Combined node and span protection strategies with node-encircling p-cycles

We develop three new design models for combined node and span protection with a single set of node-encircling p-cycles (NEPCs). In a simple reference model each node is protected by a single dedicated NEPC. In the second model NEPCs can be shared by multiple nodes as needed, and each node is allowed to make use of as many different NEPCs as needed for a capacity-efficient design. To manage the complexity, protection routing is assumed to be evenly split in both directions around the NEPC, resulting in a slight over-provisioning of NEPCs and spare capacity. The final model is the most capacity efficient by virtue of also determining in which direction protection routing should proceed around each NEPC for each light-path affected by a node failure. The models can also permit a specified level of restorability below 100% so that a carrier could offer node-failure restorability on a differentiated service basis. A significant finding is that in test case networks 25% node restorability can be achieved in addition to 100% span restorability with a single integrated set of p-cycles that require as little as 4.1% more spare capacity than for conventional protection only against span-failures.