Overview

Although the failure of a single component such as a link or a node is the most common failure scenario, it is possible to have multiple links fail simultaneously. In particular, double-link failures can happen in the following scenarios.

1. The first link fails. Th e recovery from the failure of the first link is completed within a few milliseconds to a few seconds. However, it may take a few hours to a few days for the failed physical link to be repaired. It is certainly conceivable that a second link might fail during this period, thus causing two links to be down at the same time. Suppose the link failure is a Poisson process with parameter ? and the repair times are exponentially distributed with parameter . Thus the average time to failure is 1 / ? and the average repair time is 1 / . Suppose a link fails at time t = 0, then the probability that a failure will occur on a link while the first repair is being carried out is given by

   (8.1)

   For = 9 ?, FP ? 0.1, which is large.

2. Two links may be physically routed together for some distance in real situations. A single backhoe accident may lead to the failure of both links.

In order to protect connections from link failures in the network, two paths are often assigned: a primary path on which a connection is established and a backup...