Analysis of Next Generation PON Architecture for Optical Broadband Access Networks

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1 Abstract: Analysis of Next Generation PON Architecture for Optical Broadband Access Networks Manish Choudhary and Bipin Kumar Centre For Development of Telematics, Delhi Main Mandi Road, Chatterpur, Mehrauli, New Delhi , India In this paper we have proposed a next generation hybrid WDM/TDM based PON architecture. We have analyzed the role of WDM in access networks, the challenges and strategies to enable smooth and economical migration from current TDM-PONs to future WDM-PONs. The proposed architecture is based on collector ring and distributed star networks that can support both existing TDM- PONs and new WDM-PONs. This architecture guarantees backward compatibility for users on existing TDM-PONs with only minimal upgrades on feeder network while providing higher bandwidth to users on new WDM-PON. In this paper we have reviewed the access network bandwidth upgradation scenario and scalability analysis in term of number of users and link reach. Keywords: Passive optical networks (PON), Media access control (MAC), Optical line terminal (OLT), Asynchronous transfer mode (ATM), Generic framing procedure (GFP), Optical network unit (ONU). I. Introduction Traditional optical access solutions are mostly based on point to point architectures. These solutions are expensive and unreliable, as it consists of many E/O and O/E components prone to failure, leading to high setup time and maintenance cost [1]. To address these issues and expedite the introduction of FTTX, TDM- PON is being developed including BPON, EPON and GPON. These solutions are based on similar passive tree topologies but uses different transport technologies (like ATM, Gigabit Ethernet) and Encapsulation techniques (like GFP, AAL3/5). A. The Bottleneck of current TDM-PON based architecture The current TDM-PON are bandwidth limited, when user demand outgrows the existing network capacity, then upgrading the bandwidth of existing TDM-PON will be a challenge, as its MAC protocol is highly complicated. Also the TDM-PON have only one wavelength for downstream (two wavelengths in case of video overlay) and one for upstream data, thus limiting the average bandwidth per user to few tens of Mbps [2]. In this way the available bandwidth of a single fiber is mostly wasted, moreover the tree topology of current TDM-PON lacks the features of Protection and Restoration. In the current scenario for TDM-PONs, if more users are to be added, an entirely new PON with the capability of serving up to 32 users has to be deployed on the field, and new set of transceivers at OLT side is added inside central office (CO) to serve a few additional users. This implies overprovisioning and complicated cabling, so it may not be an economical solution and more flexible design is required to solve this problem. B. Alternative architectures Alternative architectures to overcome the above limitations of TDM-PON is WDM based PON or WDM/TDM based hybrid PON. The cost of WDM components is rapidly coming down and demand for bandwidth intensive applications continues to grow. So in near future, upgradation of current TDM-PON to WDM-PON will be unavoidable. The WDM technology provides high degree of reconfigurability and scalability, thus enabling efficient sharing of resources. Thus the WDM-PON, besides eliminating the need for time multiplexing and ranging protocols also provides virtual point to point links with data transparency 1

2 and security [2] [3]. This NG-PON architecture uses several mechanisms to minimize the total system cost: 1. ONUs employ uncooled components. 2. Array waveguide grating (AWGs) are used to double the number of ONUs supported by Fibersplitter based network. 3. OLT uses tunable lasers and receivers to decrease total transceiver count and generate optical carriers on to which ONUs can modulate their up stream traffic. Rest of the paper is organized as follows: Section II describes the proposed NG-PON architecture; section III describes the functionality of NG-PON architecture and its various subsystems; Section IV discusses the optical access network bandwidth upgradation scenario, keeping track of backward compatibility; Section V describes the scalability of this architecture with respect to link reach followed by Results and Conclusion in section VI and V.III. II. Architecture of NG-PON The overall architecture of NG-PON, including TDM-PON and WDM-PON as its subsystems is shown in the Fig.1 [3]. The basic topology consists of single fiber collector ring with passive stars (PON) attached to it. The collector ring strings up remote nodes (RNs), which are the centers of the stars. The ONUs attached to the RN on west side of the ring talk and listen to the transceiver on the west side of the OLT. In the same way ONUs attached to the east side of RN talked and listens to the east side of OLT. At the logical level, there is a point to point connection between each RN and OLT. No wavelength is reused on the collector ring. When there is a fiber cut, all affected RNs will sense the signal loss and flip their orientation. Fig. 1. Architecture of NG-PON (Hybrid WDM/TDM PON) III. Functionality of NG-PON A RN has either a passive power splitter (Coupler) or an AWG inside [5]. If a RN contains a passive splitter, one dedicated wavelength on DWDM grid is used to broadcast the downstream data for the ONUs attached to the RN. Correspondingly, the ONUs have transmitters that consists of Fabry-Perot (FP) lasers send upstream data on the CWDM grids. On the other hand if RN possess an AWG, each ONUs has its own dedicated wavelength on DWDM grid to communicate with OLT as shown in the Fig. 2. Since the insertion loss of AWG is roughly 6dB regardless of the number of ports, So an AWG with more than eight ports can be used to enjoy better power budget than with a passive splitter. Each RN generally links 16 to 2

3 64 ONUs. The downstream traffic and upstream traffic belonging to the same ONU may use same wavelength, but in different directions of the same fiber. ONU OLT RN Fig. 2. Functional architecture of NG-PON with AWG One of the benefits of having both CWDM, TDM-based stars and DWDM, WDM based stars is traffic balancing. The WDM stars tend to serve corporations, while TDM stars tend to serve residential areas. Tunable components are employed to reduce transceiver counts in OLT. Tunable lasers at OLT generate both downstream frames and Continuous wave (CW) burst to be modulated by ONUs [5] [6]. No extra set of laser sources are required at ONUs. This configuration results in half duplex communication between each ONU and the OLT. Compared with traditional architecture that has two-fiber ring, two sets of light sources, and two sets of mux/demux devices to perform full duplex operation, this architecture dramatically lowers deployment cost. A. Remote network unit (RN) The basic structure of RN is shown in the Fig.3. All except two output port of an NXN AWG are connected to the distribution fibers that link to the ONUs associated with this RN. A band splitter in RN is three port device made of thin film filter that performs add/drop functionality [7]. The first band splitter on the left side of AWG perform add/drop for ONU on the right and second band splitter on left hand side of AWG perform add/drop for ONU on its left. Note that the groups of wavelengths being added/dropped by two band splitters in RN must be different. Due to this each ONU has its own dedicated wavelength for both upstream and downstream transmissions on DWDM grid to communicate with OLT. Fig. 3. Remote Node with AWG 3

4 B. Optical network Unit (ONU) Since there are optical filters and AWGs on the light path, a relatively stringent specification of the stability of the upstream wavelength is equired. The most straightforward way of implementing the ONU transmitter is using stabilized laser source. However, stabilizing the laser source requires the bulky optics and power consuming electronics and hence considerable impact on cost of network deployment. The other approach is to employ a tunable laser at ONU but this would need a wavelength monitoring functionality in the OLT, and tunable lasers are quite costly. The other alternative approach is to use semiconductor amplifier (SOA) as a modulator at ONU, as shown in the Fig.4. Fig. 4. Block diagram of the NG-PON ONU, with optical carrier provided Externally by OLT With this signal can also be amplified to increase transmission distance. So the NG-PON ONU has no local optical source and uses an SOA to modulate optical CW bursts received from an OLT for its upstream transmission. ONU also does not need to have a tunable receiver. The AWG in RN allows only specific wavelength to reach NG-PON ONU, receiver just need to have enough bandwidth to receive any DWDM channel used in the network. C. Optical Line Termination (OLT) OLT block diagram is shown in the Fig.5. The Tunable components, such as fast tunable lasers and tunable filters, are employed for DWDM channels. UP Stream Fig. 5. Block diagram of OLT for NG-PON The average load of the network is usually lower than the peak load, so we can expect statistical multiplexing gain by sharing tunable components at OLT, which reduces the total system cost by minimizing the transceiver count for a given number of ONUs. It also allows multiple ISP s to coexist on 4

5 the same network. Downstream optical signals from tunable transmitters in DWDM channels enters port 1 of the circulator and passes through port 2 of the circulator and WDM coupler, to enters into the optical access network. The WDM coupler separates the upstream CWDM channels from DWDM channels. Upstream traffic in DWDM channels from the ring passes through the same device, but in reverse order, and are separated from downstream traffic by circulator. Note that for downstream traffic, tunable transmitters at the OLT are used and CW optical burst to be modulated by the ONU for their upstream frames. With this configuration, only half-duplex communications are possible at the physical layer between OLT and each ONU. IV. Optical Access Network Bandwidth Upgradation Fig 6. Shows the bandwidth-upgrading scenario of optical access network. Fig. 6(a) shows the existing PON connected from the same CO. Each PON has its own cabling and OLT is inside the CO. Fig 6(b), shows the first migration step of existing TDM-PON network infrastructure. The passive couplers of the PONs are replaced with RNs that consists of passive couplers and thin film add/drop filters. The feeder fibers of PON are replaced with single fiber that string the RN served by this CO. Note that distribution fibers are untouched during this upgradation. From the ONU point of view, the functionality of the optical access network is exactly the same; only a short downtime for upgrade is needed. Therefore, existing ONUs can virtually work the same as before without major upgrade. Fig6(c)&(d), illustrates the second phase of upgradation. As more users demand high bandwidth for future broadband applications the choice is to insert RNs with AWGs. In this case, there is a dedicated DWDM channel between each ONU and the CO. So, In general the hybrid PON architecture smoothly upgrades optical access networks from pure TDM-PON to WDM/TDM based NG-PON in an economical manner. Fig. 6. Optical access Network bandwidth upgradation scenario from existing TDM- PON to NG-PON (WDM/TDM based Hybrid PON). 5

6 V. Scalability analysis This section analyzes the scalability of NG-PON network in terms of the number of users and link reach [8]. There are two main constraints, the link s power budget and available wavelengths. The number of user are upper bounded by the Components characteristics, such as maximal power transmission, receiver sensitivity and insertion losses. Let us consider the typical values according to common specifications as per table. I. Table I : Typical specification of optical components Symbol Description Values Unit P T Transmission Power +5 dbm P TFP Output Power of FP Laser -5 dbm Rsen Receiver sensitivity -35 dbm I LAD Insertion Loss: Thin film add/drop 1 db IL F Propagation loss: Fiber 0.3 db/km IL AWG Insertion Loss of AWG 6 db IL SA Loss: Splicing and aging on the link 2 db G SOA Gain of SOA db S Passive splitter splitting ratio 4-32 db NF Noise Figure of SOA 7 db First, assume that network supports only DWDM users, which means every RN has an AWG. We can assume that each RN can support upto 32 ONUs. Let W t be the total number of ONUs. Then, the number of RNs on the ring to support W t ONUs, X, is given by: - W t 32 X = (1) Let L be the maximal fiber length between OLT and any ONU. The power splitting ratio between receiver branch and modulation branch inside the ONU is 6 to 1.3dB [9]. The down stream traffic power budget must satisfy the following condition inorder for signal to be successfully received: P T IL AD. X IL. L IL 6 IL R Similarly the power budget for the upstream traffic [10] can be estimated as: - G SOA AD W t F AWG Note that for DWDM users, the upstream traffic has a tighter bond of power budget than that of the downstream traffic. In the same way, we can derive the bound for the number of users and link reach assuming the network only serves TDM-PON, In this scenario, The ONUs are having low power FP Laser, here we derive link reach considering to the upstream power budget: - AWG L 26 ( db ) 32 + IL. X IL 6 IL SA R sen SA Sen (2) (3) P TFP log 2 ( S ). 3 IL. X IL. L IL W t S + AD F SA 0. 3 L 28 3 log ( )( ) 2 S db R sen (4) 6

7 VI. Results and Analysis Maximum Fiber Length (Km) WDM PON, SOA Gain=25dB WDM PON, SOA Gain=10dB TDM PON, Split ratio=32 10 TDM PON, Split ratio= Number Of ONUs Fig. 7. Link reach and number of ONUs bound given the gain of SOA and the splitting Ratio of the optical splitters as parameters Simulation results in Fig. 7., Shows the bound for different values of G soa and Split ratio. Although in practical scenario, the network serves TDM and WDM user, Eq. (2) and Eq. (4) can be effective guidelines for network design and scalability analysis for NG-PON. From the above analysis we can also conclude that, for smaller number ONUs, TDM-PON are better than WDM-PON, but for large number of ONUs, WDM-PON are better in term of power budget. This analysis assumes that Rayleigh backscattering is not a limiting factor for the link power budget and also considers the worst case network scenario where the network experience fiber brake and all ONUs are served by the transceiver on one side of OLT. V.III Conclusion In this paper we have proposed the NG-PON architecture based on the topology of a collector ring and distribution stars. In this architecture state of art optical components are used in an efficient way for economical deployment of the optical access networks. For example the use of tunable components not only decrease total transceiver counts but also provide gradual upgrade to higher bandwidth by simple adding more transceiver as user demand increases. For scalability our analysis shows the number of DWDM ONUs is limited by available wavelength, while number of CWDM ONUs is limited by the power budget. The work reported in this paper is an attempt to bridge the TDM based PON with the WDM based PON, to form NG-PON architecture, assuring the seamless coexistence of old and new ONU on the same network. 7

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