A New Insight and Approach to Node Failure Protection with Ordinary p-Cycles

Since the advent of p-cycles it has been understood that, in addition to their span-protecting properties, p-cycles have the same inherent protection as a BLSR ring to "on-cycle" demands that transit a failed node. It has remained less clear how to protect straddling demand flows from node failure as well. Other work has sought to achieve full node protection with extensions of the p-cycle concept such as node-encircling p-cycles, path-segment or "flow"-protecting p-cycles, and failure independent path-protecting p-cycles. Here we report a rather simple and useful finding about the problem-that high levels of node failure protection can be achieved with an ordinary set of p- cycles which designed in a way that every demand that transits a node in a straddling manner is also intercepted at points upstream and downstream on its route by some other p-cycle. We first characterize the inherent properties of ordinary minimum- capacity p-cycle network designs when inspected for use from this new standpoint. We then alter the basic network design model to maximize node restorability and even achieve 100% protection against both single node and span failures, with little additional capacity. The practical importance is that the simplicity of basics-cycles is retained, and only one set ofp-cycles is required, while efficiently achieving node failure protection either for priority paths or for all demands in a network.