In optical WDM networks, since each lightpath can carry a huge mount of traffic, failures may seriously damage the end-user applications.As a result, fault-tolerance is an important issue in these networks. The primary path is the light path which carries traffic during normal operation. in case of a failure, the traffic is rerouted on a backup path. a reliable and fault-tolerant routing algorithm is discussed in this paper for establishing primary and backup path. load balancing is used to establish the primary path. In this method, the page link cost metrics are estimated based on the current load of the links. The blocking probability is calculated by the source through the received feedback from the destination by sending a small fraction of probe packets along the existing paths. Then the the lowest blocking probability light path is selected as the optimum path. The studies and simulations show that this algorithm reduces the blocking probability and latency and increases the throughput and channel utilization.

Wavelength-Division-Multiplexing (WDM) Networks
It involves the Simultaneous transmission of multiple streams of data with the help of the exclusive properties of fiber optics. Here, the wavelength acts as the signature address and determines the communication path. huge amount of data can be transferred at high speeds through the WDM networks. WDM is considered as the most capable technology for forming the backbone of next generation internet.

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http://scribddoc/23730529/A-Reliable-and-Fault-Tolerant-Routing-for-Optical-WDM-Networks