Optical Networks - _6_1 SONET_SDH_74

8 shows different types of sonet equipment deployed

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Figure 6.8 shows different types of SONET equipment deployed in a network. SONET is deployed in three types of network configurations: rings, linear con- figurations, and point-to-point links. The early deployments were in the form of point-to-point links, and this topology is still used today for many applications. In this case, the nodes at the ends of the link are called terminal multiplexers (TMs).
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6.1 SONET/SDH 387 ADM ADM ADM ADM ADM ADM ADM ADM ADM DCS ADM ADM ADM ADM ADM ADM ADM ADM ADM ADM DCS BLSR/2 or BLSR/4 BLSR/2 or BLSR/4 Backbone ring Backbone ring OC-12/OC-48 OC-12/OC-48 Access ring Access ring Access ring ADM Central office Central office UPSR OC-3/OC-12 UPSR OC-3/OC-12 UPSR OC-3/OC-12 TM TM Linear add/drop Point-to-point Figure 6.8 Elements of a SONET infrastructure. Several different SONET configurations are shown, including point-to-point, linear add/drop, and ring configurations. Both access and interoffice (backbone) rings are shown. The figure also explains the role of a DCS in the SONET infrastructure, to crossconnect lower-speed streams, to interconnect multiple rings, and to serve as a node on rings by itself. TMs are also sometimes called line terminating equipment (LTE). In many cases, it is necessary to pick out one or more low-speed streams from a high-speed stream and, likewise, add one or more low-speed streams to a high-speed stream. This function is performed by an add/drop multiplexer (ADM). For example, an OC-48 ADM can drop and add OC-12 or OC-3 streams from/to an OC-48 stream. Similarly, an OC-3 ADM can drop/add DS3 streams from/to an OC-3 stream. ADMs are now widely used in the SONET infrastructure. ADMs can be inserted in the middle of a point-to-point link between TMs to yield a linear configuration.
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388 Client Layers of the Optical Layer Maintaining service availability in the presence of failures has become a key driver for SONET deployment. The most common topology used for this purpose is a ring. Rings provide an alternate path to reroute traffic in the event of link or node failures, while being topologically simple. The rings are made up of ADMs, which in addition to performing the multiplexing and demultiplexing operations, incorporate the protection mechanisms needed to handle failures. Usually, SONET equipment can be configured to work in any of these three configurations: ring ADM, linear ADM, or as a terminal multiplexer. Rings are used both in the access part of the network and in the backbone (interoffice) part of the network to interconnect central offices. Given the capac- ity requirements in today’s networks, it is quite common to use multiple overlaid rings, particularly in backbone networks, each operating over a different wavelength provided by an underlying optical layer. Two types of ring architectures are used: unidirectional path-switched rings (UPSRs) and bidirectional line-switched rings (BLSRs). The BLSRs can use either two fibers (BLSR/2) or four fibers (BLSR/4). We will discuss these architectures and the protection mechanisms that they incorporate in detail in Chapter 9. In general,
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