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If of the conditions above are not met, then the protection scheme may not converge. For example, if the client layer protection is nonrevertive, it may switch over once to the protection path, discover that path is not available, and not switch back to its primary path. While it is desirable to have some sort of coordination between protection mechanisms in different layers, this may not always be possible. For example, the protection mechanisms in different layers may actually be activated by different nodes. In some cases, it may be possible to add a priority mechanism where one layer attempts to restore service rst, and only afterward does the second layer try. One automatic way to ensure this is to have the restoration in one layer happen so quickly that the other layer doesnt even sense that a failure has occurred. For example, consider a WDM network carrying IP trafc. As we saw in Section 9.3.3, it can take several seconds for the IP layer to detect a failure. It is entirely feasible for the optical layer to have completed its restoration within this time scale so that the IP layer does not detect the failure. This may not, however, be feasible when we have SONET rings operating over a WDM network. The SONET rings detect failures very quickly and can initiate protection switching as early 2.3 as s after a failure occurs. Another way to implement orderly restoration would be to impose an additional hold-off time in the higher layer before it attempts restoration so as to provide sufcient time for the lower layer to do its restoration. However, a large hold-off time would increase the overall restoration time and is therefore not highly desirable either. In general, it would make sense to have the priorities arranged so that the layer that can provide the fastest restoration tries rst.
Summary
Engineering the network for survivability is an important role in transport networks. Protection techniques are well established in SONET and SDH and include pointto-point, dedicated protection rings, and shared protection rings. These protection techniques are used or are being developed in other networks in the client layer. Point-to-point protection schemes work for simple systems with diverse ber routes between node locations. Dedicated protection rings are used primarily to aggregate trafc from remote locations to one or two hub locations. Shared protection rings can be used in the core parts of the network where the trafc is more distributed. Optical layer protection is needed to protect the data services that are increasingly being transported directly on the optical layer without the SONET/SDH layer