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Optical Networks - _Summary9_112

Optical Networks - _Summary9_112 - Summary 565 If any of...

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Summary 565 If any 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 mecha- nisms 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 first, and only afterward does the second layer try. One auto- matic way to ensure this is to have the restoration in one layer happen so quickly that the other layer doesn’t even sense that a failure has occurred. For example, consider a WDM network carrying IP traffic. 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
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