As mentioned in earlier chapters, due to the high bandwidths involved, any link failure in the form of a fiber cut has catastrophic results unless protection and restoration schemes for the interrupted services form an integral part of the network design and operation strategies. Although network survivability can be implemented in the higher layers above the optical network layer (e.g., self-healing in SONET rings and the ATM virtual path layer, fast rerouting in MPLS and changing routes using dynamic routing protocols in the IP layer), it is advantageous to use optical WDM survivability mechanisms since they offer a common survivability platform for services to the higher layers. For example, it is possible that several IP routes may eventually be routed through the same fiber. Hence the failure of a single fiber may affect multiple routes, possibly alternative paths for an IP route. Thus, protection at the IP layer requires complete knowledge of the underlying physical fiber topology.

As discussed earlier, a variety of optical path protection schemes can be designed using concepts such as disjoint dedicated backup paths, shared backup multiplexing, and joint primary/backup routing and wavelength assignment. Lightpath restoration schemes, on the other hand, do not rely on prerouted backup channels but instead dynamically recompute new routes to effectively reroute the affected traffic after link failure. Although this saves bandwidth, the timescale for restoration can be difficult to specify and can be of the order of hundreds of milliseconds. Hence in a dynamic scenario, path protection schemes are likely to be more useful and practical than path restoration schemes.