  Pre-configured cycle (p-Cycle) method has been studied in literature
extensively for optical network protection. A large p-cycle has high capacity
efficiency and can protect a large number of nodes against the single link
failure scenarios. All the links protected by such a p-cycle lose protection
when the p-cycle is consumed to restore traffic after a failure. As the
probability of multiple link failure is high for a large network, it also means
that with higher probability, on the second failure, protection may not be
there for the failed link. Thus, if the number of links protected by a p-cycle
is large, it makes the network unprotected with high probability on the advent
of the second failure. In this paper, we study the impact zone due to a first
link failure in the various configurations of the p-cycles. The study gives
insight into how to choose the p-cycle configuration to reduce the impact zone
while using minimum spare capacity. We propose few methods and compare them to
show how the impact zone analysis can be used to improve the fault tolerance in
an optical network.
