No abstract

The most common aim in designing a survivable network is to achieve restorability against all single span failures, with a minimal investment in spare capacity. This leaves dual-failure situations as the main factor to consider in quantifying how the availability of services benefit from the investment in restorability. We approach the question in part with a theoretical framework and in part with a series of computational routing trials. The computational part of the analysis includes all details of graph topology, capacity distribution, and the details of the restoration process, effects that were generally subject to significant approximations in prior work. The main finding is that a span-restorable mesh network can be extremely robust under dual-failure events against which they are not specifically designed. In a modular-capacity environment, an adaptive restoration process was found to restore as much as 95% of failed capacity on average over all dual-failure scenarios, even though the network was designed with minimal spare capacity to assure only single-failure restorability. The results also imply that for a priority service class, mesh networks could provide even higher availability than dedicated 1+1 APS. This is because there are almost no dual-failure scenarios for which some partial restoration level is not possible, whereas with 1+1 APS (or rings) there are an assured number of dual-failure scenarios for which the path restorability is zero. Results suggest conservatively that 20% or more of the paths in a mesh network could enjoy this ultra-high availability service by assigning fractional recovery capacity preferentially to those paths upon a dual failure scenario.

Abstract-The total transmission capacity required by a transport network to satisfy demand and protect it from failures contributes significantly to its cost, especially in long-haul networks. Previously, the spare capacity of a network with a given set of working span sizes has been optimized to facilitate span restoration [11], [12]. Path restorable networks can, however, be even more efficient by defining the restoration problem from an end to end rerouting viewpoint. We provide a method for capacity optimization of path-restorable networks which is applicable to both synchronous transfer mode (STM) and asynchronous transfer mode (ATM) virtual path (VP)-based restoration. Lower bounds on spare capacity requirements in span-and path-restorable networks are first compared, followed by an integer program formulation based on flow constraints which solves the spare and/or working capacity placement problem in either spanor path-restorable networks. The benefits of path and span restoration, and of jointly optimizing working path routing and spare capacity placement, are then analyzed.

We develop and test an algorithmic approach for providing p-cycle survivable transport network designs. The basic approach is to first identify a set of primary p-cycles, then to search for improvements on those cycles through various operations to create a final set of cycles of high individual and collective efficiency, before finally placing one p-cycle at a time, iteratively, until all working capacity of the network is protected. We compare the solution quality of the algorithm to optimal designs obtained with ILP methods. The primary advantage of this algorithmic approach is that it entirely avoids the step of enumerating all cycles, which is a preliminary step in both ILP and heuristic solution methods based on preselection. This method proceeds initially with no more than S "primary" p-cycles, and in the worst case will enumerate no more than S2*N other candidate cycles during its execution, where S is the number of spans in the network and N is the number of nodes. We also find that the set of candidate cycles developed by the algorithm can themselves be used as a quite small but highly effective set of eligible cycles in an ILP design model. IndexTermsp-cycles, optical mesh network design, algorithmic and heuristic design, restoration and protection. 0-7803-81 18-1/03/$17.00 0 2003 IEEE

No abstract

Recent work on restorable networks has shown experimentally that one can support 100% restoration with an optimized set of closed cycles of spare capacity while requiring little or no increase in spare capacity relative to a span-restorable mesh network. This is important and unexpected because it implies that future restoration schemes could be as capacity efficient as a mesh network, while being as fast as ring-based networks because there is no real-time work at any nodes other than the two failure nodes. This paper complements the prior work by giving a greater theoretical basis and insight to support the prior results. We are able to show in a bounding-type of argument that the proposed protection cycles ("p-cycles") have as high a restoration efficiency as it is possible to expect for any type of preconfigured pattern, and are categorically superior to preconfigured linear segments or trees. We are also able to show that the capacity efficiency of a fully preconfigured p-cycle network has the same well-known lower bound as that of a span restorable mesh network which is cross-connected on-demand. These results provide a theoretical underpinning for the efficiency of p-cycles and confirmation of the experimental observations. Index Terms-Communication system operations and management, metropolitan area networks, network fault tolerance, SONET, wide area networks.

No abstract

No abstract