P-cycle protection

The p-Cycle protection scheme is a technique to protect a mesh network from a failure of a link, with the benefits of ring like recovery speed and mesh-like capacity efficiency, similar to that of a shared backup path protection (SBPP). p-Cycle protection was discovered in late 1990s, with research and development done mostly by Wayne D. Grover, and D. Stamatelakis.

In Transport communication networks two methods were developed and introduced for restoration and recovery, one was a ring-based protection and the other was mesh restoration. The ring based protection offered a quick recovery time at the expense of higher capacity redundancy, while the mesh restoration offered better capacity-efficiency at the expense of slower recovery times. In 1998 the p-Cycle became a promising technique for recovery in mesh networks because of the combined benefits of ring network recovery speed and mesh like capacity efficiency. In a mesh network, the spare capacity is used to create the ring like structures as shown in Figure 1. Due to the nature of the rings assuming bi-directional line switched ring (BLSR), only 2 end nodes are involved in a case of a link failure to switch traffic to a pre-planned cycle (path) and recover, as it is demonstrated in Figure 2.

One of the key differences between a ring-based scheme and the p-cycle scheme is the ability of the p-cycle to protect links that are not on the p-cycle ring as shown in Figure 3. The ability to protect two channel for every spare channel that is assigned to the p-cycle allows to achieve mesh-like capacity efficiency. This feature gives the p-cycle the additional efficiency over the ring-based schemes."Another over looked feature of the p-Cycle is that working paths may be freely routed over the network graph and are not limited to follow the ring-constrained routings".

The p-cycles come in few variations depending on how they protect a given network and their underlying architecture. The types of p-cycles that are available are: Hamiltonian, Simple, Non-Simple, Span, Node encircling, Path, and Flow. The Hamiltonian, Simple, and Non-Simple are named after their underlying architecture (In relationship to the Network). The Span, Node, Path, and Flow p-cycles are named after the type of protection offered to the network.

To design p-cycle, a few methods may be used. The two main categories in which the p-cycles are formed are: Centralized or Distributed. Further categorization is based on a number of factors including order of the p-cycle and working demands based on routing. The p-cycles can be created after the working demands are routed in the network or at the same time depending on the needs and requirements. There are a number of papers dealing with the p-cycle design, and the idea that p-cycle networks are based many times on the single Hamiltonian cycle seems to float around. While the idea may be good from management simplicity, it does not mean it is the best possible solution.

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