Shared Backup Path Protection Research Articles

A matrix-based analytical approach to connection unavailability estimation in shared backup path protection

This letter introduces a matrix-based approach to connection unavailability estimation in shared backup path protection (SBPP). The proposed approach yields accurate results for networks of national size using simple matrix operations, being therefore suitable for online routing algorithms. The accuracy of the unavailability estimates is verified through simulations.

Design and performance of protected working capacity envelopes based on p-cycles for dynamic provisioning of survivable services

As an alternative to the shared backup path protection (SBPP) method, we develop a framework for dynamic provisioning of survivable services through the use of p-cycles to form a protected working capacity envelope (PWCE) within which dynamic provisioning of protected services is greatly simplified. With a PWCE, arbitrarily fast dynamic service demands can be handled with much less complexity (in terms of database maintenance and state update dissemination) than with SBPP. Only a simple open-shortest-path-first (OSPF) topology view of nonexhausted spans in the envelope is required. If a new path can be routed through the envelope, it is protected by virtue of being routable. This is in contrast to needing a full database of the network state so that the end user can set up a shared backup protection path under SBPP. In addition, dissemination of spare capacity sharing updates occurs only on the time scale of the nonstationary evolution of the demand statistics, not like SBPP, which occurs on the time scale of individual connection arrivals or departures. During statistically stationary periods there is no dissemination of spare capacity sharing updates whatsoever with an envelope that is well matched to its load. The PWCE concept thus offers some new trade-offs between operational simplicity and spare capacity efficiency. Under the PWCE concept p-cycles are of particular interest for consideration because, although many protection techniques can be the basis of PWCE operation p-cycles offer the unique combination of ring-like protection times with the capacity efficiency of shared-mesh networks. But, in addition, p-cycles offer a further important property for a transparent optical network: that of providing fully pre-cross-connected protection paths. Because all protection paths are preconnected structures, optical transmission path integrity can be validated before failure and is not of such concern as it is in schemes where optical replacement path segments of several wavelength channels would have to be assembled on the fly (without the benefit of o-e-o between stages). The main contribution of this work is the detailed implementation and simulation of test networks operating under PWCE and designed with novel envelope volume maximizing formulations. A wide range of network capacity environments were considered to find that p-cycle-based PWCE is close to SBPP in blocking performance while simultaneously offering much simpler operation and a faster restoration speed.

The protected working capacity envelope concept: an alternate paradigm for automated service provisioning

At present we see only one basic approach being considered by implementors and standards organizations for provisioning of dynamic protected lightpath services that make efficient use of shared protection capacity. This is the paradigm of a primary working path protected end-to-end by a disjoint backup path using shared spare capacity. Sharing is arranged among the backup paths associated with other primary paths that are failure-disjoint from the current primary path. This approach called shared backup path protection (SBPP) has many advantages but also provides a great deal of network state information to be available and current in every node. It also makes the arrangement of protection a per-connection task, rather than user-selectable service option supported by the network itself. We propose an alternative paradigm for consideration, partly summed up as provisioning over protected capacity rather than provisioning protection. Under a given distribution of spare capacity, locally acting protection or restoration schemes create an "envelope" of protected working channels. Dynamic provisioning within this envelope is simplified to a shortest path routing problem and (depending on the mode of operation) requires little or no dissemination of state changes on a per-connection basis. We explain how existing "static" capacity design methods can be adopted to the dimensioning of such a working capacity envelope and the envelope dimensions further adapted online to track evolution of the overall pattern of random demand. An important property is that nothing needs to be done to arrange protection for services on the per-connection timescale other than routing the service itself. Arbitrarily fast-paced demand arrivals and departures can be accommodated within a static distribution of spare capacity. Adjustments to the envelope itself are required only on the timescale on which the statistical parameters of the random demand changes. This may provide an inherently more scalable, less database-dependent, and higher-availability alternative than SBPP, or at least an additional service modality that can be offered to customers.

Extending the p-cycle concept to path segment protection for span and node failure recovery

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.