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2.1.4. The p-Cycle Protection
The p -cycle is a virtual cycle in a network and can protect working capacity of the link
that has both end points on the p -cycle. The p -cycle protection scheme uses additional
capacity in cycles to protect the links. Usually, different amount of capacity is needed on all
the links of a p -cycle. The capacity is pre-configured in such a way that when a link fails, only
the end nodes of the failed link need to do rerouting. Hence, no signaling is required. The p -
cycle protects two types of links, on-cycle links and straddling (chord) links. In Figure2, the
thick solid lines indicate the pre-configured capacity of p -cycle [14].
In on-cycle protection, it is assumed that there is another way around a cycle when a
failure occurs in order to reroute the connections over the other way. In a straddling link, the
end-nodes are on the cycle but the link is not on the cycle, and the cycle has two routes
between the end-nodes of the failed link. The p -cycle can hence protect twice the pre-
configured capacity of the p -cycle [14]. A link may be protected by several p -cycles, i.e. if a
link fails, the connections using that link may be protected by rerouting them along several
different p -cycles protecting that link [14].
Figure 2. p-cycles protection scheme: a)on-cycle link protection. b)straddling link protection.
The optimal solution is usually hard to find because we need to enumerate all the cycles
of a graph. In a planar graph that can be applied for many real networks, the scheme using
special kind of cycles would be very fast to find very good solution [15]. Pre-configuration
means that the configuration is done before happening a failure [16].
Figure 3. Protection principle of p-cycles for link protection.
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