Path segment restoration (PSR)
Fig. 1 Path segment restoration
Path segment restoration is also called sub-path restoration. It is similar to the path restoration technique, which is however more generic than the latter. Path restoration carries out restoration always at the two end nodes of a working paths, while path segment restoration can perform restoration between any node pair on a working path as long as one node is on the left-hand side and the other node is on the right-hand side of a network failure. Fig 1 shows an example of path segment restoration. Under this technique, for a link failure (3-4), we can use many protection paths as shown by dotted lines to recover the failure. Moreover, the dotted line actually represents a protection path group (more than one path) existing between the node pair. Thus, it can be seen that the path segment restoration technique is very flexible in its restoration. Since it is any node pair, the two end nodes of the working path is just one of these node pairs. Thus, path restoration can be seen as a special case of the path segment restoration. As a more generic technique, the path segment restoration technique shows advantages of better spare capacity efficiency and faster restoration speed than the path restoration technique. Of course, this is the at the cost of more complicated network control and operation as rather than a single end-to-end protection path, we have to handle all the eligible restoration paths corresponding to any working path segments. For example, in Fig. 1 there are a total of 3×2=6 protection path groups that recover the failure of link (3-4). The segment-protecting p-cycle technique is essentially also a type of path segment protection technique which, compared to path segment restoration, is more restrictive to require deploying preconfigured p-cycles for working segment recovery.
