEOS 01), vol. 1, pp , [127] Y. Horiuchi and M. Suzuki, "Optical label switch routing on wide-scale optical network using digitally encoded SCM label," IEE Electron. Lett., vol. 37, pp , [128] G. K. Chang, G. Ellinas, B. Meagher, W. Xin, S. J. Yoo, M. Z. Iqbal, W. Way, J. Young, H. Dai, Y. J. Chen, C. D. Lee, X. Yang, A. Chowdhury, and S. Chen, "Low latency packet forwarding in IP over WDM networks using optical label switching techniques," in Proc. 12 th Annual Meeting of the IEEE Lasers and Electro-Optics Society (LEOS 99), vol. 1, pp , [129] K. V. Shrikhande, I. M. White, D. Wonglumsom, S. M. Gemelos, M. S. Rogge, Y. Fukashiro, M. Avenarius, and L. G. Kazovsky, "HORNET: a packet-over-wdm multiple access metropolitan area ring network," IEEE J. Sel. Areas in Commun., vol. 18, pp , [130] K. Shrikhande, A. Srivatsa, I. M. White, M. S. Rogge, D. Wonglumsom, S. M. Gemelos, and L. G. Kazovksy, "CSMA/CA MAC protocols for IP-HORNET: an IP over WDM metropolitan area ring network," in Proc. IEEE Global Telecommunications Conference, vol. 2, pp , [131] D. Wonglumsom, I. M. White, S. M. Gemelos, K. Shrikhande, and L. G. Kazovsky, "HORNET-A packet-switched WDM network: optical packet transmission and recovery," IEEE Photon. Technol. Lett., vol. 11, pp , [132] E. Park and A. E. Willner, "Self-routing of wavelength packets using an all-optical wavelength shifter and QPSK subcarrier routing control headers," IEEE Photon. Technol. Lett., vol. 8, pp , [133] D. Wonglumsom, I. M. White, K. Shrikhande, M. S. Rogge, S. M. Gemelos, A. Fu-Tai, Y. Fukashiro, M. Avenarius, and L. G. Kazovsky, "Experimental demonstration of an access point for HORNET-A packet-over-wdm multiple-access MAN," IEEE J. Lightw. Technol. vol. 18, pp , [134] I. M. White, Y. Fukashiro, K. Shrikhande, D. Wonglumsom, M. S. Rogge, M. Avenarius, and L. G. Kazovsky, "Experimental demonstration of a media access protocol for HORNET: a WDM multiple access metropolitan area ring network," in Proc. Optical Fiber Communication Conference (OFC 00), vol. 2, pp , [135] I. M. White, M. S. Rogge, K. Shrikhande, Y. Fukashiro, D. Wonglumsom, F. T. An, and L. G. Kazovsky, "Experimental demonstration of a novel media access protocol for HORNET: a packet-over- WDM multiple-access MAN ring," IEEE Photon. Technol. Lett., vol. 12, pp , [136] I. M. White, K. Shrikhande, M. S. Rogge, S. M. Gemelos, D. Wonglumsom, G. Desa, Y. Fukashiro, and L. G. Kazovksy, "Architecture and protocols for HORNET: a novel packet-over-wdm multipleaccess MAN," in Proc. IEEE Global Telecommunications Conference, vol. 2, pp , [137] D. J. L. Blumenthal, J.; Gaudino, R.; Sangwoo Han; Shell, M.D.; Vaughn, M.D., "Fiber-optic links supporting baseband data and subcarrier-multiplexed control channels and the impact of MMIC photonic/microwave interfaces," IEEE Trans. Microwave Theory Tech., vol. 45, pp ,

64 Chapter 2 Literature Review [138] B. H. Wang, K. Y. Yen, and W. I. Way, "Demonstration of gigabit WDMA networks using parallely processed subcarrier hopping pilot-tone (P/sup 3/) signaling technique," IEEE Photon. Technol. Lett., vol. 8, pp , [139] L. G. Kazovsky, I. M. White, K. Shrikhande, and M. S. Rogge, "High capacity metropolitan area networks for the next generation Internet," in Proc. 35 th Asilomar Conference on Signals, Systems and Computers, vol. 1, pp. 3-7, [140] D. Dey, A. van Bochove, A. Koonen, D. Geuzebroek, and M. Salvador, "FLAMINGO: a packetswitched IP-over-WDM all-optical MAN," in Proc. 27 th European Conference on Optical Communications (ECOC 01), vol. 3, pp , [141] K. I. Kitayama and N. Wada, "Photonic IP routing," IEEE Photon. Technol. Lett., vol. 11, pp , [142] K. Kitayama, N. Wada, and H. Sotobayashi, "Architectural considerations for photonic IP router based upon optical code correlation," IEEE J. Lightw. Technol., vol. 18, pp ,

65 Chapter 1 Introduction networks. A PON consists of a single shared optical fibre connecting the CO and a passive branching device located at the remote node (RN). These types of double star PON architectures have several advantages such as reduced amount of fibre deployed in the outside cable plant, reduced amount of optical transceivers, and a non-requirement for an electrically powered box at the RN [22]. Different network architectural implementations of PON can be realised by referring to the PON architecture in Figure 1.1, which comprises a passive distribution network with a CO, few remote nodes (RNs) and a large number of ONUs. The RN can be a passive star coupler (SC) or a wavelength router such as an arrayed waveguide grating (AWG) multiplexer. Both directions of communication are supported using coarse wavelength division (CWDM) multiplexing. The downstream direction uses the 1.55 µm wavelength window, while the upstream direction uses 1.31 µm wavelength window. The architectures of the PON can be varied such as wavelength division multiplexed PON (WDM-PON), power splitting PON (PS-PON) and composite PON (CPON) [26]. Using the AWGs at the RN, different ONUs are assigned with unique wavelength channels for data transmission in a WDM-PON. In WDM- PON, both upstream and downstream directions use separate wavelength channels for each ONU [29-33]. Among the PON configurations, power splitting PON (PS-PON) where the optical carrier is shared by many subscribers can be a much more cost effective candidate for fibre-to-the-home (FTTH) applications [34-38]. Low cost sources and components can be used in PS-PON, however the splitting loss at the optical passive splitter can limit the number of ONUs that are connected to each splitter. Both power splitting and WDM routing can be combined in a single PON that is called composite PON (CPON). This architecture uses WDM routing for distributing downstream signals and power combining for upstream signals. The FSAN (Full Service Access Network) consortium initially proposed APON (ATM over PON) as the most cost effective architecture for the broadband FITL deployment and APON was standardised by ITU (International Telecommunication Union) in 1998 [39]. However, Ethernet technology has become more dominant since IP traffic had started dominating data traffic and Ethernet is much more optimised for the variable size IP packets. In addition, the cost of the ATM switches and network cards are more expensive than the Ethernet cards, and newly adopted QoS techniques to support voice, video and data, have also contributed to the revival of the Ethernet as a viable access transport technology [24]. Ethernet over passive - 3 -

66 Chapter 1 Introduction optical network is viewed as the future access network architecture because of the IP dominated traffic [22, 40]. Ethernet for the First Mile (EFM) group has proposed possible technologies for the EPON standardisation Emerging PS-PON technologies To date, PS-PONs have been thoroughly explored and standardised [41-43]. Due to the ever increasing growth of customer numbers and their required bandwidth, concepts for gradual upgrading of the PS-PON are required. The evolution of the optical access network can be represented through several parameters such as extension of transmission reach, upgrade in terms of data rates, increased number of customers, and new features offered to according to the demand by the end users [44, 45]. PS-PONs have been limited by the splitting loss of the SC. As the number of splits in the SC increases, the splitting loss of the SC increases as well. Therefore, power budget that is required to achieve higher transmission bit rates in both downstream and upstream directions becomes inadequate to support higher bandwidth data to the end users. Moreover, the number of users that are connected to the SC is also limited. In PS-PONs, a split ratio of 1:32 is usually considered for the deployment [22]. The proposed upgrades for the PS-PONs were motivated by several factors. The increase in transmission range could support the switching node and therefore cost savings can be made for the operation of the network. A use of amplified splitter technologies would result in longer transmission distance and support of large number of users being connected to single termination using statistical multiplexing [39, 45]. However, these amplified PON techniques require active hardware in the field and therefore add some complexities such as maintenance and redundancy of the amplifiers for the network. Next generation customer access networks must be able to deliver services based on customer demand and with guaranteed service requirements as agreed with the service provider. The use of optical networking in high data rate local area networks (LANs) such as storage area networks (SANs), high-end enterprise networks, application service providers, and web hosting sites have continually increased. Therefore, reliability of these optical access networks - 4 -

67 Chapter 1 Introduction has become paramount importance for the provision of such new value added services. In optical access networks, aggregated high speed traffic is transmitted between the customers and the central office (CO) and therefore a failure in network elements can cause serious problems. The reliability and the robustness of the access networks depend on the architectures and the topologies that were used in the design and implementation of the network. Providing protection against fibre network failures at any level is expensive due to the high cost associated with equipments and components [46]. Therefore, cost effective protection architectures have to be developed to support continuing service provisioning to the end users. The other market dynamically affecting the delivery of the data services is distribution of the users. The change to bursty data traffic patterns opens up an opportunity for an efficient use of inherent PS-PONs with distributed statistical multiplexing to increase the efficiency of the access networks. Using distributed multiplexing, virtual private networking (VPN) capabilities can be developed. However, establishing an access VPN in a broadband access network is difficult as it requires several tunnels to be established between the various involved entities. Apart from setting up a VPN on an existing broadband access network such as PS-PON, the challenge in designing a VPN is to provide security for the transmitted traffic as the traffic goes over a non private network [47, 48]. As the next generation optical access network infrastructures should be capable of providing VPN capabilities amongst the users within the network, appropriate secure transmission mechanisms have to be developed. As the PS-PONs evolves, traffic transport would become more packet oriented. One of the many issues to be considered when an optical network is implemented is the need to implement a signalling system within the network to coordinate traffic among users. It is desirable that the signalling system is implemented on the same physical network for the data paths. Many signalling protocols proposed for these kinds of networks, namely carrier sense multiple access (CSMA), aloha, and token ring, are inefficient due to the large propagation delay between the customer terminals. Moreover, the processing time contributed to the signalling purposes at each customer terminal for data reception as well as transmission is required to be considerably smaller than the data packet size. Ethernet is the most widely used link layer protocol and has the potential to yield a seamless optical network across different network boundaries, from wide area network to local access network. A physical layer - 5 -

68 Chapter 1 Introduction signalling mechanism that supports the media access control layer, which is implemented as one of the functions on an Ethernet interface card may be realised at each customer teminal to yield high throughput at low cost [49-55]. There are additional requirements and features that will be required to make a next generation optical access network more suitable for a successful deployment of the network. Appropriate techniques and protocols have to be developed to obtain dynamic bandwidth allocation amongst the users [56-58]. Moreover, the control of the network could be more distributed amongst the customers rather than performing all the management functionalities in a centralised fashion [59]. By distributing the operational functionalities amongst the users, bandwidth efficient and more intelligent distributed control protocols have to be developed [60-67]. By facilitating these additional features for the PS-PON, the access network infrastructure can be gradually upgraded towards a WDM-PON architecture [44, 68, 69]. This thesis addresses a number of requirements of a next generation optical access networks and proposes solutions and experimentally demonstrates these schemes and performs theoretical evaluations to identify the performance and scalability limitations of the schemes. 1.2 Thesis Outline The objective of this thesis is to investigate and develop novel architectures and technologies for future optical access networks incorporating WDM technologies. A number of architectures and technologies have been developed and demonstrated for the transmission of signals between the CO and the ONUs that are located at the customer premises. The next generation optical access network architecture should be capable of handling multiple requirements of the users of the network. These user requirements should be carried out on the existing access network infrastructure with minimal additional cost. This thesis aims to provide several feasible solutions to a number of user requirements such as user to user networking, protection of services without disruptions, secure transmissions of traffic in a multi-access environment, and efficient signalling for packet transport. The novel architectures and technologies for the optical access networks presented in this thesis are categorised under four main topics and they are local area network emulation in passive optical networks, protection and restoration in passive optical networks, applications of - 6 -

69 Chapter 1 Introduction electronic code division multiple access in passive optical networks, and signalling mechanism using electronic code division multiple access for packet based access networks. This thesis begins with a brief survey in chapter 2 of the proposed architectures and technologies for a PS-PON and WDM packet based access networks. A brief summary of the PONs is followed by the techniques previously proposed for the local area network (LAN) emulation in a PS-PON and protection schemes against the feeder fibre and distribution fibre breaks. This chapter also presents a brief summary of direct sequence spread spectrum (DS- SS) based CDMA techniques and its viable applications in an optical customer access network environment. A literature summary of a number techniques proposed for packet signalling mechanisms is also presented. The need for further research into the development of a next generation optical access network has motivated the work undertaken in this thesis. In chapter 3, two separate local area network (LAN) emulation schemes using RF subcarrier multiplexed transmission of LAN traffic are proposed and experimentally demonstrated. The first LAN emulation scheme uses a narrowband fibre Bragg grating (FBG) placed in the feeder fibre for the spectral filtering of LAN traffic. Two separate experiments were carried out with a single notch FBG and a double notch FBG and the results were compared. The second LAN emulation uses a secondary distribution fibre between the SC and the ONUs for the redirection of upstream signals. An experimental demonstration to verify the proposal is also presented. Both the LAN emulation schemes are compared in terms of bandwidth requirements, dispersion tolerance, optical source stability, and power budget. Theoretical analysis, simulation results and mathematical calculations were performed for these comparison studies. Chapter 4 investigates a number of protection architectures against feeder and distribution fibre breaks in a PS-PON. A feeder fibre protection scheme that uses a CWDM overlay is proposed and experimentally demonstrated. Two separate schemes for the protection against distribution fibre breaks are also proposed and experimentally demonstrated. One protection scheme uses dual distribution fibres to each ONU from the SC. This scheme is experimentally demonstrated with the protection switching being carried out at each ONU inconjunction with a LAN emulation scheme demonstrated in chapter 3. The second protection architecture that uses interconnections amongst the ONUs is also experimentally demonstrated with the LAN - 7 -

70 Chapter 1 Introduction emulation schemes that were demonstrated in chapter 3. A detailed analysis of the scalability of the protection architecture that uses interconnection amongst the ONUs based on power budget is presented. A simple experiment for the characterisation of the optical switch used in the experimental demonstrations is also presented. Chapter 5 demonstrates several applications of electronic code division multiple access (E- CDMA) in a PS-PON environment. E-CDMA has been proposed and experimentally demonstrated as an asynchronous upstream access scheme for a PS-PON. The experimental demonstration was carried out with three Fabry-Perot laser diodes (FP-LDs) and the requirement for a power control for the received E-CDMA signals is theoretically demonstrated. A theoretical scalability analysis is also carried out to identify the number of active optical sources for the E-CDMA signal transmissions in the presence of optical beat interference (OBI). Two separate schemes for secure LAN emulation within a PS-PON using E-CDMA are also presented. One scheme uses RF subcarrier multiplexed E-CDMA LAN traffic with a secondary distribution fibre for the distribution of secure LAN traffic. The second scheme uses an additional wavelength channel with a FBG placed in the feeder fibre for the reflection of dedicated wavelength channel to provide a secure LAN emulation amongst the ONUs. Both schemes were experimentally demonstrated. A multiple and secure VPN scheme using E-CDMA is also proposed. This VPN scheme is experimentally demonstrated with FP-LDs. A mathematical model was used to study the scalability of the number of simultaneous VPNs in the presence of noise sources from thermal noise, shot noise, OBI, and multiple access interference (MAI). Chapter 6 demonstrates a novel control packet signalling mechanism using E-CDMA for a WDM packet-based access network. A number of possible access network architectures, where this scheme could easily be deployed are presented. This E-CDMA signalling mechanism was experimentally demonstrated with two wavelength channels. A simulation study that shows the capabilities of this technique with the use of digital signal processing components is also presented. Possible architecture for the fast decoding of the E-CDMA signal is also presented. A theoretical model was used to analyse the scalability of the network, whereby the noise sources such as thermal noise, shot noise, relative intensity noise - 8 -

71 Chapter 1 Introduction (RIN), and MAI were considered to analyse the number of WDM channels for various modulation depths and transmission bit rates of the payload data of the packet. Chapter 7 gives a summary of the research undertaken for the thesis together with conclusions. Suggestions for future work based on the findings of this thesis are also included. 1.3 Original Contributions The original contributions to the field of optical access networks contained in this thesis can be summarised as follows. Publications arising from this work are listed in Section 1.4. Proposal and experimental demonstration of an optical layer LAN emulation scheme using RF subcarrier multiplexed transmission of LAN traffic with a narrowband FBG placed in feeder fibre for the optical spectral filtering and reflection of LAN traffic back to the ONUs. Chapter 3 Publications 1, 3, and 10 of Section 1.4. Proposal and experimental demonstration of a second optical layer LAN emulation scheme using RF subcarrier multiplexed transmission of LAN traffic with the use of a secondary distribution fibre for the redirection of LAN traffic back to the ONUs. Chapter 3 - Publications 3, 11, 16 and 17 of Section 1.4. Detailed analysis of the LAN emulation schemes outlining the benefits and drawbacks in terms of feasible deployment. Chapter 3 Publication 3 of Section 1.4. Proposal and experimental demonstration of a protection architecture against feeder fibre breaks in a PS-PON. Chapter 4 - Publication 2 and 20 of Section 1.4. Proposal and experimental demonstration of a protection scheme against distribution fibre breaks using interconnections amongst the ONUs inconjunction with a LAN emulation scheme that uses a narrowband FBG placed in the feeder fibre of a PS- PON. Chapter 4 - Publications 7, 12 and 17 of Section

72 Chapter 1 Introduction Proposal and experimental demonstration of a protection scheme against distribution fibre breaks using interconnections amongst the ONUs inconjunction with a LAN emulation scheme that uses a secondary distribution fibre for monitoring the distribution fibre state. Chapter 4 - Publications 7, 13, 16 and 17 of Section 1.4. Theoretical scalability analysis based on power budget for the distribution fibre protection schemes that use interconnections amongst the ONUs. Chapter 4 - Publication 7 of Section 1.4. Proposal and experimental demonstration of a protection scheme against feeder fibre and distribution fibre breaks inconjunction with a LAN scheme using dual fibres. Chapter 4 - Publication 5 of Section 1.4. Proposal and experimental demonstration of an upstream access scheme using E- CDMA in a PS-PON - Chapter 5. A detailed theoretical scalability analysis for E-CDMA based upstream access scheme. Chapter 5. Proposal and experimental demonstration of a secure LAN emulation scheme in a PS- PON with E-CDMA LAN traffic being carried on an RF carrier with a secondary distribution fibre. Chapter 5. Proposal and experimental demonstration of a secure LAN emulation scheme in a PS- PON with E-CDMA LAN traffic being carried on a separate wavelength channel with a FBG placed in the feeder fibre. Chapter 5. Proposal and experimental demonstration of a multiple and secure VPN scheme in a PS-PON using E-CDMA. Chapter 5 - Publications 4 and 14 of Section 1.4. A detailed theoretical analysis to demonstrate the scalability of the E-CDMA VPN scheme. Chapter 5 - Publications 19 of Section

73 Chapter 1 Introduction Proposal and experimental demonstration of a novel signalling mechanism using E- CDMA for a packet based access network. Chapter 6 - Publications 6 and 9 of Section 1.4. A simulation study of the digital implementation of the E-CDMA signalling mechanism using VPI Transmission Maker. Chapter 6. A detailed theoretical analysis to identify the limiting factors of the signalling mechanism Chapter 6 - Publication 8 of Section Publications arising from the work completed in this Thesis 1. N. Nadarajah, M. Attygalle, A. Nirmalathas, and E. Wong, A Novel Local Area Network Emulation Technique on Passive Optical Networks, IEEE Photon. Technol. Lett., vol. 17, pp , May N. Nadarajah, A. Nirmalathas, and E. Wong, Self-protected Ethernet passive optical networks using coarse wavelength division multiplexed transmission, IEE Electron. Lett., vol. 41, pp , Jul N. Nadarajah, M. Attygalle, E. Wong, and A. Nirmalathas, Novel Schemes for Local Area Network Emulation in Passive Optical Networks with RF Subcarrier Multiplexed Customer Traffic, IEEE J. Lightw. Technol., Special Issue Optical Networks 05, vol. 23, pp , Oct N. Nadarajah, E. Wong, and A. Nirmalathas, Implementation of Multiple Secure Virtual Private Networks over Passive Optical Networks using Electronic CDMA, IEEE Photon. Technol. Lett., vol. 18, pp , Feb

74 Chapter 1 Introduction 5. N. Nadarajah, E. Wong, and A. Nirmalathas, Automatic Protection Switching and LAN Emulation in Passive Optical Networks, IEE Electron. Lett., vol. 42, no. 3, pp , Feb N. Nadarajah, E. Wong and A. Nirmalathas, Packet Labeling Technique Using Electronic Code-Division Multiple-Access for WDM Packet-Based Access Networks, IEEE Photon. Technol. Lett., vol. 18, pp , Feb N. Nadarajah, E. Wong M. Attygalle, and A. Nirmalathas, Protection Switching and Local Area Network Emulation in Passive Optical Networks, IEEE J. Lightw. Technol., vol. 24, pp , May N. Nadarajah, E. Wong, and A. Nirmalathas, Performance Analysis of Electronic Code Division Multiple Access based Control Packet Signaling for WDM Access Networks, submitted for publication in IEEE Photonics Technology Letters. 9. N. Nadarajah, A. Nirmalathas, and E. Wong, Packet Labelling using Electronic Code Division Multiple Access Technique for WDM Packet Networks, in Proc. 2 nd International Conference on the Optical Internet and 28 th Australian Conference on Optical Fibre Technology (COIN/ACOFT 03), pp , N. Nadarajah, M. Attygalle, A. Nirmalathas, and E. Wong, LAN emulation in passive optical networks using subcarrier multiplexing, in Proc. 9 th Optoelectronics and Communications conference and 3 rd International Conference on the Optical Internet (OECC/COIN 04), pp , N. Nadarajah, A. Nirmalathas, and E. Wong, LAN emulation on Passive Optical Networks using RF subcarrier multiplexing, in Proc. 17 th Annual Meeting of the IEEE Lasers and Electro-Optics Society (LEOS 04), pp , N. Nadarajah, E. Wong, A. Nirmalathas, and M. Attygalle, Novel Architecture for Protection in Conjunction with Local Area Network Emulation in Passive Optical

75 Chapter 1 Introduction Networks, in Proc. 4 th International Conference on the Optical Internet (COIN 05), pp , N. Nadarajah, A. Nirmalathas, and E. Wong, Protection and LAN emulation in Ethernet Passive Optical Networks, in Proc. 31 European Conference on Optical Communications (ECOC 2005), vol. 3, pp , st 14. N. Nadarajah, E. Wong, and A. Nirmalathas, Secure E-CDMA virtual private network over passive optical networks, Proc. 18 th Annual Meeting of the IEEE Lasers and Electro-Optics Society (LEOS 05), pp , M. Attygalle, N. Nadarajah, and A. Nirmalathas, A Novel technique for Wavelength Reuse in WDM-PON Proc. 18 th Annual Meeting of the IEEE Lasers and Electro- Optics Society (LEOS 05), pp , E. Wong, N. Nadarajah, C.-J. Chae, and A. Nirmalathas, Passive Optical Network Architectures with Optical Loopbacks, Proc. 18 th Annual Meeting of the IEEE Lasers and Electro-Optics Society (LEOS 05), pp , E. Wong, N. Nadarajah, C.-J. Chae, A. Nirmalathas and M. Attygalle, Improved PON architectures for LAN emulation and Protection, Invited talk, in Proc. SPIE International symposium Microelectronics, MEMS and Nana-technology, paper , N. Nadarajah, E. Wong, and A. Nirmalathas, Performance Analysis of Electronic CDMA Signaling in WDM Packet Networks, in Proc. 11 th Optoelectronics and Communications conference (OECC 06), N. Nadarajah, E. Wong, and A. Nirmalathas, Scalability Analysis of Electronic Code Division Multiple Access based Virtual Private Networks over Passive Optical Networks, in Proc. 5 th International Conference on the Optical Internet (COIN 06),

76 Chapter 1 Introduction 20. A. Nirmalathas, N. Nadarajah, E. Wong, M. Atygalle, and C.-J. Chae, Automatic Protection Switching in Passive Optical Network Architectures, Invited talk, in Proc. 5 th International Conference on the Optical Internet (COIN 06), References [1] P. E. Green, Fiber optic networks, Prentice Hall, [2] G. P. Agrawal, Fiber optic communication systems, Wiley-Interscience, [3] R. Ramaswami, and K. N. Sivarajan, Optical Networks: A practical perspective, Morgan Kauffman Publishers, San Francisco, CA, USA, [4] R. Ramaswami, Optical fiber communication: From transmission to networking, IEEE Commun. Mag., vol. 40, pp , May [5] S. S. Wagner, and H. Kobrinski, WDM applications in broadband telecommunication networks, IEEE Commun. Mag., vol. 27, pp , [6] V. W. S. Chan, K. L. Hall, E. Modiano, and K. A. Rauschenbach, Architectures and technologies for high-speed optical data networks, IEEE J. Lightw. Technol., vol. 16, pp , Dec [7] G. Hawley, System considerations for the use of xdsl technology for data access, IEEE Commun. Mag., vol. 35, pp , Mar [8] A. I. Karshmer, and J. N. Thomas, Computer networking on cable TV plants, IEEE Network, pp , Nov [9] C. J. Brunet, Hybridizing the local loop, IEEE Spectrum, pp , Jun [10] M. Humphrey, and J. Freeman, How xdsl supports broadband service to the home, IEEE Network, pp , Jan./Feb [11] D. Veeneman, and B. Olshansky, ADSL for video and data services, in Proc. IEEE International Conference on Communications (ICC 95), pp , [12] C. A. Eldering, F. M. Gardner, N. Himayat, and E. Dickinson, Performance comparison of multiple access techniques in hybrid fibre-coax return systems, In Proc. Optical Fiber Communication conference (OFC 95), paper TuK3, [13] S. Ooghe, J. D. Clercq, I. V. d. Voorde, Y. T'Joens, and J. D. Jaegher, "Impact of the Evolution of the Metropolitan Network on the DSL Access Architecture," IEEE Commun. Mag., vol. 41, pp , [14] D. W. Faulkner, D. B. Payne, J. R. Stern, and J. W. Ballance, "Optical networks for local loop applications," IEEE. J. Lightw. Technol., vol. 7, pp , Nov [15] T. Miki, "Toward the service-rich era (optical access networks)," IEEE Commun. Mag., vol. 32, pp , Feb [16] D. E. A. Clarke and T. Kanada, "Broadband: the last mile," IEEE Commun. Mag., vol. 31, pp , Mar

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78 Chapter 1 Introduction [34] L. G. Kazovsky, and P. T. Poggiolini, "STARNET: A Multi-gigabit-per-second Optical LAN Utilizing a Passive WDM star," IEEE J. Lightw. Technol., vol. 11, pp , May/ Jun [35] P. Ossieur, X. Z. Qiu, J. Bauwelinck, D. Verhulst, Y. Martens, J. Vandewege, and B. Stubbe, "An overview of passive optical networks," in Proc. International Symposium on Signals, Circuits and Systems, vol. 1, pp , [36] N. Chand, P. D. Magill, S. V. Swaminathan, and T. H. Daugherty, "Delivery of digital video and other multimedia services (>1 Gb/s bandwidth) in passband above the 155 Mb/s baseband services on a FTTx full service access network," IEEE J. Lightw. Technol., vol. 17, pp , Dec [37] T. H. Wood, R. D. Feldman, and R. F. Austin, "Demonstration of a cost-effective, broadband passive optical network system," IEEE Photon. Technol. Lett., vol. 6, pp , Apr [38] R. D. Feldman, T. H. Wood, J. P. Meester, and R. F. Austin, "Broadband upgrade of an operating narrowband single-fiber passive optical network using coarse wavelength division multiplexing and subcarrier multiple access," IEEE J. Lightw. Technol., vol. 16, pp. 1-8, Jan [39] I. Van de Voorde, C. M. Martin, I. Vandewege, and X. Z. Oiu, "The superpon demonstrator: an exploration of possible evolution paths for optical access networks," IEEE Commun. Mag., vol. 38, pp , [40] I. Radovanovic, W. V. Ettenn and H. Freriks, "Ethernet-Based Passive Optical Local-Area Networks for fiber-to-the-desk Application," IEEE J. Lightw. Technol., vol. 21, pp , [41] Broadband optical access systems based on passive optical networks (PON), ITU-T Recommendation G [42] Gigabit-capable passive optical networks (GPON): General characteristics, ITU-T Recommendation G.984.1, [43] Ethernet in the first mile task force: IEEE 802.3ah, Draft 3.0b. [44] K.-D. Langer, J. Grubor, and K. Habel, "Promising evolution paths for passive optical access networks," in Proc. International Conference on Transparent Optical Networks (ICTON'04), pp , [45] A. J. Phillips, J. M. Senior, R. Mercinelli, M. Valvo, P. J. Vetter, C. M. Martin, M. O. Van Deventer, P. Vaes and X. Z. Qiu, "Redundancy strategies for a high splitting optically amplified passive optical network," IEEE J. Lightw. Technol., vol. 19, pp , Feb [46] M. Medard, and S. Lumetta, "Architectural issues for robust optical access", IEEE Commun. Mag., vol. 39, pp , Jul [47] R. Cohen, "On the establishment of an access VPN in broadband access networks," IEEE Commun. Mag., vol. 41, no. 2, pp , Feb [48] R. Venkateswaran, Virtual private networks," IEEE Potentials, vol. 20, no. 1, pp , [49] R. Ramaswami, "Multiwavelength lightw. networks for computer communication," IEEE Commun. Mag. vol. 31, pp , [50] S. F. Su, A. R. Bugos, V. Lanzisera and R. Olshansky, "Demonstration of a multiple-access WDM network with subcarrier-multiplexed control channels," IEEE Photon. Technol. Lett., vol. 6, pp ,

79 Chapter 1 Introduction [51] A. E. Willner, M. W. Maeda and J. R. Wullert, "Comparison of central and distributed control in a WDMA star network," In Proc. International Conference on Communications (ICC'92), vol. 2, pp , [52] M. W. Maeda, A. E. Willner, J. R. Wullert, II, J. Patel and M. Allersma, "Wavelength-division multiple-access network based on centralized common-wavelength control," IEEE Photon. Technol. Lett., vol. 5, pp , [53] K. V. Shrikhande, I. M. White, D. Wonglumsom, S. M. Gemelos, M. S. Rogge, Y. Fukashiro, M. Avenarius, and L. G. Kazovsky, "HORNET: a packet-over-wdm multiple access metropolitan area ring network," IEEE J. Sel. Areas Commun., vol. 18, pp , [54] K. Shrikhande, A. Srivatsa, I. M. White, M. S. Rogge, D. Wonglumsom, S. M. Gemelos, and L. G. Kazovksy, "CSMA/CA MAC protocols for IP-HORNET: an IP over WDM metropolitan area ring network," in Proc. IEEE Global Telecommunications Conference, vol. 2, pp , [55] D. Wonglumsom, I. M. White, S. M. Gemelos, K. Shrikhande, and L. G. Kazovsky, "HORNET-A packet-switched WDM network: optical packet transmission and recovery," IEEE Photon. Technol. Lett., vol. 11, pp , [56] G. Kramer, B. Mukherjee, and G. Pesavento, "IPACT a dynamic protocol for an Ethernet PON (EPON)," IEEE Commun. Mag., vol. 40, pp , Feb [57] E. Ringoot, N. Janssens, A. Tassent, J. Angeloupoulos, C. Blondia, and P. Vetter, "Demonstration of dynamic medium access control for APON and SuperPON," in Proc. IEEE Global Telecommunications Conference, vol. 3, pp , [58] F. J. Effenberger, H. Ichibangase, and H. Yamashita, "Advances in broadband passive optical networking technologies," IEEE Commun. Mag. vol. 39, pp , Dec [59] B. St Arnaud, J. Wu, and B. Kalali, "Customer-controlled and -managed optical networks," IEEE Lightw. Technol., vol. 21, pp , Nov [60] E. Wong and C.-J. Chae, "CSMA/CD-based Ethernet passive optical network with optical internetworking capability among users," IEEE Photon. Technol. Lett., vol. 16, pp , Sep [61] E. Wong and C.-J. Chae, "CSMA/CD-based Ethernet passive optical network with shared LAN capability," in Proc. Optical Fiber Communication Conference (OFC'04), vol. 1, [62] E. Wong and C.-J. Chae, Efficient Dynamic Bandwidth Allocation Based on Upstream Broadcast in Ethernet Passive Optical Networks, in Proc. Optical Fiber Communications Conference (OFC'05), vol. 6, [63] E. Wong and C.-J. Chae, "Support of Differentiated Services in Ethernet Passive Optical Networks via Upstream Broadcast Dynamic Bandwidth Allocation Scheme," in Proc. 4 th International Conference on Optical Internet (COIN'05), pp , [64] E. Wong, N. Nadarajah, C.-J. Chae, and A. Nirmalathas, Passive Optical Network Architectures with Optical Loopbacks, in Proc. 18 th Annual Lasers and Electro Optics Society Meeting (LEOS'05), pp , [65] C.-J. Chae, E. Wong, and R. S. Tucker, "Optical CSMA/CD media access scheme for ethernet over passive optical network," IEEE Photon. Technol. Lett., vol. 14, pp , May

80 Chapter 1 Introduction [66] B. N. Desai, N. J. Frigo, A. Smiljanic, K. C. Reichmann, P. P. Iannone, and R. S. Roman, "An optical implementation of a packet-based (Ethernet) MAC in a WDM passive optical network overlay," in Proc. Optical Fiber Communication Conference and Exhibit (OFC'01), vol. 3, pp. WN5-1 - WN5-3, [67] E. Wong, and C.-J. Chae, "Performance of differentiated services in a CSMA/CD-based Ethernet over passive optical network," in Proc. 17 th IEEE Annual Lasers and Electro Optics Society Meeting (LEOS'04), vol. 2, pp , [68] F. -T. An, Kyeong Soo Kim, D. Gutierrez, S. Yam, E. Hu, K. Shrikhande, and L. G. Kazovsky, "SUCCESS: a next-generation hybrid WDM/TDM optical access network architecture," IEEE J. Lightw. Technol., vol. 22, no. 11, pp , [69] Y. -L. Hsueh, W. T. Shaw, L. G. Kazovsky, A. Agata, and S. Yamamoto, "SUCCESS PON Demonstrator: Experimental Exploration of Next-Generation Optical Access Networks," IEEE Optical Communications, vol. 22, pp , Aug

81 Chapter 3 Local Area Network Emulation in Passive Optical Networks 3 Local Area Network Emulation in Passive Optical Networks 3.1 Introduction The passive optical network (PON) technology has been recognised as an efficient solution to facilitate high bandwidth, low cost and fault-tolerant next generation broadband access networks [1-5]. A typical PON architecture employs either a wavelength insensitive passive power splitter such as star coupler (SC) or a wavelength sensitive device such as arrayed waveguide grating (AWG) as a branching device to allow communication between the central office (CO) and the optical network units (ONUs) that are located at the customer premises. The former architecture is referred to as a power splitting PON (PS-PON) while the latter architecture is referred to as a wavelength division multiplexed PON (WDM-PON) [5]. A PS- PON system consists of a transmitter at the CO, a common feeder fibre, a SC as a power splitter/combiner, and N ONUs in various locations. In PS-PONs, the downstream transmissions from the CO to the ONUs are carried on a wavelength at 1.5 µm window while the upstream transmissions from the ONUs to the CO are carried on a wavelength at 1.3 µm window. At the CO and the ONUs, these upstream and downstream wavelength channels are combined and separated using coarse wavelength division multiplexing (CWDM) devices. Apart from the conventional upstream and downstream transmissions between the CO and the ONUs, customers of this type of PON may require private communication links between themselves for various computer applications and telecommunication services, such as distributed data processing, broadcast information systems, teleconferencing, interactive video games and data storage. Moreover, some customers leasing several floors within a building may require their own private network such as local area network (LAN) apart from the standard communication links with the CO. One possible solution to provide a customer networking amongst the ONUs is to install two kinds of networks separately, whereby one

82 Chapter 3 Local Area Network Emulation in Passive Optical Networks network is used for the public telecommunication services while the other can be used for the private communications amongst the customers. For a PS-PON in which customers share the same upstream wavelength, such intercommunication between the customers may be realised by overlaying a separate network in which each ONU is connected to all other ONUs via a point-to-point optical link. However, installing an additional network incurs heavy cost, and it also becomes impractical and inefficient to connect to each customer in the network [6, 7]. Consequently, there have been increasing interests in deploying point-to-point customer communication links via LAN emulation over an existing PON infrastructure [8, 9]. Reuse of the PON infrastructure to facilitate intercommunication links between customers of the same PON can greatly reduce the cost and management issues of the network. Overlaying a LAN on the existing PON incurs a minimum additional cost since it utilises the existing facility. The overlaid network can be used to interconnect several customers in scattered buildings to form a group of community. The resulting PON system enables fibre to the premises so that the tenants in a building can subscribe to telecommunications and internet services individually while keeping their own network. In these PS-PON architectures, bursty data traffic from each customer opens an opportunity to use the statistical multiplexing of the PON to increase the efficiency of the network. The overlaid virtual PON technologies bring multiple campuses and multitenant business buildings onto a same optical fibre facility and therefore making savings on fibre facilities capital and ongoing operational costs [10, 12]. This chapter discusses higher layer or router based LAN emulation techniques that were proposed by the EFM group and several optical layer LAN emulation schemes with experimental demonstrations. Section 3.2 gives a brief overview of the higher layer LAN emulation techniques for a PS-PON. The advantages and disadvantages of these proposals are described. Optical layer LAN emulation techniques were proposed and experimentally demonstrated to reduce the problems associated with the higher layer LAN emulation techniques. Section 3.3 briefly discusses a few of the previously proposed optical layer LAN emulation techniques and their advantages and disadvantages. Section 3.4 describes two different optical layer LAN emulation schemes, whereby the customer traffic that is transported amongst the ONUs is carried on a RF subcarrier, while the upstream access traffic to the CO is carried at baseband. As both these signals are electrically multiplexed before the

83 Chapter 3 Local Area Network Emulation in Passive Optical Networks transmissions, only one optical source is used at each ONU. Section describes the optical layer LAN emulation scheme in which a narrowband fibre Bragg grating (FBG) is used at the remote node (RN) and this scheme is experimentally verified using single and double notch FBG filters. Section describes the second optical layer LAN emulation scheme with the use of a secondary distribution fibre between the RN and the ONUs with experimental demonstrations. Section 3.5 compares both these LAN emulation schemes in terms of bandwidth requirements of the signals, dispersion tolerance of the RF LAN data signals, required stability of the optical source at each ONU and power budget of all transported signals. 3.2 Higher layer LAN emulation A number of higher layer LAN emulation schemes have been proposed to the Ethernet in the First Mile (EFM) alliance IEEE 802.3ah [13, 14]. The standard working group plans to provide a mechanism whereby the optical line terminal (OLT) and ONUs can emulate a bundle of point-to-point links within a PON. These techniques are mainly focused on higher layer protocols in which specific ONUs are assigned unique PON tags to establish point-topoint connections with each other. LAN traffic, or the customer traffic originating from one customer to be delivered to all or some customers within the same PON, is carried on the upstream wavelength channel along with the upstream access traffic to the CO. At the CO, the received upstream packets from each customer are routed back to the ONUs in the downstream direction. At the CO, bridges and/or routers are employed to separate the LAN traffic from the upstream access traffic to the CO. Figure 3.1 shows the protocol layer arrangement, which is used to emulate a shared LAN. This kind of solution allows standard Ethernet-compatible devices such as bridges, routers, and hosts to connect to the Ethernet PON-based LAN. However, this mode of point-to-point LAN emulation operation requires one particular frame from the OLT to be serially transmitted to each receiving ONU. Another possibility that has been discussed by IEEE 802.3ah is to have the OLT automatically reflect all received packet frames from any ONU to all ONUs except for the originating ONU. At the OLT, only one media access control (MAC) is present, which transmits every packet frame to all ONUs. In this scenario, the ONUs are not

84 Chapter 3 Local Area Network Emulation in Passive Optical Networks isolated from one another as each ONU receives all of the other ONUs point-to-point traffic. Furthermore, all upstream (ONU to OLT) traffic, even which not destined to any other ONU, is reflected down to the other ONUs. By employing better frame forwarding mechanism in the bridges and by avoiding unnecessary re-broadcasting of all packets, this type of naive shared LAN emulation techniques can be modified to improve the efficiency of LAN emulation and upstream and downstream channel bandwidth utilisation. OLT Emulator 2 MAC Emulator 1 PHY PHY P-P Em MAC 1 PHY P-P Em MAC 2 ONUs PHY P-P Em MAC 3 Figure 3.1: A simple protocol layer arrangement model for shared LAN Emulation. To reduce the deficiencies in the above shared LAN emulation techniques, advanced upper layer shared LAN emulation (ULSLE) techniques are also considered [15]. In the advanced ULSLE techniques, most traffic is not reflected back to the ONUs as in previous techniques. In any ULSLE techniques, customer traffic needs to be transmitted to the OLT on the upstream wavelength channel and redirected to the appropriate ONUs using the downstream wavelength channel. The bridges or routers that are equipped at the OLT need to be very complex to obtain relatively higher efficiency of transmission bandwidth of the wavelength channels. These bridges or routers must be capable of supporting higher layer protocols, thereby potentially increasing the cost and complexity of the network. Furthermore, the effective downstream channel bandwidth is reduced as the LAN traffic is routed back to the ONUs on the downstream wavelength channel. Moreover, the redirected LAN traffic needs to

85 Chapter 3 Local Area Network Emulation in Passive Optical Networks be separated from the downstream traffic using complex filtering mechanisms that are employed at the ONUs. Even though using a single fibre access facility to communicate with another user in the network is more efficient and practical, giving direct wavelength access to each customer for the transmission of LAN traffic reduces the efficiency. The higher layer routers attached to each ONU are not designed to communicate with many neighbouring routers in the network simultaneously as router designs are not intended to terminate many individual connections. Therefore, a simplified LAN that is overlaid on the existing PON with high flexibility is required for the intercommunications amongst the customers in the PON. By comparison, emulating point-to-point links amongst customers directly on the optical layer in the PON can effectively overcome several drawbacks [16-23]. In the next section, we discuss a few previously proposed optical layer LAN emulation techniques. 3.3 Optical layer LAN emulation schemes Optical layer LAN emulation techniques are more efficient in bandwidth utilisation in upstream and downstream wavelength channels as the customer traffic is physically redirected to each customer rather than via the CO. Therefore, the customer traffic does not get redirected back to the respective ONUs using the upper layer bridges or routers located at the CO. This effectively reduces the required higher layer interfaces at the CO. The entire bandwidth in the upstream and downstream wavelength channels is used for the transmission of signals between the ONUs and the CO. Moreover, no further complex filtering mechanisms are required at the ONUs to separate the LAN traffic from the downstream traffic. The use of more complex filtering and packet forwarding schemes that are used in the bridges or routers at the CO are also eliminated and therefore reduces the cost and complexities associated with the operation at the ONUs and CO. There have been a few experimental demonstrations for the optical layer LAN emulation techniques. A carrier sense multiple access with collision detection (CSMA/CD) protocol based shared LAN capability was proposed [16, 17], whereby an N N SC with secondary distribution fibres were used to redirect the upstream signals to each ONU. In this scheme, the effective upstream channel bandwidth is reduced. Moreover, the performance of this scheme is limited by the CSMA/CD protocol, whereby shorter distribution fibre lengths and low upstream transmission rates are required for higher transmission efficiency. Another optical layer LAN emulation scheme uses optical switches

86 Chapter 3 Local Area Network Emulation in Passive Optical Networks for selecting the mode of operation (customer to customer transmission or conventional upstream and downstream transmissions) [18]. In this scheme, simultaneous modes of operations cannot be carried out and therefore shows inefficient performance. Optical layer LAN emulation techniques that require an additional wavelength source at each ONU for customer intercommunication were also considered [19-21]. Nonetheless, these schemes necessitate an additional optical transceiver capable of transmitting and receiving LAN wavelength channel at each ONU. A scheme with common regenerator is also proposed to emulate a LAN over the PON [22, 23]. In this scheme, the common regenerator in the PON plant increases the cost of installation and operation of the network. As in previously proposed optical layer LAN emulation schemes, the use of an additional optical source at the ONUs and common regenerators to provide LAN emulation over PON is not effective in customer access networks, and therefore an electronic multiplexing mechanism may be implemented to simultaneously support conventional upstream access traffic and LAN traffic on a single wavelength channel. Time division multiplexing (TDM), subcarrier multiplexing (SCM) and code division multiplexing (CDM) can be used to combine two separate data streams into a single wavelength channel. TDM techniques that are used for LAN emulation are proposed [16, 17]. Electronic code division multiple access (E- CDMA) techniques that are used to provide virtual private networking (VPN) capabilities in a PON are described later [24, 25]. We have proposed to implement LAN emulation using SCM [26-29] and in the next section the detailed characteristics of RF SCM based optical layer LAN emulation is presented. 3.4 LAN emulation using RF subcarrier multiplexing RF SCM is a scheme where different signals are multiplexed in RF domain and transmitted using a single wavelength channel. A significant advantage of SCM is that microwave devices are more mature than the optical counterparts. The stability of these devices is also better than the optical devices. Moreover, advanced modulation formats can be applied to the SCM systems according to the requirements. Due to the simple and low cost implementation, SCM has been used to transmit digital optical signals for LANs [30-34]. The optical layer LAN emulation schemes that are discussed in this chapter also use SCM

87 Chapter 3 Local Area Network Emulation in Passive Optical Networks RF LAN data λ u Upstream baseband data f L f L Figure 3.2: Upstream access traffic to the CO is carried at baseband; LAN traffic to other ONUs in the PON is carried on an RF carrier that is placed outside the bandwidth of the upstream baseband data. Two separate schemes for LAN emulation using RF subcarrier multiplexed LAN traffic are described in this chapter. Figure 3.2 shows that the upstream access traffic to the CO is carried at baseband, and is referred to in this work as upstream baseband data, while LAN traffic is carried on an RF carrier that is chosen to be out-of-band from the upstream access traffic [26-29]. Scheme 1 uses a narrowband FBG placed in the feeder fibre close to the SC for the spectral filtering and reflection of the LAN traffic back to the ONUs while the upstream access traffic is carried to the CO at baseband [26-28]. Scheme 2 uses a secondary distribution fibre between the SC and each ONU for the redirection of all upstream signals. In this scheme, at each ONU, the upstream signals are detected and LAN traffic is electrically separated from the upstream baseband data [28, 29]. Compared to higher layer LAN emulation proposals and previously demonstrated optical layer LAN emulation schemes, this proposed technique is more feasible as it requires cheaper electronics at each ONU, reduces the complexity and provides efficient bandwidth utilisation of the downstream and upstream wavelength channels. In the following sections, we discuss the RF SCM based LAN emulation schemes and analyse the performances of both schemes LAN emulation using RF subcarrier multiplexing and narrowband FBG This section describes the LAN emulation scheme that employs a narrowband FBG placed close to the SC in the feeder fibre of the PON [26-28]. The schematic diagram of this scheme, denoted Scheme 1, is shown in Figure 3.3. At each ONU, upstream baseband data

88 Chapter 3 Local Area Network Emulation in Passive Optical Networks and LAN data signals are generated for the transmission in the upstream direction. LAN data is amplitude modulated onto an RF carrier that is chosen out-of-band from the upstream baseband data using a voltage controlled oscillator (VCO). These signals are then electrically combined and modulated onto the upstream wavelength channel λ u. Upstream Receiver Downstream Transmitter λ u CO WDM 1.3/1.5 µm λ d Feeder Fibre FBG λ u -f L 1xN SC Downstream Receiver WDM 2 3 LAN data 1.3/1.5 µm Receiver 1 Upstream Transmitter ONU 1 Transmitted upstream baseband data λ u Reflected RF sideband ONU N λ u f L f L f L Figure 3.3: Physical architecture for LAN emulation (Scheme 1) using RF subcarrier multiplexing with a narrowband fibre Bragg grating placed in the feeder fibre for the optical separation and loopback. A narrowband FBG is placed in the feeder fibre close to the 1 N SC, whereby N corresponds to the number of ONUs. The Bragg wavelength of the FBG is chosen such that FBG reflects one of the optically modulated RF sidebands and broadcast the LAN data to all ONUs. Alternatively, a double notch FBG may also be used such that both optically modulated RF sidebands are reflected back to the ONUs while the upstream baseband data is transmitted to the CO. Even though the optical source at the ONU needs to be wavelength specific, by choosing a FBG accordingly the practical implementation of Scheme 1 can be carried out. As LAN data is amplitude modulated on to the RF carrier, which generates amplitude shift keying (ASK) signals, it can be recovered by direct detection as will be shown through theoretical modelling. This allows the LAN data receiver to be of low bandwidth even though a high frequency RF carrier is used for the modulation of the LAN data. The

89 Chapter 3 Local Area Network Emulation in Passive Optical Networks bandwidth of the LAN data receiver may only be in the order of the transmission bit rate of the LAN data Transmission protocol in the upstream direction RF LAN data RF LAN data Upstream baseband data Upstream baseband data One time slot One time slot One time slot Figure 3.4: Upstream transmission protocol using TDMA. Upstream baseband data and LAN data are transmitted simultaneously in the preassigned timeslot. In this scheme, the downstream signal transport from the CO to the ONUs is based on TDM, whereby the packets in the downstream direction are broadcast to all ONUs and each ONU extracts the packets that are destined to it using the MAC address on the packet header. The upstream transmission of the signals is based on the time division multiple access (TDMA) protocol. As shown in Figure 3.4, LAN data may be concurrently transmitted in the same preassigned time slot that is used for baseband data transmission to the CO. Likewise, the transmission of baseband data to the CO may be carried out even in the absence of the LAN data. Similarly, LAN data transmission may be carried out in the absence of the upstream baseband data provided that a time slot is assigned to an ONU for the transmission by a request by the ONU to the CO for the LAN data transmission. If the LAN data is transmitted in the absence of the upstream baseband data, while the optically modulated RF sidebands are reflected back to the ONUs using the narrowband FBG, the residual optical carrier reaches the upstream baseband data receiver at the CO without any data encoded onto it. This leads to a flooding of light in the upstream baseband data receiver. However this could effectively be controlled as CO has the knowledge of the upstream transmissions from each ONU. The use of TDMA protocol for the transmission of both signals in the upstream direction reduces the requirement for separate complex protocol for the transmission of the LAN data. It should also be noted that dynamic bandwidth allocation (DBA) scheme for the upstream baseband data transmission from the ONUs to the CO can also be implemented [35-37]. As DBA scheme is implemented for the upstream transmissions, the allocated time slot to each ONU

90 Chapter 3 Local Area Network Emulation in Passive Optical Networks varies. In this case, the maximum allocated time duration for the transmission of the LAN data for each ONU is also limited to this time duration. This imposes a limitation on the TDMA protocol used for the transmissions of both signals in the same time slot as an ONU may have large amount of data information to be sent to other ONUs, while only smaller amount of information is required to be sent to the CO. In this scenario, an ONU would have to wait for the next transmission cycle for the transmission of the remaining LAN data. On the other hand, an ONU may have smaller amount of LAN data compared to the amount of upstream baseband data. In this scenario, the time duration allocated for the LAN data transmission is not fully utilised leading poor efficiency of the TDMA protocol used for the LAN data transmissions. However, the use of a single access control protocol for the transmission of both upstream baseband data to the CO and LAN data to other ONUs in the PON simplifies the required transmission control at the ONUs Optical combination of baseband and RF subcarrier multiplexed data In both LAN emulation schemes, the upstream access traffic to the CO is carried at baseband while the LAN data is carried on a RF subcarrier frequency. At each ONU, these signals are electrically combined and then modulated onto an optical carrier for the transmission in the upstream direction. For the electrical combination, a RF diplexer is required such that the electrical crosstalk between these signals is reduced to obtain higher efficiency in the transmission of signals. The diplexer used at each ONU for the combination of the signals introduces excess resistive loss of the combined signals. A wideband RF power combiner can also perform similar functions; however as the operating frequency band of the power combiner is larger, the crosstalk level between the combined signals may also be higher leading to higher distortion resulting in reduced receiver sensitivity. In the experiment that was carried out to demonstrate the capability of the LAN emulation scheme using a narrowband FBG, the combination of the baseband data and RF SCM signals were performed using a dual electrode Mach-Zehnder modulator (DEM). One arm of the DEM was driven by the dc biased upstream baseband data while the other arm was driven by the RF SCM LAN data. Differential driving of the two arms acts to combine these signals onto the upstream wavelength channel. Given that the modulator is driven by two different

91 Chapter 3 Local Area Network Emulation in Passive Optical Networks signals at baseband and an RF frequency, the operating point of the DEM and the driving signal amplitudes need to be carefully optimised to achieve better transmission characteristics for both signals. This can easily be demonstrated by investigation of bias points in conjunction with the modulator transfer function. The signal applied to the DEM can be written as [38, 39] V () t = V aα () t + bβ () t ( 2πf t + φ ) d ( s + c) cos Equation 3-1 where, V d is an arbitrary voltage, α ( t) = 0, 1 is the baseband data, av d represents the amplitude of baseband data, () t = 0, 1 β is the RF SCM LAN data, bv d is the amplitude of the RF SCM LAN data, cv is the bias voltage, f is the RF subcarrier frequency, and φ is the d phase of the RF carrier frequency. As the electrical bandwidth of the modulator is larger than the RF subcarrier frequency, then transfer function of the modulator can be expressed as s P P () t = Pout f ( V () t ) () t = P f V ( aα() t + bβ () t ( 2πf t + φ) out ( cos + c)) d s Equation 3-2 Here, P out is the maximum output power. Figure 3.5 illustrates the relationship between the modulator transfer function and the upstream baseband data and RF SCM LAN data. As can be seen from Figure 3.5, increasing the extinction ratio of the upstream baseband data increases the performance of that signal. As the extinction ratio of the upstream baseband data is increased, the separation of the operating points in the DEM is increased. These points decrease the power of the RF SCM LAN data and therefore induce nonlinear distortion of the RF SCM LAN data channel, reducing the overall signal to noise ratio (SNR) and therefore increasing the interchannel interference (ICI) between the upstream baseband data and the RF SCM LAN data. For the efficient transmission of both signals an optimum operating point in the DEM is required to reduce the SNR degradations, nonlinear distortions and ICI. The equation 3-2 can be expanded as a Taylor series around the point b=0 as shown in the equation 3-3, which shows the expansion up to the third order

92 Chapter 3 Local Area Network Emulation in Passive Optical Networks Optical Output P 1 P 0 V 0 V 1 V π Electrical Input Figure 3.5: Upstream baseband data and RF SCM LAN data signal operating points on the dual electrode Mach-Zehnder modulator transfer function. P P () t out f ( aα() t + c) + f ( aα( t) + c) bβ ( t) cos( 2πf t + φ) 1 + f f ( aα() t + c) b β ( t) cos ( 2πf t + φ) ( aα() t + c) b β ( t) cos ( 2πf t + φ ) s s s Equation 3-3 As the spectral components of interest are around f = 0 for upstream baseband data and f = for the RF SCM LAN data, using the relations β 2 ( t) = β 3 ( t) = β ( t) and expansions for f s cos 2 3 ( x) and cos ( x) the equation 3-3 can be rewritten as P P () t out f + ( aα() t + c) + f ( aα( t) + c) bβ ( t) cos( 2πf t + φ ) 1 4 f ( aα() t + c) b β ( t) + f ( aα() t + c) b β ( t) cos ( 2πf t + φ) 3 s s Equation 3-4 From the above equation, it could be said that the first two terms of the equation correspond to the desired signals, upstream baseband data and RF SCM LAN data. The last two terms relate to the nonlinear beating between these two signals. The desired upstream baseband

93 Chapter 3 Local Area Network Emulation in Passive Optical Networks data signal is given by f ( aα () t + c) and is dependent only on the amplitude of the upstream baseband data and the bias conditions. However RF SCM LAN data signal given by ( aα () t + c) bβ ( t) cos( 2πf t + φ) f1 s depends on the amplitude of both signals and the bias conditions. For the upstream baseband data bits 0 and 1 s, the operating points are V 0 = Vd c and V 1 = Vd ( a + c) respectively. The output upstream baseband data levels can be expressed as P ( V ) and P f ( V ) 1 Pout f 1 P0 = =. The instantaneous power of the RF SCM LAN data can be given as P f ( V ) b P f ( V )b out and [38, 39]. out 1 1 out 0 0 For the DEM, the equation 3-2 can be simplified to P π V () t = P ( ) = out f V Pout 2 Vπ cos 2 Equation 3-5 where, V is the switching voltage of the modulator and P is the output power when V = 0. π Assuming V d = V π and substituting equation (3-5) in (3-1) gives out P π = out cos 2 ( s + c) 2 () t P aα () t + bβ () t cos( 2πf t + φ ) Equation 3-6 ] The driving voltage range [ V π,0 is considered. If the RF SCM LAN data is absent, then the optimum biasing point becomes V 0 = Vπ and V 1 = 0. This gives P 1 = Pout and P 0 = 0 with infinite extinction ratio for the upstream baseband data. If the upstream baseband data is absent, then optimum point is V 0 V = V π 1 =. 2 If the upstream baseband data and the RF SCM LAN data signals are combined together, then the condition for interference being minimum would be f1 ( a + c) = f1(c) [38, 39]. This corresponds to the operating point for upstream baseband data signal being the middle point of the transfer function of the modulator with upstream baseband data signal data values being symmetric around this point. To obtain interchannel crosstalk free operation to optically

94 Chapter 3 Local Area Network Emulation in Passive Optical Networks combine upstream baseband data and RF SCM LAN data signals, the optimum bias point should be the middle point of the DEM. With this optimisation of bias points and driving amplitudes in mind, experiments were carried out to demonstrate the feasibility of the optical layer LAN emulation technique using a narrowband FBGs placed in the feeder fibre close to the SC. Two separate experiments were conducted whereby the first one uses a single notch FBG such that only one optically modulated RF side band that consists of LAN data is filtered and reflected back to all ONUs. The second experiment uses a double notch FBG for the spectral filtering of LAN data from the upstream baseband data. The following sections describe the experiment demonstrations and the results. The results from both the experiments are compared in terms of the BER measurements Experimental demonstration using a single notch FBG λ d = nm 10 km SMF MZM Circulator 1 FBG Gb/s 1.25 Gb/s Receiver 4x4 Star Coupler 2.2 km SMF 2.2 km SMF 5 GHz RF Oscillator 155 Mb/s Receiver Circulator Gb/s Receiver Mb/s λ u = nm Dual electrode modulator Bias Tee DC 1.25 Gb/s Figure 3.6: Experimental setup to demonstrate the feasibility of Scheme 1 for LAN emulation using a single notch narrowband FBG for the spectral separation of LAN data back to the ONUs. Figure 3.6 shows the experimental setup to demonstrate and verify Scheme 1, whereby a single notch FBG is used for the spectral filtering of the LAN data from the upstream

95 Chapter 3 Local Area Network Emulation in Passive Optical Networks baseband data. A downstream signal of pseudo random bit sequence (PRBS) non return to zero (NRZ) data at 2.5 Gb/s was modulated onto the downstream wavelength channel λd of nm using a DEM and transmitted to the ONUs through a 10 km standard single mode (SSMF) feeder fibre, a 4 4 SC and a 2.2 km distribution fibre. For the upstream transmission, LAN data of 155 Mb/s PRBS NRZ was amplitude modulated onto the RF frequency at 5 GHz using an RF mixer. RF amplifiers were used to compensate for the losses in the RF mixers and to boost the RF power of the upconverted RF LAN data before the optical modulation. Then, 1.25 Gb/s PRBS NRZ baseband data was optically combined with the upconverted RF LAN data using a DEM [38, 39]. However, both signals can be electrically combined and directly modulated onto the upstream wavelength channel [40-43]. The former was chosen due to the high loss of the available RF combiner at 5 GHz frequency. The upstream wavelength channel λ u was chosen to be nm such that one of the optically modulated RF subcarrier sidebands matches to the Bragg wavelength of the FBG. 0 Reflection (db) Wavelength (nm) Figure 3.7: Reflection profile of the single notch narrowband FBG used in the experiment to demonstrate the feasibility of Scheme 1. Figure 3.7 shows the reflection profile of the FBG used in the experiment. The FBG has approximately 99.7% peak reflectivity with a 3 db reflection bandwidth of GHz (0.029 nm). It should also be noted that it has a very flat response over reflecting bandwidth. Optical

96 Chapter 3 Local Area Network Emulation in Passive Optical Networks circulators were used at the OLT and ONU ends to separate the upstream and downstream wavelength channels. The use of CWDM devices instead of optical circulators would have reduced the backscattered light in the dowsntream data and upstream data receivers; however appropriate CWDM couplers were not available during the experiment. In the experiment, the unused ports of the SC were terminated with optical isolators to reduce the reflections. Both 1.25 Gb/s upstream baseband data and 2.5 Gb/s downstream data were detected using 2.5 Gb/s p-i-n receivers, while 155 Mb/s LAN data was recovered using a 2.5 Gb/s APD receiver. A series of experiments were conducted to examine the crosstalk effects of one signal on the other. Each signal was transmitted in the absence and presence of the other signals and bit-error-rate (BER) was measured for each case. When both upstream baseband data and RF LAN data were modulated for the upstream transmission, the modulation index of the upstream baseband data was reduced to approximately 53% such that the composite signal was modulated within the linear region of the DEM response to avoid nonlinear distortion of the signals. The BER meaurements of the 2.5 Gb/s downstream data were performed in the absence of the RF LAN data and vice versa since a wavelength selective device was not implemented to separate λ u and λd. If λ u is chosen in the 1.3 µm wavelength window with λ d in the 1.5 µm wavelength window, a low cost CWDM device could be used for the wavelength separation Optical Spectra Figure 3.8 shows the optical spectra at the FBG for the transmitted and reflected portions of the upstream signals measured at the FBG using an optical spectrum analyser with 2.5 GHz resolution bandwdth. Before the filtering of the RF LAN data using the FBG, the optically modulated RF sidebands that contain the LAN data have approximately 10 db less optical power compared to the main optical carrier that carries the upsteam baseband data. The filtered optically modulated RF side band of the transmitted spectrum shows 20 db suppression compared to the spectrum before the filtering. In the reflected spectrum, the suppression of the optical carrier from the the reflected optically modulated RF sideband was 17 db

97 Chapter 3 Local Area Network Emulation in Passive Optical Networks 0-10 Transmitted Reflected Optical Power (dbm) Before FBG 20 db 17 db Wavelength (nm) Figure 3.8: Observed optical spectra at the narrowband FBG used in the feeder fibre in Scheme 1. As can be seen from the optical spectrum for the transmitted upstream signals before the FBG, there are multiple RF lobes. This is a result of nonlinear phenonema of the optical modulator whereby the intermodulation products of the RF SCM LAN data occuring at multiple of the fundamental frequency of 5 GHz. In the refelected spectrum these RF sidelobes are well suppressed due to filtering used by the narrowband FBG that had a flat passband. Moreover, as a low bandwidth optical receiver is used for the detection of LAN data, the remaining low power intermodulation products are not expected to cause any significant penalty for the recovered LAN data. As shown in Figure 3.9, the optical spectra observed at the input of the downstream data receiver at ONU and the upstream baseband data receiver at the CO show low crosstalk of the signals that are due to the back scattered light in the fibre. The crosstalk in the upstream baseband data receiver and downstream data receiver were -23 db and -25 db respectively. These crosstalk levels are not expected to induce any power penalty for the recovered 2.5 Gb/s downstream data and 1.25 Gb/s upstream baseband data as can be confirmed in the BER curves. The crosstalk levels can further be suppressed using CWDM filters and by choosing the upstream and downstream wavelength channels appropriately

98 Chapter 3 Local Area Network Emulation in Passive Optical Networks Optical Power (dbm) 0-10 Downstream Receiver -20 Upstream Receiver db 23 db Wavelength (nm) Figure 3.9: Observed optical spectra at the upstream baseband data and downstream data receivers BER Results Figure 3.10 shows the measured BER curves for all three signals. Figure 3.10 (a) shows the BER measurements for the LAN data. No penalty was observed for the transmission through the entire link with and without the upstream baseband data. As the multiplexing of upstream baseband data and RF SCM LAN data signals are carried out for the modulation on the upstream wavelength channel, the interchannel interference (ICI), which occurs when the RF spectra of different signals overlap, is a potential problem that can degrade the performance of both overlapped signals and therefore resulting in power penalty [44]. The ICI caused by the upstream baseband data to the RF SCM LAN data can be severe as the ICI from a wideband signal to a narrowband signal can cause larger power penalty than vice-versa [45]. Moreover, the resulting power penalty of the recovered data also depends on the power of each signal. The ICI can be treated using bandlimiting electrical filters, whereby the upstream baseband data can be spectrally shaped using a low pass filter (LPF), while the RF modulated LAN data can be limited using a bandpass filter (BPF). In the experiment, neither a LPF was used to shape the pulse of the upstream baseband data nor a BPF was used to limit the spectral overlap of the RF LAN data. As the RF LAN data at 5 GHz was placed after the 4 th sidelobe of the 1.25 Gb/s upstream baseband data, the ICI from the upstream baseband data was negligible. As the power of the 4 th sidelobe of the upstream baseband data was lower, the ICI from the upstream baseband data to the upconverted RF LAN data was not significant

99 Chapter 3 Local Area Network Emulation in Passive Optical Networks Mb/s RF LAN data 1.25 Gb/s upstream baseband data 2.5 Gb/s downstream data (a) (b) (c) -6 Log 10 (BER) Back to back Transmission Complete link With upstream baseband data Back to back Transmission Complete link With RF LAN data With upstream & downstream signals Received Optical Power (dbm) Back to back Transmission Complete link With upstream signals Figure 3.10: Measured BER plots for 2.5 Gb/s downstream data, 1.25 Gb/s upstream baseband data and 155 Mb/s LAN data for the LAN emulation scheme using a single notch narrowband FBG placed in the feeder fibre close to the SC for the spectral separation and reflection of LAN data. The penalty for the 1.25 Gb/s upstream baseband data transmission compared to back to back (B-B) measurements was 2.1 db. This penalty is due to the reduction of modulation index as mentioned earlier. As the upconverted RF LAN data was placed at a null of the 1.25 Gb/s upstream data spectra, the ICI from the RF LAN data to the upstream baseband data is negligible. Therefore, no significant power penalty for the upstream baseband data was meaured in the presence of the upconverted RF LAN data. The penalty in the presence of the downstream data signal in the link was also negligible as the crosstalk of the backscattered light of the downstream wavelength channel into the upstream baseband data receiver was lower than -23 db from the upstream wavelength channel. A transmission penalty of 0.15 db was measured for the 2.5 Gb/s downstream data through the entire link compared to B-B measurements. An additional penalty of 0.15 db was observed in the presence of the upstream signals. These penalties are possibly a result of experimental errors. The insets of Figure

100 Chapter 3 Local Area Network Emulation in Passive Optical Networks show the respective eye patterns for the recovered data signals. The difference in the shapes are possibly due to the difference of the LPF profiles used after the detection. The LPFs used after the detection had a very sharp cutoff and nonlinear phase response and therefore the eye displays a different shape. For the 2.5 Gb/s downstream data, the shape of eye is caused by the limiting amplifier used after the detection. For the 155 Mb/s LAN data, the non-symmetric eye diagrams obtained from the output signals indicate that a nonlinear phenomena has occurred during the optical filtering process. The non-symmetrical eye diagram arises from the inter-modulation products that fall in the selected RF subcarrier signal. As can be observed from Figure 3.8, RF SCM LAN data has many side-lobes due to relatively higher modulation index values for the RF SCM LAN data signal and therefore increases the intermodulation noise that arises from the nonlinear relation between the current and the optical power. The nonlinear effect due to the optical pre-filtering can be neglected if the optical modulation index of the SCM signal is low. As occurs in optical pre-filtering in WDM systems [46], the non-flat pass-band response of the optical filter impairs the RF SCM LAN data signal that passes through it [32, 47]. The signal distortion due to linear filtering effects will reduce the aperture of the eye diagram. However, the narrowband FBG that was used in the experiment had a flat pass-band for more than adequate bandwidth for the 155 Mb/s LAN data and therefore it is expected that the optical filtering of the RF SCM LAN data does not degrade the performance of the recovered LAN data Experimental demonstration of LAN emulation using double notch FBG Section described the experimental demonstration and the results of the LAN emulation scheme, whereby a single notch FBG is used. However, a double notch FBG can also be used for the spectral filtering of both optically modulated RF sidebands that contain the LAN data. In this case, both filtered optically modulated RF sidebands that contain the LAN data reach the ONUs and therefore the optical power of the received LAN data increases twice compared to that of in the scheme whereby a single notch FBG is used. Moreover, it will be shown later that the detection of the LAN data from both the optically modulated RF sidebands using a low bandwidth receiver does not result in any power penalty caused by the dispersion through the fibre

101 Chapter 3 Local Area Network Emulation in Passive Optical Networks λ d = nm 10 km SMF MZM Circulator Double 2.5 Gb/s notch FBG 1.25 Gb/s Receiver 2.2 km SMF 4x4 Star Coupler 2.2 km SMF GHz RF Oscillator 155 Mb/s Receiver Circulator Gb/s Receiver Mb/s λ u = nm Dual electrode modulator Bias Tee DC 1.25 Gb/s Figure 3.11: Experimental setup to demonstrate the feasibility of LAN emulation using a double notch FBG placed in the feeder fibre close to the SC for the spectral separation and reflection of RF LAN data back to the ONUs. The experimental setup shown in Figure 3.11 to demonstrate the LAN emulation using a double notch FBG is the same as the one described in section except for the FBG used. In this experiment, a double notch FBG was used instead of the single notch FBG. As the notches of this double notch FBG were at different wavelengths, the wavelengths of the upstream and downstream channels were also changed. The reflection bandwidth of the double notch FBG was also different to the single notch FBG that was used earlier. Therefore, the RF carrier frequency that carries the LAN data was also increased to GHz. The transmission rates of the downstream data, upstream baseband data and LAN data remained at 2.5 Gb/s, 1.25 Gb/s and 155 Mb/s as before. Even though the RF carrier frequency is changed, the LAN data receiver setup has not changed as the LAN data is directly detected. Moreover, the receiver setups for the downstream data and upstream baseband data have not changed since the transmission rate of these signals have remained the same

102 Chapter 3 Local Area Network Emulation in Passive Optical Networks 0 Transmission (db) Wavelength (nm) Figure 3.12: Transmission profile of the double notch FBG used in the experimental demonstration for LAN emulation. Figure 3.12 shows the transmission profile of the double notch FBG used in the experiment. It has two notches at nm and nm and each notch has approximately 100% peak reflectivity with the 3 db reflection bandwidth of GHz (0.114 nm). The RF carrier frequency of GHz was chosen such that that both optically modulated RF sidebands fall in the notches of the double notch FBG Optical Spectra Figure 3.13 shows the optical spectra at the double notch FBG that shows the transmitted and reflected portions of upstream optical signals. Before the spectral separation, the optical power of each of the optically modulated RF sidebands that contain LAN data was approximately 10 db lower compared to the main optical carrier that carries the upstream baseband data. The reflected spectra shows approximately 10 db suppression of the optical carrier compared to the optically modulated RF sidebands. This suppression is lower than that of in the results obtained from the experiment using single notch FBG, whereby approximately a suppression of 17 db was obtained. This is possibly a result of the fluctuation of the suppressed main optical carrier. The transmitted spectrum shows that the filtered optically modulated RF sidebands were suppressed more than 30 db

103 Chapter 3 Local Area Network Emulation in Passive Optical Networks Optical Power (dbm) db 10 db Before FBG Transmitted Reflected Wavelength (nm) Figure 3.13: Optical spectra measured at the FBG showing the transmitted and reflected spectra. Optical Power (dbm) db Upstream Receiver Downstream Receiver 22 db Wavelength (nm) Figure 3.14: Observed optical spectra at the downstream data and upstream baseband data receivers. Figure 3.14 shows the observed optical spectra at the downstream data and upstream baseband data receivers. As a single fibre is used for the transmissions in both directions, the back scattered light of each wavelength channel causes crosstalk at each receiver. The suppression of crosstalk at the upstream baseband data receiver and downstream data receiver

104 Chapter 3 Local Area Network Emulation in Passive Optical Networks are -20 db and -22 db respectively. These crosstalk values at each receiver ports were approximately closer to the values obtained from the experiment that uses a single notch FBG as the transmission distances and the optical launch power are approximately equal in both experimental demonstrations BER Results Mb/s RF LAN data 1.25 Gb/s upstream data 2.5 Gb/s downstream data -5 Log 10 (BER) -6-7 Back to back -8 Back to back Transmission Transmission With downstream data Back to back Transmission -9 With Upstream data With RF LAN data With Upstream signals Received Optical Power (dbm) Figure 3.15: Measured BER plots for 2.5 Gb/s downstream data, 1.25 Gb/s upstream baseband data and 155 Mb/s LAN data in the LAN emulation scheme using a double notch FBG. Figure 3.15 shows the BER curves for all signals showing the transmission and interference penalties. For the 155 Mb/s LAN data, compared to B-B measurements, a penalty of 0.45 db was observed when the RF LAN data was transmitted through the link. An additional penalty of 0.4 db was observed when 1.25 Gb/s upstream baseband data was included in the upstream transmission. It shoud be noted that in the experimental BER curves obtained when a single notch FBG was used, no significant penalty was observed. The penalty increase in this

105 Chapter 3 Local Area Network Emulation in Passive Optical Networks experiment is possibly due to the ICI from the upstream baseband data. As a different RF carrier frequency was used for the modulation of the LAN data, the convertion losses in the RF mixer was also changed. Therefore, the total RF power of the upconverted RF LAN data before the optical modulation has also changed. Moreover, the operating point of the DEM may have drifted due to temperature changes. This has led to ICI between the two signals resulting in power penalty for the recovered 155 Mb/s LAN data. It should be noted that sensitivity of the LAN data in the presence of other signals is approximately dbm, which is similar to that of in the previous experiment with a single notch FBG. The BER curves for the 1.25 Gb/s upstream baseband data show that no signalificant penalty was observed when the signals were transmitted through the link. No additional penalty was observed when the downstream data signals were present. However, as the 155 Mb/s upconverted RF LAN data was added to the upstream wavelength channel, a penalty of 0.6 db was measured. This penalty can be attributed to the ICI from the upconverted RF LAN data. As BER curves for the 2.5 Gb/s downstream data show no significant penalty was observed in the presence of upstream signals LAN emulation using separate distribution fibre loopback In the previous section, an optical layer LAN emulation scheme with the use of a narrowband FBG placed in the feeder fibre for the separation of LAN data from the upstream baseband data is discussed in detail. As a narrowband FBG is utilised for the LAN emulation, the optical transmitter used at each ONU is required to be very stable to obtain successful operation of the network. Moreover, the Bragg wavelength of the narrowband FBG that is placed in the feeder fibre drifts due to temperature changes of the environment. The drifts in the wavelength of the optical source at the ONU and the narrowband FBG place higher requirements in terms of stability for the optical source used in the ONUs. Therefore, an alternative solution is required to relax the stringent requirements of the optical source used at the ONU. In this alternative scheme, LAN data to other ONUs in the PON is carried on an RF carrier while upstream data to the CO is carried at baseband. Instead of the narrowband FBG placed in the feeder fibre for the spectral filtering of LAN data, an additional distribution fibre between the SC and the ONUs is used whereby one fibre is for LAN connectivity and the other is for normal PON connectivity

106 Chapter 3 Local Area Network Emulation in Passive Optical Networks Redirection of upstream signals Ups tream R eceiver Dow nstream Tra nsmitter λ u WDM 1.3/1.5 µm λ d λ u Feeder Fibre CO Distribution λ u Fibres N+1 x N+1 Star Coupler Upstream baseband data Terminated unused port RF LAN data Receiver Downstream Receiver WDM Upstream 1.3/1.5 µm Transmitter ONU 1 ONU N RF LAN data f L f L Figure 3.16: LAN emulation architecture using RF subcarrier multiplexed LAN traffic with an additional distribution fibre between the SC and the ONUs, whereby the upstream signals are looped back to each ONU. Figure 3.16 illustrates the LAN emulation scheme in which the redirection of the optically modulated RF sidebands that contain the LAN data along with the upstream baseband data is performed by a ( + 1 ) ( N +1) N SC and additional short length distribution fibres [28, 29]. In this scheme, denoted as Scheme 2, the ( N + 1 ) ( N +1) SC replaces the ( N ) 1 SC that was used in Scheme 1. The number of ONUs that are attached to the SC is N and one of the ports facing towards the ONUs is terminated. Therefore, Scheme 2 requires an additional port in the SC to support same number of ONUs as in Scheme 1. Each ONU in the PON is connected to the SC via two short length distribution fibres as shown in Figure The signals transmitted from each ONU on upstream wavelength channel are therefore redirected back to each ONU through the second distribution fibre. The additional distribution fibre connected to each ONU from the SC is not expected to increase the cost of deployment of the access network as each fibre cable that connects the ONU to the SC usually has more than two fibres within it. In the deployment of the optical access network, the installation cost of the network dominates more than the cost of the fibres itself, which is continuously decreasing [48, 49]. Moreover, having an additional distribution fibre to each ONU, PON architectures can be improved with additional features such as efficient random access protocol such as CSMA/CD for upstream transmissions, LAN emulation, protection capabilities for the upstream transmissions etc. [16, 17, 50-53]. As in Scheme 1, upstream signals consist of

107 Chapter 3 Local Area Network Emulation in Passive Optical Networks baseband data to CO and LAN data that is modulated on an RF carrier to other ONUs. At each ONU, the looped back signals are detected and the upconverted RF LAN data is electrically separated from the upstream baseband data using an electrical BPF. Then, the RF LAN data is down-converted to baseband frequencies using a phase locked loop (PLL) containing a VCO to recover the LAN data. In this scheme, the modulation of the LAN data on the RF carrier could be done in any arbitrary signal format such as amplitude shift keying (ASK), phase shift keying (PSK) or frequency shift keying (FSK), since coherent detection of RF signals is performed at the RF LAN data receiver for the recovery of the LAN data Transmission protocol in the upstream direction As in Scheme 1, the access of the wavelength channel for the upstream signal transmissions follows TDMA protocol. Therefore, upstream baseband data and RF LAN data are transmitted simultaneously in the same time slot. In this scheme, as both upstream baseband data and RF LAN data are redirected to each ONU through the second distribution fibre, by performing carrier sensing of the redirected signals with the use of simple and low cost electronics, carrier sense multiple access with collision detection (CSMA/CD) protocol can also be adopted [16, 17, 54-56]. Optical CSMA/CD random access control protocol is very efficient for the transmissions of signals in the upstream direction in PON. As the distribution fibre length to each ONU from the SC is shorter, and the upstream transmission rates are lower compared to the downstream transmission rates, CSMA/CD protocol obtains higher efficiency in the utilisation of the upstream wavelength channel. This is because the packets from multiple ONUs that collide at the SC can be detected earlier at the ONUs than at the CO due to lower propagation delay between the SC and ONUs. Therefore, the packet transmissions from the ONUs can be stopped in an event of collision and rescheduled later. In conventional CSMA/CD, the collisions of upstream packets that occur at the SC are detected only at the CO after a long delay, and the subsequent broadcast of the collision from the CO back to all ONUs incurs an equally long time. With the optical loop-backs, the CO is eliminated from the CSMA/CD access scheme. At each ONU, the redirected packet frames are used for carrier-sensing and collision detection, and this information is used to control the upstream transmissions. Further, the reduced roundtrip propagation delay required for carriersensing and collision detection increases the efficiency of the access scheme, leading to a

108 Chapter 3 Local Area Network Emulation in Passive Optical Networks higher utilisation of the trunk fibre. Results from network performance evaluations show that with appropriate priority queuing mechanisms, low delay and manageable jitter can be achieved for real-time voice traffic and without bandwidth starvation of the best effort traffic using the optical loop-backs [57]. While the ONUs randomly access the upstream wavelength channel in the optical CSMA/CD protocol, the ONUs in upstream broadcast with dynamic bandwidth allocation (UB-DBA) scheme are dynamically allocated time on the upstream wavelength channel based on a selfpolling access scheme [50, 51]. As with the IEEE 802.3ah Multipoint Control Protocol, UB- DBA utilises control messages to request and allocate bandwidths between the OLT and ONUs, but UB-DBA benefits from the redirection of upstream packet frames to facilitate several mechanisms. A simple ranging mechanism, whereby each ONU can measure its round trip time with the SC during every transmission is performed at each ONU using the optical loop-backs. This ranging enables a simple self-synchronisation mechanism that allows highspeed dynamic TDMA protocol to be implemented. In several DBA schemes that are proposed for the PONs, the control messages are transmitted on both upstream and downstream wavelength channels [58-60]. However, in the UB-DBA scheme, the control messages are transported only on the upstream wavelength channel. Differing types of traffic are allocated to different portions of the transmission polling cycle, ensuring real-time voice traffic is transported with minimal delay and jitter, whilst maximising the throughput of the best effort traffic. Results from network performance evaluations show that stringent QoS requirements are met and high channel utilisation is achieved. Further, as UB-DBA allocates excess bandwidth to users with higher bandwidth demands, it minimises the variation in the transmission polling cycle time, and remains insensitive to the number of overloaded users. The downstream capacity of the PON is also optimised and remains constant regardless of the network size since only the upstream wavelength channel is used for bandwidth request and allocation [50, 51]. It should also be noted that in this LAN emulation scheme using RF subcarrier multiplexing, LAN data is broadcast to all ONUs. As in Scheme 1, the transmission of each signal can be carried out in the absence of each other. If LAN data is transmitted in the absence of the upstream baseband data, then flooding of light occurs at the upstream baseband data receiver. However, this can also be controlled at the burst mode receiver at the CO using control

109 Chapter 3 Local Area Network Emulation in Passive Optical Networks packets from the ONUs to the CO [61, 62]. In Scheme 2, as the entire upstream signals are redirected to each ONU using the second distribution fibre, the upstream baseband data transmitted from an ONU to the CO reaches other ONUs in the PON. There exists a security issue. However, the privacy and security of the transmission of the upstream baseband data signals can be achieved using data encryption [63-66] Experimental demonstration 2.2 km SMF λ d = nm MZM Circulator Gb/s 1.25 Gb/s Receiver 10 km SMF 2.5 Gb/s PLL & 4x4 Receiver 2.5 GHz Data Recovery Star Coupler Circulator Gb/s 2.2 km SMF Receiver GHz RF Oscillator RF mixer 155 Mb/s MZM λ u = nm 2.5 GHz 1.25 GHz Power Combiner 1.25 Gb/s BPF: Band Pass Filter, LPF: Low Pass Filter, PLL: Phase Locked Loop Figure 3.17: Experimental setup to demonstrate the feasibility of LAN emulation (Scheme 2), whereby an additional distribution fibre is used to redirect the upstream signals. The experimental setup to demonstrate the feasibility of Scheme 2 is shown in Figure The transmission bit rates for downstream data, upstream baseband data, LAN data are 2.5 Gb/s, 1.25 Gb/s and 155 Mb/s respectively as in Scheme 1. The downstream signal of PRBS NRZ data at 2.5 Gb/s was modulated onto downstream wavelength channel λ d = nm and transmitted to the ONUs. For the upstream transmission, PRBS NRZ BPSK data at 155 Mb/s was modulated onto a RF carrier frequency at 2.5 GHz to generate the upconverted RF LAN data, which was then electrically combined with 1.25 Gb/s PRBS

110 Chapter 3 Local Area Network Emulation in Passive Optical Networks NRZ data using a RF combiner. In Scheme 1, RF carrier frequency at 5 GHz was used to allow optical filtering. In Scheme 2, as no optical filtering was employed lower RF carrier frequency was used. However, higher RF carrier frequency could have been used in Scheme 2 at the expense of larger passive loss in the RF power combiner. For the experimental demonstration of Scheme 2, an additional 2.2 km distribution fibre was connected to a port (facing the CO) of the 4 4 SC to the ONU. To avoid crosstalk between the combined signals, the upconverted RF signal was band-limited using an electrical BPF with a centre freqeuncy of 2.5 GHz and bandwidth of 300 MHz before the combination. Similarly, 1.25 Gb/s upstream baseband data was band limited using a 1244 Mb/s SDH LPF before the combination with the upconverted RF LAN data. RF power (dbm) Gb/s upstream baseband data 155 Mb/s LAN 2.5 GHz RF carrier Frequency (GHz) Figure 3.18: Observed RF spectrum at the input of the transmiter at the ONU showing 1.25 Gb/s upstream baseband data and 155 Mb/s LAN data on 2.5 GHz RF carrier. Figure 3.18 shows the RF spectrum of the transmitted composite signals in the upstream direction. The modulation depth of the 1.25 Gb/s upstream baseband data was set to 59%. Thereafter, the composite signals were modulated onto upstream wavelength channel λ u = nm using a MZM and transmitted in the upstream direction. The upstream wavelength channel λ u in Scheme 2 was randomly chosen as it had no restrictions, unlike in Scheme 1, whereby the FBG defined the choice of upstream wavelength channel λ u. The upstream signals are split in the SC whereby one portion is transmitted through the 10 km feeder fibre to the CO, while the other portion is redirected back to the ONUs through the

111 Chapter 3 Local Area Network Emulation in Passive Optical Networks second distribution fibre. The redirected signals are received at the RF LAN data receiver. The detection of the upstream baseband data and downstream data signals were performed using the same 2.5 Gb/s p-i-n receivers, while the RF LAN data was detected using the 2.5 Gb/s APD receiver. For the recovery of 155 Mb/s LAN data, the detected signals were fed through a BPF centered at 2.5 GHz with a bandwidth of 300 MHz and the LAN data was recovered using the PLL based on a costas loop circuit Optical Spectra 0 Optical Power (dbm) -10 Downstream Receiver -20 Upstream Receiver db 30 db Wavelength (nm) Figure 3.19: Observed optical spectra at the upstream baseband data and downstream data receivers. Figure 3.19 shows the crosstalk of the backscattered light of the downstream and upstream data signals at the corresponding receivers and they are - 22 db and - 30 db respectively. These crosstalk values can further be reduced using WDM couplers at the ONU and CO sides for combining and separating the upstream and downstream wavelength channels BER Results As in Scheme 1, each signal was transmitted in the absence and presence of each other and the crosstalk effects were measured using BER measurements. As the secondary distribution fibre to the ONU is connected to a port of the SC facing the CO, the BER measurements for the 155 Mb/s LAN data were performed in the presence of the 2.5 Gb/s downstream data and

112 Chapter 3 Local Area Network Emulation in Passive Optical Networks vice versa. Figure 3.20 shows the measured BER curves for all signals. Figure 3.20(a) shows that the power penalty for the 155 Mb/s LAN data compared to B-B measurements and it is 0.85 db. The major cause of this penalty is due to the ICI from the 1.25 Gb/s upstream baseband data. The upstream baseband data was bandlimited using a LPF to reduce the spectral overlaps with the upconverted RF LAN data. However, the crosstalk was possibly induced by the non-optimum operating points in the MZM leading to nonlinear distortion of the RF LAN data. No additional penalty was observed for the 155 Mb/s LAN data in the presence of the downstream data transmission Mb/s RF LAN data 1.25 Gb/s upstream baseband data 2.5 Gb/s downstream data (a) (b) (c) -6 Log 10 (BER) -7-8 Back to back Transmission Complete link With upstream baseband data With upstream & downstream signals Back to back Transmission Complete link With RF LAN data With upstream & downstream signals Back to back Transmission Complete link With upstream signals Received Optical Power (dbm) Figure 3.20: Measured BER plots for 2.5 Gb/s downstream data, 1.25 Gb/s upstream baseband data and 155 Mb/s LAN data for Scheme 2. The measured receiver sensitivity for the 155 Mb/s LAN data in the presence of other signals was approximately dbm. However, in Scheme 1, whereby a narrowband FBG is used for the LAN emulation the measured receiver sensitivity for the 155 Mb/s LAN data was approximately dbm. The difference in receiver sensitivity is a result of the detection

113 Chapter 3 Local Area Network Emulation in Passive Optical Networks process. In Scheme 1, 155 Mb/s LAN data was directly detected and therefore the bandwidth of the detected signal was only 155 MHz. However, in Scheme 2, the entire upstream signals with a bandwidth of 2.5 GHz were detected and the upconverted RF LAN data was electrically separated before downconverting to baseband frequencies using a PLL. As the bandwidth of the detected signals in Scheme 2 is larger compared to that of in Scheme 1, the sensitivity is reduced. Moreover, the PLL that was used for the coherent detection of LAN data has contributed to the reduction of the receiver sensitivity. As shown in Figure 3.20(b), the power penalty for 1.25 Gb/s upstream baseband data is less than 0.1 db compared to B-B measurements. In scheme 2, the power penalty is smaller compared to that of in Scheme 1 due to different experimental implementations. As the upstream baseband data was electrically combined with the upconverted RF data, the extinction ratio was reduced due to the insertion loss in the RF combiner and the B-B measurements were done while the RF combiner was in place. Figure 3.20(c) shows the BER measurements for the 2.5 Gb/s downstream data transmissions. No significant penalty was observed for the transmission through the link and in the presence of upstream signals. The receiver sensitivites for both 2.5 Gb/s downstream data and 1.25 Gb/s upstream baseband data in both schemes are close to each other. The differences in these values are possibly due to the experimental errors. 3.5 Comparison of the proposed LAN emulation schemes The proposed LAN emulation schemes using RF SCM transmission employ two different physical layouts. Scheme 1 uses a narrowband FBG for optical filtering and reflection of LAN data, while Scheme 2 uses an additional distribution fibre for the redirection of LAN data. These schemes vary from each other in several functionalities such as detection, the required stability of the optical source at the ONUs, dispersion tolerance of the transported RF SCM LAN data signals, power budget of all the signals etc. In this section, both schemes are compared and analysed in terms of bandwidth requirements and upgradeability, dispersion tolerance in transmission of RF LAN data, stability requirements of the optical source at the ONUs, and power budget

114 Chapter 3 Local Area Network Emulation in Passive Optical Networks Bandwidth requirements The bandwidth requirements of the optical receiver for the reception of the RF LAN data differ in the two schemes. In Scheme 1, as the RF LAN data is optically separated and directly detected, a low bandwidth optical receiver is adequate to recover the LAN data. In Scheme 2, the upstream signals are looped back using a separate distribution fibre and detected before the electrical separation to recover the LAN data. Therefore, the RF LAN data receiver bandwidth is required to be higher than that of Scheme 1. As the detected bandwidth of the RF LAN data signal is different in each scheme, the receiver sensitivity of the detected RF LAN data signals also differs due to the different in noise bandwidth. Nonetheless, it should be noted that the RF carrier frequency for Scheme 2 can be made lower than that of Scheme 1 as electrical separation is performed in Scheme 2. Relatively larger RF carrier frequency may be required in practice for Scheme 1 to overcome laser stability issues and relatively larger bandwidth of the optical filters. In both schemes, the modulation bandwidth of the laser needs to large enough to modulate both upstream baseband data and RF LAN data. However, in comparison to Scheme 2, relatively larger (in the order of 5-10 GHz) RF carrier frequencies are required for the modulation of LAN data in Scheme 1. However, it is not expected to increase the cost and complexity of the ONUs as low cost and high frequency VCOs and high speed directly modulated lasers are commercially available. While the direct modulation of high frequency RF signals could cause dispersion induced RF power fading when RF LAN data signals propagate through the dispersive fibre [67-69], it is noted that the LAN data is tolerant to dispersion induced power fading as it is directly detected in Scheme 1 and practical distribution fibre distances in a PS-PON are small to be affected severely by the dispersion induced power fading in Scheme 2. This is explained further in the next section. For practical PON deployment, an upgrade in the transmission bit rate of the upstream baseband data due to customer demand should not affect the LAN emulation service provisioning. That is, a PON infrastructure must be easily upgradeable to higher bit rates without much disruption to the existing service provisioning. For Scheme 1, this is easily implemented by selecting a larger RF carrier frequency for LAN data transmission and a corresponding tuning/replacement of the FBG. Note that even though the RF carrier frequency is increased, an upgrade for the LAN data receiver is not required because the LAN data is optically separated at the FBG and directly detected at the LAN data receiver

115 Chapter 3 Local Area Network Emulation in Passive Optical Networks However in Scheme 2, as the upstream baseband data bit rate is increased, the RF carrier frequency must be increased accordingly, resulting in an increase in RF LAN data receiver bandwidth and the RF components such as RF mixer and electrical BPF for the separation of LAN data from the upstream baseband data. In both schemes, as the transmission bit rate of the upstream baseband data is increased, the modulation bandwidth of the laser is increased. Similarly, LAN data bandwidth may also be required to be increased. This increase in bandwidth results in changes in the RF and optoelectronic components at the ONUs. As the bandwidth of LAN data is increased, LAN data receiver bandwidth is increased in Scheme 1. An increase in the bandwidth of the LAN data could potentially increase the reflection bandwidth of the narrowband FBG used in the passive plant. A wider bandwidth FBG can be used to avoid changes of the FBG in the passive plant. However, as the bandwidth of the FBG is increased, the RF carrier frequency that is required for the modulation of LAN data is also increased to allow spectral filtering without affecting the upstream baseband data transmissions to the CO. In Scheme 2, the bandwidth of the LAN data receiver and the electrical BPF are increased in accordance with the increase in the bandwidth of the LAN data Dispersion tolerance The optical transmitter at each ONU is modulated with upstream signals that contain upstream baseband data to the CO and the LAN data that is carried on an RF carrier to other ONUs. During the transmission of these signals through the fibre, fibre dispersion occurs as different wavelength components propagate with different speed. In the LAN emulation whereby a narrowband FBG is used to separate the RF SCM LAN data from the upstream baseband data, both optically separated RF side bands that contain the LAN data arrive at the LAN data receiver with different propagation delays and therefore with different phase delays. As the baseband modulation component is the beat result of the sideband signals and the optically modulated RF carrier, if they meet incoherently, degradations can occur in the recovered LAN data and therefore it results in dispersion induced power penalty. When the upstream signals are transmitted through dispersive fibre, each optical signal is subjected to a phase change as it propagates. If the phase changes are defined asφ L, φ U andφ B

116 Chapter 3 Local Area Network Emulation in Passive Optical Networks for the lower, upper sideband optical signals and the optical carrier respectively, then the electric field of the composite signal can be represented as [28, 70, 71] E total ( j( ωrft + φl )) ( jφ ) + A exp j( ω t + φ ) = exp AL exp ( jω t) 0 ( + AB exp ) B U RF U Equation 3-7 where A, A and A represent the electrical field amplitudes of the lower, upper sideband L U B optical signals and the optical carrier respectively; andω is the angular frequency of the RF carrier. In Scheme 1, the lower sideband is reflected using the narrowband FBG and the reflected field at the RF LAN data receiver can be expressed as RF [ L RF L E = ( jω t) A exp( j( ω t + φ ))] reflected exp 0 Equation 3-8 At the RF LAN receiver, the generated photocurrent is given as * 2 lower = ρ EE = ρ E = ρ A L i 2 [ ] Equation 3-9 where ρ is the responsivity of the photodiode. As can be seen from Equation 3-9, the photocurrent does not depend onω RF. If a double notch FBG is used to reflect only the sidebands the electrical field of the reflected optical signal is given as E reflected ( jω t) [ A exp( j( ω t + φ )) + A ( j( ω t + φ ))] = exp 0 exp Equation 3-10 L RF L U RF U Then, the output current of the photodetector is 2 2 [ AL + AU + 2 Re( AL AU ).cos( ω RFt + φu φ L ] 1 * i = ρ 2 2 ) Equation 3-11 As a low bandwidth optical receiver is used to recover the LAN data, the 2 ωrf component falls outside the bandwidth of the photodetector. Moreover, the field amplitudes of the RF sidebands are the same and can therefore be written as A = A A. L U =

117 Chapter 3 Local Area Network Emulation in Passive Optical Networks In this scenario, equation 3-11 can further be modified to be 2 2 [ A + A ] = ρ[ A ] = ilower 1 2 i both = ρ 2 Equation In both implementations of Scheme 1, the photocurrent does not depend onω RF, and since the bandwidth of the LAN data is small, Scheme 1 is not affected by dispersion induced RF power fading. This means that if the spectra filtering performed to separate the optically modulated RF sidebands with a phase difference, it can be neglected and LAN data can be detected independently from fibre dispersion. As the detection of LAN data has no effect of the phase difference between the two optically modulated RF sidebands, higher RF subcarrier frequency can be used to carry the LAN data. However, it should be noted that phase difference between components in one RF sideband cannot be neglected if the modulation bandwidth of the LAN data is larger. However, LAN data bandwidth is not required to be higher (less than 1 Gb/s) in practical deployments leading to dispersion-free detection of LAN data even though it was carried on higher RF carrier frequency. From equation 3-12, it can be deduced that when both sidebands are detected, the photocurrent is twice as large as when a single sideband is detected and therefore available power margin for the LAN data increases by approximately 3 db. In contrast, the upstream signals are redirected to all ONUs through the second distribution fibre in Scheme 2. The received RF power of the LAN data can be written as [70] ω RF πlcd ω0 2 2 P RF LAN dataα cos Equation 3-13 where L is the length of the total transmission distance, and D is the chromatic dispersion of fibre (=17 ps/nm/km for SSMF). As shown in equation 3-13, the received RF signal power depends on the length of the distribution fibre and the RF carrier frequency. Therefore, in Scheme 2, due to the phase changes in the optically modulated RF sidebands, dispersion induced RF fading occurs resulting in power penalty. However in practice, the length of the employed distribution fibres in the PON is short as the ONUs are located close (< 2 km) to the SC. Moreover, the RF carrier frequency for scheme 2 can be made relatively low as

118 Chapter 3 Local Area Network Emulation in Passive Optical Networks mentioned earlier. Therefore the received RF LAN data power does not suffer from significant power penalty Optical source stability As optical filtering is employed at the FBG to filter out the RF LAN data from the upstream signals in Scheme 1, it necessitates tight wavelength control at each ONU. If the wavelength control is relaxed, wavelength instabilities may lead to crosstalk between the upstream baseband data and RF LAN data and also distorts the filtered signals. This will lead to reduced sensitivity of the LAN data. Scheme 1 also requires that the narrowband FBG be stable with respect to varying temperature and environmental effects as it is deployed as part of the outside plant and therefore athermal packaging of FBG should be used in practical implementations. Power Penalty (db) Mb/s 620 Mb/s Wavelength (nm) Frequency Shift (GHz) Figure 3.21: Simulated power penalty results for the LAN data with varying frequency shifts in the upstream optical transmitter. Power penalty results for 155 Mb/s LAN data and 620 Mb/s LAN data are shown. Figure 3.21 shows the simulated results for the power penalty for the recovered LAN data against varying frequency shifts of the optical source at the ONUs. The simulation was carried

119 Chapter 3 Local Area Network Emulation in Passive Optical Networks out using VPI Transmission Maker software. For this simulation, the reflection profile of the single notch narrowband FBG that was used in the experiment was used. For the simulation, optical frequency shifts of the upstream transmitter is varied in 0.5 GHz steps in both directions for the LAN data transmission rates of 155 Mb/s and 620 Mb/s. As the frequency shift increases to approximately 1 GHz, no power penalty was observed for both transmission rates. However, as the frequency shift of the optical transmitter increases beyond this frequency shift, the power penalty for the recovered LAN data increases rapidly. For the frequency shift between 1 GHz and 2 GHz, the power penalty for the LAN data is more than 3.5 db for the 155 Mb/s LAN data transmissions, while it is approximately 6 db for the 620 Mb/s LAN data transmissions. As the transmission rate of the LAN data increases, the expected power penalty also increases for a particular shift in operating wavelength. As expected, the simulation results show that wavelength drifts in the upstream transmitter need to be very tightly controlled. This places a very stringent requirement on the upstream transmitter and the narrowband FBG placed in the outside passive plant. Alternatively, these requirements can be vastly relaxed if a higher frequency RF carrier is chosen for LAN data transmission. This allows the FBG bandwidth to be increased and the stability requirement of the upstream optical source to be relaxed. However, if a higher frequency RF carrier is used for the transmission of LAN data, then the required bandwidth of the optical source at the ONU also needs to be increased. However the cost of the optical sources is increasingly becoming cheaper. Therefore, the use of tight wavelength controlled optical sources at the customer premises will become feasible. As the access networks are cost sensitive, more technological improvements have to be made for the feasible deployment of the LAN emulation technique using a narrowband FBG. The design and fabrication capabilities of these types of FBG filters have become feasible and more economical. FBGs for these types of optical networking applications are required to stand operating temperature change of 80 o C for more than 25 years. There are many experimental demonstrations on FBGs with ultra high tempearture stability, whereby the FBGs can stand for even higher temperatures for longer duration with minimal drifts. Moreover, new materials and post fabrication treatments of the FBGs have enhanced the thermal stability of the FBGs [72-75]. The athermal packaging of the FBGs can also be used so that these FBGs could stand without much drift for higher temperatures and for longer duration [76]. Moreover, wavelength locking techniques can be used to stabilise the

120 Chapter 3 Local Area Network Emulation in Passive Optical Networks wavelength sources used at the ONUs. As the LAN data is filtered using the narrowband FBG, the drift in FBG due to temperature variation causes changes to the performance of the recovered LAN data. Using the recovered LAN data, the wavelength of the source used at each ONU can be tuned. Here, the FBG is used as a sensor to track the changes in wavelength in the transmitter and the compensation is made using simple electronic circuits. As the drifts in the FBG is not very fast process, the change in the transmitter in each ONU can be carried out accordingly to compensate the drifts in FBG [77-81]. The combination of a highly temperature stable FBG and the tracking the drift using the FBG can be simultaneously used in the LAN emulation scheme. There are no additional optical components required at each ONU to track the changes in the wavelength drift and simple electronic circuits can be used to monitor the wavelength changes in the FBG and making adjustments in the wavelength source at each ONU. Moreover, wavelength sources such as DFB lasers with wavelength lockers are commercially available for lower cost these days. Therefore, using simple additional capabilities, the LAN emulation scheme with a narrowband FBG can be made a reality. In comparison, Scheme 2 does not have such stringent stability requirements on the upstream wavelength transmitter as optical filtering is not performed. Therefore, wideband sources such as light emitting diodes (LED) or Fabry Perot laser diodes (FP-LD) with high power and adequate bandwidth could be used for the transmission of both upstream baseband data and RF LAN data signals Power budget The experimental demonstration of the PS-PON supporting two types of LAN emulation schemes using RF SCM transmission of the LAN traffic has been demonstrated. As the number of ONUs increases in the PON, the loss in the network increases due to the splitting loss of the SC and therefore limiting the transmission bit rates of the downstream data, upstream baseband data and upconverted RF LAN data and the transmission distance. This section investigates the scalability of the network with increasing number of ONUs. This analysis uses the meaured parameters of the devices employed in the experiment, since this would reflect the achievable network performance using readily available devices. The maximum number of channels, N, that can be supported in the PON is limited by the power

121 Chapter 3 Local Area Network Emulation in Passive Optical Networks budget. The SC splitting loss is the primary loss limiting factor in the path of all three signals. The power loss in all three signals result in an upper limit on the number of channels that can be supported for given optical source powers and receiver sensitivities Downstream power budget The downstream data signal is modulated using an external modulator and the full modulation depth of the modulator can be used. For the transmission of downstream signals, 1.5 µm wavelength window is used and therefore the LAN data transmission on the upstream wavelength that is on the 1.3 wavelength window does not impact the performance of the downstream signal transmission. Therefore, the downstream power budget is limited by the splitting loss of the SC. The loss of the SC including the excess loss can be given as [82] SC Loss = 10 log(2( N 1)) Equation 3-14 Here, N denotes the number of splits in the SC. For a PS-PON, the number of splits in the SC is same as the number of ONUs as well. As the number of ONUs (N) increases, the loss of the SC also increases and therefore limits the number of ONUs that can be supported in a PON. As the transmission bit rate of the downstream signal increases, the receiver sensitivity decreases due to the increase in the noise bandwidth of the signal. For high transmission bit rate of the downstream data signals, the number of splits in the SC limits the number of ONUs that can be supported. By reducing the transmission rate of the downstream signals, the number of ONUs that can be supported using a single SC can be increased Upstream power budget Due to the combination of upstream baseband data and RF LAN data, the power budget of upstream baseband data is particularly significant. The power budget margin is calculated the same way as that for the downstream data. However, the upstream modulation depth is shared

122 Chapter 3 Local Area Network Emulation in Passive Optical Networks between the upstream baseband data and RF LAN data. The reduction of upstream baseband data amplitude reduces the sensitivity of the baseband signal. Moreover, the ICI between the upstream baseband data and the RF LAN data significantly reduces the sensitivity of the recovered upstream baseband data. Using a RF diplexer rather than a power combiner to get better isolation of signals in the operating frequency ranges, performance improvements can be obtained RF LAN data power budget As mentioned in the upstream data power budget, the RF LAN data power budget heavily depends on the modulation index of the RF LAN data. The sensitivity of the RF SCM data signal not only depends on its amplitude value, but also on the amplitude of the upstream baseband data and biasing points. It has been shown that an increase in the power of the RF LAN data would not increase the sensitivity of the LAN data due to the clipping and nonlinear distortion. Moreover, the ICI between the upstream baseband data and the upconverted RF LAN data could significantly reduce the sensitivity of the recovered LAN data if the biasing conditions were not carried out quite carefully. Therefore, by optimising the amplitudes of both upstream baseband data, and RF LAN data, better performance can be achieved for the LAN data. Table 3.1 shows the calculated power budget for all signals for a PON consisting of 32 ONUs using the values of the transmitted power of all signals, the sensitivity and passive loss values of the components as measured from the experimental setups described earlier, therefore, there are ample margins for improvement. The details of both Scheme 1 and Scheme 2 are outlined in Table 1. The number of splits is analogous to the number of ONUs in the PON. Referring to Table 1, the receiver sensitivity for the 2.5 Gb/s downstream data differs with each scheme. In Scheme 1, a wideband RF amplifier (DC 5 GHz) was used after the detection of the 2.5 Gb/s downstream data. In Scheme 2, a 2.5 GHz limiting amplifier was used after the detection of the downstream data. Since the noise bandwidth of 5 GHz is larger than the signal bandwidth of 2.5 GHz in Scheme 1, the sensitivity was reduced by approximately 1.25 db. As the modulation depth of the 1.25 Gb/s upstream baseband data is different in both schemes the sensitivity in Scheme 1 is 1.45 db lower than that of in Scheme

123 Chapter 3 Local Area Network Emulation in Passive Optical Networks 2. For Scheme 1, 155 Mb/s LAN data traverses through the SC twice. However, the power margin (0.2 db) was adequate to support higher bandwidth LAN data for a PON with 32 ONUs. This is because of the higher sensitivity of the LAN data at BER = 10-9 which was approximately dbm. In Scheme 2, the power margin for the LAN data is sufficiently large (1.45 db) to support higher bandwidth LAN data in a PON consisting of 32 ONUs. In this scenario, the the RF LAN data traverses through the SC only once, however the sensitivity of detected RF LAN data was approximately dbm. The sensitivity for the LAN data in Scheme 2 is lower compared to that of in Scheme 1. In Scheme 2, the entire upstream signals were detected and therefore the bandwidth of the signals were larger resulting in lower sensitivity for the recovered LAN data. Moreover, the PLL that was used for the recovery of the LAN data in Scheme 2 had limited dynamic range and therefore had contributed to the reduction in the sensitivity for the recovered LAN data. On the other hand, for Scheme 1, the sensitivity is high because the bandwidth of the received signal is lower. 2.5 Gb/s downstream data 1.25 Gb/s upstream baseband data 155 Mb/s RF LAN data Scheme 1 Scheme 2 Scheme 1 Scheme 2 Scheme 1 Scheme 2 Transmitted Power WDM coupler CO 10 km feeder fibre loss FBG loss x km distribution fibre loss WDM coupler ONU x x x Circulator loss x BER = Power Margin (32 ONUs) ~ -5 ~ -4 ~ -2.5 ~ -0.5 ~ 0.2 ~1.45 Table 3.1: Calculated power budget margin using the measured experimental parameters for both LAN emulation schemes of a PON that consists of 32 ONUs

124 Chapter 3 Local Area Network Emulation in Passive Optical Networks Using the parameters in the above table, power margin for each signal can be calculated for both schemes. Figure 3.22 shows the calculated power margin for all signals as a function of the number of ONUs. The values used in all calculations are as listed on Table 1 except for the insertion loss of the SC which varies with the number of ONUs. As expected, the results show that as the number of ONUs increases, the power margin decreases for all signals due to increasing SC insertion loss from the increase in the number of splits. The power margin curve for the LAN data shows higher power margin for lower number of ONUs in Scheme 1 than it is in Scheme 2. This is due to the higher receiver sensitivity of the LAN data in Scheme 1. However, for larger number of ONUs the power margin for the LAN data in Scheme 2 is superior. It is because, the splitting loss of the SC is high for large number of ONUs, and in Scheme 1, LAN data traverses through the SC twice, whereas it is only once in Scheme 2. Power Margin (dbm) Gb/s Downstream data - Scheme Gb/s Downstream data - Scheme Gb/s Upstream data - Scheme Gb/s Upstream data - Scheme Mb/s LAN data - Scheme Mb/s LAN data - Scheme Number of ONUs Figure 3.22: Calculated power margin for all signals in both schemes for LAN emulation using the measured parameters from experiments. It should be noted that the calculated power margins for the 2.5 Gb/s downstream data and 1.25 Gb/s upstream baseband data are not affected by the inclusion of the RF LAN data. These power margins were a result of the poor sensitivity of the p-i-n receivers used in the experiment. The power margins for all signals can be increased using higher launch power

125 Chapter 3 Local Area Network Emulation in Passive Optical Networks and optical receivers with better sensitivity and forward error correction techniques. The calculations and Figure 3.22 show that SC splitting loss severely limits the scalability of the network. The number of ONUs that can be supported in a single PS-PON is reduced with increased transmission bit rates of the signals. As the downstream data transmission rate is usually larger, the scalability of the downstream data is reduced. In Scheme 1, LAN data traverses through the SC twice and therefore experiences more loss than other signals. This limits the maximum allowable LAN data transmission rate. In scheme 2, both upstream signals are detected before the electrical separation of the LAN data. As the noise bandwidth of the optical LAN data receiver is therefore increased, the sensitivity is decreased imposing a limitation on the maximum allowable splits in the SC. Therefore, to provide more splits in the SC and support more ONUs for a extended reach, PON strategies similar to amplified PON architectures can be considered [83-84]. 3.6 Conclusions This chapter presented two novel schemes for emulating optical layer communication links between customers in a PON employing RF SCM transmission of the 155 Mb/s customer traffic in conjunction with 1.25 Gb/s upstream access traffic to CO. Both schemes eliminate the use of additional optical sources to provide LAN emulation on the PON. One scheme requires a narrowband FBG placed along the feeder fibre close to the SC for LAN emulation, whereby the optically modulated RF sidebands that contain LAN data are reflected back to the ONUs while the upstream baseband data is carried to the CO. The second scheme requires an additional distribution fibre from SC to each ONU for the loopback of the upstream signals. In this scheme, the upstream signals are detected and the RF LAN data is electrically separated from the upstream baseband data. Both optical layer LAN emulation schemes use TDMA protocol for the upstream transmissions for simpler operations. Other random access control protocols such as CSMA/CD and UB-DBA can also be implemented. The experimental results show that RF LAN data imposes minimal penalty on the conventional upstream and downstream traffic. Each scheme is compared in terms of bandwidth requirements, stability of the optical source used at the ONUs, dispersion tolerance of the transported RF SCM data signals and power budget of all signals. Scheme 1 requires

126 Chapter 3 Local Area Network Emulation in Passive Optical Networks highly stable optical transmitter at each ONU, while the required laser at each ONU in Scheme 2 is not so stringent. Even though high frequency RF carrier is used for the modulation of LAN data in Scheme 1, the direct detection of LAN data enables dispersionfree transmission. In Scheme 2, the practical distribution fibre lengths are shorter such that LAN data is not severely affected by the dispersion. Using the measured parameters from experiments, the power budget can be calculated for both schemes to find the scalability for the PON. It shows that both schemes are capable of supporting high bandwidth LAN data for a PON consisting of 32 ONUs. By increasing the launch optical power and using higher sensitivity optical receivers better power margin for the upstream baseaband data and downstream data can be obtained for increased number of ONUs. 3.7 References [1] F. J. Effenberger, H. Ichibangase, and H. Yamashita, "Advances in broadband passive optical networking technologies," IEEE Commun. Mag., vol. 39, pp , Dec [2] G. Kramer and G. Pesavento, "Ethernet passive optical network (EPON): Building a next-generation optical access network," IEEE Commun. Mag. vol. 40, pp , Feb [3] I. Radovanovic, W. van Etten, and H. Freriks, "Ethernet-based passive optical local-area networks for fiber-to-the-desk application," IEEE J. Lightw. Technol., vol. 21, pp , Nov [4] D. Kettler, H. Kafka, and D. Spears, "Driving fiber to the home," IEEE Commun. Mag., vol. 38, pp , Nov [5] R. D. Feldman, E. E. Harstead, S. Jiang, T. H. Wood, and M. Zirngibl, "An evaluation of architectures incorporating wavelength division multiplexing for broad-band fiber access," J. Lightw. Technol., vol. 16, pp , Sep [6] R. Cohen, "On the establishment of an access VPN in broadband access networks," IEEE Commun. Mag., vol. 41, no. 2, pp , [7] R. Venkateswaran, Virtual private networks," IEEE Potentials, vol. 20, no. 1, pp , [8] S.-J. Park, C.-H. Lee, K.-T. Jeong, H.-J. Park, J.-G. Ahn, K.-H. Song, "Fiber-to-the-home services based on wavelength-division-multiplexing passive optical network," IEEE J. Lightw. Technol., vol. 22, pp , [9] IEEE 802.3ah Std, Part 3: CSMA/CD access method and physical layer specifications, 2004 Edition. [10] B. Arnaud, J. Wu, and B. Kalali, "Customer-controlled and -managed optical networks," IEEE J. Lightw. Technol., vol. 21, pp , Nov [11] P. P. Iannone, K. C. Reichmann, A. Smiljanic, N. J. Frigo, A. H. Gnauck, L. H. Spiekman, and R. M. Derosier,. "A transparent WDM network featuring shared virtual rings," IEEE J. Lightw. Technol. vol. 18, pp , Dec

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131 Chapter 3 Local Area Network Emulation in Passive Optical Networks [72] G. Brambilla, and H. Rutt, Fiber Bragg gratings with ultra-high temperature-stability, in Proc. Optical Fiber Communication Conference (OFC'02), pp , [73] I. Riant, S. Borne, and P. Sansonetti, Dependence of fiber Bragg grating thermal stability on grating fabrication process, in Proc. Optical Fiber Communication Conference (OFC'96), pp , [74] E. Salik, D. S. Starodubov, V. Grubsky, and J. Feinberg, Thermally stable gratings in optical fibers without temperature annealing, in Proc. Optical Fiber Communication Conference and the International Conference on Integrated Optics (OFC-IOOC'99), vol. 3, pp , [75] Bai-Ou Guan, Hwa-Yaw Tam, Xiao-Ming Tao, and Xiao-Yi Dong, "Highly stable fiber Bragg gratings written in hydrogen-loaded fiber," IEEE Photon. Technol. Lett., vol. 12, pp , Oct [76] J. F. Brennan, P. M. Bungarden, C. E. Fisher, and R. M. Jennings, "Packaging to reduce thermal gradients along the length of long fiber gratings," IEEE Photon. Technol. Lett., vol. 16, pp , Jan [77] Youngil Park, Seung-Tak Lee, and Chang-Joon Chae, "A novel wavelength stabilization scheme using a fiber grating for WDM transmission," IEEE Photon. Technol. Lett., vol. 10, pp , Oct [78] R. Giles, R. and J. Song, "Fiber-grating sensor for wavelength tracking in single-fiber WDM access PONs," IEEE Photon. Technol. Lett., vol. 9, pp , Apr [79] M. Ichioka, J. Ichikawa, T. Sakai, H. Oguri, and K. Kubodera, Athermalized wavelength locker using fiber Bragg grating, in Proc. 4 th Pacific Rim Conference on Lasers and Electro-Optics (PR-CLEO-01), vol. 1, pp , [80] G. E. Shtengel, R. F. Kazarinov, and L. E. Eng, Simultaneous laser wavelength locking and spectral filtering using fiber Bragg grating, in Proc. IEEE 16 th International Semiconductor Laser Conference, pp , [81] D. Forbes, and A. Robinson, Laser wavelength stabilisation using fiber grating, in Proc. IEE Colloquium on Optical Fiber Gratings, pp. 13/1-13/6, [82] D. Podwika, D. Stefanski, J. S. Witkowski, and E. M. Pawlik, "Computer networks based on optical passive couplers," in Proc. 2 nd International Conference on Transparent Optical Networks (ICTON 00), pp , [83] A. J. Phillips, J. M. Senior, R. Mercinelli, M. Valvo, P. J. Vetter, C. M. Martin, M. O. Van Deventer, P. Vaes and X. Z. Qiu, "Redundancy strategies for a high splitting optically amplified passive optical network," J. Lightw. Technol., vol. 19, pp , Feb [84] I. Van de Voorde, C. M. Martin, I. Vandewege, and X. Z. Oiu, "The super PON demonstrator: an exploration of possible evolution paths for optical access networks," IEEE Commun. Mag., vol. 38, pp , Feb

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133 Chapter 4 Protection and Restoration in Passive Optical Networks 4 Protection and Restoration in Passive Optical Networks 4.1 Introduction Passive optical network (PON) based approach has emerged as the most future-proof technology for optical access networks, enabling the network operators to deliver a rich mix of conventional and new services such as high speed internet access, storage area networking, local area networking and on-demand/broadcast video services such as tele-conference and tele-teaching [1-3]. As new services are introduced, the protection and restoration of PONs to monitor and manage such networks have gained an active interest [4-7]. PONs are generally built on a star type physical topology as this topology is more cost effective than ring and bus topologies on grounds of line costs and power budgets [8]. The double star PON architectures do not have natural resilient capabilities against link failures and are clearly vulnerable to fibre link breaks. A failure in the feeder fibre link, which connects the central office (CO) and the passive branching device such as star coupler (SC) and arrayed waveguide grating (AWG) can disrupt the transmissions to and from every single optical network unit (ONU) and therefore affects the entire network operation. A break in the distribution fibre link that connects the branching device and the ONU affects a particular ONU. Full services access network (FSAN) group considers protection of the APON (Asynchronous Transfer Mode PON) systems in its proposals. In ITU T G.983.1, four protection schemes have been defined [9-13]. These architectures propose solutions against feeder and distribution fibre link breaks and equipment failures at the optical line terminal (OLT) and ONUs. Most of the proposed solutions consider duplicating the systems. Depending on the level of protection, the architectures and therefore the incurring cost are varied. These protection systems are described in detail in Chapter 2 of this thesis. In this chapter, several protection schemes for a power splitting PON (PS-PON) against feeder fibre and distribution fibre breaks are proposed and experimentally demonstrated

134 Chapter 4 Protection and Restoration in Passive Optical Networks Section 4.2 describes the protection scheme against feeder fibre breaks, whereby the protection is performed by carrying out the transmission of an affected PON overlaid on another PON using coarse wavelength division multiplexing (CWDM). Section 4.3 discusses the protection architectures against distribution fibre breaks. Section describes and experimentally demonstrates a protection scheme against distribution fibre breaks using dual distribution fibres. This protection scheme in conjunction with a LAN emulation scheme is experimentally demonstrated in section Section explains a protection scheme against distribution fibre breaks by interconnecting adjacent ONUs. Sections and describe this protection scheme with two LAN emulation techniques that are discussed in detail in Chapter 3 of this thesis. Section 4.4 discusses the scalability of the protection architectures in conjunction with LAN emulation schemes, in terms of power budget. The experiment to find the local switching speed of the opto-mechanical switch, used in the experiments, is presented in section Protection against feeder fibre breaks Upstream Receiver Downstream Transmitter λ u CO WDM 1.3/1.5 µm λ d Feeder Fibre Fibre Break 1xN SC Distribution Fibre ONU 1 λ d WDM λ u 1.3/1.5 µm Downstream Receiver Upstream Transmitter ONU N Figure 4.1: A PS-PON architecture, whereby a break in the feeder fibre link disrupts the transmissions of signals between the CO and the ONUs. Figure 4.1 shows a PS-PON architecture, whereby a break in the feeder fibre link disrupts the transmission of signals in both upstream and downstream directions between the CO and the ONUs. As the feeder fibre link in the PON between the CO and the SC is common for all ONUs, a break in the feeder fibre disconnects all ONUs from the CO. Therefore, a survivability scheme against feeder fibre link breaks is required for seamless transmission of

135 Chapter 4 Protection and Restoration in Passive Optical Networks signals to ensure quality of service (QoS) for the services provided to the customers. As described in several protection architectures, protection against feeder fibre link breaks can be performed by duplicating fibre links between the CO and the SC [9-13]. In an event of a break in the working feeder fibre link, the PON interface at the CO detects the failure and switches the transmission to the spare protection feeder fibre link. As the protection feeder fibre needs to be in a separate fibre duct, the installation costs for the fibre ducts between the CO and the SC costs become high. Moreover, hot and/or cold standby circuits have to be placed at the CO to continuously monitor the state of the feeder fibre and this increases the cost and complexity of the operation at the CO. During the protection switching, signal loss may occur. Most of the PS-PON architectures use time division multiple access (TDMA) protocol for the upstream transmission of signals. During the protection switching, the transmission of upstream signals from an ONU to the CO experience different delays and therefore re-ranging of all connected ONUs is required to synchronise the ONUs for collisionfree transmission of packet frames in the upstream direction [10, 11, 13, 14] Feeder fibre protection scheme using CWDM separation As the duplication of the feeder fibre cables from the CO to the SC in each PON is very costly, overlaying the affected transmissions on another existing PON can potentially reduce the cost on installation and maintenance. An automatic protection switching (APS) scheme that exploits the use of wavebands and simple, low loss CWDM couplers to switch the transmissions from the affected network onto a separate but similar network that uses a different set of wavelength channels for traffic transport can be used to protect the signal transmission against feeder fibre breaks. Figure 4.2 shows the proposed scheme for a PS-PON with feeder fibre protection capabilities [15]. Two OLTs serve two separate PONs with each PON using a set of wavelength channels for downstream and upstream transmissions. PON 1 uses λ d1 and λ u1 for downstream and upstream transmissions respectively, while PON 2 uses λ d2 and λ u2. However, a single wavelength channel can be used for both downstream and upstream transmissions in a PON using time compression multiplexing (TCM) protocol [16-18]. The wavelength channels used in a PON reside in a waveband (λ d1 and λ u1 are in one waveband and λ d2 and λ u2 in

136 Chapter 4 Protection and Restoration in Passive Optical Networks another waveband) such that these wavebands can be combined and separated by CWDM couplers. OLT 1 Upstream λ u1 Receiver WC Downstream Transmitter λ d1 Central Office Control OSW 1 Control Feeder Fibre 1 CWDM CWDM Coupler 1 Coupler 3 Normal path for PON 1 Protection path for PON 1 2 x N SC Distribution Fibre WC λ u1 λ d1 ONU i Upstream Transmitter Downstream Receiver OLT 2 Upstream Receiver Downstream Transmitter λ u2 WC λ d OSW 2 Feeder Fibre 2 CWDM CWDM Coupler 2 Coupler 4 2 x N SC Other PON λ d1 λ u1 λ d2 λ u2 Waveband 1 Waveband 2 WC WDM Coupler OLT Optical Line Terminal OSW Opto-mechanical switch Figure 4.2: Proposed scheme for feeder fibre protection, whereby CWDM is used to combine/separate the transmissions of the PONs. In this scheme, the CWDM couplers that are used in the OLTs and the RNs have same specifications enabling mass production of these components. Moreover, these CWDM couplers exhibit low insertion loss (less than 0.5 db) for the pass-band wavelength channels and show higher isolation (more than 25 db) for the reject band wavelength channels. These devices are also made very compact enabling simpler integration with devices such as SC. Conventionally, 1.5 µm/ 1.3 µm window wavelength channels are used for downstream and upstream transmissions respectively in PS-PONs. However, the wavelength sources at 1.5 µm window wavelengths are becoming less expensive and their use for upstream transmissions can be seen as an intermediate step towards wavelength division multiplexed PON (WDM PON) from PS-PON [19, 20]. The CWDM couplers are placed at each OLT and at each remote node (RN) that comprises a 2 N SC, whereby the number of ONUs that is served by one OLT is N. The OLTs that serve both PON1 and PON 2 reside in the CO and these OLTs are interconnected using an

137 Chapter 4 Protection and Restoration in Passive Optical Networks opto-mechanical switch (OSW) that is placed at each OLT. Similarly, the CWDM filters placed in front of the SCs are also connected using fibre. Using only one CWDM filter at each RN requires two separate interconnecting fibres between the RNs. However, both fibres can be bundled in a single cable. Feeder fibre 1 C W 1 CWDM 1 W 2 W 2 W 1 2 x N SC PON 1 CWDM 2 C C Interconnecting fibre Feeder fibre 2 C CWDM 3 W 1 W 2 W 1 2 x N SC W 2 CWDM 4 PON 2 Figure 4.3: Interconnection of two PONs with a single fibre and CWDM couplers. Figure 4.3 shows the interconnection architecture between the RNs, whereby the RNs are interconnected using one fibre. Two CWDM filters are used at each RN. The wavelength channels for PON 1 are in waveband 1 and the wavelength channels for PON 2 are in waveband 2. The functions of the CWDM filter and OSW are described below. In normal state, each OSW is in bar state such that transmissions to each PON are carried out directly. For PON 1, the transmissions in both directions are carried through feeder fibre 1, while PON 2 uses feeder fibre 2 for its transmissions. A break in the feeder fibre cable 1 of PON 1 can be identified using the upstream data loss at the OLT 1. Moreover, the location of the break can be easily identified using optical time domain reflectometry (OTDR) techniques at the CO [21, 22]. Therefore, the state of OSW 1 is changed to the cross state such that upstream and downstream transmissions to PON 1 are carried out through feeder fibre 2. In this protected state, feeder fibre 2 is used as a protection fibre for PON 1. As the wavelength channels used for PON 1 lie in a different waveband to that of PON 2, these wavelength channels can be combined and separated using CWDM couplers placed at the OLTs and the

138 Chapter 4 Protection and Restoration in Passive Optical Networks RNs. In this scheme, PON 1 and PON 2 are capable of protecting each other against feeder fibre cable breaks, yet it should be noted that the architecture cannot provide protection against breaks occurring simultaneously on both feeder fibre cables. However, the probability of both feeder fibre breaks occurring simultaneously is low. The feeder fibre protection capability simultaneously enables migration from PS-PON to WDM-PON whereby, passive SCs are added with CWDM couplers. The RNs are interconnected using the fibre such that a ring architecture interconnecting all RNs is formed. For this migration, the distribution fibre to each ONU remains untouched. Therefore, the functionality of the ONUs remains exactly the same as before. This separation of PONs using CWDM couplers enables more bandwidth being allocated to each ONU in a PON on demand. If an ONU or a group of ONUs in a PON requires more bandwidth, it can be provided by allocating a separate wavelength channel in the appropriate waveband such that the operations of ONUs in the other PON remain untouched. This type of CWDM PON can be gradually upgraded whereby an AWG can be inserted in the RN such that densed wavelength division multiplexed (DWDM) channels are used for the transmissions between the ONUs and the CO. Moreover, as each RN is interconnected in a ring topology, simultaneous feeder fibres can be protected against breaks. Therefore, multiple PONs can be protected using appropriate CWDM couplers and multi port SCs Procedure for the fast protection switching The downstream signal transported from the CO to the ONUs is based on time division multiplexing (TDM), whereby the downstream packets are broadcast to all ONUs and each ONU extracts its own packets using the media access control (MAC) address embedded in the packet header. If a break occurs in a feeder fibre of a PON, the transmissions between the OLT and the ONUs are disrupted. The service agreements signed by the customer may not be guaranteed if the network failure remains for longer period. Therefore, to guarantee the required QoS for the customers, faster protection switching is critical. The upstream transmissions to the ONUs in most PS-PONs are based on TDMA protocol. Therefore, in the event of a fibre break, the system needs to be setup appropriately for the transmission of signals in the protected path. Therefore, several traffic parameters such as ranging for the TDMA protocol, burst mode receiver setting at the CO are required to be initialised for the

139 Chapter 4 Protection and Restoration in Passive Optical Networks successful restoration of signal transmissions. As the initialisation time is usually larger for the protected architecture, several advance setups are required to reduce this time, when the operation is switched from normal state to the protected state. This includes storing of additional information at the ONUs for faster recovery. As one of the setups, ranging needs to be considered in protected state as the round trip time (RTT) is changed due to different path length. Re-ranging is time-consuming and slows down the protection switching [10, 14]. Therefore, to obtain rapid protection switching, RTT of the redundant paths are ranged during the initialisation of the system or at the period when the least traffic is running in the system and equalisation delays for various protection scenarios are locally stored at each ONU [10, 23]. As the equalisation delay to each ONU is available in a single interface at the OLT, faster protection switching can be obtained. Alternatively, uni-ranging can also be performed during the protection switching, whereby only one ONU could be ranged after protection switching [10]. As the PON is based on startree architecture, and the distance differences amongst the ONUs are intact, using the equalisation delays stored at the CO and the newly-ranged ONU, the equalisation delays for other ONUs can be calculated. In this uni-ranging scheme, as multiple ranging to all ONUs are eliminated and only one ranging is performed, the protection switching time is dramatically improved. The detailed procedure of the uni-ranging is explained below. It should be noted that the same PON interface at the OLT is used for the protected state as the protection is activated in the event of a break in the feeder fibre only. a. Choose the uni-onu: An ONU with either the shortest distance or the smallest serial number is chosen for uni-ranging. b. Calculate the distance difference: The difference in distance given by the phase equalisation data is calculated based on the results from the newly ranged ONU. Td j i = Td j Td i Here, j [1, N]; i is represented as the uni-onu and N is the number of split in the SC. c. Ranging the uni-onu: As the uni-onu s identified, the sent ranging mask should match to this ONU. Time consumed on binary tree search is avoided. After ranging, the new phase equalisation data ' Td i for ONUi can be obtained

140 Chapter 4 Protection and Restoration in Passive Optical Networks d. Calculate the new distances of ONUs: Based on the equalisation data calculated before, the distances to all other ONUs can be calculated for the protected operational path. Td Here, ' j i = Td Td ' j ' j + Td ' ji is the new phase equalisation data for ONUj in the protected state. e. Send the equalisation data: Td ' j will be sent to ONUs and ONUs use this value for distance compensation. The ONUs are set to operational status using messages between the OLT and ONUs Experimental demonstration The feeder fibre link protection scheme was experimentally demonstrated using separation and combination of upstream and downstream signals for two separate PONs using CWDM couplers. Transmission Profile Reflection Profile Figure 4.4: The transmission and reflection profiles of the red/blue CWDM filter used in the experiment. Figure 4.4 shows the transmission and reflection profiles of the red/blue (R/B) CWDM couplers used in the experiment. These CWDM couplers have flat band of approximately 13 nm from nm to nm with a low insertion loss of approximately 0.5 db. The isolation from the wavelength channels that are placed outside this band is a minimum of approximately 24 db

141 Chapter 4 Protection and Restoration in Passive Optical Networks λ d1 = nm MZM Circulator Gb/s 1.25 Gb/s Receiver 3 4 OSW R/B 1 10 km SMF R/B 2 4x4 SC 2.2 km SMF Circulator Gb/s Receiver Gb/s MZM 1.2 km SMF 10 km SMF 3 km SMF λ u1 = nm OLT 2 R/B 3 R/B 4 PON 2 λ d2 λ u2 Figure 4.5: Experimental setup to demonstrate the feeder fibre protection scheme using CWDM overlay in PONs. Figure 4.5 shows the experimental setup to demonstrate the feasibility of the proposed protection scheme against feeder fibre breaks using CWDM transmission of signals. In normal state, a downstream signal of pseduo random binary sequence (PRBS) nonreturn to zero (NRZ) data at 2.5 Gb/s was modulated onto a wavelength channel of nm (λ d1 ) using a Mach-Zehnder modulator (MZM) and transmitted to the ONUs through a 10 km feeder fibre, a 4 4 SC, and a 2.2 km distribution fibre. For the upstream transmission, 1.25 Gb/s PRBS NRZ data was modulated onto a wavelength of nm (λ u1 ) using another MZM and transmitted in the opposite direction. The wavelengths λ d1 and λ u1 reside in the red band of the CWDM R/B couplers located at the CO and RN. Similarly, the wavelengths for PON 2, ie. λ d2 ( nm) and λ u2 ( nm), were chosen to reside in the blue band of the R/B CWDM coupler, such that they can be separated from λ d1 and λ u1 using the R/B CWDM couplers. In normal state, OSW 1 was set to bar state and signal transmissions in both upstream and downstream directions were performed directly. To simulate a break in feeder fibre 1, the 10 km feeder fibre of PON 1 was disconnected. In protected state, OSW 1 was changed to cross state using an external switch controller while OSW 2 was kept at bar state. Here, the signals for PON 1 were carried between port 1 and port 4 of OSW 1, 1.2 km of fibre that interconnects OLT 2, 10 km feeder fibre 2, and 3 km of fibre that interconnects both RNs. In both normal

142 Chapter 4 Protection and Restoration in Passive Optical Networks and protected states, both upstream and downstream signals of PON 1 were transmitted and recovered ensuring that the transmissions can be restored through the protection path Optical spectra Optical Power (dbm) db Common Red Blue Wavelength (nm) Figure 4.6: Observed optical spectra at the R/B CWDM coupler - 3 showing the upstream wavelength channels in the protected state. Figure 4.6 shows the optical spectra measured at all ports of the R/B CWDM coupler-3 for the upstream signals in the protected state. It shows that λ u2 is well suppressed in red port, while more than 31 db suppression achieved in blue port. Optical Power (dbm) db Common Red Blue Wavelength (nm) Figure 4.7: Observed optical spectra at the R/B CWDM coupler - 4 showing the downstream wavelength channels in the protected state

143 Chapter 4 Protection and Restoration in Passive Optical Networks Similarly, Figure 4.7 shows the observed optical spectra at all ports of the R/B CWDM coupler-4 for the downstream signals in the protected state. It shows that λ d2 is well suppressed in red port, while more than 38 db suppression achieved in blue port. -10 Optical Power (dbm) db 19 db Downstream Normal Downstream Protection Upstream Normal Upstream Protection Wavelength (nm) Figure 4.8: Optical spectra observed at the upstream data and downstream data receivers in both normal and protected states. Figure 4.8 shows the optical spectra observed at the input of the downstream and upstream data receivers measured through an optical spectrum analyser with 2.5 GHz resolution bandwidth. In normal state, the suppressions of the backscattered light at the upstream and downstream data receivers were 19 db and 26 db respectively. In the protected state, the suppressions of the backscattered light were 19 db and 17 db respectively. Note that in protected state, the signals in both directions traverse through an additional fibre with a length of 4.2 km, and the suppression of backscattered light has not caused performance degradations BER results A series of experiments were conducted to examine the crosstalk effects of one signal on the other, and the bit error rate (BER) measurements were performed. Figure 4.9 shows the BER curves for the 1.25 Gb/s upstream data and 2.5 Gb/s downstream data. The measured BER curves for the 2.5 Gb/s downstream data shows that the maximum penalty observed compared

144 Chapter 4 Protection and Restoration in Passive Optical Networks to back to back (B-B) measurements is 0.3 db. The penalty for the 1.25 Gb/s upstream data in the presence of the downstream signals is approximately 0.1 db in normal state. No additional penalty was observed in protected state for both signal transmissions. -5 Log 10 (BER) Gb/s downstream data Back to back With other signals - Normal With other signals - Protection 1.25 Gb/s upstream data Back to back With other signals - Normal With other signals - Protection Received Optical Power (dbm) Figure 4.9: BER measurements for the 2.5 Gb/s downstream data and 1.25 Gb/s upstream data in both normal and protected states. The protection architecture is not envisioned to limit the scalability of the network as long as the loss of the CWDM couplers and that from the additional span of interconnected fibres are kept low. As mentioned earlier, this protection architecture using CWDM is an intermediate step migrating towards WDM-PON. In a WDM-PON, using the cyclic properties of an AWG, the upstream and downstream wavelength channels can be allocated within a single waveband. Using the CWDM separation of these WDM channels, a protection architecure for a WDM-PON can also be developed. This protection architecture against feeder fibre breaks was experimentally demonstrated [24]

145 Chapter 4 Protection and Restoration in Passive Optical Networks 4.3 Protection against distribution fibre breaks This section focuses on the protection against distribution fibre breaks in a PS-PON. Several previous proposals considered duplicating distribution fibre cables between the CO and ONUs [9-13]. These protection architectures use additional distribution fibres from a separate splitter/ combiner such as SC for the protection against distribution fibre breaks. In these protection architectures, if tree switching is performed at the CO, the entire network operation has to be changed to protected mode if a distribution fibre link of an ONU is broken. Moreover, the switching to the redundant network is carried at the CO and therefore potentially increases the complexity of the operation and cost of the network. However, branch switching has advantages whereby the switching is performed at the affected ONUs and therefore is independent of the operation of other ONUs in the network. In the following sections, several novel architectures for the protection against distribution fibre link breaks are described in detail and experimentally demonstrated. One of the architectures uses two distribution fibre links to each ONU to perform switching of signal transmissions using an OSW placed at each ONU. The second architecture uses interconnections amongst the ONUs such that the transmissions to and from an affected ONU can be carried through the interconnected link. These protection architectures are experimentally demonstrated in-conjunction with the LAN emulation schemes that were investigated in Chapter Protection using two distribution fibres to each ONU Figure 4.10 shows the architecture that is proposed for the protection against distribution fibre link breaks. In this scheme, each ONU is connected to a ( N + 1 N + 1) SC via two distribution fibres, whereby the number of ONUs in the network is N. An OSW is placed at each ONU for switching the transmission path of the signals. A separate feeder fibre between the CO and the SC is also placed as shown in Figure In normal state, the OSW in the ONU operate in bar state whereby the transmissions of upstream and downstream signals are carried through distribution fibre 1. The upstream and downstream transmissions to and from the CO are thus carried through the working feeder fibre. If a break in the distribution

146 Chapter 4 Protection and Restoration in Passive Optical Networks fibre of an ONU is detected, then the state of the OSW at the ONU is changed to cross state such that the transmissions to and from this ONU are carried through the distribution fibre 2 and the protection feeder fibre. Central Office Optical Switch Redirection of upstream signal Feeder Fibre Distribution Fibres N+1 x N+1 Star Coupler 2 1 Control Electronics Optical Switch ONU 1 Downstream Receiver WDM Upstream Transmitter Protection Feeder Fibre ONU N Figure 4.10: Protection of distribution fibre of an ONU using dual distribution fibres. This scheme simultaneously enables the protection against feeder fibre cable breaks. As a break in the working feeder fibre is observed, using the OSW placed at the CO, the transmissions to the ONUs are carried through the protection feeder fibre. As dual feeder and distribution fibres are used for the protection of services against fibre breaks, these fibre cables should be placed in separate fibre ducts to prevent both fibre cables being disconnected at the same time. This makes the protection architecture more expensive as having dual fibre ducts to each ONU increases the installation cost [25, 26]. In several PON architectures, the distribution fibre cables can be placed on poles as the distance between the SC to the ONUs are usually less than 1-2 km. Therefore, deploying fibre to each ONU does not significantly increase the cost. This protection scheme has the capability to support the protection against distribution fibre breaks without affecting the operation of the other ONUs. Observe that if an ONU is in protected state, the transmissions to and from CO to this particular ONU are carried through the protection feeder fibre. In an event of a break of the distribution fibre of an ONU, the transmission path from the ONU to the CO is changed and therefore requires reranging for the successful and collision-free transmission of signals. However, as described in section , pre-ranging can be carried out at the system setup so that once the failure of

147 Chapter 4 Protection and Restoration in Passive Optical Networks the distribution fibre of an ONU is identified, appropriate equalisation delays can be used to perform collision free transmissions. Similarly for the feeder fibre link break, pre-ranging or uni-ranging can be used to reduce the time delays for the protection switching such that faster protection switching can be carried out and therefore the required QoS for the services can be obtained. Generally, operation and maintenance (OAM) message sequences are used between the OLT and ONUs for the setup and operation [27]. Moreover, as the distribution and feeder fibres experience breaks, the backscattered light from the non-terminated point could cause degradations for the received upstream and downstream signals. However, the use of these OSWs at the ONU and CO, reduces the effects of backscattering as the states of these OSWs are changed to cross from bar in protected state Experimental demonstration λ d = nm 2.2 km SMF OSW Circulator 2 10 km 2 SMF Gb/s MZM Circulator 1 OSW 2 4x4 2 1 Receiver SC 2.2 km 1 TTL SMF signal Gb/s 2.5 Gb/s km MZM 1.25 Gb/s SMF Receiver λ u = nm Figure 4.11: Experimental setup to demonstrate protection using dual feeder and distribution fibres. Figure 4.11 shows the experimental setup to demonstrate the feasibility of the proposed protection scheme. The downstream signal of PRBS NRZ data at 2.5 Gb/s was modulated onto a wavelength of nm using a MZM and transmitted to the ONUs through a 10 km feeder fibre, a 4 4 SC, and a 2.2 km distribution fibre. For the upstream transmissions, 1.25 Gb/s PRBS NRZ data was modulated onto a wavelength of nm and transmitted in the upstream direction. In normal state, both OSWs were set to bar state using a TTL signal driven by a controller and the signal transmissions in both directions were carried out directly. To simulate a fibre break, distribution fibre 1 was disconnected. Therefore, in protected state, both OSWs were changed to cross state and the

148 Chapter 4 Protection and Restoration in Passive Optical Networks transmissions were carried through the 2.2 km distribution fibre 2. Both 1.25 Gb/s upstream data and 2.5 Gb/s downstream data were detected using 2.5 Gb/s p-i-n receivers. A series of experiments were conducted to examine the crosstalk effects of one signal on the other, and the BER and optical spectra at each receiver ports were measured. It should be noted that both upstream and downstream signals were recovered in both normal and protected states confirming the protection capabilities of this scheme Optical spectra Optical Power (dbm) db 19 db Upstream Normal Upstream Protection Downstream Normal Downstream Protection Wavelength (nm) Figure 4.12: Observed optical spectra at the input of upstream data and downstream data receivers showing the suppression of the backscattered light. Figure 4.12 shows the optical spectra observed at the input of the downstream and upstream data receivers. In normal state, the suppressions of the reflected light at the downstream and upstream data receivers were more than 19 db and 25 db, while they were 19 db and 27 db in protected state BER Results Figure 4.13 shows the measured BER curves for 1.25 Gb/s upstream data and 2.5 Gb/s downstream data. For the 1.25 Gb/s upstream data, the measured penalty in the protected state in the presence of the downstream data is negligible. In the normal state, the penalty

149 Chapter 4 Protection and Restoration in Passive Optical Networks observed for the 2.5 Gb/s downstream data compared to the B-B measurements was 0.2 db. An additional penalty of 0.3 db was observed when the signals were transmitted in protected state. These penalties are probably due to the experimental errors Gb/s upstream data Gb/s downstream data Back to back Transmission - Normal Transmission - Protect With upstream - Normal With upstream - Protect Log 10 (BER) Back to back Transmission - Normal Transmission - Protect With downstream - Normal With downstream - Protect Received Optical Power (dbm) Figure 4.13: Measured BER curves for 1.25 Gb/s upstream data and 2.5 Gb/s downstream data. Protection against feeder fibre and distribution fibre can also carried out in a WDM-PON using dual fibre cables. In this scheme, the upstream wavelength and downstream wavelength channels are chosen according to the free spectral range (FSR) of the AWG. This protection scheme has been experimentally demonstrated [28] Protection and LAN emulation using dual distribution fibres In section 4.3.1, the protection architecture uses two distribution fibres to interconnect each ONU with the passive SC. In this protection architecture, a 1 x 2 OSW is used at each ONU to switch the transmissions to the reduandant distribution fibre in an event of a break in the

150 Chapter 4 Protection and Restoration in Passive Optical Networks distribution or feeder fibre cable. Instead of a 1 x 2 OSW, a 2 x 2 OSW may also be used at the ONUs with the advantage of simultaneously providing LAN emulation capabilities amongst the ONUs within the PON. In this LAN emulation scheme, LAN traffic that is carried amongst the ONUs within the PON are modulated on an RF carrier and the upstream access traffic to the CO is carried at baseband on a single wavelength channel. LAN emulation amongst the ONUs using a secondary distribution fibre between the ONUs and SC is provided in-conjunction with protection against distribution fibre breaks. Up stream Re ceiver Down stream Tran smitter λ uwdm λ d CO Optical Switch 2 Redirection of upstream signals Normal Feeder Fibre Protection Feeder Fibre λ u Distribution Fibres N+1 x N+1 Star Coupler Upstream baseband data 2 1 Control RF LAN data Electronics Receiver 4 3 Downstream Receiver WDM 2 1 Optical Upstream Transmitter Switch 1 ONU 1 RF LAN data ONU N f L f L Figure 4.14: Proposed scheme for LAN emulation and protection using dual distribution fibres. The proposed scheme for protection switching and LAN emulation is shown in Figure 4.14 [29]. A 2 x 2 OSW is placed at the ONU to switch the transmissions to the protected path in an event of a fibre break. Each ONU generates two signals, namely the baseband data to the CO and the LAN data for other ONUs in the PON. The LAN data is modulated onto an RF carrier that is chosen to be out of band of the baseband data. The baseband data and the upconverted RF LAN data are electrically combined and the composite signal is then modulated onto the upstream wavelength channel λ u. As each ONU is connected to the SC via two distribution fibres in the way shown, the signals from each ONU that are transmitted in the upstream direction are redirected back to all ONUs through the second distribution fibre. In normal state, the OSW operate in bar state whereby the transmissions of upstream and downstream data signals are carried through distribution fibre 1 while a portion of upstream

151 Chapter 4 Protection and Restoration in Passive Optical Networks signals are redirected to each ONU through distribution fibre 2. In this state, the upstream transmissions to CO are carried through normal feeder fibre. If both the distribution fibres are placed in a single fibre conduit, then a break in the fibre cable results in both distribution fibres being disconnected from the ONUs. On the other hand, if these distribution fibre links are placed in separate fibre conduits, then failure of one distribution fibre does not isolate the ONUs as the transmissions can be carried through the second distribution fibre. If distribution fibres are placed in separate fibre conduits, then using the redirected LAN signals, each ONU is capable of identifying the state of the distribution fibres. If the transmitted LAN data by an ONU does not arrive at that particular ONU, then the distribution fibre break is assumed. If distribution fibre 2 is broken, transmissions between the CO and the ONU are nondisrupted. In this case, LAN data from other ONUs are not received by the affected ONU even though it is capable of sending LAN data to other ONUs within the PON. If distribution fibre 1 is broken, then the transmissions to and from an ONU are disrupted. In this case, the state of the OSW is changed to cross state such that the transmissions are carried through distribution fibre 2 and protection feeder fibre. In the protected state, the downstream data and upstream data transmissions between the CO and the ONU are restored, but not the LAN data transmissions as distribution fibre 1 is disconnected from the affected ONU. As explained in section 4.3.1, this scheme is capable of restoring the transmissions between the CO and the ONUs in the event of the feeder fibre break as well. If a break in the normal or working feeder is noticed, then the upstream and downstream transmissions between the CO and the ONUs are performed through the protection feeder fibre. The upstream transmission of this PON is based on TDMA protocol, whereby both upstream baseband data and RF LAN data are transmitted simultaneously in the same time slot. In the protected state, the round trip delay (RTT) of transmitted signals in the upstream direction is changed due to different path length. Therefore, to obtain fast protection switching, RTT of the redundant paths are ranged during the initialisation of the system and equalisation delays for the protected path are locally stored at each ONU (pre-ranging). In conventional PONs, the ranging information is transmitted from the CO to each ONU frequently [30-32]. However, in this scheme that supports LAN emulation and protection, LAN data that is transmitted amongst the ONUs can be used to carry ranging information of each ONU. Therefore, to monitor the state of the distribution fibre and continuously update the ranging information, an ONU can send short packet frames to the ONUs encapsulated within the LAN

152 Chapter 4 Protection and Restoration in Passive Optical Networks data and therefore the transmission period of an ONU can be automatically adjusted in both normal and protected states. If an ONU experiences a break in its distribution fibre, then that particular information can be easily relayed to other ONUs using the LAN data. Using the information, each ONU could automatically adjust their transmission period such that no collision of the transmitted packets in the upstream direction occurs. Conventionally, the distribution fibre break is identified at the CO and this information is conveyed to each ONU with a new transmission cycle. This process consumes a lot of time due to large propagation delays between the ONUs and the CO and the processing complexities at the CO due to multiple ranging of ONUs. Therefore QoS may not be guaranteed for the services provided to the customers. However, LAN data transmission simplifies the process of ranging as the ONUs are intelligent enough to adjust the transmission period for a collision-free upstream transmission. Moreover, as the upstream signals are redirected back to each ONU, a random access control protocol such as carrier sense multiple access with collision detection protocol (CSMA/CD) can also be used for the transmission of signals in the upstream direction [33-36]. The redirection of upstream signals enables simpler transmission protocols while reducing the processing complexities associated with the operation at the CO obtaining better QoS for the services provided [37, 38]. Moreover, the ONUs could also participate in a selfpolling access scheme for the upstream channel access with dynamic bandwidth allocation capabilities [39, 40] with the protection capabilities Experimental demonstration λ d = nm Circulator 1 MZM OSW Gb/s 1.25 Gb/s Receiver km SMF OSW Gb/s 10 km SMF 4x4 Star Coupler Receiver 2.5 GHz km PLL & SMF TTL Data Recovery signal Circulator 2 10 km Gb/s SMF Receiver Power 1 RF mixer Combiner 155 Mb/s 2.5 GHz MZM 2.5 GHz RF Oscillator 1.25 Gb/s λ u = nm Figure 4.15: Experimental setup to demonstrate protection and LAN emulation using dual fibres

153 Chapter 4 Protection and Restoration in Passive Optical Networks Figure 4.15 shows the experimental setup to demonstrate the feasibility of the proposed scheme for protection and LAN emulation in a PS-PON using dual fibres. The downstream signal of PRBS NRZ data at 2.5 Gb/s was modulated onto a wavelength of nm using a MZM and transmitted to the ONUs through a 10 km feeder fibre, a 4 4 SC, and a 2.2 km distribution fibre. For the upstream transmissions, PRBS NRZ binary phase shift keying (BPSK) data at 155 Mb/s was modulated onto a 2.5 GHz RF carrier to generate upconverted RF LAN data, which was then electrically combined with 1.25 Gb/s PRBS NRZ data using a RF combiner. RF Power (dbm) Gb/s upstream baseband 155 Mb/s LAN data on 2.5 GHz RF carrier RF Frequency (GHz) Figure 4.16: Observed RF spectra of 1.25 Gb/s upstream baseband data and 155 Mb/s LAN data on 2.5 GHz RF carrier at the transmitter output. The observed RF spectra of the upstream signals is shown in Figure The modulation depth of the upstream baseband data was measured to be 67 %. The composite signal was then modulated onto a wavelength of nm and transmitted in the upstream direction. In normal state, OSW 1 was set to bar state using a TTL signal driven by a controller and the signal transmissions were carried out directly. To simulate a fibre break, distribution fibre 1 was disconnected. Therefore, in protected state, OSW 1 was changed to cross state and the transmissions were carried through distribution fibre 2. Both 1.25 Gb/s upstream baseband data and 2.5 Gb/s downstream data were detected using 2.5 Gb/s p-i-n receivers while the RF LAN data was detected using a 2.5 Gb/s APD receiver. The detected RF LAN data was fed through a band pass filter (BPF) centred at 2.5 GHz with a bandwidth of 300 MHz and using a costas loop based phase locked loop (PLL), 155 Mb/s LAN data was recovered. A series of experiments were conducted to examine the crosstalk effects of one signal on the other, and

154 Chapter 4 Protection and Restoration in Passive Optical Networks the BER and optical spectra at each receiver ports were measured. It should be noted that both upstream and downstream signals were recovered in both states, while RF LAN data could not be recovered in the protected state Optical spectra Optical Power (dbm) db 16 db Downstream Normal Downstream Protect Upstream Normal Upstream Protect Wavelength (nm) Figure 4.17: Observed optical spectra at the downstream data and upstream data receivers. Figure 4.17 shows the optical spectra observed at the input of the downstream and upstream data receivers. In normal state, the suppressions of the reflected light at the downstream and upstream data receivers were more than 16 db and 27 db, while the suppressions of the reflected light at these receivers in the protected state were 16 db and 26 db respectively. These suppression values were closer to those obtained from the expreiment that was carried out without LAN traffic BER results Figure 4.18 shows the measured BER curves for all three signals. In normal and protected operating states, the maximum penalty for the 2.5 Gb/s downstream data compared to B-B measurements was 0.5 db. Similarly, the maximum penalty for the 1.25 Gb/s upstream data was 0.1 db compared to B-B measurements. The measured BER curves for the 155 Mb/s LAN data show that the maximum penalty compared to B-B measurements was 0.75 db, and it is largely due to the interchannel interference (ICI) crosstalk from the 1.25 Gb/s upstream data. As the BER curves confirm, the upstream baseband data and downstream data

155 Chapter 4 Protection and Restoration in Passive Optical Networks transmissions to and from the CO can be restored in the event of the distribution fibre break with minimal penalty. However, LAN data cannot be recovered if the distribution fibre breaks. Similarly, the experiment can also be carried out to demonstrate the protection capabilities against the feeder fibre breaks. In this case, at an ONU, LAN data can also be recovered in addition to the upstream baseband data and downstream data as the distribution fibres of the ONUs are not considered broken Gb/s downstream data Back to back Transmission - Normal Transmission - Protect With upstream - Normal With upstream - Protect 1.25 Gb/s upstream baseband data Mb/s RF LAN data Back to back Transmission With upstream data With other signals Log 10 (BER) Back to back 10-9 Transmission - Normal Transmission - Protect With LAN data - Normal With LAN data - Protect With RF & down Normal With RF & down Protect Received Optical Power (dbm) Figure 4.18: Measured BER curves for 2.5 Gb/s downstream data, 1.25 Gb/s upstream data and 155 Mb/s LAN data Protection using the interconnections amongst the ONUs In Section 4.3.2, protection of upstream and downstream transmissions for an ONU is carried out using dual distribution fibres connected to each ONU. Providing this protection capability to each ONU requires both distribution fibres laid in separate fibre ducts. As the distribution fibre length between the SC and ONUs is practically shorter (typically less than 1-2 km), the installation cost is relatively cheap. However, providing protection of ONUs without having

156 Chapter 4 Protection and Restoration in Passive Optical Networks to connect each ONU with separate fibre ducts can provide a cost effective and more feasible solution. Practically, the ONUs at the customer premises in the PON are located closer to each other. Therefore, the ONUs that demand protection, can be interconnected and protection against distribution fibre link breaks can be provided, whereby the transmissions to and from the affected ONU can be carried through the interconnected ONU. This architecture provides a capability that all ONUs do not have to be interconnected at once. The interconnections for protection could be made on customers demand and the interconnections can be expanded into a star-ring architecture. In comparison to the previously proposed star-ring architectures [41-43], the protection architecture that is described here shows many advantages. In the previously proposed star-ring protection architectures, the RN is active as it contains an OSW, while the traversing signals are regenerated at each network terminal and therefore increasing the cost of the network. On the other hand, the protection architecture explained in this section does not add any significant cost to the ONUs and can be applied to a double star PON architecture. The following section describes the architecture that uses interconnection of adjacent ONUs for resilient fast protection switching in an event of a distribution fibre break. Central Office Feeder Fibre Distribution Normal path for Fibre 1 transmission for ONU 1 1 x N Star Coupler To previous ONU Control WDM 2 1 Coupler 50:50 Coupler 3 Optical Switch Downstream Receiver Upstream Transmitter ONU 1 Protection path for Distribution Fibre 2 Control 2 1 WDM Coupler Downstream Receiver transmission for ONU 1 50:50 Coupler 3 Optical Switch Upstream Transmitter ONU 2 To next ONU Figure 4.19: Protection architecture against distribution fibre breaks using the interconnections amongst the ONUs. Figure 4.19 shows the protection architecture for the PS-PONs, whereby interconnections between the ONUs are used to protect the ONUs against distribution fibre breaks. A

157 Chapter 4 Protection and Restoration in Passive Optical Networks 1 x 2 OSW is placed at each ONU. Two ONUs that are located closer to each other are connected using a fibre as shown. The interconnections between the ONUs can be expanded on the customers demand to provide protection to other ONUs. The ONUs that require protection can be interconnected in a Bus or Ring topology. In the next paragraph, the operation of an ONU in an event of a distribution fibre break is presented. In normal state, the OSWs are in bar state such that the transmissions between the ONU and the CO are performed directly. For example, the transmissions of signals to ONU 1 are performed through distribution fibre 1 while it is through distribution fibre 2 for ONU 2. As all the OSWs operate in bar state, the signals do not reach other ONUs through the interconnected fibre. Consider that distribution fibre 1 experiences a break and therefore the transmissions between the ONU 1 and CO are disrupted. Using the loss of signal or packet frames, ONU 1 identifies the break in the distribution fibre 1 and therefore the OSW in ONU 1 is changed to cross state. In this protected state, the transmitted downstream signals from the CO reach ONU 2 through distribution fibre 2. As the distribution fibres of other ONUs do not experience breaks in the respective distribution fibre links, the state of the OSW in these ONUs remain in bar state. The downstream signals that reach ONU 2 are split using a 3 db coupler and one portion of the signals go to ONU 1 while the other portion of signals reach ONU 1 through the interconnected fibre link. The downstream signals do not reach the ONU that interconnects ONU 1 as the OSW in that particular ONU is in bar state. The upstream signals from ONU 1 go to ONU 2 through the interconnected fibre link and the signals reach the CO via the distribution fibre 2. As the transmission of upstream signals follow TDMA protocol, there will not be any collision of signals at the 3 db coupler. In the protected operation, the signal path is changed and therefore appropriate delays in the transmission period have to be employed to perform collision-free transmission of signals in the upstream direction Experimental demonstration Figure 4.20 shows the experimental setup to demonstrate the capabilities of the proposed protection architecture against distribution fibre breaks. For the experiment, the downstream signal of PRBS NRZ data at 2.5 Gb/s was modulated onto a wavelength of nm (λ d ) using a MZM and transmitted to the ONUs through a 10 km feeder fibre, a 4 4 SC, a

158 Chapter 4 Protection and Restoration in Passive Optical Networks km distribution fibre and an OSW. For the upstream transmission, PRBS NRZ data at 1.25 Gb/s was modulated onto the upstream wavelength of nm and transmitted in the opposite direction of the downstream signal transmissions. The unused ports of the SC were terminated with isolators to prevent reflections. λ d = nm MZM Circulator Gb/s 1.25 Gb/s Receiver 10 km SMF 2.2 km SMF 4x4 Star Coupler 3 km SMF 2 Isolator Circulator :50 1 Coupler 4 3 Optical switch 1 Isolator 1.2 km SMF 2.5 Gb/s Receiver MZM 1.25 Gb/s λ u = nm 2 1 Isolator 50:50 Coupler 4 3 Optical Isolator switch 2 Isolator Figure 4.20: Experimental setup to demonstrate the protection architecture against distribution fibre breaks. Initially for the normal state, OSW 1 was set to bar state using a TTL signal driven by a switch controller and signal transmissions in both upstream and downstream directions were performed directly. To simulate the distribution fibre break, the 2.2 km fibre that interconnects OSW 1 and SC was disconnected. In protected state, OSW 1 was changed to cross state, while OSW 2 remained at bar state. Here, the signals to and from ONU 1 were carried through 1.2 km fibre that connects ONU 1 with ONU 2, OSW 2, and 3 km distribution fibre of ONU 2. Both 1.25 Gb/s upstream data and 2.5 Gb/s downstream data were detected using 2.5 Gb/s p-i-n receivers. In both normal and protected states, both signals were transmitted and recovered ensuring that all data transmissions can be restored through the protection path in the event of the distribution fibre break Optical spectra Figure 4.21 shows the optical spectra observed at the input of the downstream and upstream data receivers. The suppressions of the reflected light were 18 db and 20 db respectively at both receivers in normal state. In protected state, the suppressions were reduced to 14 db and

159 Chapter 4 Protection and Restoration in Passive Optical Networks 16 db respectively in both receivers. In protected state, the signals in both directions additionally travel through a fibre with a length of 1.2 km, an OSW and a 3 db coupler that introduce more losses. Moreover, as the signals travel through 1.2 km additional fibre, the backscattering increases and therefore the suppression of the backscattered light in each receiver port is lower in protected state compared to that of in normal state. -10 Optical Power (dbm) db 16 db Downstream Normal Downstream Protect Upstream Normal Upstream Protect Wavelength (nm) Figure 4.21: Observed optical spectra at the upstream data and downstream data receivers BER results Figure 4.22 shows the measured BER curves for both signal transmissions. For 2.5 Gb/s downstream data, a penalty of 0.4 db was observed when the signals were transmitted through the link, compared to B-B measurements. No additional penalty was observed when the upstream signals were present in the normal state. However, in protected state, an additional penalty of 0.5 db was observed. This penalty can be attributed to the crosstalk from the backscattered light that was present in the downstream data receiver. A penalty of 0.2 db was measured for the 1.25 Gb/s upstream data in the presence of downstream data signals in the normal state. In the protected state, the penalty was increased by an additional 0.2 db, which is possibly a result of the reduced suppression of the backscattered light that was present in the upstream data receiver

160 Chapter 4 Protection and Restoration in Passive Optical Networks 2.5 Gb/s downstream data 1.25 Gb/s upstream data -5 Back to back Transmission - Normal Transmission - Protect 10-9 With upstream - Normal With upstream - Protect -6 Log 10 (BER) Back to back Transmission - Normal Transmission - Protect With downstream - Normal With downstream - Protect Received Optical Power (dbm) Figure 4.22: Measured BER curves for 2.5 Gb/s downstream data and 1.25 Gb/s upstream data in normal and protected states. In the previous section, protection architectures against distribution fibre link breaks using the interconnections amongst the ONUs in a PS-PON are discussed. Interconnecting ONUs to provide protection in a WDM-PON was proposed and experimentally demonstrated [44-47]. In these WDM-PON architectures, protection against distribution fibre cuts is carried out by grouping two ONUs and interconneting them. Using the FSR of the AWG used at the RN, upstream and downstream wavelength channels of a group of ONUs use only one port of the AWG for the transmissions. One of these protection architectures use distributed protection switching, whereby the switching is performed at the ONUs [44, 45] while the second architecture demonstrates a centrally controlled protection switching capability, whereby an OSW placed at the CO performs the switching of signal transmissions in an event of a failure in the distribution fibres [46]

161 Chapter 4 Protection and Restoration in Passive Optical Networks Protection using interconnections amongst the ONUs with LAN emulation In the previous section, protection of ONUs against distribution fibre breaks using the interconnections of adjacent ONUs is described and experimentally demonstrated. In this section, the protection scheme in-conjunction with two different LAN emulations is demonstrated. LAN traffic that is transmitted amongst the ONUs, is used for continuously monitoring the state of the distribution fibre. This configuration emulates a LAN over the existing PON, while facilitating the switching of signal transmissions to a predetermined protection path in an event of a distribution fibre break. The proposed protection mechanism is combined with two separate optical layer LAN emulation schemes that use a single optical transmitter at each ONU for the transmission of upstream traffic to the CO and LAN traffic to other users in the PON [48-51]. In both schemes, LAN traffic is transported on a radio frequency (RF) subcarrier, while upstream traffic to the CO is transported at baseband. The protection mechanism in conjunction with the LAN emulation schemes are experimentally verified with 2.5 Gb/s downstream data, 1.25 Gb/s upstream baseband data, and 155 Mb/s LAN data transmissions. Scheme 1 uses a dual notch narrowband fibre Bragg grating (FBG) placed in the feeder fibre close to the SC for the reflection of the optically modulated RF subcarrier sidebands that consist of LAN data back to the ONUs while Scheme 2 uses a secondary distribution fibre between the SC and the ONUs for the redirection of the upstream signals back to the ONUs. Here, the reflected/redirected LAN data is exploited not only as a channel for intercommunication amongst the customers in the PON, but also provides a simple means for monitoring each distribution fibre by the respective ONU. If LAN data sent by an ONU is not received, then a break in the corresponding distribution fibre is assumed and signal transmissions for that particular ONU are switched to the predetermined protection path. Adjacent ONUs are interconnected using a fibre through the 1 2 OSWs that are placed at each ONU. In normal state, the OSW at each ONU is set to bar state whereby the signals in both upstream and downstream directions are transported directly. In the absence of the transmitted LAN data from an ONU, distribution fibre break is assumed and therefore the OSW of the affected ONU is changed to cross state such that the signal transmissions are performed through the interconnected ONU. In the following section, the operation of the

162 Chapter 4 Protection and Restoration in Passive Optical Networks protection switching mechanism with the two LAN emulation scheme is described Protocol for upstream transmissions and ranging The upstream transmission of this PON is based on TDMA protocol, whereby upstream baseband data and RF LAN data are transmitted simultaneously in the same time slot. Using TDMA protocol for the transmission of signals in the upstream direction reduces the requirement for separate complex protocol for the transmission of the LAN data. The transmissions of each of these signals may be carried out independently in the absence of each other. The transmission of upstream data to the CO may be carried out in the absence of LAN data. Similarly, LAN data transmission can be carried out in a time slot in the absence of the upstream data, provided that a time slot is assigned to the ONU by request to the CO. In protected state, the path for the signal transmissions to and from ONU 1 is changed and the signal transmissions are carried out through ONU 2. As the upstream transmission follows the TDMA protocol, in the event of a fibre break, the system needs to be setup appropriately for the transmission of signals in the protected path. Pre-ranging or uni-ranging techniques can be performed during the protection switching. It is expected that an ONU does not necessarily have LAN data to transmit to other ONUs in the PON. However, to continuously monitor the state of the distribution fibre by each ONU, an ONU could send short packet frames to continuously update the ranging information between the ONUs. Therefore, in each timeslot, LAN data from each ONU could carry information about the state of its distribution fibre and changes in ranging. This information is continuously updated and used by all ONUs to automatically adjust the transmission period in both normal and protected states. Therefore, the transmission of LAN data amongst the ONUs in the PON is used as a distributed control and management channel of the PON while carrying data and therefore reducing processing complexities associated at the CO. The flow chart in Figure 4.23 gives the operation of an ONU in an event of a failure in the distribution fibre and feeder fibre in the PON. Using the LAN data, distribution fibre failure can be identified. To carry out the transmissions of upstream data, downstream data, and LAN data, the state of the OSW is changed to cross state and the new delay in the transmission period is sent to all ONUs. To confirm the operation of the SC, ranging messages are sent to other ONUs. If the ranging messages are not received, SC is not operational. In this case, it is assumed that adjacent distribution fibres are not simultaneously broken. Therefore, the operator is informed about the possible failure

163 Chapter 4 Protection and Restoration in Passive Optical Networks of the SC. If own ranging messages are received, while the gate message from the CO is not received, a break in the feeder fibre is recognized and this information is relayed to the operator. Normal State No Receive LAN data? Yes Receive downstream data? Receive ranging info? Yes No Transmit report message to CO No Continue Transmissions Update transmission period Wait for gate message from CO Receive gate message? No Recognize as distribution fiber failure, change switch state Transmit ranging info to ONUs Receive message? Yes No Recognize as Splitter failure, inform operator Wait for gate message from CO Receive gate message? No Recognize as feeder fiber failure, inform operator Yes Continue Transmissions Figure 4.23: A flow chart showing the operation of the ONU in an event of failure in distribution fibre branch

164 Chapter 4 Protection and Restoration in Passive Optical Networks Protection with LAN emulation using narrowband FBG Upstream Receiver Downstream Transmitter λ u WDM 1.3/1.5 µm λ d Transmitted Upstream baseband c λ u CO Distribution Normal path for Fibre 1 transmission for ONU 1 Feeder Fibre FBG c b Protection path for transmission for ONU 1 a 1 x N Star Coupler Distribution Fibre 2 Reflected RF sidebands b To previous ONU Control WDM Downstream /1.5 µm Receiver λ d 50: LAN data Coupler λ Receiver 3 u OSW 1 1 Upstream Transmitter Control :50 Coupler 3 OSW 2 To next ONU a λ u ONU 2 ONU 1 Upstream baseband data RF LAN data f L f L f L f L Figure 4.24: Protection against distribution fibre breaks using interconnections amongst ONUs in conjunction with LAN emulation using a narrowband FBG placed in the feeder fibre. Figure 4.24 shows the schematic diagram of the protection mechanism in conjunction with the LAN emulation scheme using a dual notch narrowband FBG placed in the feeder fibre close to the SC [52, 53]. In this scheme, upstream data to the CO and LAN data to other ONUs in the PON are generated at each ONU for the transmission in the upstream direction. The upstream data is carried at baseband, while LAN data is carried on an RF carrier that is chosen to be outside the bandwidth of the upstream baseband data. LAN data is amplitude modulated onto the RF carrier using a voltage controlled oscillator (VCO). These signals are then electrically combined and modulated onto the upstream wavelength channel λ u. A dual notch FBG is placed in the feeder fibre close to a 1 N SC whereby N corresponds to the number of ONUs, such that both optically modulated RF subcarrier sidebands that contain the

165 Chapter 4 Protection and Restoration in Passive Optical Networks LAN data are reflected back and broadcast to all ONUs. As LAN data is amplitude modulated onto the RF carrier, the reflected optically modulated RF sidebands that contain the LAN data can be recovered by direct detection. This allows the bandwidth of the LAN data receiver to be in the order of the transmission bit rate of the LAN data. Moreover, as the upstream baseband data and the LAN data are separated using the FBG, no further filtering mechanisms are required at the CO and ONUs to separate these signals and this scheme only uses low cost electronics at each ONU for the transmission of RF LAN data. The OSWs in adjacent ONUs in the PON are interconnected using a fibre as shown in Figure In normal state, downstream, upstream and LAN data signals traverse through ports 1 and 2 of OSW 1 and therefore signal transmissions between the CO and ONU 1 are performed through distribution fibre 1. In case of a break in distribution fibre 1, the transmitted LAN data will not be received at ONU 1 and therefore distribution fibre cable break is assumed. Subsequently, the OSW 1 is changed to cross state such that signals will traverse through ports 1 and 3 of the OSW 1. Therefore, in protected state, the signals to and from ONU 1 (the affected ONU) are carried through ONU 2 and distribution fibre 2. Note that OSW 2 that is located in ONU 2 remains in bar state and the operation of other ONUs is not affected. In the protected state, downstream signals and the reflected LAN signals that reach the OSW 2 of the ONU 2 are split by the 3 db coupler where one portion goes to ONU 2 while the other reaches ONU 1 through the interconnected fibre. The upstream transmission for ONU 1 is also carried out through ONU 2 in the opposite direction of the downstream transmissions Experimental demonstration Figure 4.25 shows the experimental setup to demonstrate the feasibility of Scheme 1. The downstream signal of PRBS NRZ data at 2.5 Gb/s was modulated onto a wavelength of nm (λ d ) using a MZM and transmitted to the ONUs through a 10 km feeder fibre, a dual notch FBG filter, a 4 4 SC, an OSW, and a 2.2 km distribution fibre. The FBG placed between the feeder fibre and the 4x4 SC has two notches at nm and nm and each notch has approximately 100% peak reflectivity with the 3 db reflection bandwidth of GHz (0.114 nm). For the upstream transmission, NRZ data at 155 Mb/s was amplitude modulated onto a 7.5 GHz RF carrier using a RF mixer to generate the upconverted RF LAN data. The upconverted RF LAN data was then optically combined with 1.25 Gb/s PRBS NRZ data using a dual electrode modulator (DEM) [54, 55]. These signals can be

166 Chapter 4 Protection and Restoration in Passive Optical Networks electrically combined and directly modulated onto the upstream wavelength [56-59]. In the electrical combination, proper isolation of signals was required to minimise the electrical ICI crosstalk and distortion that occur in the passive RF power combiners. A good duplexer that combines the signals in two different frequency bands was not available during the experiment. Moreover, optical combination of baseband and RF signals avoids the passive losses that occur in RF power combiners and by driving the DEM appropriately the interference between these signals can be reduced. λ d = nm MZM Gb/s 1.25 Gb/s Receiver 10 km SMF FBG 2.2 km SMF 155 Mb/s Receiver Isolator Gb/s 2.2 km 50:50 Receiver SMF Coupler 1 4x4 Star Coupler 4 3 OSW 1 Isolator 1.25 Gb/s 3 km SMF 1.2 km SMF DEM λ u = nm 2 1 Isolator 50:50 RF 155 Mb/s Coupler mixer 4 3 OSW GHz Isolator Isolator RF Oscillator Figure 4.25: Experimental setup to demonstrate Scheme 1 for protection against distribution fibre breaks. The measured modulation depth of the upconverted RF LAN data was approximately 64%. The upstream wavelength (λ u ) was chosen to be nm such that both optically modulated RF subcarrier sidebands are reflected back to the ONUs. The unused ports of the SC were terminated with isolators to prevent reflections. Initially, for the normal state, OSW 1 was set to bar state using a TTL signal driven by a switch controller and signal transmissions in both upstream and downstream directions were performed directly. To simulate the distribution fibre break, the 2.2 km fibre that interconnects OSW 1 and SC was disconnected. In protected state, OSW 1 was changed to cross state, while OSW 2 remained at bar state. Here, the signals to and from ONU 1 were carried through 1.2 km fibre that connects ONU 1 with ONU 2, OSW 2, and 3 km distribution fibre of ONU 2. Both 1.25 Gb/s

167 Chapter 4 Protection and Restoration in Passive Optical Networks upstream baseband data and 2.5 Gb/s downstream data were detected using 2.5 Gb/s p-i-n receivers while LAN data was detected using a 2.5 Gb/s APD receiver followed by a 160 MHz low pass filter (LPF). In both normal and protected states, all three signals were transmitted and recovered ensuring that all data transmissions can be restored through the protection path in the event of the distribution fibre break. A series of experiments were conducted to examine the crosstalk effects of one signal on the other, and the BER and optical spectra at each receiver ports were measured. It should be noted that BER measurements of 2.5 Gb/s downstream data were performed in the absence of LAN data and vice versa as a CWDM coupler was not implemented to separate λ d and λ u Optical spectra Optical Power (dbm) db 12 db 12 db Downstream Normal Downstream Protect Upstream Normal Upstream Protect 16 db Wavelength (nm) Figure 4.26: Observed optical spectra at the upstream baseband data and downstream data receivers showing the suppression of the backscattered light. Figure 4.26 shows the optical spectra observed at the input of the downstream data and upstream baseband data receivers. The suppressions of the reflected light were 16 db and 12 db respectively at both receivers in normal and protected states. In protected state, the signals in both directions additionally travel through a fibre with a length of 1.2 km, an OSW and a 3 db coupler that introduce more losses. Moreover, as the signals travel through 1.2 km additional fibre, the backscattering increases and therefore the suppression of the backscattered light in each receiver port is lower in protected state compared to that of in normal state

168 Chapter 4 Protection and Restoration in Passive Optical Networks Optical Power (dbm) db Normal Operating State 11 db Before FBG Transmitted Reflected Optical Power (dbm) db Protected Operating State 11 db Before FBG Transmitted Reflected Wavelength (nm) Wavelength (nm) Figure 4.27: Observed optical spectra at the FBG in both normal and protected states. Figure 4.27 shows the observed optical spectra at the FBG for the transmitted and reflected portions of the upstream signals measured using an optical spectrum analyser with 2.5 GHz resolution bandwdth. Figure 4.27(a) and 4.27(b) show the optical spectra for the normal and protected states respectively. In both states, the suppression of optically modulated RF sidebands to the optical carrier was measured to be more than 11 db before the FBG. In the reflected optical spectra, the suppression of the optical carrier from the reflected RF sidebands was approximately 11 db. In the protected state, the measured optical power level of each signal component is approximately 4 db lower than that of in the normal state, as the signals traverse through an additional 3 db coupler and an OSW (insertion loss approximately 1 db). -30 Normal Optical Power (dbm) db 12 db Protection Wavelength (nm) Figure 4.28: Observed optical spectra at the LAN data receiver

169 Chapter 4 Protection and Restoration in Passive Optical Networks Figure 4.28 shows the optical spectra observed at the LAN data receiver in both normal and protected states. The suppressions of the optical carrier from the optically modulated RF sidebands in normal and protected states were 10 db and 12 db respectively BER results Gb/s downstream data 1.25 Gb/s upstream baseband data 155 Mb/s RF LAN data -6 Log 10 (BER) Back to back Transmission - Normal Transmission - Protection With upstream data - Normal With upstream data - Protection Back to back Transmission - Normal Transmission - Protect With down - Normal With down - Protect With LAN data - Normal With LAN data - Protect Received Optical Power (dbm) Back to back Transmission - Normal Transmission - Protect With upstream data- Normal With upstream data - Protect Figure 4.289: Measured BER curves for 2.5 Gb/s downstream data, 1.25 Gb/s upstream baseband data, 155 Mb/s LAN data in both normal and protected states. Figure 4.29 shows the measured BER curves for all signals. For 2.5 Gb/s downstream data, a penalty of 0.15 db was observed when the upstream signals were present compared to B-B measurements. No additional penalty was observed in protected state compared to normal state even though the suppression of the backscattered light of the upstream signals that were present at the downstream receiver was only 12 db. A penalty of approximately 0.38 db was measured for the 1.25 Gb/s upstream baseband data in the presence of the downstream signals in normal state compared to B-B measurements. In protected state, it was increased to 0.55 db. An additional penalty of 0.55 db was observed when RF upconverted LAN data was included with the upstream baseband data in both normal and protected states. This penalty

170 Chapter 4 Protection and Restoration in Passive Optical Networks was due to electrical ICI crosstalk from the RF upconverted LAN data. The measured BER curves for the 155 Mb/s RF LAN data show that the penalty observed in normal states in the presence of 1.25 Gb/s upstream baseband data was 0.5 db compared to B-B measurements, and this was due to the electrical ICI crosstalk from the 1.25 Gb/s upstream baseband data. In protected state, the penalty was increased by 0.38 db. The use of narrowband FBG requires highly stable optical transmitter at each ONU. However, higher frequency RF carrier is used for the transmission of LAN data, and therefore the stringent requirements for the transmitter in terms of stability can be relaxed. As discussed in Chapter 3 of this thesis, the use of highly stable FBG [60-63] in conjunction with wavelength locking techniques with the FBG acting as a sensor [64-68] can be utilised to compensate for the drifts of the FBG and the optical transmitter used at each ONU to relax the requirements of the optical source at the ONUs Protection with LAN emulation using fibre loop back Feeder Fibre (N+1) x Central (N+1) Office Star Coupler λ RF LAN u Upstream data baseband data f L f L Redirected Upstream signals Distribution Normal path for Fibre 1.2 transmission for ONU 1 Protection path for transmission for ONU 1 To previous ONU ONU 1 WDM Control Coupler 2 1 Distribution 50:50 Fibre 1.1 Coupler 3 OSW Distribution Fibre 2.1 Control :50 Coupler 3 OSW To next ONU LAN data Receiver Downstream Receiver Upstream Transmitter ONU 2 Figure 4.30: Protection architecture against distribution fibre breaks using interconnections amongst ONUs in conjunction with LAN emulation using dual distribution fibres. Figure 4.30 shows Scheme 2, whereby LAN emulation is carried out using a secondary distribution fibre to each ONU [53, 69]. In this scheme, a (N+1 N+1) SC replaces the (1 N)

171 Chapter 4 Protection and Restoration in Passive Optical Networks SC that was used in Scheme 1. The number of ONUs that are attached to the SC is N and one of the ports of the SC facing towards the ONUs is terminated. Note that in Figure 4.30, each ONU in the PON is connected to the SC via two distribution fibres. Such a physical connection enables upstream signals transmitted from each ONU on λ u to be redirected back to all ONUs via the respective second distribution fibre. For example, for ONU 1, the redirected upstream signals are detected through distribution fibre 1.2. Note that the RF upconverted LAN data is separated from the upstream baseband data using an electrical BPF and then down-converted to baseband frequencies to be recovered. As in Scheme 1, an OSW is placed at each ONU and adjacent ONUs are interconnected through the OSWs using a fibre. In normal state for ONU 1, OSW 1 remains in bar state and the transmissions are carried out directly through ports 1 and 2 of OSW 1. The upstream signals and downstream signals are carried through distribution fibre 1.1 while LAN data is received through distribution fibre 1.2. As both distribution fibres of an ONU reside in a single fibre conduit, in an event of a cable break, both distribution fibres will be disconnected from ONU 1. In this scenario, the transmitted LAN data by ONU 1 would not be received and therefore distribution fibre cable break is assumed and the transmissions are switched to the protection path. In protected state, OSW 1 is changed to cross state and the signals to and from ONU 1 are transmitted through ports 1 and 3 of OSW 1, the interconnected ONU 2 and distribution fibre 2.1. However, OSW 2 that is located in ONU 2 remains in bar state. In protected state, the transmitted downstream signals are split by the 3 db coupler that lies beside OSW 2 whereby one portion is delivered to ONU 2 while the other portion is carried to ONU 1 through the interconnected fibre link. In this scheme, only the upstream and downstream transmissions are restored in the event of the distribution fibre break. Even though ONU 1 is capable of transmitting LAN data to other ONUs in the PON, it is unable to receive LAN data from others. Therefore the restoration of LAN data is not feasible in the protected state Experimental demonstration Figure 4.31 shows the experimental setup to demonstrate the feasibility of Scheme 2. The downstream signal of PRBS NRZ data at 2.5 Gb/s was modulated onto a wavelength of nm (λ d ) using a MZM and transmitted to the ONUs through a 10 km feeder fibre, a 4 4 SC, and a 2.2 km distribution fibre. For upstream transmission, a 155 Mb/s signal of PRBS NRZ BPSK data was modulated onto a 2.5 GHz RF carrier to generate upconverted

172 Chapter 4 Protection and Restoration in Passive Optical Networks RF LAN data, which was then electrically combined with a baseband signal of PRBS NRZ data at 1.25 Gb/s using an RF combiner. The modulation depth of upstream baseband data was measured to be 54%. 2.2 km SMF 2.5 Gb/s Receiver 2.5 GHz PLL & Data Recovery λ d = nm MZM Gb/s 1.25 Gb/s Receiver 10 km SMF km SMF 3 4x4 Star Coupler 3 km SMF Isolator Gb/s 50:50 Receiver Coupler OSW MZM 1 Isolator λ u = nm 1.2 km SMF Power Combiner 2 1 Isolator 1.25 Gb/s 50:50 Coupler RF mixer 4 3 OSW 2 Isolator 155 Mb/s Isolator 2.5 GHz 2.5 GHz RF Oscillator Figure 4.31: Experimental setup to demonstrate the feasibility of Scheme 2. RF power (dbm) Gb/s Upstream baseband data 155 Mb/s LAN 2.5 GHz RF carrier RF frequency (GHz) Figure 4.292: Observed RF spectra at the input of the upstream transmitter showing 1.25 Gb/s baseband data and 155 Mb/s LAN data on 2.5 GHz RF carrier

173 Chapter 4 Protection and Restoration in Passive Optical Networks Figure 4.32 shows the measured RF spectra, whereby the 155 Mb/s LAN data is bandlimited by a BPF with a bandwidth of 300 MHz before the combination with 1.25 Gb/s upstream baseband data to reduce the electrical ICI crosstalk between the combined signals. The composite signals were then modulated onto an upstream wavelength (λ u ) at nm and transmitted in the upstream direction. OSW 1 and OSW 2 were used in ONU 1 and ONU 2 respectively and these ONUs were interconnected using a 1.2 km fibre. The transmitted upstream signals from ONU 1 were redirected back using a second 2.2 km fibre for the detection of RF upconverted LAN data. In normal state, OSW 1 was set to bar state and signal transmissions in both upstream and downstream directions were performed directly. To simulate the distribution fibre cable break, both the 2.2 km fibres that connects ONU 1 and SC were disconnected. In protected state, OSW 1 was changed to cross state, while OSW 2 remained at bar state. Here, the upstream and downstream signals between ONU 1 and CO were carried through OSW 1, 1.2 km interconnecting fibre, OSW 2 and 3 km distribution fibre. The transmitted downstream signals for ONU 1 and ONU 2 are split by the 3 db coupler after OSW 2. One portion of the downstream signals reaches ONU 2 while the other goes to ONU 1 through the 1.2 km fibre. As both distribution fibres of ONU 1 were disconnected, LAN data will not be received at ONU 1 in protected state. Yet, 2.5 Gb/s downstream data and 1.25 Gb/s upstream data transmissions can be restored in the protected state. Both 1.25 Gb/s upstream baseband data and 2.5 Gb/s downstream data were detected using 2.5 Gb/s p-i-n receivers while LAN data was detected using a 2.5 Gb/s APD receiver. The detected upconverted RF LAN data was separated from the upstream baseband data using a BPF with a bandwidth of 300 MHz and using a PLL, the 155 Mb/s LAN data was recovered. In normal state, all three signals were transmitted and recovered, while in protected state, only 2.5 Gb/s downstream data and 1.25 Gb/s upstream baseband data were recovered. A series of experiments were conducted to examine the crosstalk effects of one signal on the other, and the BER and optical spectra at each receiver ports were measured Optical spectra Figure 4.33 shows the optical spectra observed at the input of the downstream and upstream data receivers. In normal state, the suppressions of the reflected light at the upstream baseband data receiver and downstream data receiver were 20 db and 17 db respectively. In protected state, they were reduced to 16 db and 13 db respectively due to the increase in

174 Chapter 4 Protection and Restoration in Passive Optical Networks Rayleigh backscattered light as the signals travelled through an additional 1.2 km of fibre. -10 Optical Power (dbm) db 16 db 13 db Downstream Normal Downstream Protection Upstream Normal Upstream Protection 17 db Wavelength (nm) Figure 4.303: Observed optical spectra at the upstream data and downstream data receivers BER results Gb/s downstream data 1.25 Gb/s upstream baseband data 155 Mb/s RF LAN data -6 Log 10 (BER) Back to back Transmission - Normal Transmission - Protect With upstream - Normal With upstream - Protect Back to back Transmission - Normal Transmission - Protect With LAN data - Normal With LAN data - Protect With LAN & downstream signals Received Optical Power (dbm) Back to back Transmission With upstream data With upstream & downstream signals Figure 4.314: Measured BER curves for 2.5 Gb/s downstream data, 1.25 Gb/s upstream baseband data, 155 Mb/s LAN data in both normal and protected states

175 Chapter 4 Protection and Restoration in Passive Optical Networks Figure 4.34 shows the measured BER curves for all signals. For the 2.5 Gb/s downstream data, in normal state, a penalty of 0.47 db was observed in the presence of 1.25 Gb/s upstream baseband data compared to B-B measurements and an additional 0.28 db penalty was observed in protected operating state. The penalty for the transmission of 1.25 Gb/s upstream baseband data in the presence of RF LAN data in both normal and protected states compared to B-B measurements was 0.22 db. An additional penalty of 0.12 db was observed when the 2.5 Gb/s downstream data was present in both normal and protected states. The measured BER curves for the 155 Mb/s LAN data show that a maximum penalty of 1 db was observed in the presence of both upstream baseband data and downstream data compared to B-B measurements, and this is largely due to the electrical ICI crosstalk from the 1.25 Gb/s upstream baseband data. 4.4 Scalability of the protection architectures Distribution fibre 1 ONU 1 2 SC ONU 2 3 Figure 4.325:. ONU 3 Serial interconnection of ONUs in protected state in PS-PONs. In the protection architectures that use interconnections of adjacent ONUs, the restoration of the signal transmissions between the CO and several ONUs can be carried out against distribution breaks. As shown in Figure 4.36, if the distribution fibre 1 of ONU 1 is broken, then transmissions to and from ONU 1 could potentially be protected by ONU 2. If the distribution fibres of ONU 1 and ONU 2 are simultaneously broken, then the transmissions to both affected ONUs could be protected by ONU 3. However, in protected state, the signals traverse through additional 3 db coupler and an OSW experiencing approximately 4 db

176 Chapter 4 Protection and Restoration in Passive Optical Networks insertion loss at each ONU. As the signals traverse through many ONUs in the protected state, the required power budget may not be adequate for error free data transmissions. 2.5 Gb/s Downstream data 1.25 Gb/s upstream baseband data 155 Mb/s RF LAN data Scheme 1 Scheme 2 Scheme 1 Scheme 2 Scheme 1 Scheme 2 Transmitted Power Circulator ONU WDM coupler CO 10 km feeder fibre loss x x x x x FBG loss km distribution fibre loss 0.25 x x x x x x 6 OSW loss db coupler loss WDM coupler ONU Connectors/ splices loss x x sensitivity Dispersion Penalty Ageing and safety margin Table 4.1: Typical parameters of the 2.5 Gb/s downstream data, 1.25 Gb/s upstream baseband data, 155 Mb/s LAN data signals. The downstream signal transmission is continuous and an APD receiver, which has a receiver sensitivity of -32 dbm, is used to recover the downstream data. The upstream baseband data

177 Chapter 4 Protection and Restoration in Passive Optical Networks and the RF LAN data transmissions follow TDMA protocol and therefore burst mode receivers are required for the reception of these signals. A burst mode receiver operating at 1.25 Gb/s with a sensitivity of dbm [70, 71] is used for the 1.25 Gb/s upstream baseband data recovery, while the 155 Mb/s burst mode receiver shows -36 dbm for scheme 1. For scheme 2, the burst mode receiver operating at 2.5 Gb/s for the 155Mb/s LAN data reception gives -28 dbm sensitivity. It should also be noted that the transmission power for the upstream baseband data is 1 dbm while it is -5 dbm for the RF LAN data. The SC loss can be calculated using the formula given in Table 1 [72]. From the parameters of Table 2, the power margin for this system can be calculated for various number of SC splits and therefore the number of ONUs that could be protected by a single ONU can be obtained. Number of protected ONUs Gb/s downstream data - Scheme Gb/s downstream data - Scheme Gb/s upstream data - Scheme Gb/s upstream data - Scheme Mb/s LAN data - Scheme Mb/s LAN data - Scheme Number of ONUs Figure 4.336: Calculated number of protected ONUs for the protection architecture that uses interconnections amongst ONUs with both LAN emulation capabilities. Figure 4.36 shows the number of consecutive ONUs that can be protected by a single ONU with varying number of ONUs in the PON. Generally, for both schemes, as the number of ONUs in the PON increases from 4 to 16, the number of consecutive ONUs that could be protected decreases. Increasing the number of ONUs will increase the number of SC splits and therefore the splitting loss of SC increases. Further, due to the 4 db additional loss, which contributed from the due to 3 db coupler and the OSW, experienced by the signals in the

178 Chapter 4 Protection and Restoration in Passive Optical Networks protected state, the number of consecutive ONUs supported by a single ONU is decreased. In a PON that consists of 16 ONUs, in an event of a distribution fibre break, 2.5 Gb/s downstream data transmission can be protected by another ONU in both schemes. Similarly, the 1.25 Gb/s upstream baseband data transmissions can also be protected by another ONU in a PON of 16 ONUs. However, 155 Mb/s LAN data can only be protected in a PON with 4 ONUs in scheme 2. For larger splits in the SC, the LAN data cannot be protected in Scheme 1. This is because the LAN data traverses through the SC twice and therefore experiences higher losses, and the transmission power for the LAN data is low as well. To protect the signal transmissions against distribution fibre breaks for larger PON, forward error correction techniques can be used [73]. 4.5 Switching time In the above experiments, the switching was conducted in normal and protected operating states using the OSWs driven by the TTL controller. A separate experiment was carried out to find the local switching time with a much simplified setup. λ MZM 1.25 Gb/s 2.2 km SMF 50:50 Coupler Optical Switch TTL Driver 4x4 Star Coupler Isolator Low speed Photodetector 2.2 km 50:50 SMF Coupler Control Circuit Optical Switch PD 1 PD 2 Oscilloscope Figure 4.347: Experimental setup to measure the remote switching speed of the opto-mechanical switch used in the experiments. Figure 4.37 shows the experimental setup to measure the local switching time of the OSW used in the experiment. A modulated signal was split into 2 different paths using a 3 db coupler and these signals were transmitted through the OSW. The OSW was driven by a TTL driver. On the other side of SC, the signal path was divided using a 3 db coupler and one

179 Chapter 4 Protection and Restoration in Passive Optical Networks portion of signals was fed into low speed photo detector operating at 100 MHz. The detected signals were low pass filtered to generate the envelope of the signals and this signal was fed into a triggering circuit. This triggering consisted of a pattern generator, which generates TTL signals. If signals were present, then the OSW remains in bar state. While in the absence of signals, the switch is changed to cross state using the TTL signal. Using PD 1 and PD 2, the signals in both operating states can be observed using the oscilloscope. Port 1 Scale: 500 µs Switching time: ~ 2 ms Port 3 Figure 4.358: Observed oscilloscope traces of detected data showing the remote switching time of the opto-mechanical switch. Figure 4.38 shows the oscilloscope traces of the outputs from the photo-detectors and it can be seen that the local switching time of the OSW is approximately 2 ms. 4.6 Conclusions In this chapter, several architectures for the protection against the feeder fibre and distribution fibre breaks in a PS-PON have been proposed and experimentally demonstrated. The feeder fibre protection for a PS-PON can be performed by overlaying the transmissions of the affected PON on another similar PS-PON using CWDM separation. The proposed feeder fibre protection architecture can be expanded for WDM-PONs and provides an easier migration path from a PS-PON to a WDM-PON. A protection architecture against distribution fibre breaks in a PS-PON have been developed using dual fibres and interconnecting adjacent ONUs for the restoration of signal transmisisons in an event of the fibre breaks. These architectures perform branch switching, whereby the protection switching is carried out at each ONU. The use of dual fibres to each ONU to provide protection requires a separate fibre

180 Chapter 4 Protection and Restoration in Passive Optical Networks conduit and therefore potentially increases the cost of the network. On the other hand, the architectures that use the interconnections amongst the ONUs does not require additional distribution fibres for protection. These architectures can be expanded to form a star-ring architecture enabling protection to all ONUs in the network. Moreover, these architectures have been experimentally demonstrated with LAN emulation capabilities, whereby the LAN traffic is intelligently used at each ONU to monitor the state of the distribution fibre. These protection architectures were experimentally demonstrated with 2.5 Gb/s downstream data transmisison, 1.25 Gb/s upstream baseband data transmission and 155 Mb/s LAN data transmission on an RF carirer. The measured BER curves for all signals show minimal penalty for the transmission of signals in the protected path as compared to the normal path and therefore confirms that the protection architectures do not add any transmission penalties. Further, for the protection architectures that uses interconnections amongst the ONUs, theoretical calculations were carried out to calculate the scalability of the network in terms of power budget. It is shown that any ONU is capable of protecting at least one ONU in terms of downstream data and upstream data transmissions for a PON consisting of 16 ONUs. LAN data transmisisons in both LAN emulation schemes can not be restored in a PON with 16 ONUs as the network is unable to provide adequate power budget for a successful transmission. 4.7 Reference [1] P. E. Green, "Fiber to the home: the next big broadband thing," IEEE Commun. Mag., vol. 42, no. 9, pp , Sep [2] G. Kramer, and G. Pesavento, "Ethernet passive optical network (EPON): building a next-generation optical access network," IEEE Commun. Mag., vol. 40, pp , Feb [3] D. Kettler, H. Kafka, and D. Spears, "Driving fiber to the home," IEEE Commun. Mag., vol. 38, pp , Nov [4] J. Davidson, I. Hawker, and P. Cochrane, The evolution of service protection in the BT network, in Proc. Global communications conference (GLOBECOM 89), vol. 2, pp , [5] Tsong-Ho Wu, A novel architecture for optical dual homing survivable fiber networks, in Proc. IEEE International Conference on Communications (ICC 90), vol. 2, pp , [6] M. Medard, and S. Lumetta, "Architectural issues for robust optical access," IEEE Commun. Mag., vol. 39, pp ,

181 Chapter 4 Protection and Restoration in Passive Optical Networks [7] M. Gerla, P. Camarda, and G. Chiaretti, Fault tolerant PON topologies, in Proc. 11 th Annual Joint IEEE Conference on Computer and Communications, vol. 1, pp , [8] O. K. Tonguz, and K. A. Falcone, "Fiber-optic interconnection of local area networks: physical limitations of topologies," IEEE J. Lightw. Technol. Vol. 11, pp , May [9] Broadband optical access systems based on passive optical networks (PON), ITU-T, Recommendation G.983.1, [10] D. J. Xu, W. Yen, and E. Ho, Proposal of a new protection mechanism for ATM PON interface, in Proc. IEEE International Conference on Communications (ICC 01), vol.7, pp , [11] C. Xue, S. Shuhe, Z. Xu, L. Dong, and D. Yu, Proposal of a novel protection mechanism for Ethernet PONs, in Proc. IEEE Region 10 Conference on Computers, Communications, Control and Power Engineering, vol. 2, pp , [12] Y.-m. Kim, J. Y. Choi, J.-H. Ryou, H.-M. Baek, O.-S. Lee, H.-S. Park, and M. Kang, Cost effective protection architecture to provide diverse protection demands in Ethernet passive optical network, in Proc. International Conference on Communication Technology, vol. 1, pp , [13] Y. Okumura, S. Aoyagi, and E. Maekawa, Duplex system configuration in passive double star system, in Proc. IEEE Global Telecommunications Conference (GLOBECOM 94), vol. 3, pp , [14] M. Schelp, X. Wang, W. Yen, and E. Ho, The Ranging Protocol for ATM Passive Optical Networks: Analysis and improvements, in Proc. Annual Multiplexes Telephony Conference, [15] N. Nadarajah, A. Nirmalathas, and E. Wong, Self-protected Ethernet passive optical networks using coarse wavelength division multiplexed transmission, IEE Electron. Lett., vol. 41, no. 15, pp , Jul [16] C. -J. Chae, L. Seung-Tak, K. Geun-Young, and P. Heesang, "A PON system suitable for internetworking optical network units using a fiber Bragg grating on the feeder fiber," IEEE Photon. Technol. Lett., vol. 11, pp , Dec [17] H. Yamada, K. Sato, and K. Okada, Switching characteristics of LD transceivers for optical TCM transmission, IEEE J. Lightw. Technol., vol. 12, pp , Jun [18] N. Kashima, Properties of commercial 1.3-µm Fabry-Perot laser modules in a time compression multiplexing system, IEEE J. Lightw. Technol., vol. 9, no. 7, pp , Jul [19] F. -T. An, Kyeong Soo Kim, D. Gutierrez, S. Yam, E. Hu, K. Shrikhande, and L. G. Kazovsky, "SUCCESS: a next-generation hybrid WDM/TDM optical access network architecture," IEEE J. Lightw. Technol., vol. 22, no. 11, pp , [20] Y. -L. Hsueh, W. T. Shaw, L. G. Kazovsky, A. Agata, and S. Yamamoto, "SUCCESS PON Demonstrator: Experimental Exploration of Next-Generation Optical Access Networks," IEEE Optical Communications, vol. 22, pp , Aug [21] K. Tanaka, H. Izumita, N. Tomita, and Y. Inoue, "In-service individual line monitoring and a method for compensating for the temperature-dependent channel drift of a WDM-PON containing an AWGR using a 1.6 µm tunable OTDR," in Proc. 11 th International Conference on Integrated Optics and Optical Fiber Communications and 23 rd European Conference on Optical Communications (IOOC- ECOC 97), vol. 3, pp ,

182 Chapter 4 Protection and Restoration in Passive Optical Networks [22] F. Caviglia, and V. C. Di Biase, "Optical maintenance in PONs," in Proc. 24 th European Conference on Optical Communication (ECOC 98), vol.1, pp , [23] A. J. Phillips, J. M. Senior, R. Mercinelli, M. Valvo, P. J. Vetter, C. M. Martin, M. O. Van Deventer, P. Vaes and X. Z. Qiu, "Redundancy strategies for a high splitting optically amplified passive optical network," J. Lightw. Technol., vol. 19, pp , Feb [24] E. S. Son, K. H. Han, J. H. Lee, and Y. C. Chung, "Survivable Network Architectures for WDM PON," in Proc. Optical Fiber Communications Conference (OFC 05), vol. 5, [25] D. B. Meis, Centralized versus Distributed Splitting in Passive Optical Networks, in Proc. Optical Fiber Communications Conference (OFC05), paper NW 14, [26] M. D. Vaughn, D. Kozischek, D. Meis, A. Boskovic, R. E. Wagner, "Value of reach-and-split ratio increase in FTTH access networks," IEEE J. Lightw. Technol., vol. 22, pp , Nov [27] T. Yokotani, K. Murakami, and T. Yasushi, Simplified PON protected mechanism using L2 control protocols, in Proc. 10 th Optoelectronics and Communications conference (OECC 05), pp , [28] S.-B. Park, D. K. Jung, D. J. Shin, H. S. Shin, S. Hwang, Y. J. Oh, and C. S. Shim, Bidirectional Wavelength-Division-Multiplexing Self-Healing Passive Optical Network, in Proc. Optical Fiber Communication Conference (OFC 05), vol. 3, [29] N. Nadarajah, E. Wong and A. Nirmalathas, Automatic Protection Switching and LAN emulation in Passive Optical Networks, IEE Electron. Lett., vol. 42, no.3, pp , [30] S. Topliss, D. Beeler, and L. Altwegg, Synchronization for passive optical networks, IEEE J. Lightw. Technol., vol. 13, pp , May [31] S. Culverhouse, R. A. Lobbett, and P. J. Smith, Optically amplified TDMA distributive switch network with Gb/s capacity offering interconnection to over 1000 customers at 2 Mb/s, IEE Electron. Lett., vol. 28, pp , [32] B. Miah, and L. Cuthbert, An economic ATM passive optical network, IEEE Commun. Mag., pp , [33] C.-J. Chae, E. Wong, and R. S. Tucker, "Optical CSMA/CD media access scheme for Ethernet over passive optical network," IEEE Photon. Technol. Lett., vol. 14, pp , May [34] E. Wong, N. Nadarajah, C.-J. Chae, and A. Nirmalathas, Passive Optical Network Architectures with Optical Loopbacks, in Proc. 18 th Annual Lasers and Electro Optics Society Meeting (LEOS 05), pp , [35] E. Wong, N. Nadarajah, C.-J. Chae, A. Nirmalathas, and M. Attygalle, Improved PON architectures for LAN emulation and Protection, in Proc. SPIE International symposium Microelectronics, MEMS and Nana-technology, paper , [36] B. N. Desai, N. J. Frigo, A. Smiljanic, K. C. Reichmann, P. P. Iannone, and R. S. Roman, "An optical implementation of a packet-based (Ethernet) MAC in a WDM passive optical network overlay," in Proc. Optical Fiber Communication Conference and Exhibit (OFC 01), vol. 3, [37] E. Wong, and C.-J. Chae, "Performance of differentiated services in a CSMA/CD-based Ethernet over passive optical network," in Proc. 17 th IEEE Annual Lasers and Electro Optics Society Meeting (LEOS 04), vol. 2, pp ,

183 Chapter 4 Protection and Restoration in Passive Optical Networks [38] S.R. Sherif, A. Hadjiantonis, G. Ellinas, C. Assi, and M.A. Ali, A novel decentralized ethernet-based PON access architecture for provisioning differentiated, IEEE J. Lightw.. Technol., vol. 22, pp , Nov [39] E. Wong and Chang-Joon Chae, "Efficient dynamic bandwidth allocation based on upstream broadcast in Ethernet passive optical networks," in Proc. Optical fiber Communication Conference (OFC 05), vol. 5, paper OF16, [40] E. Wong and Chang-Joon Chae, "Support of Differentiated Services in Ethernet Passive Optical Networks via Upstream Broadcast Dynamic Bandwidth Allocation Scheme," in Proc. 4 th International Conference on Optical Internet (COIN 05), pp , [41] Wen-Piao Lin, Ming-Seng Kao, and Sien Chi, "The modified star-ring architecture for high-capacity subcarrier multiplexed passive optical networks," IEEE J. Lightw. Technol., vol. 19, pp , Jan [42] Wen-Piao Lin, Ming-Seng Kao, and Sien Chi, A star-bus-ring architecture for DWDM/SCM passive optical networks, in Proc. 4 th Pacific Rim Conference on Lasers and Electro-Optics (PR-CLEO 01), vol. 2, pp , [43] Wen-Piao Lin, Ming-Seng Kao, and Sien Chi, "A DWDM/SCM Self-Healing Architecture for Broad- Band Subscriber Networks," IEEE J. Lightw. Technol., vol. 21, pp , Feb [44] Tsan-Jim Chan, Chun-Kit Chan, Lian-Kuan Chen, and Frank Tong, A Self-Protected Architecture for Wavelength-Division-Multiplexed Passive Optical Networks, IEEE Photon. Technol. Lett., vol. 15, no. 11, pp , [45] T.-J. Chan, Y. C. Yu, C. K. Chan, L. K. Chen, and F. Tong, A novel bidirectional wavelength division multiplexed passive optical network with 1:1 protection, in Proc. Optical Fiber Communication Conference (OFC 03), vol. 15, pp , [46] Z. Wang, X. Sun, C. Lin, C. K. Chan, and L. K. Chen, "A novel centrally controlled protection scheme for traffic restoration in WDM passive optical networks," IEEE Photon. Technol. Lett., vol. 17, pp , Mar [47] Xiaofeng Sun, Zhaoxin Wang, Chun-Kit Chan, and Lian-Kuan Chen, A novel star-ring protection architecture scheme for WDM passive optical access networks, in Proc. Optical Fiber Communications Conference (OFC 05), vol. 3, paper JWA53, [48] N. Nadarajah, M. Attygalle, A. Nirmalathas and E. Wong, "A novel local area network emulation technique on passive optical networks," IEEE Photon. Technol. Lett., vol. 17, pp , May [49] N. Nadarajah, M. Attygalle, A. Nirmalathas and E. Wong, "LAN emulation in passive optical networks using subcarrier multiplexing," in Proc. 9 th Optoelectronics and Communications Conference and 3rd International Conference on Optical Internet (OECC-COIN 04), pp , [50] N. Nadarajah, A. Nirmalathas, and E. Wong, LAN emulation on passive optical networks using RF subcarrier multiplexing, in Proc. 17 th Annual Meeting of the IEEE Lasers and Electro-Optics Society (LEOS 04), pp ,

184 Chapter 4 Protection and Restoration in Passive Optical Networks [51] N. Nadarajah, M. Attygalle, E. Wong and A. Nirmalathas, "Novel schemes for Local Area Network Emulation in Passive Optical Networks with RF Subcarrier Multiplexed Customer Traffic," IEEE J. Lightw. Tech., vol. 23, pp , Oct [52] N. Nadarajah, A. Nirmalathas, E. Wong and M. Attygalle, Novel Architecture for Protection in Conjunction with Local Area Network Emulation in Passive Optical Networks, in Proc. 4 th International Conference on the Optical Internet (COIN 05), pp , [53] N. Nadarajah, E. Wong M. Attygalle, and A. Nirmalathas, Protection Switching and Local Area Network Emulation in Passive Optical Networks, IEEE J. Lightw. Technol., vol. 24, pp , May [54] D. J. L. Blumenthal, R. Gaudino, Sangwoo Han, M. D. Shell, and M. D. Vaughn, "Fiber-optic links supporting baseband data and subcarrier-multiplexed control channels and the impact of MMIC photonic/microwave interfaces," IEEE Trans. Microwave Theory Tech., vol. 45, pp , Aug [55] R. Gaudino, and D. J. L. Blumenthal, "A novel transmitter architecture for combined baseband data and subcarrier-multiplexed control links using differential Mach-Zehnder external modulators," IEEE Photon. Technol. Lett., vol. 9, pp , Oct [56] P. J. Heim, and C. P. McClay, "Frequency division multiplexed microwave and baseband digital optical fiber link for phased array antennas," IEEE Trans. Microwave Theory Tech., vol. 38, pp , May [57] A. Kaszubowska, P. Anandarajah and L. P. Barry, "Multifunctional operation of a fiber Bragg grating in a WDM/SCM radio over fiber distribution system," IEEE Photon. Technol. Lett., vol. 16, pp , Feb [58] A. Kaszubowska, P. Anandarajah, L. P. Barry, Enhanced performance of an optically fed microwave communication system using a directly modulated laser transmitter with external injection, in Proc. 14 th Annual Meeting of the IEEE Lasers and Electro-Optics Society (LEOS 01), vol. 1, pp , [59] A. Kaszubowska, P. Anandarajah and L. P. Barry, Multifunctional operation of a fiber Bragg grating in a WDM/SCM radio over fiber distribution system, in Proc. 16 th Annual Meeting of the IEEE Lasers and Electro-Optics Society (LEOS 03), vol. 2, pp , [60] I. Riant, S. Borne, and P. Sansonetti, Dependence of fiber Bragg grating thermal stability on grating fabrication process, in Proc. Optical Fiber Communication Conference (OFC 96), pp , [61] E. Salik, D. S. Starodubov, V. Grubsky, and J. Feinberg, Thermally stable gratings in optical fibers without temperature annealing, in Proc. Optical Fiber Communication Conference and the International Conference on Integrated Optics and Optical Fiber Communication (OFC-IOOC 99), vol. 3, pp , [62] Bai-Ou Guan, Hwa-Yaw Tam, Xiao-Ming Tao, and Xiao-Yi Dong, "Highly stable fiber Bragg gratings written in hydrogen-loaded fiber," IEEE Photon. Technol. Lett., vol. 12, pp , Oct [63] J. F. Brennan, P. M. Bungarden, C. E. Fisher, and R. M. Jennings, "Packaging to reduce thermal gradients along the length of long fiber gratings," IEEE Photon. Technol. Lett., vol. 16, pp , Jan

185 Chapter 4 Protection and Restoration in Passive Optical Networks [64] Y. Park, S.-T. Lee, and C.-J. Chae, "A novel wavelength stabilization scheme using a fiber grating for WDM transmission," IEEE Photon. Technol. Lett., vol. 10, pp , Oct [65] R. Giles, R. and J. Song, "Fiber-grating sensor for wavelength tracking in single-fiber WDM access PONs," IEEE Photon. Technol. Lett., vol. 9, pp , Apr [66] M. Ichioka, J. Ichikawa, T. Sakai, H. Oguri, and K. Kubodera, Athermalized wavelength locker using fiber Bragg grating, in Proc.4 th Pacific Rim Conference on Lasers and Electro-Optics (PR-CLEO 01), vol. 1, pp , [67] G. E. Shtengel, R. F. Kazarinov, and L. E. Eng, Simultaneous laser wavelength locking and spectral filtering using fiber Bragg grating, in Proc. IEEE 16 th International Semiconductor Laser Conference (ISLC 98), pp , [68] D. Forbes, and A. Robinson, Laser wavelength stabilisation using fiber grating, in Proc. IEE Colloquium on Optical Fiber Gratings, pp. 13/1-13/6, [69] N. Nadarajah, A. Nirmalathas, and E. Wong, Protection and LAN emulation in Ethernet Passive st Optical Networks, in Proc. 31 European Conference on Optical Communication (ECOC 05), vol. 3, pp , [70] X.-Z. Qiu, P. Ossieur, J. Bauwelinck, Y. Yi, D. Verhulst, J. Vandewege, B. De Vos, and P. Solina, Development of GPON Upstream Physical-Media-Dependent Prototypes, IEEE J. Lightw. Technol., vol. 22, pp , Nov [71] P. Ossieur, D. Verhulst, Y. Martens, W. Chen, J. Bauwelinck, X.-Z. Qiu, and J. Vandewege, A Gb/s Burst-Mode Receiver for GPON Applications, IEEE J. Solid state circuits, vol., 40, pp , May [72] D. Podwika, D. Stefanski, J. S. Witkowski, and E. M. Pawlik, "Computer networks based on optical passive couplers," in Proc. 2 nd International Conference on Transparent Optical Networks (ICTON 00), pp , [73] K. H. Han, E. S. Son, H. Y. Choi, K. W. Lim, and Y. C. Chung, "Bidirectional WDM PON using lightemitting diodes spectrum-sliced with cyclic arrayed-waveguide grating," IEEE Photon. Technol. Lett., vol. 16, pp , Oct

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187 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks 5 Applications of Electronic CDMA in Passive Optical Networks 5.1 Introduction Over the last decade or so, optical transmission technologies had been widely deployed in many local area network (LAN) implementations [1-5]. With the rising demand for broadband connections to homes, optical access networks have become the choice for service providers. Optical fibre based access networks offer the advantage of unprecedented bandwidth for both downstream and upstream traffic in comparison to other conventional access technologies. Furthermore, high bandwidth offered by optical access can help the network to cope with increasing demand for bandwidth over a longer time frame. In the fibre to the premises (FTTP) networks, several ways to connect the customers with the network had been considered. Customers can be connected with the central office (CO) via point-to-point fibre links or an active optical network (AON), or a passive optical network (PON). Out of all these architectures, PON architecture is considered as more future-proof and cost-effective technology to implement optical access networks [6, 7]. A typical PON architecture uses a star coupler (SC) as a branching device for the communication between the optical network units (ONUs) that are located in the customer premises and the CO [8]. These types of power splitting PON (PS-PON) networks have been thoroughly explored and standardised [9-11]. There are many multiple access techniques such as wavelength division multiple access (WDMA), frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA) that can be used for the transmission of signals in a PS-PON. In this chapter, we explore the application of direct sequence spread spectrum (DS-SS) based electronic CDMA transmission technique to facilitate a number of enhanced functions within the PS-PON architecture. For simplicity, we refer the DS-SS-CDMA implemented in the

188 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks electronic later as electronic CDMA (E-CDMA) from hereon. We propose and demonstrate the use of E-CDMA to support asynchronous transmission of signals from ONUs to provide upstream access, realise secure LAN emulation schemes and support multiple virtual private networking (VPN) capabilities within a PS-PON. The discussion of E-CDMA applications in PS-PON covered in this chapter are structured as follows. Section 5.2 gives a brief overview of the previously proposed multiple access schemes for upstream transmission in a PS-PON using SCMA and TDMA. The advantages and disadvantages of these proposals are also described. Section 5.3 describes the upstream access scheme using E-CDMA in a PS-PON, followed by the experimental demonstration which is described in Section The performance analysis for a synchronous E-CDMA system and the scalability analysis in the presence of optical beat interference (OBI) are performed. A secure LAN emulation scheme using E-CDMA in-conjunction with the RF subcarrier multiplexed LAN traffic and a secondary distribution fibre for the redirection of LAN traffic is discussed and experimentally demonstrated in Section 5.4. Another secure LAN emulation scheme using an additional optical transceiver at the ONUs with the use of a fibre Bragg grating (FBG) is described and experimentally demonstrated in Section 5.5. Section 5.6 discusses a multiple and secure virtual private networking (VPN) capability within the PON using E-CDMA. This VPN capability is experimentally demonstrated in Section The limitations of the E-CDMA for the multiple and secure VPN capabilities such as OBI, multiple access interference (MAI) are addressed in Section Upstream access in PS-PONs The transmission of signals in the downstream direction from the CO to the ONUs in a PS- PON is usually based on time division multiplexing (TDM) techniques [12, 13]. Due to sharing nature of physical plant in the PON, there needs to be some form of access control protocol that allows each ONU to send data to the CO in the upstream direction without any collisions at the SC. There have been several experimental demonstrations on the access control techniques for a PON such as TDMA, SCMA, CDMA, and WDMA. WDMA is typically used in wavelength division multiplexed PON (WDM-PON) architectures [14-17], whereby a wavelength channel is allocated to each ONU for the transmission of signals in the

189 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks upstream direction. Therefore, a media access control is not required for collision-free transmissions. On the other hand, the upstream channel is shared in PS-PONs and therefore a control for the wavelength channel access is required. The multiple access techniques using TDMA and SCMA for the transmission of signals are discussed in the next section TDMA enabled upstream access in PS-PONs Time division multiplexing is a mature and established electronic access technique and is most commonly used for the PS-PONs [18-20]. In the PS-PON architectures that use TDMA protocol for the upstream access, data on an appropriate wavelength channel assigned for upstream transmission is transmitted during a time slot designated for that particular ONU. As all ONUs use a single wavelength channel for the transmission of data in the upstream direction, and the transmissions from each ONU are carried out during designated timeslots, only one optical receiver is required at the CO. The receiver at the CO is a burst mode optical receiver that is capable of receiving the packets with varying optical power from different ONUs. To avoid data collisions between the upstream signals during multiplexing into the single fibre at the SC, the transmission from each ONU is synchronised to a global clock. This becomes complicated when the distribution fibre lengths from the SC to each ONU are different due to different ONU locations. The solution to this problem is achieved by incorporating a ranging protocol that determines the correct timing for the upstream data transmission at each ONU [18, 21]. As each ONU performs the data transmission to the CO in the allocated timeslots, the efficiency of the protocol in responding to variation in demand in a dynamic fashion is not high. This is because some ONUs in the PON may not have data to be transmitted to the CO, while some ONUs require larger timeslots as they have large amount of data to be transmitted. If the allocated timeslots to each ONU are equal in size for the same upstream data rate, this type of static TDMA protocol does not obtain higher efficiency for the transmissions in the upstream direction. However, dynamic TDMA protocol can also be employed in this type of PON architectures whereby CO allocates different size timeslots to each ONU on requests and therefore dynamic bandwidth assignment (DBA) can also be incorporated with the TDMA protocol [22 26]. To dynamically allocate bandwidth to each ONU, REPORT messages are sent to CO from the ONUs while GATE messages are sent to the ONUs from the CO. As these control messages consume bandwidth in both upstream

190 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks and downstream wavelength channels, a distributed polling mechanism is proposed [27, 28]. In this distributed control protocol, DBA is performed in a distributed manner by each ONU whereby the control messages are sent only in the upstream wavelength channel and therefore the utilisation of the downstream wavelength channel is not reduced. A distributed random access control protocol such as carrier sense multiple access with collision detection (CSMA/CD) can also be used for the transmission of upstream signals [29-32]. In this scheme, the channel access for the transmission of signals by each ONU is randomly performed and if a collision of packets at the SC is observed, the transmission is stopped and rescheduled for a later time. As the transmissions are randomly carried out, a ranging protocol is not required to synchronise all ONU to a common clock. It has also been shown that this access protocol shows high efficiency for low transmission bit rates, shorter distribution fibre lengths and longer packet sizes SCMA enabled upstream access in PS-PONs Subcarrier multiplexing (SCM) can also be used for the upstream data transmission in PONs [33-36]. Each channel is defined by a particular RF subcarrier frequency and discrimination between channels is accomplished using RF filters and local oscillators after detection of the optical signal. Multiple RF signals can be transmitted using the same wavelength channel and therefore the RF signals are detected at the CO and the signals are demultiplexed using RF electronics. Synchronisation is not required between the channels in the SCM networks because they are not multiplexed in time. The cost consideration of the RF electronic subsystems can be an issue with this technique since each ONU must have unique or selectable RF electronic subsystem operating at the assigned frequency to receive and transmit data while CO requires a bank of RF mixers, filters and local oscillators to demultiplex the upstream data across all SCM channels from each ONU [33]. However, the RF components are more mature and therefore less expensive compared with optical counter parts. A disadvantage of this scheme is that the possible throughput is not high as all the SCM channels reside within the bandwidth of a single optical carrier. To transmit megabits per second rate of payload data from each ONU, the RF carrier frequencies in the order of gigahertz range are required at each ONU. If the PON consists of a large number of ONUs

191 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks (more than 32 ONUs), the RF carrier frequency becomes very high. As the RF carrier frequency increases, the cost of the RF components also becomes higher. Moreover, at higher RF frequencies, the fabrication of the RF components becomes quite difficult. Higher frequency RF components require higher bandwidth optical components such as laser and photodetector (PD) for the modulation and detection of data respectively. Again, this places higher cost on the SCM network. In addition, as the number of RF frequencies increases, second and third order inter-modulation products of these RF frequencies can potentially distort the received signals and therefore requiring careful assignment of the RF carrier frequencies at each ONU and the linearisation of RF components. As the optical mixing between the signals from each ONU occurs at a single PD at the CO, the received signals are degraded due to OBI [37-42]. Several techniques including the use of wideband incoherent sources such as light emitting diodes (LEDs) [41], and the use of spread spectrums techniques [43, 44] are used to reduce the penalty induced by OBI. There are several disadvantages of using TDMA and SCMA in PS-PONs as discussed previously. The use of E-CDMA for the transmission of signals in the upstream direction from the ONUs to the CO can potentially bring more benefits to the network. Following on the discussions on PON architectures based on TDMA and SCMA for the upstream access, next section presents a PON architecture based on E-CDMA for the upstream access. 5.3 E-CDMA enabled upstream access in PS-PONs As in previous proposals, this multiple access scheme also uses the same double star topology for the upstream data transmissions from the ONUs to the CO. The PONs that use E-CDMA for the upstream data transmissions, use unique electronic codes allocated to each ONU. As each ONU uses a unique electronic code for the modulation of the upstream data, the resulting direct sequence spread spectrum (DS-SS) signals occupy the same bandwidth. In the E- CDMA scheme, no guard time or guard bands are required. Unlike TDMA schemes, no synchronisation is required between the signals since each upstream channel employs a different code for communication. In TDMA PONs, as obtaining accurate synchronisation of timeslots is difficult, a guard time is allowed between the timeslots. At slower data rates, the guard time can be insignificant. However at high transmission bit rates, time wasted on guard

192 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks time could potentially degrade the throughput of the system. In E-CDMA PONs, guard time is not required and the users perform transmissions randomly. In TDMA PONs, a burst mode optical receiver is used at the CO for the reception of packets from the ONUs. In E-CDMA PONs, as the transmissions from ONUs to the CO are carried continuously, a complex burst mode receiver is not required. In SCMA systems, unused frequency space is allocated between the frequency bands to make optical or electrical filtering easy as it is difficult to maintain precise control of the frequency of the sources. These guard bands represent wasted frequency space. However, E-CDMA does not require guard band as the E-CDMA signals from each ONU are transmitted in the same frequency band. Furthermore, as E-CDMA signals from each ONU are sent at baseband, the cost of using high speed electronics is avoided. As the bandwidth of the signals are lower compared to that used in the SCMA PONs, high speed optical components such as optical source, and receiver are also not required. Moreover, the OBI that is present due to the beating of signals from multiple optical sources can also be reduced using E-CDMA [43, 44]. Down stream Tr ansmitter Cen tral Offi ce WDM λ u λ 1 x N λ d 1.3/1.5 µm WDM d 1.3/1.5 µm SC λ u E-CDMA Receiver Feeder ONU 1 Fibre E-CDMA Transmitter Downstream Receiver λ d downstream wavelength λ u upstream wavelength ONU N Figure 5.1: A PS-PON architecture that uses E-CDMA for the upstream access. The physical architecture to demonstrate the E-CDMA enabled upstream access for a PON is presented in Figure 5.1. A 1 N SC is placed at the remote node (RN) as a branching device for the transmission of signals between the CO and ONUs. The upstream channel from an ONU to the CO is established using a unique electronic code assigned to that particular ONU. The wavelength sources used for the transmission of E-CDMA upstream data at each ONU is similar to each other. Moreover, these optical sources do not required to be wavelength stabilised. Therefore, this scheme enables the use of inexpensive broadband sources such as

193 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks LEDs or Superluminescent diodes (SLEDs) at each ONU and therefore potentially an attractive solution for the cost sensitive customer access networks. At the CO, the E-CDMA signals from several ONUs are simultaneously detected using a single PD and out of the multiple streams of DS-SS formatted E-CDMA signals, data from each ONU is decoded using parallel E-CDMA DS-SS decoders. Within each decoder assigned to receive data from a particular ONU, data is recovered or despread using the appropriate spreading sequences identical to those used to spread at respective ONU. During the decoding process, E-CDMA DS-SS data from other ONUs appears as spread spectrum noise within the E-CDMA decoder. It is important to note that the channel noise is also spread, thereby giving further improvements for the signal-to-noise-ratio (SNR). An important factor in the use of E-CDMA for the transmission of signals is the choice of high data rate spreading sequences onto which data is mapped. Two important properties that are required for these spreading sequences are listed below [45]. Any sequence in the code set is easily distinguished from a time shifted version of itself. To achieve this, the autocorrelation function of each chip sequence to be used in the system should have a maximum at the zero shift and minima at all other shifts to allow detection of the desired signal. Any sequence in the code set is easily distinguished from any other possibly time shifted sequence in the code set. To achieve this, the cross correlation function between the pairs of sequences in the code set used should be as small as possible for all shifts, thereby minimising the interference of other users during extraction of a desired signal and allowing a multiple access capability. The choice of code sequences depends on the system constraints such as processing methods. DS-SS systems could take advantage of binary (0, 1) sequences or bipolar (-1, 1) sequences. In the E-CDMA based upstream access PONs, all the E-CDMA signals from the ONUs are detected and electronically processed. The use of the electronic correlation methods limits the channel sequences to unipolar sequences due to square law nature of the optical detectors. However, data may be recovered by direct correlation of the unipolar channel sequences with a bipolar reference. The resulting correlation functions are scaled down versions of those obtained by bipolar baseband CDMA. Several spreading sequences such as maximal length

194 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks sequences, Gold sequences, Kasami sequences, and orthogonal sequences can be used for these applications [46]. The use of E-CDMA for the upstream transmission requires proper synchronisation between the received E-CDMA signals and the local electronic code at the CO to correctly decode the data. The use orthogonal Walsh codes for the transmission of signals is not ideal as these orthogonal Walsh codes are orthogonal only when all channels are synchronised. As the ONUs are located apart in a PS-PON, these orthogonal Walsh sequences cannot be synchronised. Therefore, pseudo noise (PN) codes can be used for the upstream transmission of signals. If a PN sequence is used, the receiver at the CO integrates only a part of the long PN code for the data decision. In this process, the amount of crosstalk from other E-CDMA channels increases as the PN code rate decreases. Integrating the entire long PN code consumes a lot time for synchronisation and requires more hardware capacity for processing. To alleviate these problems, a scheme that uses a combination of PN and Walsh sequences were proposed, whereby the PN code is used for timing synchronisation and the Walsh code is used for channel identification [47]. However, the use of spreading codes such as Gold codes asynchronous transmission can be performed in conjunction with digital processing technologies. Using efficient receiver architectures and E-CDMA decoding algorithms, an alldigital E-CDMA transmission can be performed. In these digital transceiver setups, the transmitter is frequency agile, and supports fine adjustment of signal power and chip delay. The digital receiver has corresponding received signal strength indicator and frequency offset and chip offset modules. To obtain fast acquisition, parallel correlation filter architecture can be used at each E-CDMA decoder [49-53] Experimental demonstration A simple experiment was carried out to demonstrate the feasibility of the PS-PON system based on E-CDMA for the upstream transmissions. Figure 5.2 shows the experimental setup to demonstrate proposed scheme. The downstream signal of pseudo random binary sequence (PRBS) non-return-to-zero (NRZ) data at 2.5 Gb/s was modulated onto downstream wavelength channel λ d = 1527 nm using a Mach-Zehnder modulator (MZM) and transmitted through a 10 km standard single mode feeder fibre link (SSMF) and a 4 4 SC. At the SC, the

195 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks signal is split and broadcast to ONU 1 through a 2.2 km distribution fibre link. For the transmission of the E-CDMA upstream signals, PRBS NRZ data at 40 Mb/s was electronically multiplexed with 16-bit, 640 Mb/s bipolar Walsh code 1 using an RF mixer. 2.2 km SMF 1.5 µm 2.5 Gb/s Receiver λ d =1527 nm MZM O 2.5 Gb/s WC P C 10 km SMF 4x4 Star Coupler 3 km SMF WC ONU µm 1.3 µm Central Office E-CDMA Receiver 1.3 µm E-CDMA 1 encoder E-CDMA Decoder E-CDMA 2 encoder ONU 2 Figure 5.2: Experimental setup to demonstrate the feasibility of upstream transmission based on E- CDMA in a PS-PONs. The resulting DS-SS signals were directly modulated onto a Fabry-Perot laser diode (FP-LD) operating in 1.3 µm wavelength window and transmitted through the 2.2 km distribution fibre in the upstream direction. Here, the transmission rate of the bipolar Walsh code 1 was limited by the bandwidth of the FP-LD. The data rate of the E-CDMA signals can be increased by using a FP-LD with larger bandwidth. 1.3 µm/1.5 µm coarse wavelength division multiplexing (CWDM) couplers were used to separate and combine these E-CDMA signals with λ d. To demonstrate multiple access capability using E-CDMA, another 16-bit bipolar Walsh code at 640 Mb/s was electronically multiplexed with PRBS NRZ data at 40 Mb/s and the resulting DS-SS signals were directly modulated onto another FP-LD operating in 1.3 µm wavelength window and transmitted through the 3 km fibre in the upstream direction. The power of both E-CDMA signals was approximately equalled to reduce the multiple access interference (MAI). A third free running FP-LD was connected to another port of the SC to measure the performance degradations due to OBI. The downstream signals were detected using 2.5 Gb/s p-i-n receiver, whereas the E-CDMA signals were detected using an 2.5 Gb/s APD receiver. The detected E-CDMA signals were fed through an E- CDMA decoding circuit and upstream data was recovered. For the decoding of the desired E

196 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks CDMA signal, an appropriate electrical delay was employed to synchronise the local electronic code with the incoming E-CDMA signals. A series of experiments were conducted to examine the crosstalk effects, and bit error rates (BER) for both signals were measured Optical and RF spectra -30 Optical Power (dbm) Undesired E- CDMA channel Desired E- CDMA channel Wavelength (nm) Figure 5.3: Observed optical spectra at the receiver showing the optical fields from both FP-LDs. Figure 5.3 shows the observed optical spectra showing two optical fields from two separate FP-LDs that are received at the receiver at the CO. RF Power (dbm) db more interference power 2 db more interference power 3 db more interference power 4 db more interference power RF Frequency (GHz) Figure 5.4: Observed RF spectra of the desired E-CDMA signal at the receiver showing varying interference power

197 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks Figure 5.4 shows the RF spectra of the received baseband E-CDMA signals for varying received interfering optical power. Even though the received power of the interfering signals was increased, huge changes were not observed in the resulting RF spectra BER results -3-4 Back to back Transmission With upstream signals Log 10 (BER) Received Optical Power (dbm) Figure 5.3: Measured BER curves for the 2.5 Gb/s downstream data. Figure 5.5 shows the measured BER curves for the 2.5 Gb/s downstream data. Compared with back to back (B-B) measurements, a penalty of less than 0.15 db was observed in the presence of the upstream E-CDMA signals. Figure 5.6 shows the measured BER curves for the 40 Mb/s upstream data. A penalty of 0.3 db was observed when the E-CDMA signals were transmitted through the link compared to B-B measurements. This penalty can be attributed to the imperfect synchronisation with the incoming E-CDMA signals with the local bipolar Walsh code. When the desired E-CDMA were recovered in the presence of an unmodulated optical field from an optical source, an additional penalty of was 1.3 db measured. This penalty can be attributed to the OBI from the second optical source

198 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks -4-5 Back to back One channel Two channels Two channels with DS-SS Three channels Log 10 (BER) Normalised received Optical Power (dbm) Figure 5.4: Measured BER curves for the 40 Mb/s E-CDMA upstream data. In the presence of the interfering E-CDMA signals, this penalty was further increased by 0.15 db. Even though the MAI is low due to use of Walsh codes, imperfect synchronisation may have contributed a marginal penalty for the recovered 40 Mb/s upstream data. Since the periodic cross correlation at zero shift of the pair of bipolar Walsh codes that were used in the experiment is equal to zero [47, 54], the MAI is equal to zero. When the third optical source was operating, the penalty due to OBI was further increased by 1.1 db. It has been previously shown that that the effects of OBI from several FP-LDs can be suppressed by the E-CDMA [55]. However, the penalty observed from the experiment is possibly a result of the higher optical power from the second and third FP-LDs as beat noise increases with optical power Requirement for power control In the experimental demonstration for the upstream transmission using E-CDMA, the received power from each ONU was maintained approximately equal. As the ONUs in the PON could perform transmissions of E-CDMA signals in the upstream direction with arbitrary optical power, the performance of the recovered data from the ONUs is varied

199 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks However, the DS-SS receiver located at the CO cannot distinguish a signal if its strength is very low compared to other undesired DS-SS signals. Moreover, higher optical power from another optical source results in larger beat noise and therefore performance of the recovered upstream data is degraded. Therefore a power control mechanism is required such that the signal strengths from each ONU at the receiver are approximately equalled to minimise performance degradations of the recovered upstream E-CDMA data. If there are K active optical beat noise sources and if there is no difference in optical source wavelengths then the beat noise power for the Lorenzian spectrum lightwaves can be expressed as [56] P beat 2 ( ( K 1) P P + ( K 1)( K 2) P ) 2 = R Bτ C 2 w l l Equation 5.1 Here, R is responsivity of the PD, B is bandwidth the E-CDMA signal, P w is the optical power of the received desired E-CDMA signal, P l is the received optical power of the one of the ( K 1) undesired E-CDMA signals and τ C is the coherence time of the optical source. Assuming that the coherence time of the optical sources used at each ONU and the bandwidth of the E-CDMA upstream signals are the same, the normalised beat noise can be calculated for various interfering optical power levels against the number of active beat noise sources. Normalised beat noise = Beat noise power with optical power of Beat noise power with optical power of received signals are equal received signals are unequal The received optical power of the desired E-CDMA signals is changed and normalised beat noise can be calculated for varying ratios of the received optical power for the desired E- CDMA signal compared to undesired E-CDMA signals. As Figure 5.7 shows, as the number of optical source increases, the normalised beat noise decreases. The differences in normalised beat noise values become smaller for larger number of optical sources. It can be seen from equation 5.1 that as the number of optical beat sources increases, the beat noise power also increases. However, as the received optical power of the desired E-CDMA signal decreases compared to that of the undesired E-CDMA signals, the normalised beat noise increases. However, for large number of optical sources, this becomes insignificant

200 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks Normalised beat noise power Pw=Pl/2 Pw=Pl/4 Pw=Pl/8 Pw=Pl/16 Pw=Pl/32 Pw=Pl/ Number of beat optical sources Figure 5.5: Calculated normalised beat noise against the number of optical beat noise sources for varying optical power of the interfering E-CDMA signals. As the beat noise increases, the resulting penalty for the recovered upstream data also increases. As the normalised beat noise decreases with the increasing number of optical sources the power penalty difference also decreases when many optical sources are operating. This was confirmed by the experimental results, whereby 1.3 db penalty was observed for two optical sources, and an additional penalty of 1.1 db was observed with three sources Scalability in the presence of optical beat interference For the transmission of upstream E-CDMA signals, broadband optical sources such as LEDs are used at each ONU. LEDs operating in 1.3 µ m wavelength windows are more efficient than the LEDs operating in 1.5 µ m wavelength windows due to the maturity of the devices. Therefore LEDs operating at 1.3 µ m wavelength window can be used to maximise the upstream power budget. The number of upstream channels that can be simultaneously supported is limited by certain factors such as power budget and OBI. The upstream power budget is determined by the number of upstream channels that can be simultaneously supported. The upstream signals pass through a 1 N SC, whereby the total insertion loss depends on the number of splits in the SC. For a PON with 32 ONUs, the SC loss is typically

201 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks more than 18 db. Moreover, the broadband optical sources such as LEDs that are used at the ONUs have relatively low output power that could potentially limit the upstream budget. However, there have been recent advances in integrated LED-SOA devices that could potentially give larger optical output power [57-59]. The effect of OBI in the upstream E-CDMA signal s performance depends on the relative polarisation states of the signals from different encoders at a decoder. The OBI is at a minimum when half of the optical fields are in one polarisation state, while the other half is in the orthogonal polarisation state. On the other hand, the worst performance is obtained when the polarisation states of all signals are exactly aligned to each other resulting in beating between the optical fields. As the directly modulated upstream source outputs are unpolarised, the optical beat noise is halved relative to that caused by the polarised light with the same optical bandwidth because only the optical fields with the same polarisation state cause the OBI [60]. However, the OBI effect is reduced by using a larger bandwidth optical source [61, 62]. Despite the reduced OBI, an increase in spectral width of the optical source increases the susceptibility of the upstream data to dispersion effects [63, 64]. Since the practical transmission distances from the ONUs to the CO are in the order of a few kilometres (less than 10 km), and the use of LEDs that operate around the dispersion zero of standard fibre, the effects of dispersion can often be neglected. Moreover, forward error correction (FEC) techniques can also be used to recover the data error-free [65-67]. Therefore, the limiting factor for the number of channels that can be simultaneously supported is primarily the OBI. If two optical signals, photocurrent can be written as [47] E 1 () t and E 2 ( t) are detected simultaneously, then the resulting () t k. R[ E () t + E () t ] E () t E ( t) * 1 2.[ 1 2 I PD = + ] Equation 5.2 I PD () t 2 A1 = k. R + 2 A.cos 2 ( 1+ m1x1 () t ) + A2 ( 1+ m2 x2 () t ) A 1+ m x () t 1+ m x () t ^ ^ [( ) () ()] () () ω1 ω2 t + φ1 t φ2 t P1 t. P 2 t 2 2 Equation

202 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks Here, k is the scale factor, R is the responsivity, E is the electric field, A is the amplitude,φ is the instant phase, m is the modulation index, x(t) is the signal, ω is the angular frequency of the laser diode, P^ () t is the instant polarisation vector. If the amplitude and modulation index of both signals are assumed equal then [47] A A = A, and m = m = m ^ P 1 = ^. 2 P = 1 2 Let k. R. A = k. R A = I () t = I. mx () t + I. mx () t + I cos( ω ) t. 1+ mx ( t) + mx ( t) ω Equation 5.4 I PD Here, ω1 ω2 is the centre frequency of the beat noise. () t b() t c() t x. = Equation 5.5 Here, b () t and c() t are the input signal and CDMA sequence respectively. The receiver output can be written as S T () t = I () t c() t 0 PD. dt Equation 5.6 S T () t = I. mb () t. c () t. c () t dt + I. mb () t. c () t. c () t 0 + T 0 1 I cos 1 T dt ( ω ω ) t. 1+ mx ( t) 1+ mx () t. c () t dt Equation 5.7 Here, T is the bit duration. The first term in equation 5.7 is the desired signal. The second term is the MAI, which is neglected in this analysis. The last term is the OBI and it can be approximated as

203 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks OBI = T 0 [ I cos( ω ) t + I cos( ω ω ) t. mx( t) c () t ω ] dt Equation The first term of the OBI is the beat noise from the DC components of the two optical sources. The second term is the beat noise from the spread spectrums from the two signals. The maximum beat noise amounts to [47, 68] 2 ( 2 ) 2 I + m 2 σ beat = Equation 5.9 8N B c 2 s Here, N is the processing gain, and B is bandwidth of the data. c s The Q-factor of the received signal can be given as [47, 69] 2I. mt. m 8N c Q = = Equation σ 2 beat 2 + m As the number of optical sources N increases, the Q factor decreases as follows. Q 1 N m 8N c = Equation m Using the equation 5.11, the number of optical sources that can be activated simultaneously can be found. Figure 5.8 shows the calculated number of optical beat sources for the processing gain value of 128, and modulation index values 0.8 and 1. For a Q-factor greater than 6 (BER less than 10-9 ), the number of active optical sources is less than 7 for a modulation index of 0.8 and it is less than 9 for the modulation index of 1. If upstream data transmission rate is considered to be the 40 Mb/s, then the required chip rate is 40 Mb / s 128 = 5.12 Gb / s if binary phase shift keying (BPSK) is considered for the modulation of the LAN data. Even at this transmission line rate, less than 10 ONUs are simultaneously supported

204 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks 20 Q factor m=1 m= Number of optical beat sources Figure 5.6: Calculated number of optical beat sources for a processing gain of 128. An increase in the upstream data bit rate increases the total transmission bit rate and therefore high bandwidth opto-electronic components are required at the ONUs and the CO. However, by using higher order modulation schemes such as QPSK, QAM, the upstream data bit rate can be increased without increasing the total transmission rate of the system. However, these higher order modulation schemes require sophisticated electronics at the ONUs and CO for the encoding and decoding of the data. 5.4 Secure LAN emulation using E-CDMA with fibre loopback In chapter 3, a LAN emulation scheme using the RF subcarrier multiplexed transmission with a secondary distribution fibre for the redirection of upstream signals was proposed and experimentally demonstrated [70, 71]. In that particular scheme, LAN traffic is broadcast to all ONUs within the allocated timeslot. However, there exists a security problem whereby some of the ONUs in the PON that are not participating in the LAN emulation can receive the

205 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks LAN traffic. Secure transmission of LAN traffic can be performed using encryption schemes [72-75]. Since encryption is inherently a randomisation process, several encryption schemes could yield suboptimal results. The problem of generating random unpredictable encryption keys is one that is always overlooked. The use of secure tokens that emit cryptographically secure streams and noisy diodes that oscillate with unpredictable patterns have been used for encryption key generations. However, these encryption key generation schemes add complexity to the operation of the network. In this section, a physical layer security for the transmission of the LAN traffic is provided using E-CDMA. Improving the security of the transmitted signals at both higher layer and physical layer can lead to much improved performance of the network. In our proposal, the LAN data is electronically coded to generate an E-CDMA signal for the transmission to the selected ONUs in the PONs. The ONUs that have the knowledge of the E-CDMA spreading code, which was used for the encoding of the LAN data, can decode the LAN data. Redirection of upstream signals Upstream Receiver Downstream Transmitter λ u WDM 1.3/1.5 µm λ d λ u Feeder Fibre CO Distribution λ u Fibres N+1 x N+1 Star Coupler Upstream baseband data Terminated unused port RF CDMA LAN data Receiver Downstream Receiver WDM Upstream 1.3/1.5 µm Transmitter ONU 1 ONU N RF CDMA LAN data f L f L Figure 5.7: Proposed secure LAN emulation scheme using E-CDMA LAN data transmission and a secondary distribution fibre. Figure 5.9 illustrates the secure LAN emulation scheme using E-CDMA in which the redirection of the optically modulated RF sidebands along with the upstream baseband data is performed by a (N+1 N+1) SC and additional short length distribution fibres. The number of ONUs that are attached to the SC is N and one of the port facing towards the ONUs is terminated. Signals transmitted from each ONU on upstream wavelength channel λ u are

206 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks redirected back to each ONU through the second distribution fibre. The upstream signals consist of the baseband data to CO and E-CDMA LAN data that is modulated on an RF carrier to other ONUs. At each ONU, the looped back signals are detected and the upconverted RF E-CDMA LAN data is electrically separated from the upstream baseband data using an electrical band pass filter (BPF). Then, the RF E-CDMA LAN data is downconverted to baseband frequencies using a phase locked loop containing a voltage controlled oscillator (VCO) and using appropriate E-CDMA spreading code, the LAN data is decoded. The transmission of upstream signals from each ONU follows TDMA protocol. Therefore, the upstream baseband data to the CO and the RF E-CDMA LAN data are transmitted simultaneously in the same timeslot. An ONU may require sending two separate LAN data streams to a group of ONUs within the PON. In this case, using two different spreading codes the data streams can be electronically coded and then electrically combined before the RF carrier modulation. At the receiving ONUs, using the appropriate spreading codes, LAN data can be recovered. Therefore, this scheme simultaneously enables LAN data transmissions to different groups of ONUs while providing physical layer security for the transmitted signals Experimental demonstration 2.2 km SMF λ d = nm MZM Circulator Gb/s 1.25 Gb/s Receiver 10 km SMF 2.5 Gb/s PLL & 4x4 Receiver 2.5 GHz E-CDMA decoder Star Coupler Circulator Gb/s 2.2 km SMF Receiver GHz RF Oscillator RF mixer 160 Mb/s Code 10 Mb/s LAN data 2.5 GHz MZM λ u = nm 1.25 GHz Power Combiner 1.25 Gb/s

207 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks Figure 5.8: Experimental setup to demonstrate the secure LAN emulation scheme using E-CDMA transmission of LAN data on an RF subcarrier with a secondary distribution fibre loopback. The experimental setup to demonstrate the feasibility of this secure LAN emulation scheme is shown in Figure The downstream signal of PRBS NRZ data at 2.5 Gb/s was modulated onto λ d = nm and transmitted to the ONUs through a 10 km SSMF, a 4 4 SC and a 2.2 km distribution fibre. For the upstream transmission, PRBS NRZ data at 10 Mb/s was electronically multiplexed with 16-bit, 160 Mb/s bipolar Walsh code 1 using a RF mixer and the resulting DS-SS signal was modulated onto a 2.5 GHz RF carrier to generate the upconverted RF E-CDMA LAN data, which was then electrically combined with 1.25 Gb/s PRBS NRZ data using a RF combiner. E-CDMA spreading code LAN data DS-SS signal Figure 5.9: Observed oscilloscope traces showing 160 Mb/s spreading code, 10 Mb/s LAN data and the DS-SS signal. Figure 5.11 shows the measured oscilloscope traces of the 160 Mb/s bipolar Walsh code, 10 Mb/s LAN data, and the resulting DS-SS signal respectively. The DS-SS signal shows that there is a phase change at the transition between 1s and 0s of the LAN data. To avoid crosstalk between the combined baseband and RF signals, the RF E-CDMA LAN data was band-limited using a BPF with a centre frequency of 2.5 GHz and bandwidth of 300 MHz before the combination. Similarly, 1.25 Gb/s upstream baseband data was band limited using

208 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks a 1244 Mb/s low pass filter (LPF) before the combination with the upconverted RF E-CDMA LAN data. RF power (dbm) Gb/s upstream baseband data Frequency (GHz) 10 Mb/s E-CDMA LAN data on 2.5 GHz RF carrier Figure 5.10: Observed RF spectra at the upstream transmitter showing the 1.25 Gb/s upstream baseband data and 10 Mb/s E-CDMA LAN data on 2.5 GHz RF carrier. Figure 5.12 shows the observed RF spectra at the upstream transmitter showing the 1.25 Gb/s upstream baseband and the 10 Mb/s E-CDMA LAN data on 2.5 Gb/s RF carrier. Thereafter, the composite signals were modulated onto λ u = nm using a MZM and transmitted in the upstream direction. The upstream signals were split in the SC whereby one portion was transmitted through the 10 km feeder fibre to the CO, while the other is redirected back to the ONUs through the second distribution fibre. The redirected signals were received at the RF E- CDMA LAN data receiver. The detection of the upstream and downstream baseband signals was performed using 2.5 Gb/s p-i-n receivers, while the RF LAN data was detected using the 2.5 Gb/s APD receiver. For the recovery of 10 Mb/s LAN data, the detected signals were fed through a BPF centred at 2.5 GHz with a bandwidth of 300 MHz and the E-CDMA signal was down converted to baseband frequencies using the PLL based on a costas loop. For the decoding of the desired E-CDMA signal, appropriate electrical delay was employed to synchronise the local E-CDMA code with the incoming E-CDMA signals and 10 Mb/s LAN data was recovered

209 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks Optical spectra Optical Power (dbm) Downstream Receiver Upstream Receiver 30 db 28 db Wavelength (nm) Figure 5.11: Observed optical spectra at the upstream baseband data and downstream data receivers showing the suppression of the backscattered light. As shown in Figure 5.13, the optical spectra observed at the input of the downstream data receiver at ONU and the upstream baseband data receiver at the CO show low levels of crosstalk signals which is due to the back scattered light in the fibre. The crosstalk levels in the upstream data and downstream data receivers were -30 db and -28 db respectively. These crosstalk levels were not expected to induce any power penalty for the recovered 2.5 Gb/s downstream data and 1.25 Gb/s upstream baseband data as can be confirmed in the BER curves. The crosstalk levels can further be suppressed using CWDM filters and choosing the upstream and downstream wavelength channels appropriately BER results Figure 5.14 shows the measured BER curves for all signals. For the 10 Mb/s LAN data, a penalty of 0.2 db was observed compared to B-B measurements when the RF E-CDMA signals were transmitted through the transmission link. An additional penalty of 1.2 db was observed when 1.25 Gb/s upstream baseband data was included. This penalty can be attributed to the interchannel interference (ICI) crosstalk from the 1.25 Gb/s upstream

210 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks baseband data. This crosstalk was induced by the non-optimum operating points in the MZM leading to nonlinear distortion of the RF E-CDMA LAN data. In the presence of the downstream signals, the penalty was further increased by 0.2 db. The BER curves for the 1.25 Gb/s upstream baseband data show a penalty of less than 0.3 db compared to B-B measurements. No significant penalty was observed for the 2.5 Gb/s downstream data in the presence of other signals Mb/s E-CDMA LAN data 1.25 Gb/s upstream baseband data 2.5 Gb/s downstream data -6 Log 10 (BER) -7 Back to back -8-9 Back to back Transmission With upstream baseband data Transmission With RF CDMA With RF CDMA & downstream data Back to back Transmission With upstream signals With other signals Received Optical Power (dbm) Figure 5.12: Measured BER curves for the 2.5 Gb/s downstream data, 1.25 Gb/s upstream baseband data, 10 Mb/s E-CDMA LAN data Power budget Table 5.1 shows the measured experimental parameters for all signals showing the values of the transmitted power of all signals, the sensitivity and passive loss values of the components. As the number of ONUs increases in the PON, the loss in the network increases due to the splitting loss of the SC and therefore limiting the transmission bit rates of the downstream

211 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks data, upstream baseband data and upconverted RF E-CDMA LAN data and the transmission distance. 2.5 Gb/s downstream data 1.25 Gb/s upstream baseband data 155 Mb/s RF E-CDMA LAN data Transmitted Power WDM coupler CO km feeder fibre loss km distribution fibre loss x 0.69 WDM coupler ONU Connectors / splices loss BER = Dispersion Penalty Ageing and safety margin Table 5.1: Measured experimental parameters for the transmission of all three signals. This section investigates the scalability of the network as the number of splits in the SC is increased. For the transmission bit rates of the signals shown in Table 1, the power loss in all three signals result in an upper limit on the number of ONUs, N, that can be supported for given optical source powers and receiver sensitivities. The insertion loss of the SC can be given as [76] SC Loss = 10 log(2( N 1)) Equation 5.12 Figure 5.15 shows the calculated power margin for all signals as a function of the number of splits in the SC. As expected, the results show that as the number of splits in the SC increases, the power margin decreases for all signals due to increasing SC insertion loss from the increase in the number of splits. The power margin for the E-CDMA LAN data is higher compared to that of the other data streams and it is because of better sensitivity for the recovered E-CDMA LAN data. It should be noted that the calculated power margins for the 2.5 Gb/s downstream data and 1.25 Gb/s upstream baseband data are not affected by the

212 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks inclusion of the RF LAN data. They were resulted due to the poor sensitivity of the p-i-n receivers and lower launch powers used in the experiment. The power margins for all signals can be increased using higher launch power and optical receivers with better sensitivity. Power Margin (db) Gb/s downstream data 1.25 Gb/s upstream baseband data 155 Mb/s LAN data Splits of SC Figure 5.13: Calculated power margin for all signals in the secure LAN emulation scheme using the measured parameters from the experiment. 5.5 Secure LAN emulation using E-CDMA with a FBG and an additional optical transceiver In section 5.4, a secure LAN emulation scheme using E-CDMA was proposed whereby the transmission of the E-CDMA LAN data was carried out on an RF carrier along with the upstream baseband data to the CO. As the LAN data is electronically spread using a high bit rate E-CDMA spreading code, and then modulated on an RF carrier that is placed outside the bandwidth of the upstream baseband data, higher bandwidth opto-electronic components are required at each ONU for the transmission of signals. An increase in the bandwidth of the upstream baseband data and the LAN data increases the total bandwidth of the upstream signals and therefore increases the cost of the ONUs. Moreover, the transmission of upstream baseband data and RF E-CDMA LAN data are performed simultaneously in the designated timeslots, the performance of the network is limited. As TDMA is used for the upstream

213 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks transmission of signals, the maximum allocated time duration for the transmission of the LAN data for each ONU is limited. This imposes a limitation on the TDMA protocol used for the transmissions of both signals in the same time slot as an ONU may have large amount of data information to be sent to other ONUs, while only smaller amount of information is to be sent to the CO. In this scenario, an ONU would have to wait for the next cycle for the remaining E-CDMA LAN data transmission. On the other hand, an ONU may have smaller amount of LAN data compared to the amount of upstream baseband data. In this scenario, the time duration allocated for the LAN data transmission is not fully utilised leading to poor efficiency of the protocol used for the LAN data transmissions. Therefore, an alternative secure LAN emulation scheme with the use of an additional optical transceiver and a FBG is proposed and experimentally demonstrated. This scheme effectively overcomes the protocol limited poor efficiencies of the transmissions of the signals. Central Office Upstream Receiver WDM Downstream 1.3/1.5 µm Transmitter Feeder Fibre FBG λ LAN 1xN SC WDM 2 1.3/1.5 µm ONU 1 Downstream Receiver 3 LAN data Receiver 1 λ LAN & λ u Transmitters ONU N λ d - downstream wavelength λ u upstream wavelength λ LAN LAN wavelength ONU optical network unit FBG fiber Bragg grating SC star coupler Figure 5.14: Proposed scheme for secure LAN emulation using E-CDMA in conjunction with a FBG and an additional optical transceiver at the ONUs. Figure 5.16 shows the schematic of the proposed scheme for secure LAN emulation using E- CDMA with an additional optical transceiver. In this scheme, a separate wavelength channel is used for the transmission of E-CDMA signal and the E-CDMA signal from each ONU is transported at baseband. Therefore, an additional optical transceiver is required at each ONU for the transmission and reception of E-CDMA signals. As a separate wavelength channel is used, the transmission bit rate of the LAN data can be increased according to the

214 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks requirements. Moreover, the upstream baseband data transmission can be carried out independent of the LAN data transmission. A fibre Bragg grating (FBG) is placed in the feeder fibre close to the 1 N SC, whereby N corresponds to the number of ONUs. The Bragg wavelength of the FBG is chosen such that FBG reflects the wavelength channel that carries the E-CDMA LAN data and broadcast to all ONUs. At the ONUs, the E-CDMA signals are detected and electronically decoded to recover the LAN data. The upstream transmission to the CO from ONUs could follow any MAC protocol such as TDMA protocol or CSMA/CD protocol. Similarly, the transmission of E-CDMA signals could follow a MAC protocol for collision-free transmissions. As the use of E-CDMA in this LAN scheme is to provide physical layer security for the transmitted LAN data, only the ONUs that have the knowledge of the E-CDMA spreading code could decode the E-CDMA LAN data. One ONU could send two separate LAN data streams to different groups of ONUs within the PON using separate E-CDMA spreading codes. Here, LAN data streams are separately electronically coded and the resulting DS-SS signals are combined and modulated onto the wavelength channel for the transmission Experimental demonstration 2.2 km SMF R/B filter 10 km SMF O MZM C R/B O λ d = nm R/B C P 4x4 P FBG 2 Star Coupler Gb/s Gb/s Receiver 3 db Coupler 2.5 Gb/s Receiver 2.5 Gb/s Receiver 155 Mb/s E-CDMA decoder 155 Mb/s E-CDMA encoder MZM λ u = nm λ LAN = nm 1.25 Gb/s Figure 5.15: Experimental setup to demonstrate the secure LAN emulation scheme using additional optical transceiver at the ONUs

215 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks Figure 5.17 shows the experimental setup to demonstrate and verify the proposed scheme. A downstream signal of PRBS NRZ data at 2.5 Gb/s was modulated onto a wavelength channel λd of nm using a MZM and transmitted to the ONUs through a 10 km feeder fibre, a 4 4 SC and a 2.2 km distribution fibre. LAN data at 155 Mb/s PRBS NRZ was electronically multiplexed with a 16-bit bipolar Walsh code at 2.48 Gb/s and the resulting DS- SS signal was modulated onto a wavelength channel λ LAN of nm. For the upstream transmission, 1.25 Gb/s PRBS NRZ data was directly modulated onto a wavelength channel λ u of nm. λ LAN and λ u were combined using a 3 db coupler and transmitted in the upstream direction. λ d is separated from λ LAN and λ u using a red/blue (R/B) CWDM couplers. The unused ports of the SC were terminated with optical isolators to reduce the reflections. Both 1.25 Gb/s upstream data and 2.5 Gb/s downstream data were detected using 2.5 Gb/s p-i-n receivers, while E-CDMA LAN data was recovered using a 2.5 Gb/s APD receiver. The recovered E-CDMA signal was electronically decoded with the appropriate local E-CDMA code and the 155 Mb/s LAN data was recovered. A series of experiments were conducted to examine the crosstalk effects of one signal on the other. Each signal was transmitted in the absence and presence of the other signals and BER was measured for each case Optical spectra -10 Optical Power (dbm) db Before FBG After FBG Wavelength (nm)

216 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks Figure 5.16: Observed optical spectra at the FBG. Figure 5.18 shows the observed optical spectra at the FBG measured using an optical spectrum analyser with 2.5 GHz resolution bandwidth. For the optical spectra observed after the filtering by the FBG, a suppression of 26 db was observed for λ LAN compared to λ u. However, this level of crosstalk is not expected to add any significant penalty for the recovered 1.25 Gb/s upstream data. -20 Downstream Receiver Spectrum Optical Power (dbm) db Wavelength (nm) Figure 5.17: Observed optical spectra at the downstream data receiver. Figure 5.19 shows the observed optical spectra at the downstream data receiver showing the backscattered light from λ LAN and λ u. The crosstalk from λ LAN is less than -29 db. The crosstalk of backscattered light from λ u is less than -38 db. Optical Power (dbm) Upstream Receiver Spectrum 22 db Wavelength (nm)

217 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks Figure 5.18: Observed optical spectra at the upstream data receiver. Figure 5.20 shows the observed optical spectra at the upstream data receiver showing the backscattered light from λ LAN. The crosstalk from λ LAN is less than -22 db. The crosstalk from λd is very low as it is separated from λ LAN and λ u using R/B CWDM couplers. Optical Power (dbm) LAN data Receiver Spectrum 15 db Wavelength (nm) Figure 5.19: Observed optical spectra at the LAN data receiver. Figure 5.21 shows the observed optical spectra at the LAN data receiver and the crosstalk from λu is less than -15 db. The low suppression of crosstalk level is possibly caused by the reflection from the connectors BER results Figure 5.22 shows the measured BER curves for the signals. A transmission penalty of 0.1 db was measured for the 2.5 Gb/s downstream data through the entire link compared to B-B measurements. The penalty for the 1.25 Gb/s upstream data transmission compared to B-B measurements was less than 0.3 db. From the BER curves for the LAN data, 0.35 db penalty was observed for the transmission through the entire link compared to B-B measurements. This penalty is possibly a result of non-perfect synchronisation of the E-CDMA LAN data when the E-CDMA signals were transmitted through the link. An additional penalty of 0.1 db was observed in the presence of other signals

218 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks Gb/s downstream data 1.25 Gb/s upstream data 155 Mb/s E-CDMA LAN data Back to back Transmission -5 With E-CDMA LAN data Log 10 (BER) Back to back Transmission With LAN data With other signals Back to back Transmission With upstream data With other signals Received Optical Power (dbm) Figure 5.20: Measured BER curves for the 2.5 Gb/s downstream data, 1.25 Gb/s upstream data, 155 Mb/s E-CDMA LAN data. 5.6 Multiple and secure virtual private networking using E-CDMA In the secure LAN emulation schemes described in sections 5.5 and 5.6, LAN data transmission is performed in the allocated timeslots to the ONUs. E-CDMA is used only to provide physical layer security for the transmitted signals. In this section, simultaneous E- CDMA signal transmissions within a PON were carried out to provide multiple virtual private networking (VPN) capability. A VPN is a private data network that uses a non-private network infrastructure, for example, a PON deployed by a local community, to carry its traffic [77, 78]. Each VPN may incorporate a number of ONUs that belong to the PON, and data is exchanged between these ONUs. Some ONUs may also belong to multiple VPNs. In the

219 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks previously described secure LAN emulation schemes, the transmission of data from an ONU is allowed only during timeslots designated to that ONU, thereby reducing the overall transmission efficiency when some ONUs are idle. Moreover, a burst mode receiver is required at each ONU for the reception of E-CDMA LAN data and therefore adds cost and complexity to the ONUs. However, in this VPN scheme, the complexities associated with the secure LAN emulation schemes are reduced. Each VPN is allocated a unique E-CDMA code, which is multiplexed with the data transmitted on the VPN. Likewise, reception of data from a particular VPN is only possible at ONUs that have access to the unique code for decoding. Therefore, physical layer security of the transmitted VPN signals is ensured as the E-CDMA signals that belong to a particular VPN cannot be decoded by ONUs belonging to other VPNs. This scheme shows that multiple VPN transmissions can be performed at any given time, rather than only during designated time slots for each ONU. Central Office Redirection of Distribution Fibres E-CDMA signals E-CDMA ONU 1 Upstream Receiver Feeder Fibre 1.3 µm WDM terminated Downstream λ d 1.3/1.5 µm Transmitter λu WDM 1.5 µm 1.3/1.5 µm N+1 x N+1 Star Coupler λ d downstream wavelength 1.5 µm λ u upstream wavelength 1.3 µm ONU N E-CDMA signal wavelength µm Receiver Upstream λ u Transmitter λ d 3 WDM 2 Downstream 1.3/1.5 µm 1 4 Receiver E-CDMA Transmitter Figure 5.21: Proposed scheme for VPN over PON using E-CDMA. The proposed scheme for implementing secure VPNs over a PON is shown in Figure 5.23 [79, 80]. A (N+1) (N+1) SC is used to split/combine optical signals to/from each ONU, whereby the number of ONUs connected to the SC is N. Each ONU is connected to the SC via two distribution fibres as shown in Figure The transmitted E-CDMA signal from an ONU on one distribution fibre is redirected back to each of the ONUs through the second distribution fibre. The E-CDMA signal transmission is performed using wideband optical sources such as FP-LDs or LEDs in the 1.5 µm wavelength window while the upstream transmission to the CO is carried out using a wavelength source at the 1.3 µm wavelength

220 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks window. Therefore, a 1.5 µm/1.3 µm CWDM coupler is used at each port of the SC to separate these wavelength channels. The downstream signal from the CO to the ONUs is carried out using a distributed feedback (DFB) laser operating at 1.5 µm wavelength window. A FBG with a Bragg wavelength at the downstream wavelength is used in-conjunction with a circulator at each ONU to prevent reflections of the E-CDMA signals entering the downstream data receiver. Using the assigned E-CDMA spreading code, VPN data can be transmitted to other ONUs within the same VPN. As all ONUs within the same VPN use same E-CDMA spreading code, a access control protocol is required for the transmission of VPN data within the same VPN. Even though this scheme is capable of supporting multiple VPNs simultaneously, an unwanted side effect from using the multiple access capability of E- CDMA to implement simultaneous and multiple VPN transmissions is OBI from several optical sources. However, it has been shown that the OBI can be reduced by the use of incoherent light sources such as LED [61, 62] and the use of E-CDMA [43, 44, 55]. Moreover, as the expected transmission distance between ONUs is not large in a PON and the use of E-CDMA [81] to carry VPN traffic, the dispersion induced penalty due to the use of broadband light sources is negligible Experimental demonstration 2.2 km SMF MZM R/B filter O λ d =1527 nm R/B Central Office P 2.5 Gb/s C 1.25 Gb/s Receiver 1.3/1.5 µm 2.2 km SMF CWDM 3 km SMF 10 km 4x4 SMF Star Coupler 1.3 µm E-CDMA 2 encoder ONU 2 E-CDMA E-CDMA Receiver Decoder R/B filter WC C R/B 1.3 µm P 1.5 µm O ONU Gb/s Receiver 1.3 µm λ u = 1551 nm E-CDMA 1 encoder 1.25 Gb/s Figure 5.22: Experimental setup to demonstrate the feasibility of the proposed VPN scheme

221 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks Figure 5.24 shows the experimental setup to demonstrate our proposed scheme. The downstream signal of PRBS NRZ data at 2.5 Gb/s was modulated onto downstream wavelength λ d = 1527 nm using a MZM and transmitted through a 10 km feeder fibre link and a 4 4 SC. At the SC, the signal is split and broadcast to ONU 1 through a 2.2 km distribution fibre link. For the upstream transmission, PRBS NRZ data at 1.25 Gb/s was directly modulated on the upstream wavelength λ u = 1551 nm and transmitted in the opposite direction. The wavelengths, λ d and λ u were chosen such that they can be combined and separated by the R/B CWDM filters placed at the CO and ONU 1. For the transmission of the E-CDMA VPN signals, PRBS NRZ data at 40 Mb/s was electronically multiplexed with 16-bit, 640 Mb/s bipolar Walsh code 1 using a RF mixer. Here, the transmission rate of the Walsh code 1 was limited by the bandwidth of the FP-LD. The data rate of the VPN signals can be increased by using a FP-LD with larger bandwidth. The resulting DS-SS signals were directly modulated onto a FP-LD operating at 1.3 µm wavelength window and transmitted through the 2.2 km fibre in the upstream direction. 1.3 µm/1.5 µm CWDM couplers were used to separate and combine these E-CDMA signals with λ d and λ u. To demonstrate multiple VPN capability using E-CDMA, another 16-bit bipolar Walsh code 2 at 640 Mb/s was electronically multiplexed with PRBS NRZ data at 40 Mb/s and the resulting DS-SS signals were directly modulated onto another FP-LD operating at 1.3 µm wavelength window and transmitted through the 3 km fibre in the upstream direction. The transmitted E-CDMA signals in the upstream direction were separated from λ u using a 1.3 µm/1.5 µm CWDM coupler placed at the SC and the E-CDMA signals were redirected to ONU 1 using another 2.2 km fibre. The power of both E-CDMA signals was approximately equalled to reduce the MAI Gb/s upstream data signals and 2.5 Gb/s downstream data signals were detected using 2.5 Gb/s p-i-n receivers, whereas the E-CDMA signals were detected using an 2.5 Gb/s APD receiver. The detected E-CDMA signals were fed through an E-CDMA decoding circuit and 40 Mb/s VPN data was recovered. For the decoding of the desired E-CDMA signal, appropriate electrical delay was employed to synchronise the local E-CDMA code with the incoming E-CDMA signals. A series of experiments were conducted to examine the crosstalk effects of one signal on the other, and BER for all signals were measured Oscilloscope traces Figure 5.25 shows the oscilloscope traces for the transmitted E-CDMA signals observed after

222 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks encoding. Phase changes in both coded traces can be observed when the bipolar Walsh codes were multiplexed with the data. The shapes of both traces are different because the desired channel was multiplexed with the E-CDMA code of while that of the undesired channel was multiplexed with Desired CDMA channel Phase Changes Undesired CDMA channel Figure 5.23: Observed oscilloscope traces for the desired and undesired CDMA channels Optical spectra -10 Optical Power (dbm) Downstream data Receiver Upstream data Receiver Wavelength (nm) Figure 5.24: Observed optical spectra at the downstream data and upstream data receivers Figure 5.26 shows the optical spectra observed at the downstream and upstream data receivers. At both receivers, more than 40 db suppression of the backscattered light was observed. The low suppression of the backscattered light was obtained by the use of R/B

223 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks CWDM filters to separate λ d and λ u BER results Gb/s downstream data 1.25 Gb/s upstream data 40 Mb/s VPN data Log 10 (BER) Back to back Transmission With Upstream signals Back to back Transmission With LAN data With LAN & downstream signals 10-9 Back to back Transmission One Channel With upstream signals With other signals Transmission - Two Channels Received Optical Power (dbm) Figure 5.25: Measured BER plots for 2.5 Gb/s downstream data, 1.25 Gb/s upstream data and 40 Mb/s VPN data. Figure 5.27 shows the measured BER for all signals. For the 2.5 Gb/s downstream data, a penalty of less than 0.5 db was observed in the presence of upstream data and E-CDMA data transmissions when compared to B-B measurements. No significant penalty was observed for the 1.25 Gb/s upstream data in the presence of downstream data and E-CDMA data transmissions. For the 40 Mb/s E-CDMA VPN data, 0.28 db penalty was observed compared to B-B measurements when only the desired channel was transmitted. This penalty can be attributed to imperfect synchronisation at the decoding of the E-CDMA signal. An additional penalty of 0.15 db was observed in the presence of upstream and downstream signals. When the undesired E-CDMA signal from ONU 2 was included, an additional 0.2 db penalty was

224 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks observed. This penalty can be attributed to OBI, shot noise and MAI from the undesired E- CDMA signals. As multiple VPNs can be simultaneously supported in the PON, each VPN may have different transmitting power leading to higher OBI, therefore resulting in a larger power penalty for the recovered E-CDMA VPN data. To measure the penalty due to changing interfering signal power level, the optical power of the desired E-CDMA signal was kept to a constant value while the optical power of the interfering E-CDMA signal was increased. Figure 5.28 shows the observed optical spectra showing the increase in the received optical power of the interfering E-CDMA channel from the second FP-LD Interfering channel With interference - 0 db power difference With interference - 8 db power difference Optical Power (dbm) Desired channel Wavelength (nm) Figure 5.26: Observed optical spectra at the receiver showing the desired and interfering wavelength channels with varying interfering power. Figure 5.29 shows the observed RF spectra showing the increase in the resulting detected RF power of the interfering E-CDMA channel compared to that of the desired E-CDMA signal. An increase in 8 db optical power has resulted in 13 db increase in the RF power of the undesired E-CDMA signal

225 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks RF power (dbm) db With same interference power With 8 db more interference power Interfering E-CDMA channel RF Frequency (GHz) Figure 5.27: Observed RF spectra of the detected E-CDMA VPN signals showing the desired and interfering E-CDMA channels with varying interfering power Power Penalty (db) Interference Power Change (db) Figure 5.28: Measured power penalty for 40 Mb/s VPN data against varying interference power. Figure 5.30 shows the measured power penalty for the 40 Mb/s E-CDMA VPN data against the increase in the optical power of the interfering E-CDMA signal. As the optical power of

226 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks the interfering E-CDMA signal increases, the power penalty also increases. When the optical power of the interfering E-CDMA signal is more than 8 db than the desired E-CDMA signal, approximately 3 db power penalty was observed Theoretical analysis of scalability Following on the discussions on using E-CDMA for the VPN capability within a PON, the feasibility of this scheme is now discussed. This scheme is not without theoretical and practical limitations. The OBI caused by the incoherent beating of optical fields at the receiver fundamentally limits the network capacity. Secondly, the MAI from the undesired E-CDMA signals also limits the number of simultaneous VPNs and the maximum allowed bit rate of the data. For the transmission of the VPN signals within a PON, inexpensive broadband optical sources such as LEDs are used at each ONU. As described before, the power budget of the VPN signals is determined by the number of channels that can be simultaneously supported. However, the use of integrated LED-SOA devices could potentially give larger output power [57-59] enabling adequate power margin for the transmitted signals. The use of broadband optical sources reduces the effect of OBI and therefore potentially increases the number of VPNs that can be simultaneously supported. The transmission distances between the ONUs in the PON are shorter and therefore the use of broadband optical source for the transmission of VPN signals is not expected to increase the penalty induced by the dispersion. Therefore, the dispersion is neglected in this study to calculate the scalability of the network. In this scalability study, the noise factors that are present in the E-CDMA VPN receiver are considered. Thermal noise, shot noise, MAI and OBI are considered with varying values to calculate the number of simultaneous VPNs that can be simultaneously supported with a particular data rate. The VPN data signal with a bandwidth of B is given as g ( t). When this signal is multiplied with the E-CDMA spreading code () t c having the values ± 1, the DS-SS is given by () t g()() t c t s = Equation 5.13 The bandwidth of the DS-SS is B G B, whereby G is the processing gain. = SS P P

227 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks When the optical source is intensity modulated with the DS-SS signal with a modulation index of 1, the intensity of the optical signal is given by I() t = PS ( 1 + c() t g() t ) Equation 5.14 Here, P S is the average transmitted optical power. At the ONUs, the redirected E-CDMA VPN signals from K ONUs are detected using a single PD and the received optical signal can be written as I r K () t = P ( + c() t g() t ) k = 1 r 1 Equation 5.15 PS Here, the received average optical power P r =. F l gives the total loss for the E-CDMA F VPN signal. l The output current of the PD can be written as i K () t R P ( 1 + c() t g() t ) n( t) Equation 5.16 = k =1 r + Here, R is the responsivity of the PD, n ( t) gives the noise current. n () t = i () t i () t, whereby i ( t) i ( t) th + shot current respectively. th and are the thermal noise current and shot noise shot The output of one correlator (k) to recover the E-CDMA VPN data is given as i k () t = i() t c() t = RP g r k K () t + RP g () t c () t c () t + n() t c () t r i i= 1, i k i k k Equation

228 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks The first term shows the reproduced E-CDMA VPN signal, the second term is the MAI, and the third term is the additive noise. The MAI is given as [82] i I K () t = P g () t () t R γ Equation 5.18 r i i= 1, i k ik Here γ () t c () t c () t γ ik = ( τ ) ( τ ) g i k R and R are the autocorrelation functions of ik autocorrelation function of i I () t is given by γ and ( t) g k respectively. The R 2 ( ) ( K 1)( RP ) R ( τ ) R (τ I τ r γ g = ) Equation τ f PN Here, ( ) R γ τ = e 1 Rg 2 2 ( τ ) cos( πf τ ) c R I 1 r g 2 2 τ f PN ( τ ) ( K 1)( RP ) R ( τ ) e = Equation Here, f PN is the chip rate. f PN = 1 B SS From equation 5.20, the power spectral density of the i I ( t) is given by

229 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks S I 1 2 ( f ) = ( k 1)( RP ) r 2 f 2 PN f + π PN 2 PN ( f f ) 2 f 2 PN + π 2 ( f + f ) 2 c + f c Equation 5.21 The interference power I, is given by ( K 1)( RP r ) G I = P Equation E-CDMA VPN signal power, shot noise, and thermal noise are given by ( ) 2 RP S = r Equation N = erkp B Equation 5.24 shot 2 r ( einc) B N thermal 2 = Equation 5.25 Here, R, e, and einc are responsivity of the PD, electron charge, and the equivalent input noise current respectively. As multiple optical fields also beat in the PD creating the OBI, the beat noise power is also required to calculate the total signal-to-noise ratio. The beat noise power for the Lorenzian spectrum light waves can be given as P beat 2 ( ( K 1) P P + ( K 1)( K 2) P ) 2 = R Bτ C 2 w l l Equation 5.26 Here, τ C is the coherence time optical source. Pw and Pl are the received optical power of the desired E-CDMA signal and interfering E-CDMA signal respectively. For the calculations, these optical powers are assumed to be equal. Therefore, equation 5.26 can further be modified as

230 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks P 2 2 beat = R Bτ C Pl ( 2( K 1) + ( K 1)( K 2)) Equation 5.27 The coherence time can be calculated from the spectral width of the broadband optical source using the following equation. v τ C = G ( f ) df = df = Equation 5.28 v v v 2 For the optical sources with Gaussian spectral profiles, the relationship between v andτ has a scaling factor that takes the different spectral shapes into account, resulting in the equation given below. C τ = C Equation 5.29 v The number of simultaneous VPNs that can be simultaneously supported can be calculated using the above equations. For these calculations, a PON consisting of 32 ONUs is considered. Therefore, the number of SC splits is also 32. The architecture shown in Figure 5.23 is taken as the PON architecture to provide multiple VPNs. The processing gain of E- CDMA signal, the spectral width of the optical source, and the transmitted optical power varied and the number of VPNs is plotted against the SNR values. Table 5.2 shows the parameters and the corresponding values that are fixed for the theoretical analysis

231 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks Parameter Value Unit Total fibre length 5 km Attenuation of fibre 0.2 db/km Circulator loss 0.75 db WDM coupler loss 0.75 db SC loss 18 db einc 10 x A/ Hz R 0.85 A/W e 1.6 x C B 2500 MHz Table 5.2: 38 The parameters and the values that are used for the calculations to verify the number of simultaneous VPNs. 34 SNR-Shot 30 SNR (db) SNR-OBI SNR-MAI SNR-Thermal 18 SNR-Total Number of simultaneous VPNs Figure 5.29: Calculated number of simultaneous VPNs in the presence of noise factors showing the mot and least dominant noises

232 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks Figure 5.31 shows the number of simultaneous number of VPNs in the presence of all noise parameters against the SNR curves. It is assumed that the spectral width of the optical sources is 60 nm, the launched optical power is 3 dbm, and the processing gain of E-CDMA signal is 200. As can be seen from Figure 5.31, the shot noise is the least dominant of all noise. OBI is the most dominant noise. As the number of VPNs increases, the MAI also increases. This is because the received optical power increases with the number of VPNs. It is assumed that BPSK is used for the modulation of the DS-SS signals. Generally, ignoring roll-off, 12.6 db SNR is required a BER at 10-9 for BPSK modulation format [83, 84]. Considering BPSK modulation is used for the DS-SS, atleast 7 simultaneous VPNs can be supported. 21 SNR (db) P = -6 dbm P = -3 dbm P = 0 dbm P = 3 dbm Number of VPNs Figure 5.30: Calculated number of simultaneous VPNs with varying transmitted power Figure 5.32 shows the calculated number of simultaneous VPN with varying transmitted optical power at each ONU. For these calculations, the spectral width of the optical sources is taken as 60 nm, and the processing gain of E-CDMA signal as 200. As the transmitted power from each VPN increases, the incident power at the PD also increases. Therefore the shot

233 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks noise, MAI and OBI increase with the increasing optical power. For lower values of transmitted power, the resulting SNR is also lower and therefore lower number of VPNs is supported. As the optical power increases, SNR increases and therefore the number of simultaneous VPNs is also increased. For the transmitted power of -6 dbm, the received optical power is not adequate to recover the E-CDMA VPN data. As the transmitted power is increased to -3 dbm from each VPN, the resulting SNR is not adequate to support more than one VPN. However, as transmitted optical power increases to 0 dbm and 3 dbm, a maximum of six and seven VPNs are simultaneously supported. Therefore, high power optical sources such as SLEDs and LED-SOAs are required at each ONU to satisfy the power budget requirements and to obtain adequate SNR to support multiple VPNs. 21 SNR (db) nm 40 nm 60 nm 80 nm Number of VPNs Figure 5.31: Calculated number of simultaneous VPNs against varying spectral width of the optical source used at each ONU. The OBI depends on the spectral width of the optical source used at each ONU as the coherence time is inversely proportional to the spectral width. As the spectral width of the optical source increases, the OBI decreases. As can be seen from Figure 5.33, as the spectral width increases, the number of simultaneous VPNs is also increased. For these calculations, it is assumed that the transmitted optical power is 3 dbm, and the processing gain is 200. For the spectral width of 20 nm optical source, a maximum of five VPNs can be supported while

234 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks this value increases to seven when the optical source with 80 nm spectral width is used. Even though the spectral width of the optical sources is increased by a factor of 4, the number of VPNs is increased by just 2. This is because the as the number of VPNs increases, the MAI also increases and therefore limits the number of VPNs. As the spectral width of the optical source increases, the susceptibility of the VPN data to dispersion effects also increases. Therefore, increasing the spectral width of the optical source is not necessarily a feasible solution to support many simultaneous VPNs SNR (db) Number of VPNs G = 50 G = 100 G = 200 G = 500 Figure 5.32: Calculated number of simultaneous VPNs against varying processing gain of the E- CDMA signals. Figure 5.34 shows the number of VPNs against SNR for varying processing gain values of the E-CDMA signals. As the processing gain increases, the MAI decreases. As expected, as the processing gain increases, MAI decreases and therefore SNR for the recovered E-CDMA data also increases for the same number of VPNs. For the processing gain values of 50, a maximum of three VPNs can be simultaneously supported, while this values increases to nine, when the processing gain is increased to 500. However, the number of VPNs cannot be continuously increased by increasing the processing gain values. This is because, the OBI starts limiting the performance of the network

235 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks It has been shown that several parameters such as OBI and MAI limit the performance and the scalability of the VPN within a PON. Trying to increase the scalability of the network, one noise or the interference factor could be taken care of. However, this does not necessarily improve the scalability. Therefore, the parameters such as transmitted optical power, spectral width and the processing gain have to be chosen according to the requirements of network. Moreover, using appropriate electronic coding and decoding mechanisms [85, 86], with digital signal processing techniques [48-53], better scalable VPNs within a PS-PON can be obtained. 5.7 Conclusions In this chapter, several applications for E-CDMA in the PS-PON have been presented. E- CDMA provides physical layer security for the transmitted signals, enables multiple access, and provides resilience against interference. These properties of E-CDMA are exploited in the schemes demonstrated in this chapter. E-CDMA has been proposed and experimentally demonstrated for upstream access in a PS-PON. Even though the upstream transmission is performed using broadband optical sources, the number of ONUs that could be performing transmissions simultaneously is mainly limited by the OBI. A theoretical analysis of scalability has also performed to show the number of active sources. For the transmissions with full modulation index, less than nine ONUs could be performing transmissions simultaneously. The number of ONUs could be increased by reducing the electrical bandwidth of the signals and therefore effective upstream data rate is also reduced. Two separate optical layer secure LAN emulation schemes were experimentally demonstrated using E-CDMA. In both these schemes, E-CDMA is used only to provide physical layer security for the transmitted signals. One scheme uses RF subcarrier multiplexed transmission of the E-CDMA LAN traffic redirected using a secondary distribution fibre to all ONUs, while the second scheme uses a separate wavelength channel for the transmission of E- CDMA LAN traffic. Scheme 1 requires higher bandwidth opto-electronic components at each ONU. Scheme 2 requires additional optical transceiver at each ONU and a FBG in the passive plant. The BER measurements of the recovered signals show minimal penalty in the presence of other signal transmissions

236 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks A scheme for secure and multiple VPNs over a PON using E-CDMA is also demonstrated. The transmission of VPN data is performed using broadband optical sources and this scheme could potentially be overlaid on an existing PS-PON infrastructure. As in the upstream access scheme, the number of active VPNs at the same time is limited by the OBI. Theoretical analyses of scalability have been performed to calculate the number of active optical sources. It is shown that not only OBI limits the scalability, but also MAI from the undesired E- CDMA signals. Improving one noise factor does not necessarily improve the total performance or the scalability of the network because of the limitations by other noise factors. However, for a PS-PON consisting of 32 ONUs, the number of simultaneous VPNs is not usually very high. As the number of required VPNs depends on the parameters such as transmitted optical power, processing gain of E-CDMA signals, spectral width of the optical source and the electrical bandwidth of the E-CDMA signals, the values for these parameters should be chosen according to the requirements by the network. 5.8 References [1] Y. Mochida, "Technologies for local-access fibering," IEEE Commun. Mag., vol. 32, pp , Feb [2] N. J. Frigo, Recent progress in optical access networks, in Proc. Optical fiber communication conference (OFC 96), vol. 2, pp , [3] N. J. Frigo, "Local access optical networks," IEEE Network, vol. 10, pp , no. 6, [4] I. Yamashita, and N. Shibata, Fiber to the home: reality and dream, in Proc. 11 th International Conference on Integrated Optics and 23 rd European Conference on Optical Communications (ECOC 97), vol. 4, pp , [5] H. Frazier, and G. Pesavento, "Ethernet takes on the first mile," IT Professional, vol. 3, pp , no. 4, [6] F. J. Effenberger, H. Ichibangase, and H. Yamashita, "Advances in broadband passive optical networking technologies," IEEE Commun. Mag., vol. 39, pp , Dec [7] F.-T. An, K. S. Kim, D. Gutierrez, S. Yam, E. Hu, K. Shrikhande, and L. G. Kazovsky, SUCCESS: a next-generation hybrid WDM/TDM optical access network architecture," IEEE J. Lightw. Technol., vol. 22, pp , Nov [8] R. D. Feldman, E. E. Harstead, S. Jiang, T. H. Wood, and M. Zirngibl, An evaluation of architectures incorporating wavelength division multiplexing for broad-band fiber access," IEEE J. Lightw. Technol., vol. 16, pp , Sep

237 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks [9] Broadband optical access systems based on passive optical networks (PON), ITU-T Recommendation G [10] Gigabit-capable passive optical networks (GPON): General characteristics, ITU-T Recommendation G.984.1, [11] Ethernet in the first mile task force: IEEE 802.3ah, Draft 3.0b. [12] I. M. McGregor, G. J. Semple, and G. Nicholson, Implementation of a TDM passive optical network for subscriber loop applications, IEEE J. Lightw. Technol., vol. 7, pp , Nov [13] A. M. Hill, D. B. Payne, K. J. Blyth, D. S. Forrester, A. Silvertown, J. W. Arkwright, D. W. Faulkner, and J. W. Balance, An experimental broadband and telephony passive optical network, in Proc. Global Communications Conference (GLOBECOM 90), pp , [14] K. H. Han, E. S. Son, H. Y. Choi, K. W. Lim, and Y. C. Chung, Bidirectional WDM PON Using Light-Emitting Diodes Spectrum-Sliced With Cyclic Arrayed-Waveguide Grating, IEEE Photon. Technol. Lett., vol. 16, pp , Oct [15] D. K. Jung, S. K. Shin, C. H. Lee, and Y. C. Chung, Wavelength-division-multiplexed passive optical network based on spectrum-slicing techniques, IEEE Photon. Technol. Lett., vol. 10, pp , Sep [16] J. B. Stark, M. C. Nuss, W. H. Knox, S. T. Cundiff, L. Boivin, S. G. Grubb, D. Tipton, D. DiGiovanni, U. Koren, and K. Dreyer, Cascaded WDM passive optical network with a highly shared source, IEEE Photon. Technol. Lett., vol. 9, pp , Aug [17] R. Monnard, M. Zirngibl, C. R. Doerr, C. H. Joyner, and L. W. Stulz, Demonstration of a 12*155 Mb/s WDM PON under outside plant temperature conditions, IEEE Photon. Technol. Lett., vol. 9, pp , Dec [18] S. Topliss, D. Beeler, and L. Altwegg, Synchronization for passive optical networks, IEEE J. Lightw. Technol., vol. 13, pp , May [19] S. Culverhouse, R. A. Lobbett, and P. J. Smith, Optically amplified TDMA distributive switch network with Gb/s capacity offering interconnection to over 1000 customers at 2 Mb/s, IEE Electron. Lett., vol. 28, pp , [20] B. Miah, and L. Cuthbert, An economic ATM passive optical network, IEEE Commun. Mag., pp , [21] M. Schelp, X. Wang, W. Yen, and E. Ho, The Ranging Protocol for ATM Passive Optical Networks: Analysis and improvements, in Proc. Annual Multiplexes Telephony Conference, [22] F. J. Effenberger, H. Ichibangase, and H. Yamashita, Advances in broadband passive optical networking technologies," IEEE Commun. Mag., vol. 39, pp , Dec [23] G. Kramer, B. Mukherjee, and G. Pesavento, IPACT a dynamic protocol for an Ethernet PON (EPON), IEEE Commun. Mag., vol. 40, pp , Feb [24] C. M. Assi, Y. Ye, S. Dixit, and M. A. Ali, "Dynamic bandwidth allocation for quality of service over Ethernet PONs," IEEE J. Sel. Areas Commun. vol. 21, pp , Nov [25] M. Ma, Y. Zhu, and T. H. Cheng, "A bandwidth guaranteed polling MAC protocol for Ethernet passive optical networks," in Proc. 22 nd Annual Joint Conference of the IEEE Computer and Communications Societies, vol. 1, pp ,

238 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks [26] J. Jang, and E. K. Park, "Dynamic resource allocation for quality of service on a PON with home networks," IEEE Commun. Mag., vol., pp , Jun [27] E. Wong and Chang-Joon Chae, "Efficient dynamic bandwidth allocation based on upstream broadcast in Ethernet passive optical networks," in Proc. Optical Fiber Communication Conference (OFC 05), vol. 5, [28] E. Wong and Chang-Joon Chae, "Support of Differentiated Services in Ethernet Passive Optical Networks via Upstream Broadcast Dynamic Bandwidth Allocation Scheme," in Proc. 4 th International Conference on Optical Internet (COIN 05), pp , [29] IEEE 802.3ah Std, Part 3: CSMA/CD access method and physical layer specifications, 2004 Edition. [30] E. Wong and C.-J. Chae, "CSMA/CD-based Ethernet passive optical network with optical internetworking capability among users," IEEE Photon. Technol. Lett., vol. 16, pp , Sep [31] E. Wong and C.-J. Chae, "CSMA/CD-based Ethernet passive optical network with shared LAN capability," in Proc. Optical Fiber Communication Conference (OFC 04), vol. 1, [32] C.-J. Chae, E. Wong, and R. S. Tucker, Optical CSMA/CD media access scheme for Ethernet over passive optical network, IEEE Photon. Technol. Lett., vol. 14, pp , May [33] T. E. Darcie, "Subcarrier multiplexing for multiple-access lightwave networks," IEEE J. Lightw. Technol., vol. LT-5, pp , [34] W.-P. Lin, M. S. Kao, S. Chi, The modified star-ring architecture for high-capacity subcarrier multiplexed passive optical networks, IEEE J. Lightw. Technol., vol. 19, pp , Jan [35] C. R. Giles, R. D. Feldman, T. H. Wood, M. Zirngibl, G. Raybon, T. Strasser, L. Stulz, A. McCormick, C. H. Joyner, and C. R. Doerr, "Access PON using downstream 1550-nm WDM routing and upstream 1300-nm SCMA combining through a fiber-grating router," IEEE Photon. Technol. Lett., vol. 8, pp , Nov [36] S. L. Woodward, G. E. Bodeep, T. E. Darcie, G. Huang, Z. Wang, and J. J. Werner, "A passive-optical network employing upconverted 16-CAP signals," IEEE Photon. Technol. Lett., vol. 8, pp , Sep [37] C. Desem, Measurement of optical interference due to multiple optical carriers in subcarrier multiplexing, IEEE Photon. Technol. Lett., vol. 3, pp , Apr [38] T. H. Wood, and N. K. Shankaranarayanan, Operation of a passive optical network with subcarrier multiplexing in the presence of optical beat interference, IEEE J. Lightw. Technol., vol. 11, pp , Oct [39] C. Desem, Optical interference in subcarrier multiplexed system with multiple optical carriers, IEEE J. Sel. Areas Commun., vol. 8, pp , [40] W.-P. Lin, "Reducing multiple optical carriers interference in broad-band passive optical networks," IEEE Photon. Technol. Lett., vol. 9, pp , Mar [41] R. D. Feldman, K.-Y. Liou, G. Raybon, and R.F. Austin, "Reduction of optical beat interference in a subcarrier multiple-access passive optical network through the use of an amplified light-emitting diode," IEEE Photon. Technol. Lett., vol. 8, pp , Jan

239 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks [42] S. L. Woodward, X. Lu, T. E. Darcie, and G. E. Bodeep, Reduction of optical beat interference in subcarrier networks, IEEE Photon. Technol. Lett., vol. 3, pp , Jan [43] C.C. W. Hsiao, B. H. Wang, and W. I. Way, "Multiple access in the presence of optical-beat and gochannel interference using Walsh-code-based synchronized CDMA technique," IEEE Photon. Technol. Lett., vol. 9, pp , Aug [44] F. Yamamoto, and T. Sugie, "Reduction of optical beat interference in passive optical networks using CDMA technique," IEEE Photon. Technol. Lett., vol. 12, pp , Dec [45] M. J. Parham, C. Smythe, and B. L. Weiss, Code division multiple access techniques for use in opticalfiber local-area networks, J. Electronics and Communication Engineering, vol. 4, no. 4, pp , Aug [46] E. H. Dinan, and B. Jabbari, Spreading codes for direct sequence CDMA and wideband CDMA cellular networks, IEEE Commun. Mag., pp , Sep [47] B. Ahn and Y. Park, "A symmetric-structure CDMA-PON system and its implementation," IEEE Photon. Technol. Lett., vol. 14, no. 9, pp , [48] S. K. S. Chan, and V. C. M. Leung "An FPGA receiver for CPSK spread spectrum signaling," IEEE Transactions on Consumer Electronics, vol. 45, no. 1, pp , [49] G. Yunfeng, Z. Zhaoyang, C. Yanyan, J. Xiangfeng and Q. Peilang, "An All Digital Packet DS-CDMA Transceiver for HFC Network Upstream Transmission," in Proc. IEE Asia-Pacific Conference on Circuits, Systems and Electronic Communication Systems, pp , [50] B. Y. Chung, C. Chien, et al. "Performance analysis of an all-digital BPSK direct-sequence spreadspectrum IF receiver architecture," IEEE J. Sel Areas in Commun., vol. 11, no. 7, pp , [51] J. Ramos, C. Crespo, E. Carballo, M. Burgos, J. M. De Blas, and J. L. Alonso, Digital DS-spread spectrum receiver with reduced computation cost, in Proc. 8 th Mediterranean Electrotechnical Conference, vol. 2, pp , [52] Q. Shi, S. R. Korfhage, R. J. O Dea, A low cost receiver design for DSSS location system, in Proc. IEEE International Conference on Communications, Circuits and Systems and West Sino Expositions, vol. 1, pp , [53] A. Mehrnia, K. Shakeri, A. A. Eftekhar, F. Soheili, M. Nassiri, and A. Fotowat, An all digital spread spectrum processor featuring multiplier free zero-if down conversion, decimation, and multiplexed despreading, in Proc. 12 th International Conference on Microelectronics, pp , [54] P. M. Lam, and K. Sripimanwat, Synchronous optical fiber code division multiple access networks using Walsh codes, in Proc. IEEE 7 th International Symposium on Spread Spectrum and Applications, vol. 2, pp , [55] N. Minato, M. Kashima, K. Matsuno, A. Sasaki, and S. Oshiba, A Novel Optical Access System using CDM/SCM Channel Control, in Proc. 6 th Optoelectronics Communications Conference (OECC 01), pp , [56] T. Demeechai, and A. B. Sharma, Beat noise in a non-coherent optical CDMA system, in Proc. 8 th International Conference on Communication Systems, vol. 2, pp ,

240 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks [57] B. Glance, K. Y. Liou, U. Koren, E. C. Burrows, G. Roybon, C. A. Burrus, K. Dreyer, and C. Doerr, A single-fiber WDM local access network based on amplified LED transceivers, IEEE Photon. Technol. Lett., vol. 8, pp , Sep [58] K. Y. Liou, B. Glance, U. Koren, E. C. Burrows, G. Roybon, C. A. Burrus, and K. Dreyer, Monolithically integrated semiconductor LED-amplifier for applications as transceivers in fiber access systems, IEEE Photon. Technol. Lett., vol. 8, pp , Jun [59] K. Y. Liou, U. Koren, E. C. Burrows, J. L. Zyskind, and K. Dreyer, A WDM access system architecture based on spectral slicing of an amplified LED and delay-line multiplexing and encoding of eight wavelength channels for 64 subscribers, IEEE Photon. Technol. Lett., vol. 9, pp , Apr [60] G. J. Pendock, and D. D. Sampson, Capacity of coherence-multiplexed CDMA networks, Optical Communications, vol. 143, pp , [61] J. S. Lee, Y. C. Chung, and D. J. DiGiovanni, Spectrum sliced fiber amplifier light source for multichannel WDM applications, IEEE Photon. Technol. Lett., vol. 5, pp , [62] R. D. Feldman, K.-Y. Liou, G. Raybon and R. F. Austin, Reduction of optical beat interference in a subcarrier multiple-access passive optical network through the use of an amplified light-emitting diode, IEEE Photon. Technol. Lett., vol. 8, pp , [63] G. J. Pendock, M. J. L. Cahill, and D. D. Sampson, Multi-gigabit per second demonstration of photonic code division multiplexing, IEE Electron. Lett., vol. 31, pp , [64] G. J. Pendock, and D. D. Sampson, Transmission performance of high bit rate spectrum-sliced WDM systems, IEEE J. Lightw. Technol., vol. 14, pp , [65] K. H. Han, E. S. Son, H. Y. Choi, K. W. Lim, and Y. C. Chung, "Bidirectional WDM PON using lightemitting diodes spectrum-sliced with cyclic arrayed-waveguide grating," IEEE Photon. Technol. Lett., vol. 16, pp , Oct [66] J. -G. Zhang, G. Picchi, Forward error-correction codes in incoherent optical fibre CDMA networks, IEE Electron. Lett., vol. 29, no. 16, pp , Aus [67] H. Lundqvist, G. Karlsson, On error-correction coding for CDMA PON, IEEE J. Lightw. Technol., vol. 23, pp , Aug [68] A. W. Lam, and S. Tantaratana, Theory and Applications of spread spectrum systems, Piscataway, NJ: IEEE Press, [69] G. P. Agrawal, Fiber Optic Communication systems, New York: Wiley-Intersci., [70] N. Nadarajah, M. Attygalle, E. Wong and A. Nirmalathas, "Novel schemes for Local Area Network Emulation in Passive Optical Networks with RF Subcarrier Multiplexed Customer Traffic," IEEE J. Lightw. Tech., vol. 23, pp , Oct [71] N. Nadarajah, A. Nirmalathas, and E. Wong, LAN emulation on passive optical networks using RF subcarrier multiplexing, in Proc. 17 th annual meeting of the IEEE lasers and electro-optics society (LEOS 04), pp , [72] S. S. Roh, and S.-H. Kim, "Security model and authentication protocol in EPON-based optical access network," in Proc. 5 th International Conference on Transparent Optical Networks (ICTON 03), pp ,

241 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks [73] P. D. Townsend, S. J. D. Phoenix, K. J. Blow, and S. M. Barnett, "Design of quantum cryptography systems for passive optical networks," IEE Electron. Lett., vol. 30, pp , Oct [74] P. D. Kumavor, A. C. Beal, S. Yelin, E. Donkor, and B.C. Wang, "Comparison of four multi-user quantum key distribution schemes over passive optical networks," IEEE J. Lightw. Technol., vol. 23, pp , Jan [75] P. D. Townsend, "Secure communications on passive optical networks using quantum cryptography, " in Proc. 22 nd European Conference on optical Communications (ECOC 96), vol. 3, pp , [76] D. Podwika, D. Stefanski, J. S. Witkowski, and E. M. Pawlik, "Computer networks based on optical passive couplers," in Proc. 2 nd International Conference on Transparent Optical Networks (ICTON 00), pp , [77] R. Cohen, "On the establishment of an access VPN in broadband access networks," IEEE Commun. Mag., vol. 41, no. 2, pp , [78] R. Venkateswaran, Virtual private networks," IEEE Potentials, vol. 20, no 1, pp , [79] N. Nadarajah, E. Wong and A. Nirmalathas, Implementation of Multiple Secure Virtual Private Networks over Passive Optical Networks using Electronic CDMA IEEE Photon. Technol. Lett., vol. 18, pp , Feb [80] N. Nadarajah, E. Wong, and A. Nirmalathas, Secure E-CDMA virtual private network over passive optical networks, in Proc. 18 th Annual Meeting of the IEEE Lasers and Electro-Optics Society (LEOS 05), pp , [81] G. H. Smith, A. Nirmalathas, J. Yates, and D. Novak, Millimetre-wave fiber-radio system incorporating broadband radio CDMA in Proc. 24 th European Conference on Optical Communications (ECOC 98), pp , [82] S. Kajiya, K. Tsukamoto, and S. Komakai, Proposal of fiber-optic radio highway networks using CDMA method, in Proc. 4 th IEEE International Conference on Universal Personal Communications, pp , [83] J.-S. Baik, and C.-H. Lee, Capacity analysis of a hybrid WDM/SCMA-PON, in Proc. 9 th Optoelectronics and Communications Conference (OECC 05), pp , [84] F. G. Stremler, Introduction to communication systems, Addison-Wesley Publishing company, [85] M. M. Mustapha, and R. F. Ormondroyd, Effect of multi-access interference on code synchronization using a sequential detector in an optical CDMA LAN, IEEE Photon. Technol. Lett., vol. 12, pp , Aug [86] J-. P. Laflamme, and L. A. Rusch, Multiple access interference suppression in an optical DS-CDMA LAN using fractionally-spaced equalization, in Proc. IEEE Canadian Conference on Electrical and Computer Engineering, pp ,

242 Chapter 5 Applications of Electronic CDMA in Passive Optical Networks

243 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks 6.1 Introduction Most of today s data networks are supported using the Internet Protocol (IP) and it is widely believed that the IP will remain as the dominant network layer technology. As the Internet continues to evolve, the data traffic carried by local area networks (LANs) is on the rise. Consequently, future access and LAN architectures should be optimised for IP transport to provide broad bandwidth at affordable cost. Packet based access networks based on high performance Ethernet and/or Multi Protocol Label Switching (MPLS) solutions are being currently investigated for deployment [1-4]. Using this kind of packet based approach allows the consolidation of application around IP and these new interfaces are more bandwidth efficient compared to the asynchronous transfer mode (ATM) cell switching techniques. MPLS can be seen as an extension of the all-ethernet packet based architecture, where it allows use of Ethernet interfaces but making use of the label-switching paradigm instead of Ethernet switching [1]. Conventional customer access networks are usually built using Ethernet over copper cables, however the demand to deliver higher bandwidth, rich mix of conventional and new services such as high speed internet access, storage area networking, local area networking and on-demand/broadcast video services such as tele-conference and tele-teaching, file transfer across the computer terminals (e.g. digital video storage and processing) has increased the inability of conventional copper based Ethernet networks to reliably offer the required peak data rates (~1 Gb/s) and the expected throughputs at an acceptable cost. With the recent standardisation of Gigabit Ethernet technologies, fibre optic links between key network routers or gateways can be deployed to realise reliable high data rate networks for a local/wide area networking applications [5-10]. Optical networking all the way to the computer desktops will soon become important in realising fibre-to-thedesktop (FTTD) capabilities combined with wavelength division multiple access (WDMA) as

244 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks a way of offering cost-effective, reliable and highly efficient network infrastructure for distributed data processing applications. One of the many issues to be considered when an optical customer access network is implemented is a signalling system within the network that coordinates the traffic among customer network terminals. It is desirable that the signalling system is implemented on the same physical network for the data paths. Many signalling protocols proposed for these kinds of networks such as carrier sense multiple access (CSMA), aloha, and token ring are inefficient due to the large propagation delay between the terminals [11-14]. Moreover, the processing time contributed to the signalling purposes at each customer terminal for data reception and transmission is required to be considerably smaller than the data packet size. Ethernet is the most widely used link layer protocol and has the potential to yield a seamless optical network across the different network boundaries, from a wide area network to a local access network, a physical layer signalling mechanism that supports the media access control (MAC) layer, which is implemented as one of the functions on an Ethernet interface card may be realised at each customer network terminal to yield high throughput at low cost. This chapter discusses some of the previously proposed physical layer signalling mechanisms and their disadvantages in Section 6.2. Section 6.3 describes the proposed signalling mechanism for a packet based access network using electronic code division multiple access (E-CDMA). Several architectures including star and ring networks with centralised and distributed control for operation that employ E-CDMA control packet signalling are discussed in Section 6.4. Section presents the receiver architectures that are used in WDM star networks incorporating E-CDMA control packet signalling scheme and WDM ring networks incorporating this signalling scheme is discussed in Section A description of the E- CDMA access control interface is summarised in Section 6.5. Section 6.6 presents a carrier sense multiple access with collision avoidance protocol using E-CDMA signalling in a WDM ring network. The simulation results for the E-CDMA control packet signalling technique are given in Section The experimental demonstration of this scheme is presented in Section Theoretical scalability analysis for the E-CDMA signalling in a WDM network is presented in Section 6.7. Section 6.8 gives results from the theoretical analysis that was performed to identify optimum power budget for successful transmission of E-CDMA control and baseband payload data of a packet

245 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks 6.2 Control packet signalling schemes As the state of the actual network infrastructure is changing its direction from a circuit switched network to a packet switched network with the capability of digital processing of signals in electronics [15], several experimental demonstrations have been performed for signalling mechanisms for packet based networks. These signalling mechanisms allow a smooth evolution path from current technologies to a multi-service access network, which provides more flexibility, data rate/format transparency and bandwidth efficiency [16, 17]. A few of the proposed techniques for control packet signalling are optical subcarrier multiplexed (SCM) signalling, dedicated wavelength channel signalling and optical code division multiplexed (OCDM) signalling. In the SCM technique, low bit rate header data is RF carrier modulated and transmitted in the same time slot as the payload data of the packet [19-24]. In the SCM signalling scheme, there is no need for synchronisation between the header and the payload of the packet as both of them are transmitted in the same wavelength channel. However, this technique adds complexity at each network terminal in the forms of active microwave components at the modulation and detection. In addition, the modulation depth of the payload data of the packet is decreased and hence a power penalty is paid. In a WDM packet network, increasing the SCM signals will lead to nonlinear effects in the RF and optical domain [25]. Moreover, the use of multiple RF subcarriers limits the performance due to dispersion [26, 27], optical beat interference (OBI) [28-30] and the scalability. Moreover, SCM signal processing requires higher bandwidth optoelectronic components. The performance of the SCM signalling technique is also reduced by the analog subcarrier detection circuits. It is also very difficult to upgrade a network that uses SCM signalling technique due to the electronic hardware complexities that are associated with. In the dedicated wavelength channel signalling [31, 32], header of a packet is carried on a dedicated wavelength channel. This scheme increases the throughput of the network since most of the wavelength channels are occupied with the payload data of the packet. However, due to the fibre dispersion, the synchronisation between the header and payload of a packet may be lost. Therefore, realignment of the payload data and header data at each network terminal is required that could make the system more complex. Moreover, the requirement of an additional wavelength channel to carry the header data of the packet in the access network is very costly. OCDM signalling [15, 16] has been demonstrated using bit level coding and

246 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks header processing. Moreover, packet routing and switching are also performed using this technique. Although 80 Gb/s photonic packet routing based on OCDM has been experimentally demonstrated, it is too difficult and costly to perform low-level functions using this technique in customer access networks. Bit serial coding is another signalling mechanism that had widely been used to carry header bits of a packet in the packet based networks. Bit serial coding can be performed at bit level, whereby each bit carries routing information and at packet level, where a string of bits are transported with the payload data. In the packet-level serial coding, header data and payload data of the packet can be transported at the same bit rate or the header data transmission rate can be lower than that of the payload data [35-37]. On the other hand, there have been demonstrations, whereby the header data and payload data transmission bit rates are same [38, 39]. A different approach whereby additional control packets at considerably low data rates can be used for signalling for routing of packets and several such proposals of WDM packet networks have been demonstrated [40, 41]. In many packet based access networks, out of band signalling techniques are more popular as they pave the way for keeping the payload of the packet in optical domain from the origin to the destination. Combining the MPLS technology, whereby the control bits of the packet are transmitted using an out of band signalling method, an optically transparent access network can be realised. In the next section, we discuss novel approaches for a signalling that uses electronic code division multiple access (E-CDMA) for a WDM packet based optical access network. 6.3 Packet signalling using electronic code division multiple access A control packet signalling technique based on E-CDMA has been developed for the WDM packet based access networks [42]. In the E-CDMA control packet signalling technique, each wavelength channel in the access network is assigned a unique electronic code allowing a particular wavelength channel to be identified anywhere in its path by its predefined code. The header of the packet is electronically multiplexed with the electronic code, which is higher in bit rate to produce the DS-SS signal. The resultant DS-SS signal, which is termed as

247 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks E-CDMA hereon, is then modulated onto a RF carrier frequency, whereby the RF carrier frequency is chosen to be outside the bandwidth of the baseband payload data of the packet. This RF carrier frequency is common to all WDM channels as can be seen from Figure 6.1. The ability to use a single RF carrier frequency makes the possibility to use one standard E- CDMA access control interface card at all customer network terminals and therefore reduces the complexity and provides seamless connections of the network nodes for faster physical layer signalling. λ 1 λ 2 H 1 H 2 λ f H f H Figure 6.1: Optical spectrum of CDMA coded header and baseband payload. By multiplexing the baseband payload data and the RF up-converted E-CDMA coded header data, it s possible to get the header and payload of a packet transmitted within the same time slot as shown in Figure 6.2. Header Payload H 1 λ 1 Header Payload H 2 λ 2 One Fibre Figure 6.2: Header and payload of the packet transmission in the same time slot. Using E-CDMA as an out of band signalling technique shows many advantages compared to the SCM signalling technique. The performance limitations that arise in SCM signalling due

248 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks to the RF mixing of the multiple RF subcarriers, RF bandwidth constraints on the photonic components, OBI, dispersion due to the double side band modulation (DSB) of RF subcarriers, scalability problems that arise due to the above mentioned constraints can vastly be reduced in this E-CDMA control packet signalling technique. Moreover, to process the E- CDMA control signals in the intermediate nodes, wavelength demultiplexing is not required. Therefore, WDM interfaces are not required, which reduces the cost of the network terminals. The bandwidth requirements for E-CDMA control packet signalling technique is not higher than that required for the SCM control packet signalling technique since one RF carrier is used to carry all E-CDMA control signals. Several digital modulation formats such as binary phase shift keying (BPSK), quaternary phase shift keying (QPSK), and complementary code keying (CCK) can be used in-conjunction with the DS-SS without increasing the spread spectrum bandwidth. E-CDMA control packet signalling technique shows better performance compared to SCM control packet signalling technique in the presence of interference. Therefore, the modulation depth of E-CDMA control signals is not required to be high as the required carrier to noise (CNR) ratio is not large. In comparison to SCM control packet signalling technique, better performance for the E-CDMA control packet signalling can be obtained. E-CDMA control signals are carried in the same wavelength channel as the payload data signals and therefore the cost savings are high compared to the dedicated wavelength channel signalling mechanism. Each network terminal is equipped with the low-cost E- CDMA access control interface card and these E-CDMA access control interface cards may be integrated with the Gigabit Ethernet optical transceivers allowing an easier upgrade path for an existing optical access network infrastructure. The requirement for large number of external electronics can be minimised using this technique and enables the use of an existing technology with reduced use of additional custom designed RF and optical interfaces. This E- CDMA control packet signalling technique is capable of being digitally implemented [43-50] to ease the hardware complexity, bandwidth constraints and reduced nonlinearity to get better CNR for error-free transmission. Moreover, it is shown that the use E-CDMA could vastly reduce the performance limitations induced by fibre dispersion [51] and OBI [52, 53]. The architectures whereby the E-CDMA control packets can also be used to facilitate a range of network signalling functions such as medium access using both in a centralised or decentralised approaches are discussed in the next section. This E-CDMA control packet signalling technique could effectively be used in any arbitrary topology (star, bus or ring) WDM network with the E-CDMA access control interface cards at each network terminal

249 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks 6.4 Optical network architectures incorporating E-CDMA control packet signalling Electronic Data Network Interface Photodetector E-CDMA Access Control Interface MAC Controller & Electronics Laser Figure 6.3: Architecture of an optical network terminal using E-CDMA control signalling. Figure 6.3 shows the architecture of a customer network terminal, whereby an E-CDMA access control interface card and electronic data network interface card (e.g. Gigabit Ethernet interface card) are interfaced with a photodetector (PD) and a laser. The ability to use the E- CDMA signalling on an E-CDMA access control interface card for the entire network overlaid on the same physical network infrastructure for the data transport using the data network interfaces such as Gigabit Ethernet interfaces makes this signalling technique very attractive to implement on an existing optical access network. To make the signalling network operational, an E-CDMA access control interface card can simply be attached to the optical interfaces of the customer network terminal for the transmission and reception of E-CDMA signalling channels. The E-CDMA control signals generated from the E-CDMA interface card are on an RF carrier whereas the Gigabit Ethernet signals are baseband signals and therefore these signals can be electronically combined and separated. The E-CDMA control signals are processed at the network terminals using the multi channel E-CDMA processor and media access (MAC) controller, which could be a built-in electronic functionality of the E-CDMA access control card. The E-CDMA control signalling channels carry information about the baseband payload data that is carried on a particular wavelength channel, the transmitting terminal and other MAC information of the network that are required for the seamless connection between terminals. In simpler terms, the E-CDMA access control interfaces provide the required intelligence for each customer network terminal to enable high capacity data transport in multiple access optical networks in a cost effective manner

250 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks Consider an optical network of arbitrary topology (star, bus, and ring) and each customer network terminal uses one wavelength channel for the Gigabit Ethernet data transport to other terminals in the network. Using unique electronic identification codes in the form of E- CDMA, each network terminal can be identified. The ability to identify the source or the destination of a particular data signal using the out-of-band E-CDMA control signals that are allocated to each terminal using a E-CDMA access control interface leads to seamless transportation of data, whereby optical paths are dynamically configured at each network terminal and thus provides higher network efficiency in data transport, whereby there is a higher probability of collisions, contentions and configuration setup delays in the optical paths or receivers are avoided. Using the E-CDMA control signalling network, transmission coordinations between network nodes can be arbitrated. If there were contentions or collisions of the data signals, the source network nodes can be easily identified a lot faster compared to conventional signalling protocols, whereby the control signals are transmitted with the data and the identification of transmission terminals become more difficult. Using the multiple access capability of the E-CDMA, and multi channel E-CDMA processing capability of the E-CDMA access control interfaces, each network node can be identified. The signalling mechanism using E-CDMA that is implemented on the E-CDMA access control interfaces whereby the E-CDMA signals can be processed faster to provide a viable solution for signalling in a WDMA network environment. The E-CDMA access control interface could be built using Field Programmable Gate Arrays (FPGA) [43-50], which can be reconfigurable according to the requirements at the customer network terminals. As the number of network terminals and the bandwidth requirement within the network increases, the signalling mechanism needs to be able to scale better to provide easier upgrade path for seamless data transport and E-CDMA can easily be scaled to larger WDM channels Passive star architectures incorporating E-CDMA control signalling Two architectural options for WDM access networks that use E-CDMA control signalling are considered. They are centralised WDM access networks and distributed (decentralised) WDM access networks. In a centralised WDM network, a central controller coordinates the traffic flow and the channel access for each terminal for transmission of channels in the network,

251 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks whereas in a decentralised or distributed WDM network, each terminal manages its own traffic flow. Computer Terminal 1 C 1, C 2, C N Computer Terminal N Computer Terminal 2 λ 1, C 1 C 1, C 2, C N λ N, C N Computer Terminal N-1.. λ 2, C 2 NxN Star coupler.. C 1, C 2, C N Computer Terminal i λ i, C i C 1, C 2, C N Computer Terminal i+1 Figure 6.4: Distributed passive optical star network architecture incorporating E-CDMA control signalling and packet transport. Figure 6.4 shows a typical distributed WDM star network where each network terminal is interconnected with another using a passive star coupler in a distributed fashion. It is assumed that each network terminal uses a fixed wavelength channel for the transmission of packets to other terminals in the network. Therefore, each terminal is equipped with a tunable wavelength receiver for the reception of data. If a network terminal requires data transport with another terminal, it sends the data on its fixed wavelength channel and the reception node can receive the data. The transmitting node also sends the information regarding the payload data that is carried on the wavelength channel along with the information in the form of E-CDMA control signal. Using this E-CDMA control signal, the receiving node identifies the contents of the received wavelength channels and directs the detected signal s path to the data interface card. This way, every single network terminal is capable of communicating with another terminal in the network. E-CDMA control signalling is required to coordinate the functions of the access controller in the receiving terminal to receive the data traffic from

252 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks a designated transmitter. In this architecture, multiple network terminals are able to receive data from a single transmitter enabling multicasting. Wideband photodetector Optical signals Access Controller Gigabit Ethernet Interface E-CDMA Access Control Interface Band pass f H Multi channel CDMA processor Decision circuit Overlapped data f H E-CDMA coded Signalling channels λ 1, λ 2 λ N f H 1, H 2.. H N Figure 6.5: Processing of the out of band E-CDMA control signals and baseband payload data signals in a network terminal of a distributed passive star network. The E-CDMA processing technique that is used in this distributed passive star architecture is shown in Figure 6.5. Using a PD with sufficient bandwidth to detect all E-CDMA control signals and baseband payload data, and a multi-channel E-CDMA processor, each network terminal is capable of receiving payload data in a distributed fashion from any other terminal in the network. After photodetection, the E-CDMA control signals are separated from the baseband payload data using an electrical bandpass filter (BPF). The multi-channel E-CDMA processor that is built within the E-CDMA access control interface card processes all E- CDMA channels from all terminals. Each E-CDMA control channel identifies the wavelength channel and the source of the signals. If the receiving terminal decides to receive the payload data, the access controller is activated so that the recovered payload data is sent to the data interface for further decoding. Multiple transmitting terminals could send data on their fixed wavelength channels to one particular network terminal simultaneously. This leads to contentions or collisions at the receiver as only one PD is used receive the signals. However

253 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks using the out of band E-CDMA control signals that are sent along with the payload data, each wavelength channel and therefore the source of the signals can be individually identified. An E-CDMA control signal can be sent to all other nodes regarding the contentions and collisions at the receiver so that the transmissions can be re-scheduled at later time. Computer Terminal 1 λ 1, C 1 Central Controller C C Computer Terminal N λ N, C N Computer Terminal 2 C 1, C 2, C N C C Computer Terminal N λ 2, C 2 NxN Star coupler C C C C Computer Terminal i Computer Terminal i+1 λ i, C i Figure 6.6: Centralised passive optical star network architecture incorporating E-CDMA control signalling and packet transport. The E-CDMA control signalling technique could also be used in a centralised WDM star network as shown in Figure 6.6, whereby a central controller performs most of the E-CDMA processing and broadcasts a control channel in the form of an E-CDMA control signal to each network terminal to choose the required payload data from the desired wavelength channel. If a network terminal requires transmission to another terminal in the network, it sends a request to the central controller regarding the access of the receiving terminal. The central controller processes all E-CDMA control signals from all network terminals. Then a control signal is sent to all other network terminals on a separate E-CDMA control signal (C C ). Each network terminal monitors this E-CDMA control signal from the central controller and allocates a time period to receive the payload data from a particular terminal. This way, contentions or collisions at the receiver are avoided at the receiver

254 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks In the centralised or distributed WDM star network, each node can be equipped with a wavelength tunable wavelength transmitter and a fixed wavelength receiver (TT-FR). In this architecture, each network node is assigned a unique E-CDMA code such that every node can be individually identified. The ability to identify each E-CDMA control signal in the presence of the other E-CDMA control signals using multi-channel E-CDMA processor has several advantages in a multiple access optical network compared to the use of in-band signalling mechanisms in a multiple access environment. Based on the out-of-band E-CDMA control signalling network, the wavelength channel access can be coordinated such that the contentions can be avoided. In the centralised network, the central controller could make the decisions based on the polling by the network terminals and could allocate required time slots in a wavelength channel. The central controller needs to monitor the multiple E-CDMA control signals from several terminals to send the request to other terminals. In a decentralised network, the reception terminal itself could make decisions on the channel use by other terminal for the transportation of data depending on its requirements. There are a few significant differences between the centralised star network and distributed star network that employ E-CDMA control signalling for transmission coordination. In the centralised network, the network terminals do not need to have multi-channel E-CDMA processors to monitor and process all E-CDMA control signals from every other terminal in the network, but requires only one E-CDMA processor to process the E-CDMA control signal from the central controller. Therefore the electronic hardware complexity can be reduced at each network terminal. In conventional centralised WDMA star network, which employs inband signalling mechanisms, multiple WDM interfaces are required to detect each wavelength channel separately and carry out the signal processing functionalities. However, using E- CDMA implementations over E-CDMA access control interfaces, there is no need for several WDM interfaces as a single PD is adequate to detect all E-CDMA control signals and process them using a multi channel E-CDMA processor. In the centralised WDM star network, after the E-CDMA signal processing, one E-CDMA control signal is broadcast to all network terminals carrying the required information about the wavelength accessibility. Therefore, the RF bandwidth requirement of the PD used in each network terminal for the detection of the payload data signals and E-CDMA signal from the central controller can be smaller in the centralised star network. In the distributed star network, the bandwidth of the PD needs to be large enough to detect all E-CDMA signals and payload data from other network terminals

255 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks WDM ring architectures incorporating E-CDMA signalling Wideband photodetector Gigabit Ethernet Interface Band pass f H Multi channel CDMA processor Decision circuit Wavelength Tunable transmitter Figure 6.7: Receiver structure of a network terminal with tunable transmitter-fixed receiver architecture in a distributed WDM ring network. Consider a WDM ring architecture whereby each terminal in the network is equipped with TT-FR as shown in Figure 6.7. Each terminal is assigned a unique E-CDMA code and therefore the transmitting terminal can be identified by the particular E-CDMA code. As each node in the ring network receives data from other terminals on a fixed wavelength channel, the E-CDMA control signal from each terminal can be used to coordinate the traffic flow in each wavelength channel. The packet availability in each wavelength channel can be easily identified using the out-of- band E-CDMA control signal, whereby the payload data remains in the optical domain from the source to the destination. As the terminals are in a ring network and each terminal is capable of monitoring each wavelength channel in the network using the E-CDMA control signals and therefore collisions of packets are avoided. In the similar architecture with the centralised network operation, each node could notify to a particular network terminal or multiple terminals using an E-CDMA control signal associated with the particular node about its intention to transmit data. The central controller could then process the E-CDMA control signals and allocate time slots on a particular wavelength channel to the required network terminal. Again, the central controller does not require multiple WDM interfaces to monitor and process all E-CDMA control signals from all wavelength channels in the network terminals. At the network terminals, the E-CDMA control signal is monitored and timeslots on the reception wavelength channel can be allocated to other terminals for data transfer. This feature of the E-CDMA control packet signalling can be used to develop a MAC protocol such as CSMA/CA protocol for the transmission of packets in a wavelength

256 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks channel within a ring network. This feature will be explored in detail in section 6.6 of this chapter. 6.5 E-CDMA access control interface The E-CDMA access control interface card provides the intelligence to the network terminal by processing the E-CDMA control signals to setup a network path between two network terminals such that data transmissions are carried out. To enable this, seamless communication between the E-CDMA access control interfaces over the fibre needs to be constructed. We address a few issues that are to be considered in an E-CDMA access control interface card to enable successful E-CDMA control signalling. In optical transmission between network terminals, the propagation delay is not the largest of the end-to-end delay. The algorithms that are chosen for the E-CDMA processing need to be faster such that the processing delay does not dominate the end-to-end delay in a packet transport. The algorithms that use serial search for the E-CDMA control signal decoding are generally slower and take a large amount of time (in ms) to decode the E-CDMA information. This duration can be extremely large in optical transport of high bandwidth data. Therefore an appropriate algorithm that provides parallel search for decoding E-CDMA information needs to be chosen for the implementation in the E-CDMA access control interfaces. In the E-CDMA access control interface card, if the parallel search mechanism is implemented, then the required signal processing resources in the processor are high. However, high speed FPGAs with high performance signal processing blocks are commercially available and therefore they could be used at the E-CDMA access control interface. The MAC processor within the E-CDMA access control hardware performs multiple operations such as adaptive power monitoring and control as well as sending acknowledgement messages to the transmitter on the receipt of a correct packet. Moreover, as the transmission of E-CDMA control packets are based on asynchronous transmission, multiple access interference (MAI) must be controlled. As the E- CDMA control packets are bursty, MAI would not be as severe as in cases where continuous E-CDMA control signal transmissions are performed. However, MAI that exists could be significant in the presence of large number of E-CDMA control packets with high power. Adjustments in the MAC controller are required to achieve higher transmission efficiency. For example, power control of the E-CDMA control signals in the optical path is an important factor. The E-CDMA control signal that has got the higher power induces significant MAI to

257 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks the E-CDMA control signal with the lower power [49, 55-57]. Therefore power control of E- CDMA control signals are monitored and dynamically adjusted to keep the MAI low at each network terminal especially in the WDM ring networks. Moreover, the E-CDMA control packets are required to support several MAC functionalities of the WDM packet-based access network and the digital processing architectures at the transmitter and receiver should be capable of performing these functions appropriately. A brief summary of the E-CDMA control packet encoding and decoding capabilities is given below. At the transmitter, baseband E-CDMA control signal generation is initially carried out. To modulate the baseband E-CDMA control signals onto the chosen RF carrier, a frequency synthesiser in phase locked loop (PLL) block to offer high precision RF carrier frequency control is used [45, 49]. The receiver may consist of analog and digital circuits. The analog circuits are used for code tracking and the digital circuits are used for code acquisition and baseband E-CDMA decoding. SCM Control signal Frequency Synthesizer VCO Clock IF signal mixer A/D Converter Digital IF signal Figure 6.8: Analog circuit to perform coarse synchronisation. Figure 6.8 shows a typical analog circuit that is used in the receiver for the coarse synchronisation of the E-CDMA control signals for successful E-CDMA decoding [49]. As the E-CDMA control signals may have been generated from several terminals in the network, there will be frequency offset in the RF carriers used. To down convert the E-CDMA control packets from the RF carrier frequency to the baseband or intermediate frequencies, a coarse synchronisation is performed. This process is termed as code tracking, whereby using a feedback loop containing a voltage controlled oscillator (VCO), the RF carrier frequency at the local node is continuously changed. An analog to digital (A/D) converter is used to identify the frequency offsets of the RF carrier frequencies and the E-CDMA control signals

258 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks are down converted to intermediate frequency (IF) or baseband frequencies for further fine tuning. PRBS Generator Shift register Tap select Correlator Correlator V th Decision device Data Reassembler V th Figure 6.9: Code acquisition process for finer synchronisation for the E-CDMA decoding. Figure 6.9 shows the code acquisition process, whereby finer synchronisation process is carried for the E-CDMA decoding at the receiver at each network terminal [48]. The random spreading sequences are generated using the pseudo random binary sequence (PRBS) generator. These sequences are stored at the shift register. A tap select function is used to select a particular spreading sequence with a number of different delays. Most generally, the sampling rate at which the processing is carried out is twice the chip rate of the spreading sequences [45, 48]. Therefore, each tap may have a delay that is equivalent to half a chip. The incoming E-CDMA control signals are fed to a bank of correlators with each operating with half a chip delay. Using a threshold voltage, the timing of E-CDMA control signal is performed and the decision device is used to identify the correlator with the highest correlation peak and lock the code acquisition process at that particular correlator. Finally, using a data re-assembler, the E-CDMA coded header data information is recovered. As mentioned earlier, the received E-CDMA control signal strength impacts the performance of the recovered header data as the MAI varies with the received power of each E-CDMA control packet of each wavelength channel. Therefore, a proper power control mechanism is adopted at the receiver before the E-CDMA is performed. At the receivers, a received signal strength indicator (RSSI) is used to estimate the received power of each E-CDMA control packet and then can be submitted to MAC functionalities for power controlling [49]. It should

259 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks be noted all the processing functionalities described above for the E-CDMA control interface can be performed using FPGAs and therefore making the entire electronic processing functionalities built into a single processing unit. 6.6 CSMA/CA protocol for WDM packet ring networks using E-CDMA signalling In a multiple access WDM packet ring network, bandwidth in each wavelength channel is shared by many different nodes in the ring. In a resilient packet ring (RPR) network, packets in each wavelength channel go through an O/E/O process at each intermediate node before they arrive at the destination nodes [58, 59]. However in this architecture, the performance of the network can be degraded if the packets undergo many O/E/O conversions at the nodes. The increasing bit rate of the payload of the packet could also be limited by the electronic processing speed at the nodes. Therefore, by making an all-optical connectivity between origin and destination nodes of packets, better performance can be achieved. In the multiple access WDM ring networks, where all-optical connectivity between origin and destination is provided, there could be transmission collisions or receiver contentions of packets leading to poor efficiency of the network. Therefore, there is a need for a MAC protocol for the ring network to prevent packet collisions and arbitrate the bandwidth access among the nodes. Using a statically assigned reservation protocol, optical packet contentions can be avoided. However in access networks, the fluctuations in network traffic are quite large and much more dynamic protocol responsive to network traffic changes may be more suitable. An example of this approach would be carrier-sense multiple access with collision avoidance (CSMA/CA) protocol, whereby wavelength channels in the WDM packet ring network are monitored and packet transmission activity in each wavelength channel is identified to insert a packet into the ring network. Baseband carrier sensing techniques [60-64] and RF carrier sensing techniques [20, 65-68], dedicated wavelength channel signalling [31, 32] have been proposed and experimentally demonstrated to verify the concept of CSMA/CA protocol for a WDM packet ring network. E-CDMA control packet signalling can also be used to monitor the packet transmission activity across wavelength channels and thus leading to CSMA/CA protocol for a WDM packet ring network [69]. The architecture of the

260 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks network terminal in the WDM ring network incorporating E-CDMA control packet signalling mechanism is shown in Figure By electronically processing the E-CDMA control of each packet from all wavelength channels, the need for a demultiplexer to monitor each wavelength channel separately is eliminated. Tap λ drop Fibre delay Add CDMA Header Payload λ drop Packet Receiver FBG Photo detector Band pass f H Multi channel CDMA processor Decision circuit Wavelength tunable transmitter f H Overlapped payload data λ 1, λ 2 λ N f CDMA coded header H 1, H 2.. H N Figure 6.10: The architecture of a node in the ring network incorporating E-CDMA signal processing. The network terminal is based on TT-FR architecture, whereby each terminal is associated with a fixed drop wavelength channel on which it can receive packets from other terminals in the network. The tunable transmitter equipped at each node is used to send packets to other nodes in the ring network. The channel availability for packet insertion at each node can simply be found by monitoring the E-CDMA control packet on each wavelength channel as both E-CDMA coded header and payload of each packet are sent within the same timeslot. In this E-CDMA signalling technique, header can be encoded with information such as packet length. In that case, it is not essential that the E-CDMA coded header and payload data should be within the same time slot. In any case, it is vital that the starting bits of both payload and E-CDMA coder header of the packet are aligned. However, by using much higher bandwidth E-CDMA spreading code and header data, it is possible to keep the E-CDMA coded header size smaller (in time), while the information from the header data can be used to find out the packet availability duration. To achieve the collision-free packet insertion, a small portion of optical power is tapped off and all the wavelength channels are detected using a wide-band

261 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks PD. The inset of Figure 6.10 shows the RF spectrum of the detected signal containing the baseband data from all wavelength channels and the multiple E-CDMA header channels at f H. For monitoring packet activity across wavelength channels, overlapping payload channels are ignored. However, each E-CDMA coded header at f H can be recovered and identified appropriately using a multi-channel E-CDMA processor, which has knowledge of the unique electronic codes assigned for each wavelength channel and the wavelength availability can be found and a packet can be inserted into the network Simulation The proposed control packet signalling technique using E-CDMA is verified through a simulation process using VPI Transmission-Maker software. The generation of packet including the E-CDMA control signal can be done using logic gates. Implementation of circuits using digital logic gates exhibit much more linearity compared to the analog devices and they would be able to operate at high frequencies with good performance [43, 44]. The digital modulations (BPSK & ASK) are performed using logic gates AND and XNOR to generate and decode E-CDMA control packets at the transmitter and the receiver respectively Mb/s Label 2 Gb/s subcarrier XNOR BPSK signal 1 Gb/s Payload AND Recovered Payload 1 GHz LPF Recovered Label 62.5 MHz LPF 2 GHz XNOR 2 GHz Oscillator Figure 6.11: Simulation setup using digital logic gates

262 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks Figure 6.11 shows the simulation setup in VPI Transmission-Maker software for the generation and recovery of signals for achieving E-CDMA as a signalling technique. As this simulation is to study and understand the performance of the E-CDMA control signals with digital processing capabilities, optical fibre transmission was not included in the simulation. The header data was spread with a 62.5 Mb/s electronic code using a XNOR gate and the resultant E-CDMA control signal was then modulated onto a digital RF carrier frequency at 2 GHz using another XNOR gate to generate a BPSK signal. The BPSK signal was amplitude modulated with 1 Gb/s payload signal using an AND gate to generate a composite signal that signifies a packet at the transmitter. E-CDMA coded header data 1 Gb/s Baseband Payload Figure 6.12: Observed RF spectrum of the composite signals at the transmitter. Figure 6.12 shows the RF spectrum of the composite signals, consisting of 1 Gb/s baseband payload data and the E-CDMA control signal modulated on an 2 GHz digital RF carrier. At the receiver, using a low pass filter (LPF) at 1 GHz, the baseband payload is recovered. For the recovery of the E-CDMA coded header, the composite signal is passed through a BPF centred at 2 GHz and down converted to baseband frequencies using another XNOR gate and then de-spread using the same electronic spreading code and the header data was recovered

263 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks Glitches (a) (b) Figure 6.13: (a) Eye diagram of the recovered E-CDMA coded header, (b) Eye diagram of the recovered baseband payload data. The eye diagram of the recovered E-CDMA coded header is shown in Figure 6.13 (a) and the eye diagram of the recovered payload at 1 Gb/s is shown in Figure 6.13 (b). Clear eye opening was obtained for the recovered E-CDMA coded header data. The observed timing jitter in the eye for the recovered header data can be removed by retiming the signal. The eye diagram of recovered baseband payload data shows some glitches that are undesirable. It is understood that during the simultaneous transition of 0 to 1 or 1 to 0 bits of both digital RF subcarrier at 2 GHz and the E-CDMA control signal at a bit-period creates the glitches in the low pass filtered payload data. Therefore, digital processing functionalites were used to eliminate the glitches that arise due to the filtering of the baseband payload data. Figure 6.14 shows the simulation setup to eliminate the glitches that occur during the lowpass filtering of the baseband payload data. Using a clock recovery circuit, the clock of the low pass filtered payload can be recovered. Using the recovered clock and the low pass filtered baseband data the signal can be retimed and regenerated using a D-flip flop. Using the D-flip flop and the clock recovery circuit, the signal can be recovered that does not show any glitches or time-jitter problems

264 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks D-Flip Flop Clock Recovery Figure 6.14: Simulation setup for payload recovery that eliminates glitches in the low pass filtered baseband payload data. Low-pass Filtered Payload Recovered Payload Recovered Clock Figure 6.15: Eye diagram of recovered payload using D-flip flop eliminating the glitches after low pass filter

265 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks Figure 6.15 shows the eye diagrams of the baseband payload data that has glitches and jitter (1 st eye) and the eye of the regenerated payload data. The eye of the regenerated payload data is very clear (2 nd eye) and does not show timing jitters as well as the glitches after it has been retimed with the flip flop. If the E-CDMA control packets and the baseband payload data are digitally generated and recovered, then appropriate clock and data recovery (CDR) circuits have to be employed to recover the signals error-free Experimental demonstration Transmitter 2 λ 2 ( nm), C 2 Transmitter 1 λ 1 ( nm), C GHz 160 Mb/s Code (C 1 ) A RF Oscillator C λ 1 10 km SMF Optical Filter 3 db Coupler MZM 2.5 Gb/s Photo detector 1.25 GHz LPF Recovered payload 2.5 GHz 10 Mb/s Header (H 1 ) B Transmitter 1.25 Gb/s Payload Receiver RF carrier Recovery & CDMA decoding Recovered header Figure 6.16: Experimental setup to demonstrate the feasibility of the signalling scheme using E- CDMA. The feasibility of the E-CDMA signalling technique is verified via experiments with a setup shown in Figure A 10 Mb/s, PRBS non-return-to-zero (NRZ) header data was multiplexed with a 160 Mb/s bipolar Walsh code (C 1 ) using a RF mixer, resulting in a E- CDMA control signal with a processing gain of 16. In practical implementations, asynchronous Gold codes can be used for multiplexing the header data [70, 71]. Figure 6.17 shows the oscilloscope traces of the bipolar Walsh code (A), header (B) and E-CDMA signal (C) respectively. Phase changes in the E-CDMA control signal was observed when the phase of the header data was changed. Then the E-CDMA control signal was modulated on to an RF carrier at 2.5 GHz to generate a BPSK signal. RF amplifiers were used to compensate for the conversion losses in the RF mixers. Thereafter, the BPSK signal was electrically combined

266 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks with a 1.25 Gb/s, PRBS NRZ baseband payload data using a RF combiner. Figure 6.18 shows the observed RF spectrum of the composite signals consisting of 1.25 Gb/s baseband data and the 10 Mb/s E-CDMA coded header at the transmitter. A B C Phase Changes Figure 6.17: The observed the oscilloscope traces of the Walsh code (A), header (B) and E-CDMA signal (C) Gb/s baseband payload data 10 Mb/s header Figure 6.18: Observed RF spectrum at the transmitter showing the 1.25 Gb/s baseband payload data and 10 Mb/s E-CDMA coded header on 2.5 GHz RF carrier. Thereafter, the composite signal was then modulated onto a wavelength channel at nm (λ 1 ) using a Mach-Zehnder modulator (MZM). Similarly, using another bipolar Walsh

267 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks code (C 2 ), a second composite signal was generated and modulated onto another wavelength channel at nm (λ 2 ). Even though baseband payload data for each wavelength channel was generated from a single pulse pattern generator (PPG), delays in each arm were varied to ensure asynchronous transmission of the baseband signals. These two wavelength channels were multiplexed via a 3 db coupler and transmitted through 10 km of standard single mode fibre (SSMF). The RF-upconverted E-CDMA control signal power of each channel was approximately equalised to minimise the interference from the undesired E-CDMA control signal (from λ 2 ) as the MAI is power dependant. The first goal of the experiment was to detect header H 1 (of λ 1 ) in the presence of the interfering E-CDMA channel of λ 2. For this demonstration, the optical filter was not placed in the signal path and λ 1 and λ 2 were detected using a 2.5 Gb/s optical receiver. The baseband payload data at 1.25 Gb/s could not be recovered as it was overlapped with that of from λ 2. Figure 6.19: Observed RF spectrum at the receiver showing 1.25 Gb/s overlapped baseband payload data and 10 Mb/s E-CDMA coded header on 2.5 GHz RF carrier. Figure 6.19 shows the observed RF spectrum of the detected signals at the receiver from wavelength channels. It shows that the 1.25 Gb/s baseband payload data from both wavelength channels are overlapped as asynchronous transmission of baseband payload data was performed

268 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks Figure 6.20: Observed RF spectra of the E-CDMA signals at the receiver. The detected E-CDMA signals were separated from the overlapped payload data using a BPF at 2.5 GHz with a bandwidth of 300 MHz. Figure 6.20 shows the observed RF spectra of the E-CDMA control signals from both wavelength channels. Using a RF phase shifter, the RF upconverted E-CDMA control signals were down-converted to baseband E-CDMA control signals. For the decoding of header (H 1 ), appropriate electrical delay was employed to synchronise the local bipolar Walsh code (C 1 ) with the incoming E-CDMA control signals and H 1 was recovered. The second goal was to drop λ 1 using a fibre Bragg grating (FBG) and recover both header data (H 1 ) and baseband payload data of λ 1. In this case, λ 1 was considered as the reception wavelength channel of this particular node. The 1.25 Gb/s baseband payload data was recovered using a 1.25 GHz LPF and header H 1 was recovered using the same method mentioned previously, but in the absence of the interfering E-CDMA control signal from λ BER Results Figure 6.21 illustrates the measured bit error rate (BER) curves for 10 Mb/s header data (H 1 ) and 1.25 Gb/s payload data showing the transmission and interference penalties. A penalty of 0.25 db was measured for the payload data on λ 1 for the 10 km SMF transmission compared to back to back (B-B) measurements. For the header data, the transmission penalty was negligible. However, a penalty of 0.5 db was observed in the presence of undesired E-CDMA control signal. Since the periodic cross correlation at zero shift of bipolar Walsh codes are equal to zero, the observed penalty is possibly a result of non-perfect synchronisation and

269 Chapter 6 Signalling Mechanism using E-CDMA for Packet-Based Access Networks therefore caused by MAI from the undesired E-CDMA control signal from λ 2 [72, 73]. By increasing the spreading code length, the interference penalty can be reduced in asynchronous conditions [74] Log 10 (BER) Baseband data, λ 1 Back to Back 10 km SMF Header, H 1 λ 1 Back to back 10-9 λ 1 10 km SMF λ 1 & λ 2 Back to back λ 1 & λ 2 10 km SMF Received Optical Power (dbm) Figure 6.21: Measured BER characteristics for 10 Mb/s E-CDMA coded header data and 1.25 Gb/s baseband payload data recovered from λ 1. a b c Figure 6.22: Eye diagrams of the recovered header data (H 1 ). The measurements correspond to a : electrical back to back, b : 10 km single channel (λ 1 ) transmission and c : 10 km double channel (λ 1 and λ 2 ) transmission