Some concepts related to network protection
Working route and protection path
In network protection, each protected service is often made up of a pair of path called working and protection paths. At normal time, the working path takes user traffic, while the protection path either standbys or takes other lower-priority services. Only when there is a failure occurred with the working path, then should the protection path be taken over to carry affected user traffic.
Double failures
Double failures means that there are two network failures that simultaneously occur with a network. Such two failures can occur sequentially; that is one failure occurs first, and the second one follows. Or the two failures can occur at the same time.
Restorability
Restorability can be defined in two ways. One is defined as how much percentage of affected traffic can be recovered for a network failure. The second is from the pointview of network failure types that can be recovered. If a network can be recovered from link failures, we may say it has link-failure restorability; similarly, if a network can recover from node failures, we may say it has node-failure restorability.
Restoration time
Restoration time is referred to as the time taken from the moment of a failure occurrence to the moment of failure recovery. This time spans the times for failure detection, failure message transfer, and failure recovery. Sometimes, restoration time is also called restoration speed. Typically, for a SDH/SONET transport network, the required restoration time is within 50 ms. For the new generation of MPLS-based network, the required restoration time is relaxed to be within 150~200ms.
