Overview

A subgraph-based routing strategy attempts to provide a passive form of redundancy to optical networks in the event of a given set of failure scenarios such as a single link failure or a single node failure. It is passive in that, before a connection is established, it is subjected to the constraints that it can be routed in the network as a fault free of any given set of failures, and is thus guaranteed in the event of a failure from the set. The end user experiences nominal interruption in service due to network state restoration and it is characterized in [194]. The key characteristics of the subgraph-based routing strategy are as follows.

• The redundancy in routing is provided using the resources available in the network.

• Fault recovery network states are maintained throughout the operation of the network.

• Subgraph fault tolerance provides a 100% guarantee for recovery from all possible predefined failure states.

• Subgraph routing is a pro-active path-based fault tolerance strategy.

• The network state must be altered to accommodate the new topology of the network caused by a failure state.

The first characteristic highlights one of the most important aspects of the strategy; it does not require the allocation of system transmission resources to ensure recoverability after the detection and location of a link failure. Simply put, there is no explicit link capacity lost due to the routing of backup connections because no active backup connections exist in this strategy. The second...