Overview

Providing resilience against failures is an important requirement for many high-speed networks. As these networks carry more and more data, the amount of disruption caused by a network-related outage becomes more and more significant. A single outage can disrupt millions of users and result in millions of dollars of lost revenue to users and operators of the network.

As part of the service-level agreement between a carrier and its customer leasing a connection, the carrier commits to providing a certain availability for the connection. A common requirement is that the connection be available 99.999% (five 9s) of the time. This requirement corresponds to a connection downtime of less than 5 minutes per year.

A connection is routed through many nodes in the network between its source and its destination, and there are many elements along its path that can fail. The only practical way of obtaining 99.999% availability is to make the network survivable, that is, able to continue providing service in the presence of failures. Protection switching is the key technique used to ensure survivability. These protection techniques involve providing some redundant capacity within the network and automatically rerouting traffic around the failure using this redundant capacity. A related term is restoration. Some people apply the term protection when the traffic is restored in the tens to hundreds of milliseconds, and use the term restoration to schemes where traffic is restored on a slower time scale. However, we do not distinguish between protection and restoration in this chapter.