Distributed path restoration based on optical cross-connects can provide highly capacity-efficient real time restoration for WDM-based optical networking. However, to obtain an assured restoration level with the theoretically very low amounts of spare capacity that path restoration allows one must solve, or closely approximate a solution to, the integer multicommodity maximum flow (MCMF) problem. MCMF is, however, a hard combinatorial optimization problem due to what is called the "mutual capacity" aspects of the problem: which of many competing Origin-Destination pairs should be allowed paths over the finite spares on each span? Integer MCMF is further complicated by the non uni-modular nature of the problem, i.e. fractional flows are forbidden but would arise if solved by Linear Programming. This paper presents a heuristic principle that tests well against Integer Programming solutions of MCMF routing. The heuristic is first characterized in a centralized program, then adapted for use in a distributed path restoration protocol. In all test cases the protocol obtains over 97% of the paths found in an optimal MCMF solution in the same network. Via OPNET simulation it is also predicted that the protocol will run in well under 2 seconds which means it could be used directly in real-time, or in distributed pre-failure self-planning, for restoration. The significance is that network operators could aggressively optimize their spare capacity, towards theoretical minimums, while still assuring 100% restorability.

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 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.

This paper studies the unavailability of mesh-restorable networks from the standpoint of dual-span failures as the main source of outage in networks that are efficiently designed to fully restore any single failure. Computer emulation of the detailed routing behavior of three progressively more adaptive restoration protocols is used to find the dual-failure restorability (R 2 ) under modular and nonmodular capacity assumptions. This is supplemented with two new optimization formulations to study the implications of designing for R 2 =100% and to observe the trade-off of achievable R 2 versus the total allowed investment in spare capacity. I.

this report, contact: Wayne D. Grover TRLabs 800 Park Plaza, 10611 - 98 Avenue Edmonton, Alberta, Canada, T5K 2P7 Tel.: (780) 441-3815 Fax: (780) 441-3600 World Wide Web URL: http://www.trlabs.ca Network Restorability Design Using Preconfiguration TR-1999-05 TRLabs Network Systems Table of Contents Table of Contents 1. Executive Summary ..............................................................................................................1 2. Introduction ..........................................................................................................................2 2.1. Background ..................................................................................................................2 2.2. Prior Work ...................................................................................................................4 2.3. Outline .........................................................................................................................7 3. Terms, Concepts, and Methodology .....................................................................................7 3.1. Terminology .................................................................................................................7 3.2. Methods and Test Networks ........................................................................................9 4. Pre-configuration Design Based on Spanning Trees ............................................................10 4.1. Spanning Tree Pre-configuration Heuristic .................................................................10 4.2. Tree Pre-configuration Results ....................................................................................11 5. Genetic Algorithms for Mixed-Pattern Pre-configuration Design ..............

Abstract not found

We describe an approach for economically optimized co-design of ring and mesh network components in a single hybrid transport network. It is based on a proposed planning principle, called "forcer-clipping", to strategically embed rings into a mesh network in a way that can lower total design cost below either pure architecture. Results show significant advantage depending on the test network and relative ring-mesh equipment costs. A Mixed Integer Programming formulation is also presented and solved to provide a performance benchmark for the heuristic on suitably small test cases and to test for validity of the forcer-clipping hypothesis. I.

Traffic routing is a key component in a network planning system. This paper concentrates on the routing algorithms and follows their evolution over multiple releases of a planning tool during a period of six years. The algorithms have grown from the initial stage of finding shortest paths with Dijkstra's algorithm to cover more complex routing tasks such as finding protected and unprotected routes and capacity limited routing. We present the algorithms and in particular emphasize the practical aspects: how previous algorithms were reused and what were the practical experiences of using the algorithms. A conclusion of the study is that algorithms should be considered with an engineering attitude. It is not enough to focus on selecting the most sophisticated state-ofthe -art algorithm for a given problem. Evolution capability, potential for reuse, and the development cost over the system lifetime are equally important aspects. 2001 Elsevier Science Inc. All rights reserved.

of "roll to protection" in rings. The basic methods can be adapted for other maintenance models or models of the restoration process or adapted to handle multiple maintenance scenarios. The work is aimed at understanding and managing the effects of maintenance in a mesh-restorable network and ultimately providing new operational and design capabilities for future mesh-based network planning and operations systems. I. Motivation and Background One of the motivations for mesh-restorable networking is the greater capacity efficiency of mesh networks relative to ring-based transport networks. The capacity efficiency comes from the more direct routing of working paths, the need for less spare capacity for restoration, and the avoidance of "stranded capacity" effects in rings where one or more ring spans may exhaust while other spans of the ring have valuable but unusable remaining working capacity. Of course capacity efficiency is not the only consideration in a choice of n

this document will be updated and grow in its coverage of additional schemes. The next update will include the addition of design for shared backup path protection schemes Wayne D. Grover, John Doucette TRLabs / University of Alberta Design of mesh-based transport networks Version 1, November 2001 2 I.

Abstract not found