7.8 Shared Protection Based on Pre-cross-connected Bandwidth

The predeployed-subconnection protection scheme avoids problems with optical amplifier transients; however, it does require a small amount of switching to concatenate subconnections together to form the appropriate backup path. It is worthwhile to discuss a class of shared protection schemes based on pre-cross-connected bandwidth, where the need for any switching at the time of failure is eliminated except at the endpoints of the failed connection. Because of the minimal amount of switching required, these schemes are likely to be somewhat faster than the subconnection method, although the issue of transients does need to be addressed in these schemes.

The origin of this protection class is pre-connected protection cycles, or p-cycles, where the spare capacity is pre-connected to form cycles [GrSt98]. Each cycle protects against failures on the cycle itself, as well as failures on links that straddle the cycle. Restoration requires switching only at the nodes on either side of the failure. (The initial p-cycle proposal was for link-based protection; however, the idea was later extended to path protection [KoGr05].) P-cycle link-based protection is illustrated in Fig. 7.21, where there are two working paths as shown by the dotted lines: B-F-E-D and B-F-G. Only one cycle of protection capacity is shown, A-B-C-D-E-F-A, as indicated by the dashed line. This one cycle is not sufficient to protect against all possible working-path failures; however, for simplicity, the other cycles are not shown. (Note that because this is a closed protection ring, lasing