All-Optical Switches The Evolution of Optical Functionality Roy Appelman, Zeev Zalevsky, Jacob Vertman, Jim Goede, Civcom Introduction Over the last few years, significant changes in optical networking have occurred. The ever-increasing demand for data services has been the driving force behind the wide deployment of new technologies to increase network capacity. Traditional optical networks were designed for efficient bandwidth utilization for voice applications, while not incorporating true optical layer functionality. Today, service providers are seeking to deploy new, intelligent optical networks based on a new generation of optical components in order to increase capacity, while decreasing operational costs and increasing revenues. These components are characterized by new optical functionality, as well as multi-function integration. One such component is the all-optical switch. The switch has evolved from a simple opto-mechanical device to an integrated component operating at higher speeds and incorporating new functionality. The use of optical switches for wavelength routing and optical cross connect applications has been explored extensively. However, the addition of new capabilities, such as dynamic variable optical attenuation (VOA) and optical multicast, into the optical switch component, together with the increase in operation speed, offers significant advantages and enables a variety of new applications. This white paper explores the different applications enabled using this breed of components. Table-1 illustrates a comparison between next generation optical switches and traditional devices. Next generation components are characterized by better performance (see Fig.-1) and additional optical functionality. This improved performance enables a plethora of new telecommunication applications (see Fig.-2). Applications range from the introduction of new networking technologies to new novel system implementations. This paper explores some of the new applications that stand to benefit from these new, next-generation optical components. Characteristic Traditional Optical Switches Next Generation Optical Switches Switching Speed >1ms <1µsec Multicast Not available Dynamic power partition between ports Integrated VOA Not available High dynamic range VOA functionality Reliability Based on mechanical devices (~10 Million cycles) Based on opto-electronic elements (~10 Billion cycles) Insertion loss Low Low Cross talk High 1 Low Scalability (in port count) Low Medium - High 1 While operating at sub ms switching speeds. Table-1 A comparison between traditional and next generation optical switches Figure-1 Typical switching performance of a next generation all-optical switch 1 Figure-2 Switching requirements from telecommunication applications
Optical Protection Schemes The establishment of SONET/SDH as the dominant transport technology has introduced some de-facto standards in network protection schemes. The emerging practice has tended towards the use of dual homing (or no protection) in the network access, self healing SONET/SDH rings in the MAN, and 1:1 or 1:N (primarily) linear protection in the longhaul [1]. The evolution towards intelligent optical networks has lead to the need for new, all-optical protection schemes to address a wide variety of network architectures, such as rings, linked rings, hybrid mesh-ring and mesh architectures. The integration of switching, multicast, and attenuation control is extremely useful when designing such protection schemes. For example, some of the most well-known ring protection schemes are Unidirectional Path Switched Ring (UPSR) and 2- Bi-directional Line Switched Ring (BLSR/2). Recently these schemes have been mapped into the optical domain [2] to create optical channel dedicated path protection rings (OCh-DPRING - based on O-UPSR) and optical multiplexed section shared protection rings (OMS- SPRING - based on O-BLSR) optical protection schemes (see Fig.-3). In the OCh-DPRING scheme (1+1 approach), each fiber carries wavelength channels in counter-propagating directions. The wavelengths are bridged at the head-end, which provides the ability to perform receiver-based protection switching with little or no signaling (see Fig-3a). The use of the dynamic multicast function in the integrated optical switch can help in the design of an OCh-DPRING architecture. The power distribution capability compensates for the power loss incurred from bridging. Since the optical paths in both directions of the ring are seldom equal, a dynamic power distribution approach is more efficient. Another method for improving efficiency is to lower the signal level below the standard operating SNR. Bridge function This allows for relatively fast and simple receiver-initiated protection schemes with minimal power penalty. In the OMS-SPRING architecture, protection is performed at the fiber span (OMS) level. This is more cost-effective in heavily multiplexed DWDM systems. The OMS-SPRING scheme is based on performing fiber loopback at the node adjacent to the failure (see Fig.-3b). In this scheme two virtual fibers are created from the two physical ones. A wavelength assignment/numbering scheme is used and the working and protection wavelengths are divided between the fibers. This implies that the traffic in each fiber (both working and protection) travels in opposite directions. The wavelength assignment ensures a reserved, protected wavelength for each working one. This scheme has the advantage of protecting the fiber rather than the optical channel. However, it suffers from the need to address the additional distance of the protection path, and a more complex signaling protocol. The combined switching and attenuation control functions are necessary in this architecture to support a flexible wavelength assignment mechanism and to create an efficient fiber loopback mechanism. These added bonuses increase when applying OMS-SPRING to 4 fiber rings. Integrated optical switches make ring-based schemes scalable to linked-ring, mesh-ring and mesh architectures. Using fast optical switches with integrated gain control and multicast functionality enables the utilization of virtual protected ring architectures as an overlay [3] over physical mesh networks (see fig. 4). The result is fast restoration, enabling carriers to offer differentiated service(s). It s also possible to mix different types of protection schemes. For example, it s possible to mix 1:N linear protection with ringbased protection mechanisms on one segment of the optical path so that a high-risk link can be protected with more costly 1+1 ring based protection (such as OMS-SPRING) while using a more economical 1:N scheme for the rest of the path. Loopback protection A D A D Protection channel Cut Protection channel Cut B C B C (A) DPRING protection scheme (B) OMS-SPRING protection scheme Figure-3 Ring based optical protecion schemes 2
B Dynamic switch and VOA function E Dynamic multicast function A Cut F Protection channel C D (A) DPRING protection scheme over mesh B E loopback - dynamic switch and VOA function Dynamic switch and VOA A Cut F Protection channel C D (B) OMS-SPRING protection scheme over mesh Figure-4 Ring based virtual optical protecion schemes overlaying physical mesh architecture 3
Dynamic All-Optical Networks The rising customer demand for high bandwidth dataoriented services, coupled with recent advances in optical technology, has led to the introduction of dynamic, intelligent optical networks. The intelligent optical layer offers various services and performance enhancements such as dynamic channel provisioning, optical power monitoring, optical layer protection and optical burst switching. For both all-optical and O-E-O based networks, these new features create a DWDM network capable of dynamic wavelength allocation. The control plane for these types of networks may be based on network management systems, GMPLS or an optical burst switching variant, however the optical functionality required in all cases is similar. Dynamic wavelength allocation creates the need to better adapt to changes in the physical layer in the network. To illustrate, the network model in Figure-5 will be used. A steady state network topology, that includes single wavelength connectivity between nodes A and G via nodes C and F, is shown. At a given time, an additional wavelength is lit between nodes D and G. The effects of introducing a new wavelength differ for an O-E-O based network and an alloptical one. In O-E-O networks the wavelength is terminated at every node. This limits the optical effects to each link traversed by the wavelength. It also allows for simple equalization of optical power at every link (transmitter output power). Therefore, in O-E-O based networks the transient effects are limited to the optical amplifiers. For example, the new wavelength between nodes D and G causes an increase of optical power at the input to amplifier 1. This results in 3dB less gain to the connected channel. The decrease in optical power doubles at amplifiers 2 and 3, causing a significant decrease in power at the receiver in node F. As a result, a LOS alarm and protection switch may occur. To avoid this problem, the amplifiers along the link must be able to adapt to changing input power. This can be done using electronic control of the amplifier pump, by integrating a closed loop VOA into the amplifier design to control the output power level, or through the use of an amplifier design such as the one described in Fig.-8. Each one of the options fits a different networking scenario. For example, the amplifier design described in Fig.-8 offers significant cost advantages as well as increased flexibility in wavelength provisioning, however it results in reduced performance. In contrast electronic control of the optical pump offers increased amplifier performance, however it is considerably more expensive and offers limited flexibility. In the all-optical case, the new wavelength that arrives from node D follows a different optical path than the one from node A. Hence, a difference in optical power results at the input to node C. This yields several possible effects: An increase of optical power at the input to amplifier one, resulting in decrease of gain to connected channels. Cross-talk between channels due to nonlinear effects both in the optical amplifier and optical fiber. Cross-talk between channels in the optical cross-connect due to insufficient isolation. To manage these effects, gain control of both the optical signal and amplifiers is required. Gain control at the optical cross-connects (OXC) assures operation within a desired range of optical power. A larger operational range results in better optical reachability, but is more difficult to achieve. Automatic gain controlled amplifiers are necessary to address amplifier gain changes in the network. The integrated switch may be used either in the amplifiers (see Fig.-8) or in the OXCs. Switch operation speed is also important. Lighting new wavelengths must be performed either very quickly or very slowly and incrementally. Fast operation allows the optical path to stabilize before protection mechanisms are triggered. This requires stabilization times in the sub-millisecond range (system implementation dependent), which may not be possible with all-optical components. Incremented operation lights up the wavelength in small power increments. The effect of each individual increment on the previously connected channels is negligible; therefore stabilizing times per increment may be relaxed. The drawback is the length of time needed for setting up a connection and the added complexity to the over-all process. Dynamic optical switch C Optical Amplifiers F A Amp. 1 Amp. 2 Amp. 3 Figure-5 Dynamic optical network example topology B G D Provisioned channel E 4
All-Optical Multicast The multicast concept for packet-oriented networks has been widely studied during the past years due to the exponentially increasing number of bandwidth-intensive applications. By extending the concept to the optical domain, packet-based applications such as Broadband Video, HDTV, Storage Area Networks and Multimedia can be provided with enhanced performance. In addition, other benefits, such as optimizing the network (minimizing transceiver usage in the network, maximization of the virtual connectivity between the network nodes, wavelength grooming, minimizing of the number of wavelengths, etc.) can be realized. This has led to recent interest in optical multicast-capable networks [4], [5], [6]. Optical multicast refers to point-to-multipoint connections that are created using light-trees (see Fig.-6). All-optical multicast refers to distribution of the optical input power between the various output ports of the node. To illustrate some of the benefits that may be gained from optical multicast, the example illustrated in Fig.-6 will be used. A single wavelength is shown connecting nodes {A, D, F, and G} (notice that node C is simply splitting the light while node F is performing a drop-andcontinue function). Assuming adequate optical budget, this same wavelength could connect node E as well (upon reception of a join request). Only one transmitter and 4 receivers are needed for this configuration. To achieve the same connectivity using an O-E-O configuration would require 5 transmitters and 5 receivers as well as highbandwidth electronic switching. An all-optical connection using point-to-point light paths would require 4 transmitters and 4 receivers as well as 4 different wavelengths due to the shared link between nodes A and C. These benefits have a cost. The realization of an optical multicast architecture must balance several conflicting design and performance criteria [4], including: 1. Minimizing the number of nodes traversed. 2. Minimizing some combination of the number of transceivers, optical amplifiers and O-E-O regenerators in the network. 3. Maximizing the virtual connectivity between the network nodes. 4. Maintaining an operable optical power budget. 5. Solving the Routing and Wavelength Assignment (RWA) problem for both unicast and multicast connections that may exist side-by-side. 6. Minimizing the wavelength blocking probability. The optical switch with integrated multicast and gain control is a key device in the implementation of dynamic, all-optical, multicast-capable networks. They can help solve one of the primary design problems in optical multicast networks: managing the optical power budget. Since optical multicast inherently involves the distribution of the optical power among several client nodes, a power penalty is incurred. Flexible, dynamic power distribution keeps these losses to a minimum, while maximizing the efficiency of the network. In addition, when adding or removing a node from the multicast tree, the reaction time of the switch is important. For example, in the network scenario described in Fig.-6, upon reception of a join request from node E, power must be allocated along the optical path (in the tree) from node A to E. To assure network stability, it s imperative that the active connections to nodes {D, F, and J} are not disturbed. A closed-loop device (see Fig.-7) operating at submicrosecond speeds can minimize the effects of optical power transients on the active connections. A multicast enabled node C F A B Split added upon join request Working optical channel Point-to-point link G D Figure-6 Optical multicast network scenario E 5 Figure-7 Close loop gain controlled multicast device The device uses one of the multicast ports to monitor the optical power and changes the optical attenuation or multicast ratio accordingly
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Optical Amplifiers Optical amplifiers (either EDFA or Raman) are usually built out of two main modules. A pump laser is used to supply the needed exciting power for the nonlinear optical effect providing the gain. A fiber gain module is used as the interaction medium for the amplification. The evolution of the optical layer, as detailed in the previous section, has not left the optical amplifiers behind. Today many different amplifier architectures are being developed to meet the evolving network s needs. Some architectures use electronic manipulation of the optical pump to provide increased dynamic range and to manage fast transients in the network. Some architectures are based on driving a single gain medium with several pump lasers, while other architectures provide low cost multi channel amplifiers by splitting a single pump laser between several gain modules. The evolved optical switch may be integrated into many of these architectures. It can be used as a fast gain control device either at the input or output of the amplifier to manage transients in the network and to enable the use of amplification modules with reduced dynamic range. It can also be used to create optical amplifier architectures where both wavelengths and fibers are provisioned dynamically. Figure-8 shows amplifier architecture that details such an adaptation. A single pump laser is used to excite a number of doped fibers. The optical switch with multicast and gain control functionality manages the power distribution between the interaction modules. This allows for the provisioning of gain modules for dark fibers. Using appropriate control-plane signaling, gain may be provided to these dark fibers at will. This architecture allows for gain control of existing optical channels in lit fibers as well as economic provisioning of dark ones. To illustrate, in order to provide gain to M active fibers as well as provision N-M dark ones, only one pump laser coupled to a 1xN-integrated switch is required (Figure-9). Using traditional amplifiers would require N pump lasers as well as N variable attenuators to manage the gain control. DWDM fibers Pump Laser 1xN switch Amplified DWDM fibers Gain medium Figure-8 Example of next generation optical amplifiers cable M Lit fibers N-M Dark fibers Single pump module A single next generation amplifier cable M Lit fibers M-N Dark fibers N traditional amplifiers with VOA Figure-9 provisioning scenario 7
OADMs Sonet/SDH rings are the most common topology in MANs today. The large installed base of fiber rings requires that any development in metro communication systems take this into account. This means that DWDM technology will initially be deployed over ring architectures with more economic hybrid mesh-ring and mesh architectures installed as overlays. Reconfigurable Optical Add/Drop Multiplexers (R- OADMs) are essential for deploying dynamic DWDM systems in ring architectures. The ability to reconfigure wavelengths quickly with no constraints allows carriers to dynamically provision their networks, thereby quickly realizing new revenue. All-optical reconfigurability also provides an easy migration path to more complex ring-mesh and mesh architectures. The evolved optical switch provides much needed optical functionality to the R-OADM. The switch provides the add/drop functionality. The gain control assists in wavelength reconfiguration, while the multicast functionality provides a Input much needed drop-and-continue function. Drop-and-continue is essential for provisioning wavelengths along interconnected rings, sharing a single wavelength s bandwidth between nodes, and for some protection architectures. Additional Applications As seen in this document, the evolved all-optical switch serves as an enabling technology for many telecommunication applications. However, the switch can be used in non-telecom applications as well. Using the switch in the optical set-up described in figure-10 results in a tapped delay line that may be used either as an optical buffer, temporal switch between packets, or the simulation of temporal multipath for RF-testing applications. Optical switches may also be used in a variety of testing equipment. When performing time consuming tests, such as PDL or insertion loss measurements on a multi-port optical system, significant time savings can be realized during manufacture by using fast, reliable optical switches (with or without multicast capability) to switch between the systems ports and the test equipment. Output 2x2 switching elements Data Packets The Result: Possible rearrangement of data time Data Packets time Input 1x2 switching elements 1xN Coupler Optical Signal Output Optical signal T in time The Result: A configurable optical Figure 10 A tapped optical delayline/buffer T in T out time Conclusion The evolution of the optical switch into a multi-functional alloptical device has enabled numerous novel applications that could not have been realized previously. These applications range from new networking applications, such as dynamic all-optical networks and optical burst switching, to test equipment and delay lines. Next generation infrastructures built upon these applications will provide new and useful services at a significantly lower cost. 8
References [1] Ayandeh S., Veitch P., Dynamic Protection and Restoration in Multilayer Networks, OIF2001.166, April 2001. [2] Ghani N., et al., Architectural Framwork for Automatic Protection Provisioning in Dynamic Optical Rings, OIF2001.041, January 2001. [3] Doverspike R., Yates j., Challenges for MPLS in Optical Network Restoration, IEEE Com. Magazine, Feb. 2001. [4] Papadimitriou D., et al., Optical Multicast A Framework, OIF2001.093, April 2001. [5] Mukherjee B., et al., Light Trees: Optical Multicasting fo Improved Performance in Wavelength Router Networks, IEEE Com. Magazine, Feb. 1999. [6] Papadimitriou D., et al., Optical Rings and Optical Hybrid Mesh-Rings Topologies, Internet Draft, Work in Progress, draft-papdimitiou-optical-rings-00.txt, Feb 2001. 9