Path restoration (PR)
Fig. 1(a) Path restoration (with the first protection path)
Fig. 1(b) Path restoraiton (with the second protection path)
Path restoration (PR) is a type of path-based network protection technique. The protection or restoration is carried out at the two end nodes of a working flow. PR is similar to shared backup path protection (SBPP) . However, the key difference between them is that the SBPP network protection technique is a type of failure-independent technique; that is, no matter where the failure is on the working route, the recovery would always use the same protection route and perform switching-over operation at the two end nodes of the working flow. In contrast, path restoration is a type of failure-dependent network protection technique, which allows multiple protection paths for failure restoration. The protection paths are not required to be link or node disjoint from the working path. Also, for a specific link or node failure, a specific set of protection paths are used to recover the failure. Fig. 1(a) and (b) show that two different protection paths are used to protect the working path. The two routes are sharing some common links with the working path. Thus, for a specific link failure, only those unaffected protection paths can be used to protect the working path. Path restoration also allows spare capacity sharing among different protection paths. The key condition to ensure full failure recovery is that a fiber link should reserve an amount of protection capacity that is maximal among all the link failure situations.
Fig. 1(c) Path restoration with stub release
To have even better capacity utilization, under the PR technique, there is a concept called stub release, which releases all the capacity that was originally used by the working flow, which is interrupted and switches all its traffic onto its protection paths. Fig. 1(c) just shows an example of the stub under the PR network protection technique. In general, the PR technique with stub release is the most capacity-efficient network protection technique among all the network protection techniques including ring techniques, span restorable networks, SBPP, and so on.
The good spare capacity efficiency of the PR technique is the at the cost of more complicated network control and operation and a longer restoration time. This tradeoff becomes even stronger when the step of stub release is also taken, which needs to free those capacity on the stub of an interrupted working path first and then signal to establish corresponding protection paths for failure recovery.
