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Dwdm Topologies

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Dwdm Topologies CHAPTER 16 DWDM TOPOLOGIES 16.1 INTRODUCTION Dense wavelength division multiplexing (DWDM) networks are classified into four major topological configurations: DWDM point-to-point with or without add-drop multiplexing network, fully connected mesh network, star network, and DWDM ring network with OADM nodes and a hub. Each topology has its own requirements and, based on the application, different optical components may be involved in the re- spective designs. In addition, there are hybrid network topologies that may consist of stars and/or rings that are interconnected with point-to-point links. For example, the Metropolitan Optical Network project (MONET) is a WDM network developed for and funded by a number of private companies and by U.S. government agencies. It consists of two sub- networks, one located in New Jersey and one in the Washington, D.C./Maryland area; the two are interconnected with a long-distance point-to-point optical link. 16.2 POINT-TO-POINT TOPOLOGY Point-to-point topology is predominantly for long-haul transport that requires ultrahigh speed (10-40 Gb/s), ultrahigh aggregate bandwidth (in the order ofseveral ter- abits per second), high signal integrity, great reliability, and fast path restoration capa- bility. The distance between transmitter and receiver may be several hundred kilome- ters, and the number of amplifiers between the two end points is typically less than 10 (as determined by power loss and signal distortion). Point-to-point with add-drop mul- tiplexing enables the system to drop and add channels along its path. Number of chan- nels, channel spacing, type of fiber, signal modulation method, and component type se- lection are all important parameters in the calculation of the power budget. 197 198 Part IV Dense Wavelength Division Multiplexing Figure 16.1 A DWDM point-to-point with add-drop multiplexing enables the system to drop and add channels along its path. In DWDM, each channel is carried over a specified wavelength (hi) also known as "optical channel." Different channels may carry different data (e.g., voice, data, video, data packets) at different bit rates. The transmitter-receiver optical link has several optical components: fiber(s), optical amplifiers, OADM, optical filters, cou- plers, laser sources, and modulators and receivers. Each has its own signal-affecting characteristic, as described in Part II. An end-to-end simplistic view of a DWDM point-to-point system that includes lasers, an optical multiplexer and demultiplexer, fibers, optical amplifiers (OA), and an optical add-drop multiplexer is shown in Figure 16.1. 16.3 RING-CONFIGURED MESH AND STAR NETWORKS A variety of proprietary ring DWDM networks have been deployed. In general, a DWDM ring network consists of a fiber in a ring configuration that fully intercon- nects nodes; some systems have two fiber rings for network protection. Such a ring may cover a local or a metropolitan area and span a few tens of kilometers. The fiber ring may contain few (4) to many wavelength channels, and few to many nodes. The bit rate per wavelength channel may be 622 Mb/s or lower, or 1.25 Gb/s or higher. One of the nodes on the ring is a hub station where all wavelengths are sourced, ter- minated, and managed; connectivity with other networks takes place at this hub sta- tion. Each node and the hub have optical add-drop multiplexers (OADM) to dropoff and add one or more designated wavelength channels. In DWDM ring networks, the hub station may source and terminate several types of traffic [e.g., synchronous transport module (STM) IP, video] . The hub manages all channels (wavelengths) assigned to a path between nodes and also the traffic type. At an OADM, one (or more) optical frequency is dropped off and added, whereas the remaining frequencies pass through transparently. However, as the number of OADMs increases, the signal is subject to losses and optical am- Chapter 16 DWDM Topologies 199 IP _~~,,~~ lJ~ STM -- ...; Figure 16.2 A DWDM ring network; the hub station sources and terminates payloads of several types. plification (not shown) may be required. The number of nodes is typically less than the number of wavelengths in the fiber. Figure 16.2 depicts a basic configu- ration but does not address network survivability or ring fault avoidance. In the ring topology, the hub station manages channel (wavelength) assign- ment so that a fully connected network of nodes with OADM is accomplished. The hub may also provide connectivity with other networks . In addition, an OADM node may be connected with a multiplexer/demultiplexer where several data sources are multiplexed. A simple ring topology with a hub and two nodes, A and B, linked via wavelength Ak is shown in Figure 16.3, where node A also multi- plexes several data sources. All data sources are terminated by the corresponding OADM node (node B), however, since they are on the same channel (and the same wavelength). Figure 16.3 In a DWDM ring topology, channel (wavelength) assignment may be managed by the hub station. 200 Part IV Dense Wavelength Division Multiplexing FiberVista: A high-level view The FiberVista research project employs both dense and coarse wavelength division multi- plexing. At the curb, where coax meets fiber, coarse WDM supports a two-way link between an optical-electrical converter and the nearest hub in downstream transmission. Dense WDM simplifies delive ry of different levels of customer-specific content, which are received by everyone the system serves .Demultiplexing at a hub separates out narrowcast content targeted at nearby towns. Video or data destined for a particular home is combed out of the spectrum and delivered in much the same way. Downstream Figure 16.4 View of LUCENT Technologies' project FiberVista is illustrative of a DWDM and CWDM system that delivers all service types to the home. (From LUCENT Technologies, Bell Labs Technology, vol. 2, no. 2, 1998, p. 13. Reprinted with permission.) A project that is illustrative of a coarse WDM (CWDM) system applied in the access area (residential) is illustrated in Figure 16.4. This project, dubbed FiberVista, reuses TV technology to open up the fiber capacity to residential users and offers to the home all types of service-IP, video, analog, and digital. With FiberVista, each hub on a fiber ring can serve 10,000-30,000 homes; hubs can be as far apart as 125 km. From the master head, a transceiver (an optical-electrical con- verter) converts the optical signal to electrical via a coax cable with taps that feed in- dividual homes where cable modems and set-t boxes can select among TV, Internet, and voice services. Such a system would transmit in the upstream direction (from the home) more than 4 Mb/s per home, and in the downstream direction (to the home) about 1 Obis . At the curb, where coax meets fiber, bidirectional CWDM support (1550 nm downstream and 1300 nm upstream) links the transceiver with the hub. Similar ring architectures are also studied for metropolitan (large city) and for en- terprise (business community, high rises) networks. Chapter 16 DWDM Topologies 201 16.4 A DWDM HUB The area of DWDM node and DWDM hub is currently evolving. Thus in this sec- tion we attempt to provide stimulating discussion without any effort to provide sys- tem solutions. 16.4.1 Transmit Direction A hub, in general, accepts various (electrical) payloads, such as communications transport protocoVInternet Protocol (TCPIIP), asynchronous transfer mode (ATM), STM, and high-speed Ethernet (l Gb/s, 10 Gb/s). Each traffic type (channel) is sent to its corresponding physical interface, where a wavelength is assigned and is mod- ulated at the electrical-to-optical converter. The optically modulated signals from each source are then optically multiplexed and launched into the fiber (Figure 16.5). Wavelength manager Modulators and r·······························..·······.. ·· ..··• ········· .. ··· ..·.. ~pi ·i·~~·I ·t;~nsmitters TCP/IP r TCP/IP i ATM r ATM Single-mode fiber STM STM PHY i Transmit direction .lI ljii s Electronic regime Photonic regime Figure 16.5 The hub (in the transmit direction) receives a variety of traffic types (TCPfIP, ATM, STM, etc.). Each type is launched into the fiber on a separate wavelength . 16.4.2 Receive Direction When a hub receives a WDM signal, it optically demultiplexes it to its component wavelengths (channels) and converts each optically modulated signal to a digital electrical signal. Each digital signal then is routed to its corresponding electrical in- terface: TCPIIP, ATM, STM, and so on (Figure 16.6). Notice, however, that each 202 Part IV Dense Wavelength Division Multiplexing _ . ............... .................. Detedtors TCP/IP 4 '-- -' ~====~ ATM 1 Single-mode fiber STM 1+-- Receive direction .lI! PHY iii · .. Electronic regime Photonic regime Figure 16.6 The hub (in the receive direction) demultiplexes the optical signal to its component, wavelength channels, and it converts each channel to a traffic type, TCP/lP, ATM, STM, etc. channel requires its own clock recovery circuitry (only one is shown) because all channels may be at different bit rates. 16.5 FAULTS DWDM networks must be able to detect faults on the link or on the ring (broken fiber, faulty port unit, inoperable node) and to isolate a fault. The objective is to of- fer continuous transmission (service) or service with the minimum disruption possi- ble, as recommended in the standards. Depending on network topology and archi- tecture, fault avoidance may be accomplished with dual counterrotating rings (in ring networks), similar to the fiber-distributed data interface (FOOl). When a fault is detected in a counterrotating ring architecture, the neighboring OADMs avoid the fault by rerouting traffic via a Ll-turn optical cross-connect (Figure 16.7).
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