ARCHITECTURE AND PERFORMANCE REQUIREMENTS OF OPTICAL METRO RING NODES IN IMPLEMENTING OPTICAL ADD/DROP AND PROTECTION FUNCTIONS www.enablence.com
1. INTRODUCTION Wavelength Division Multiplexing (WDM) has become the preferred transmission technology in transport networks of long distance operators. WDM was initially deployed in point-to-point configurations to offer capacity relief on congested links suffering form fiber exhaustion. The combination of a multitude of wavelengths on a single fiber allows for sharing of network elements such as amplifiers, resulting in considerable cost savings. WDM is used today in virtually all network topologies, including metropolitan area (metro) rings. With the increasing volume of traffic passing through network nodes, routing functionality at the optical layer provides clear benefits. Optical Add/ Multiplexers (OADMs) and Optical Cross-Connects (OXCs) allow terminating signals on individual wavelength channels in a node and replacing them with other signals (on the same channels), while transparently passing through the remaining wavelengths. This capability enables to set up end-to-end wavelength paths that eliminate the need for expensive optical-electrical-optical (OEO) conversions. We propose an integrated Tunable Ring Node (TRN) module that offers flexible scalability for 2-fiber metro ring nodes. Supported functions include reconfigurable wavelength add and drop, dedicated and shared protection, power monitoring, automatic optical power equalization, and alarm generation. 2. METRO WDM RINGS 2.1. Topology Metro optical communication networks are built as transmission fiber lines that connect nodes, forming rings. Traffic in metro rings typically travels over a pair of fibers, where one fiber transports the signals in one direction and the other carries the contra-directional traffic. Figure 1 shows a schematic representation for a 2-fiber metro WDM ring and a functional diagram for an OADM node. 1 1 2 2 Figure 1. A 2-fiber metro ring and a generalized OADM node. 2 P age
2.2. OADM The basic building blocks in WDM rings are the OADMs. these elements, multiplexed signals can be extracted on a per-wavelength basis from either of the input fibers into the port, while the remaining wavelengths pass through to the output fibers. The Add port can transmit on either or both of the output fibers. Figure 2 shows a diagram and a connectivity table for the OADM functionality. OADM From Add To Figure 2. An OADM with 2 multiplexed put fibers, 2 multiplexed put fibers, and for each wavelength an Add and a port, and a connectivity table for the OADM functionality. 2.3. Protection Switching Providing recovery at the optical layer becomes inherently attractive as the network throughput increases. Reconfigurable OADMs and OXCs support recovery actions as protection switching in the optical domain. The first step towards a survivable optical layer has been the use of WDM rings. These rings can be divided into unidirectional and bidirectional signal flows, the former using dedicated protection and the latter using shared protection, as described below. 2.3.1. Dedicated Protection path switching, restoration of traffic is handled by the source node and the destination node. A Unidirectional Path Switched Ring (UPSR) that uses dedicated protection is referred to as a Dedicated Protection Ring (DPRing). A two-fiber DPRing uses one fiber as the working fiber and the other fiber as the protection fiber. 1+1 dedicated protection, a (variable) splitter and a selector switch are used (Figure 3), and in 1:1 dedicated protection, switches are used on both the transmitter and the receiver side (Figure 3). Splitter Switch Switch Switch 3 P age Figure 3. Dedicated Protection Switching: shows the working state and the protection state for 1+1 protection using a (variable) splitter and a selector switch, shows the 1:1 protection states using switches on both the transmitter and the receiver side. the two WDM fibers in a DPRing, each wavelength demand is protected using a main path along one side of the ring and a backup path along the opposite side of the ring.
Figure 4. An OCh-DPRing consisting of 6 nodes connected by 6 working (white) and 6 protection (gray) fiber lines. The possible protection paths are shown as dashed lines. When a link or node failure occurs within the ring, the affected traffic is switched over to the protection path, as illustrated in Figure 5. Figure 5. OCh-DPRing after failure: traffic is re-routed from the working fibers onto the protection fibers and the connection is restored. Figures and show different locations for a fiber cut. There are essentially three states in which the nodes can be for each wavelength: - Receiving signals from one of the put fibers - Transporting signals from put to put for both fibers - Transmitting a signal from the Add port, onto both the protection and the working fiber for 1+1 protection or on one of the two fibers for 1:1 protection switching. For a WDM system, all states and connections are on a per-wavelength basis. The nodes therefore consist of a demultiplexer (demux), a switching fabric, and a multiplexer (mux). Each of the nodes should therefore be capable of supporting, for each wavelength, the states shown in Figure 6. 1+1 dedicated protection From Add To ½ ½ 1:1 dedicated protection From Add To Figure 6. States supported by each node in a DPRing for 1+1 dedicated protection and 1:1 dedicated protection. 4 P age
2.3.2. Shared Protection A metro ring that utilizes shared protection is referred to as a Shared Protection Ring (SPRing). The protection switching occurs on a per wavelength basis in the so-called Optical Channel Shared Protection Ring (OCh-SPRing), or on a multiplex section level as Optical Multiplex Section Shared Protection Ring (OMS-SPRing). an OCh-SPRing, path switching is used (restoration of traffic is handled by the source node and the destination node), and the ring is referred to as a Bidirectional Path Switched Ring (BPSR). an OMS-SPRing, line switching is used (restoration of traffic is handled by the nodes at the ends of the failed link), and the ring is referred to as a Bidirectional Line Switched Ring (BLSR). a SPRing, 50% of the ring capacity (half the wavelengths) is dedicated for protection purposes, which allows sharing this pool of protection capacity amongst different wavelength demands routed on the ring (Figure 7). The working channels in one fiber are protected by the protection channels in the other fiber, traveling in the opposite direction around the ring. Working Bandwidth Protection Bandwidth λ 1 λ 2 λ 2 λ 1 Figure 7. A 2-fiber SPRing with 4 connections on 2 different wavelengths. Half the bandwidth on each fiber is used for protection switching; the wavelengths used in the clockwise and counter-clockwise direction have to be different. A SPRing can use the protection fiber for low priority traffic under normal conditions, has higher fiber link utilization, can deal with multiple failure scenarios, but is complicated to implement. 2.3.2.1. Optical Multiplex Section Shared Protection Ring For an OMS-SPRing (BLSR), in the event of a failure condition, the OADMs adjacent to the failure loop back the affected light paths on the protection channels of the ring (Figure 8). It is important to note that bidirectional traffic has to use different sets of wavelengths in order to avoid wavelength conversion. the TRN, λ 1 and λ 2 each designate either a single wavelength or multiple different wavelengths. 5 P age
2.3.2.2. Optical Channel Shared Protection Ring For an OCh-SPRing (BPSR), when a failure occurs, the affected connections are individually switched at the terminating OADMs to the opposite side of the ring, using the protection capacity on the other fiber (Figure 9). An important advantage of the OCh-SPRing over the OMS-SPRing is the reduced length of the protection path, thus minimizing optical power loss and signal distortion. Figure 9. OCh-SPRing fault recovery of 4 connections on 2 different wavelengths λ 1 (dotted) and λ 2 (solid). and show different locations for a fiber cut. Each node has to support the states shown in Figure 10. OMS-SPRing From To Add OCh-SPRing From Add To Figure 10. States supported by each node in a SPRing for OMS-SPRing and OCh-SPRing. 3. IMPLEMENTATION: TUNABLE RING NODE MODULE 3.1. TRN Architecture The TRN is a switching fabric that, together with a demux / mux pair, makes up a 2-fiber OADM. For simplicity, the working principles are shown in Figure 11 for two wavelengths per fiber. 6 P age { Add{ { } } } Figure 11. Implementation of the TRN switching matrix. Since the on-chip signal flow cannot be contra-directional as in, the inputs and outputs are configured on the left and the right side, respectively.
Channel 1 (λ 1 ) (λ 1...λ Ν ) (λ 1...λ Ν ) Add λ 1 Add λ Ν (λ 1...λ Ν ) λ 1 λ Ν (λ 1...λ Ν ) Channel Ν (λ Ν ) Figure 12. An Ν-Channel TRN, where the Ν different wavelengths are depicted by λ 1 (solid) and λ Ν (dotted). As illustrated in Figure 12, the signals from the input are demultiplexed and switched/attenuated on a perwavelength basis. After signal processing, the signals are fed into multiplexers. 3.2. TRN As A Node The TRN all-optical switches do not depend on protocol or transmission speed, therefore they are immune to system upgrades for years to come. This advantage saves carriers considerable upgrade costs in the core transmission systems as advances are made at the system edges. With a response time of about 5 ms, the thermo-optic switches in a TRN are adequate for system restoration, especially for SONET, where a 50 ms budget is allocated to restoration and restructuring of multiple circuit connections. On a per-wavelength basis, while being fully wavelength insensitive, the TRN enables high flexibility in upgrading a system to more wavelengths or different bands. Protection and restoration in the WDM ring topology is practical, survivable and uses existing technology. A back-to-back configuration of protection switches, as in the TRN, is necessary. The integrated polymeric switches in an TRN have the same 2D optical waveguide based connectivity as the well-known bubble switch, with the advantage of using reliable solid-state low-power thermo-optic technology for its switching mechanism. this case, light is tuned from input waveguides to output waveguides by adiabatic modal transformation or by phase manipulation. Consequently, features such as multicasting and power equalization or attenuation are already embedded in this technology. This is an important advantage over the MEMS and bubble on/off digital switching technologies, where additional devices must be added to the system to achieve these functionalities. The TRN includes the following functionalities on a single chip: Reconfigurable OADM. Dedicated Protection in UPSR. Shared Protection in BPSR/BLSR. Power monitoring of the demultiplexed signals at the ports and at the put ports. Automatic Load/Power Balancing. This capability eliminates the need for external photodiodes and Variable Optical Attenuators (s), thereby reducing equipment cost, power consumption, and space requirements, all the while increasing reliability. The TRN s dynamic attenuation, based on network conditions, improves signal quality and makes for easier provisioning. The absolute power of all channels is equalized to a referenced channel from either the or the direction. The power of the Add channel is set to match the channel plus an adding offset. These measures ensure a uniform power distribution across the channels exiting the put ports. Alarm generation. The module monitors the through signal level and generates alarms in the event of signal degradation. 7 Page
3.3. TRN States The following examples of the TRN states are based on the reconfiguration of one of the two wavelengths (solid line λ), while the other wavelength (dotted line λ) remains in the same state (except in the Through State example): from, Add to, through from to. 3.3.1. Through State The Through state transports the signals through the node (Figure 13). this state, signals are present at the put ports but not at the Add ports. This state is present in both dedicated and shared protection schemes in unidirectional and bidirectional rings. Add λ 1, λ Ν λ 1, λ Ν Figure 13. State for - Through and - Through in an Ν-Channel TRN, where the Ν different wavelengths are depicted by λ 1 (solid) and λ Ν (dotted). 3.3.2. OADM and 1:1 Dedicated Protection The two states depicted in Figures 14 and 15 support Reconfigurable OADM from both directions. These states can also be used to protect the paths based on a 1:1 dedicated protection configuration. The Added or ped signals can come from the protection part (Figure 14) or from the working part (Figure 15) of the ring. These states are present in both unidirectional and bidirectional rings. Add λ 1, λ Ν Figure 14. First state supporting Reconfigurable OADM and 1:1 dedicated protection, representing - Through, and Add to, in an Ν-Channel TRN, where the Ν different wavelengths are depicted by λ 1 (solid) and λ Ν (dotted). Add 8 P age λ 1 λ Ν λ Ν λ 1 Figure 15. Second state supporting Reconfigurable OADM and 1:1 dedicated protection, representing - Through, and Add to, in an Ν-Channel TRN, where the Ν different wavelengths are depicted by λ 1 (solid) and λ Ν (dotted).
3.3.3. 1+1 Dedicated Protection The two states depicted in Figures 16 and 17 support 1+1 dedicated protection. When either the (Figure 16) or the (Figure 17) signals are ped, the Added signal is sent to both and using a splitter that can be fixed (50/50) or variable to adjust for different path lengths to the destination node. Add λ 1 λ Ν λ Ν λ 1 Figure 16. First state supporting 1+1 dedicated protection, representing and Add to and, in an Ν-Channel TRN, where the Ν different wavelengths are depicted by λ 1 (solid) and λ Ν (dotted). Add λ Ν λ 1 Figure 17. Second state supporting 1+1 dedicated protection, representing and Add to and, in an Ν-Channel TRN, where the Ν different wavelengths are depicted by λ 1 (solid) and λ Ν (dotted). 3.3.4. Shared Protection The two states depicted in Figures 18 and 19 support shared protection. They represent ping the put and Adding a signal to the put (Figure 18), and ping the put and Adding a signal to the put (Figure 19), thereby enabling the - and - U-Turns needed in an OMS-SPRing. Add λ 1 λ Ν Figure 18. First state supporting shared protection, representing - U-Turn, in an Ν-Channel TRN, where the Ν different wavelengths are depicted by λ 1 (solid) and λ Ν (dotted). λ Ν Add λ Ν λ Ν λ 1 9 P age
3.4. TRN Layout A 4-channel TRN was fabricated. At the core of it is an optical chip that has the configuration shown in Fig. 20. The optical waveguiding circuitry is polymeric, and the thermal actuation heaters are resistive strips of metal. The detectors are photodiode arrays flip-chip mounted on top of 45 mirrors. The mirrors were fabricated by Excimer laser ablation of the polymer followed by smoothing and metalization. λ 1 Add Silicon Chip Waveguide planar polymer on chip glass fiber off chip λ 2 Add 1x2 Switch 2x2 Switch Variable 1x2 Splitter λ 3 Add Variable Optical Attenuator Optical Power Tap tegrated Photodiode λ 4 Add Figure 20. Layout of the optical chip that constitutes the core of an integrated 4-channel TRN. addition to the optical chip, the TRN module includes fiber pigtails and packaging for the optical chip, and an electronic control board. The chip measures 1.2 cm x 2.6 cm. The chip package measures 2.5 cm x 7.5 cm x 1 cm. The full module (including the chip package, the electronic control board, and the fiber management) measures 7.5 cm x 12.5 cm x 1.65 cm. Today s implementation of a non-integrated ring node uses 48 discrete elements (8 1x2 switches, 8 2x2 switches, 8 s, 12 taps, 12 photodiodes). The proposed integrated solution is an exemplary embodiment of the benefits of optical integration as it provides, when compared to the discrete solution, significant cost reduction, space savings, lower electrical power consumption, higher reliability (fewer devices, runs cooler), and fewer board level fiber interconnects. 3.5. TRN Performance The integrated TRN has performance characteristics that compare favorably with those of the discrete solution, as the integration of a large number of functions on a single chip minimizes the main drawback of optical integration, namely the fiber-to-chip pigtail loss, as pigtailing is done only once in the module. 10 Page
Table 1. Key performance characteristics of the integrated TRN. Parameter Value sertion Loss 2.5 db Attenuation Range 15 db Shut-Off Attenuation 25 db Polarization Dependent Loss 0.35 db Power Monitoring Accuracy 0.5 db Optical Power Range -30 to +10 dbm Isolation 50 db Power Consumption 1.5 W Switching Time 5 ms Tap Ratio 5% Photodiode Responsivity 0.85 A/W 4. CONCLUSION The TRN module based on our planar lightwave circuit technology offers the highest level of integration available in the industry, with performance, stability, and accuracy adequate for demanding optical communication applications. The device consists of an array of 4 independent optical add-drop circuits each with automatic load balancing and supporting shared and dedicated protection protocols in 2-fiber metro ring optical networks. 11 P age