A new approach to node-failure protection with span-protecting p-cycles

Recent work has revealed a new, relatively simple and possibly cost-effective, approach to achieve combined protection of optical networks against both node and span failures. The resulting network designs use only a single set of p-cycle structures that have the same or only slightly more capacity than a corresponding optimal set of p-cycles for span protection. The new principle is based on a generalization of how nodes in a BLSR-ring or p-cycle (to date) derive survivability through loop-back at the nearest two neighbour-nodes on the same ring. The generalization views any combination of node failure and an affected transiting path from the standpoint of the two-hop segment defined by the failure node, and the nodes immediately adjacent on the affected path. We then ask whether these nodes are found together within the same p-cycle as the failure node, or another p-cycle entirely. In any case where they are, we show that the transiting path affected by the node failure is inherently restorable by ordinary p-cycle switching actions whether the respective two-hop segment is on-cycle, straddling, or partially on-cycle and partially straddling. We explain the principle and characterize its effectiveness in terms of network-wide single node failure restorability (R1-node) in networks designed only for minimum spare capacity, networks designed for enhanced R1-node (at min capacity) and networks designed strictly for R1-node = 1.