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Unformatted text preview: 518 Network Survivability 9.2 Protection in SONET/SDH A major accomplishment of SONET and SDH network deployment was to provide a significant improvement in the availability and reliability of the overall network. This was done through the use of an extensive set of protection techniques. Similar schemes are used in both SONET and SDH, but their nomenclature is different. We will specify both nomenclatures but use the SONET nomenclature for the most part. A taxonomy of the different protection schemes is given in Table 9.1. We will start by describing the different types of protection mechanisms that are used for simple point-to-point links, and then discuss how these can be applied for networks. Each protection scheme can be associated with a specific layer in the network. As we saw in Chapter 6, the SONET layer includes a path layer and a line layer. Both path layer and line layer protection schemes are used in practice. Equivalently, SDH networks use both channel layer and multiplex section (MS) layer protection schemes. A path layer protection scheme operates on individual paths or connections in the network. For example, in an OC-48 ( 2 . 5 Gb/s) ring supporting STS-1 ( 51 Mb/s) connections, a path layer scheme would treat each STS-1 connection independently and switch them independently of each other. A line layer scheme on the other hand, operates on the entire set of connections at once and generally does not distinguish between the different connections that are part of the aggregate signal. In the former example, a line layer protection scheme in an OC-48 ring would switch all the connections within the OC-48 together. (There are some exceptions to this statement. The bidirectional line-switched rings (BLSRs) that we will study later do allow bits to be set for each connection. In the event of a failure, only those connections that are specified are switched. This is needed to ensure that some connections can be left unprotected if so desired, and also to handle node failures, as we will see in Section 9.2.4.) 9.2.1 Point-to-Point Links Two fundamental types of protection mechanisms are used in point-to-point links: 1 + 1 protection and 1 : 1 or, more generally, 1 : N protection, as shown in Figure 9.3. Both operate in the line or multiplex section layer. In 1 + 1 protection, traffic is transmitted simultaneously on two separate fibers (usually over disjoint routes) from the source to the destination. Assuming unidirec- tional protection switching, the destination simply selects one of the two fibers for reception. If that fiber is cut, the destination simply switches over to the other fiber and continues to receive data. This form of protection is very fast and requires no 9.2 Protection in SONET/SDH 519 Source Destination Switch Switch Source Source Destination Destination Working fiber Protection fiber Switch Switch Switch Switch Switch Switch Low-priority data Protection fiber 1 2 N (a) (b) (c) . . ....
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- Spring '09