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                    Optical Switching Systems and Technologies                                   133



                    Figure 6-4
                    The ONI
                    transport system
                    (ONI Systems)



                                                       Access        Metro Core
                                                      Optical Ring
                                                                     Optical Ring




                                       The Need for Metropolitan DWDM Networks


                                       The task of interconnecting SAN islands is one that is naturally
                                       suited to optical networking. SONET/SDH-based network architec-
                                       tures were designed for voice and private-line services (for example,
                                       DS-1, DS-3) and cannot scale to support the gigabit channels
                                       required for storage transport.These networks also cannot transport
                                       storage data natively and in addition suffer from long provisioning
                                       times.
                                         The solution lies in the next-generation metropolitan DWDM net-
                                       works that are being built out rapidly today. These networks meet
                                       the requirements of metropolitan carriers for capacity, transparency,
                                       scalability, dynamic provisioning, manageability, and survivability
                                       for data-centric services at the lowest “first cost.” Next-generation
                                       metropolitan DWDM networks, designed to offer wavelength ser-
                                       vices, will enable service providers to leverage the existing network
                                       infrastructure to offer SAN services and other user services (Fig-
                                       ure 6-5).



                                       Dynamic Optical Add-Drop Multiplexing

                                       As the enterprise market evolves, Gigabit Ethernet and Fiber Chan-
                                       nel will play increasingly important roles in all facets of metropoli-
                                       tan optical networks. Metropolitan optical cross-connects can
                                       provide dynamic reconfiguration of metropolitan enterprise net-
                                       works, with optical layer survivability compatible with SONET
                                       recoverability. In the reconfiguration/survivability scenario, optical
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