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of some intra-area traffic. When fiber link < i,j > along р w fails, instead of shutting down the
entire working path р w and switching the traffic to р b , this scheme only turns down the sub-
path р wk and switches the traffic to р bk . Therefore, other sub-paths that do not traverse fiber
link < i,j > along р w are not affected. Because of the independence of sub-paths, sub-path
protection can survive from up to α failures as long as there is at most one failure per area,
where α is the number of areas.
Ten major protection schemes (see Figure10) that support unicast traffic in WDM mesh
networks are reviewed in this section.
Figure 10. Reviewed unicast protection algorithms.
3.1. Dynamic Hop Constrained Sub-Path Protection (HCSPP) [22]
The HCSPP can improve cooperation between the blocking probability and failure
recovery time. In this way, the straddling path and hop count are used to subdivide a
protection cycle. When a request connection arrives, in the first step, the primary and the end-
to-end backup paths should be calculated. After computing the costs of primary path and its
corresponding end-to-end backup path and also forming the cycle, the hop counts of the cycle
is computed. If the hop count is lower than a threshold, then the corresponding wavelength
resources are recorded. For every node along primary and end-to-end backup paths
(excluding the source and destination), the preliminary straddling path which should be link-
disjoint with primary and end-to-end backup paths is computed. The shortest hop count
straddling paths which start from each node are selected and added to a list. Then, the list is
sorted based on an ascending order of node numbers. Therefore, the protection cycle which is
formed through the primary path and end-to-end backup path is divided by the selected
straddling paths from the list. Finally, redundant straddling paths are removed.
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