Path protection

Abstract: This study investigates the problem of fault management in a wavelength-division multiplexing (WDM)-based optical mesh network in which failures occur due to fiber cuts. In reality, bundles of fibers often get cut at the same time due to construction or destructive natural events, such as earthquakes. Fibers laid down in the same duct have a significant probability to fail at the same time. When path protection is employed, we require the primary path and the backup path to be duct-disjoint, so that the network is survivable under single-duct failures. Moreover, if two primary paths go through any common duct, their backup paths cannot share wavelengths on common links. This study addresses the routing and wavelength-assignment problem in a network with path protection under duct-layer constraints. Off-line algorithms for static traffic is developed to combat single-duct failures. The objective is to minimize total number of wavelengths used on all the links in the network. Both integer linear programs and a heuristic algorithm are presented and their performance is compared through numerical examples.

Abstract: This paper is an introduction to survivability of WDM networks. All the main optical protection techniques proposed as far as now for the WDM layer are classified and reviewed. In particular, commonly adopted protection strategies for ring and mesh networks are explained. Moreover, off-line planning of WDM networks able to support path protection is briefly introduced. Finally, an example of heuristic network-capacity optimization is presented, discussing results obtained by considering a case-study network.

Abstract: The paper introduces an extension to the method of p-cycles for network protection. The p-cycle concept is generalized to protect path segments of contiguous working flow, not only spans that lie on the cycle or directly straddle the p-cycle. The original span protecting use of the p-cycle technique is extend to include path protection or protection of any flow segment along a path. It also gives an inherent means of protecting working flows that transit a failed node. We use integer linear programming to study the new concept and determine its inherent capacity requirements relative to prior p-cycle designs and other types of efficient mesh-survivable networks. Results show that path-segment-protecting p-cycles ("flow p-cycles") have capacity efficiency near that of the shared backup path-protection (SBPP) scheme currently favored for optical networking. Because its protection paths are fully preconnected and because it protects path segments (not entire paths), it has the potential for both higher speed and higher availability than SBPP. We also develop capacity optimization models to support 100% restoration of transiting flows through failed nodes. Only a very small additional spare capacity is needed to achieve both 100% span and intermediate node-failure restorabilities, and a very high transiting traffic restorability can be accomplished for node failure restorability given spare capacity only for span-failure protection. An immediate practical application is to suggest the use of flow p-cycles to protect transparent optical express flows through a regional network.

Abstract: We propose a new technique for optical-network protection called failure-independent path-protecting (FIPP) p-cycles. The method is based on an extension of p-cycle concepts to retain the property of full preconnection of protection paths, while adding the property of end-to-end failure-independent path-protection switching against either span or node failures. An issue with applying the popular method of shared-backup path protection (SBPP) to an optical network is that spare channels for the backup path must be cross connected on the fly upon failure. It takes time and signaling to make the required cross connections, but more importantly, until all connections are made, it is not actually known if the backup optical path will have adequate transmission integrity. Thus, speed and optical-path integrity are important reasons to try to have backup paths fully preconnected before failure. With fully preconnected protection, not only can very fast restoration be attained, but the optical-path engineering can also be assured prior to failure. Regular p-cycles are fully preconnected, but are not end-to-end path-protecting structures. SBPP is capacity efficient and failure independent-failures only need to be detected at the end nodes and the end nodes activate and switch over to one predefined backup route for each working path-but the backup paths are not preconnected. FIPP p-cycles support the same failure-independent end-node-activated switching of SBPP, but with the fully preconnected protection-path property of p-cycles. As a fully preconnected and path-oriented scheme, FIPP p-cycles are, therefore, potentially more attractive for optical networks than SBPP. Results confirm that FIPP p-cycle network designs will exhibit capacity efficiency that is characteristic of path-oriented schemes and may be as capacity efficient as SBPP, but more conclusive comparisons on larger scale networks await further study.