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Optical Networks - _9_5 Optical Layer Protection Schemes_110

Optical Networks - _9_5 Optical Layer Protection...

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9.5 Optical Layer Protection Schemes 549 Table 9.3 A summary of optical protection schemes operating in the optical mul- tiplex section (OMS) layer. Both dedicated protection rings (DPRings) and shared protection rings (SPRings) are possible. Protection Scheme 1 + 1 1:1 OMS-DPRing OMS-SPRing Type Dedicated Shared Dedicated Shared Topology Point-point Point-point Ring Ring As we have seen over the history of the Internet, new customer applications and services are being created and developed all the time, and they have varying requirements of availability and costs. It is important to have a collection of protec- tion services that meets not only the requirements of current applications but also unforseen future applications. 9.5 Optical Layer Protection Schemes We next look at the different types of optical layer protection schemes. Conceptually, the schemes are largely similar to their SONET and SDH equivalents. However, their implementation is substantially different, for several reasons: the equipment cost for WDM links grows with the number of wavelengths to be multiplexed and terminated; link budget constraints need to be taken into account when designing the protection scheme; and one may to deal with wavelength conversion constraints. We saw in Chapter 8 that the optical layer consists of the optical channel (OCh) layer (or path layer), the optical multiplex section (OMS) layer (or line layer), and the optical transmission section (OTS) layer. Just as SONET protection schemes fit into either the line layer (for example, BLSR) or the path layer (for exam- ple, UPSR), optical protection schemes also belong to the OCh or OMS layers. An OCh layer scheme restores one lightpath at a time, whereas an OMS layer scheme restores the entire group of lightpaths on a link and cannot restore individ- ual lightpaths separately. Table 9.3 provides an overview of schemes operating in the optical multiplex section layer. Table 9.4 summarizes schemes operating in the optical channel layer. These schemes have not yet been standardized, and there are many variants. We have attempted to use a nomenclature that is consistent with SDH terminology. In SONET, there is not a significant cost associated with processing each connec- tion separately in the path layer instead of processing all the connections together in the line layer. The reason is that the processing is done using application-specific
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550 Network Survivability Table 9.4 A summary of optical protection schemes operating in the optical channel layer. Protection Scheme 1 + 1 OCh-SPRing OCh-Mesh Type Dedicated Shared Shared Topology Mesh Ring Mesh integrated circuits, where the incremental cost of processing the path layer com- pared to the line layer is not significant. In contrast, a significant difference in cost can be associated with OCh layer schemes relative to OMS layer schemes. An OCh layer scheme has to demultiplex all the wavelengths, whereas an OMS layer scheme operates on all the wavelengths and thus requires less equipment.
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