7.3.1 Ring Protection

In ring protection, the protection capacity is organized into ring structures. A ring is a simple survivable topology, such that if a single link or node failure occurs, all traffic that was routed over the failed link/node is routed in the reverse direction around the ring to avoid the failure. Note that a network with a mesh topology can use ring-based protection; a working path that traverses multiple rings is protected by spare capacity in each of the individual rings. There are many variations of ring-based optical protection, as described in [LSTN05]. For example, the implementations may differ on whether they use two-fiber rings or four-fiber rings, or whether they restore each path individually or several multiplexed wavelengths at once.

Dedicated ring protection is fairly straightforward: a bi-directional working path combined with its protection capacity occupies one wavelength around both directions of the ring. Shared ring protection allows greater capacity efficiency, although it may be more challenging to implement.

Network-side shared ring protection is described here in more detail to illustrate some of the interesting features, especially with regard to optical bypass. An example of this type of protection is shown in Fig. 7.7, where the ring is assumed to be a two-fiber ring. One fiber...