Heuristic Solution to Protect Communications in WDM Networks using P-cycles

Hamza Drid 1, Bernard Cousin 1 and Miklos Molnar 2 IRISA 1 Université de Rennes I – Campus de Beaulieu 35042 Rennes Cedex, France 2 INSA Département informatique F35043 Rennes Cedex, France {hdrid, bcousin, molnar}@irisa.fr

Abstract

Optical WDM mesh networks are able to transport huge amount of information. The use of such technology however poses the problem of protection against failures such as fibre cuts. One of the principal methods for link protection used in optical WDM networks is pre-configured protection cycle (p-cycle). The major problem of this method of protection resides in finding the optimal set of p-cycles which protect the network for a given distribution of working capacity. Existing heuristics generate a large set of p-cycle candidates which are entirely independent of the network state, and from then the good sub-set of p-cycles which will protect the network is selected. In this paper, we propose a new algorithm of generation of p-cycles based on the incremental aggregation of the shortest cycles. Our generation of p-cycles depends on the state of the network. This enables us to choose an efficient set of p- cycles which will protect the network. The set of p-cycles that we generate is the final set which will protect the network, in other words our heuristic does not go through the additional step of p-cycle selection.

I. Introduction

Survivability becomes an important issue and well studied topic in wavelength division multiplexing (WDM) optical mesh networks. The WDM technology allows the transmission of huge amounts of data over the same fibre. Currently an optical fibre of 140 wavelengths is able of transmitting up to 14 Tbit/s over a distance of about 160 kilometres [11]. Consequently, any cut of such a fibre may lead to huge data loss and a lot of traffic being blocked. For this reason, methods and mechanisms of protection should be implemented to minimize the data loss when the failure occurs. Several methods for network protection have been proposed. One of the principal protection methods used in optical WDM networks is pre-configured protection cycles (p-cycle). The concept of p-cycle was introduced by Grover and Stamatelakis [2]. The basic idea of p-cycle protection has been inspired from the ring1 protection. The main difference between p-cycle and ring protection is that, p-cycle protection not only protects the cycle links but protect all links whose end nodes belong to the p-cycle. In other words, p-cycle protects the cycle links and the straddling links in the cycle. A straddling link is a link which does not belong to the p-cycle but whose end-nodes are both on the p-cycle. Each p-cycle can offer two restoration paths to the failed straddling links without requiring any additional spare capacity. This propriety reduces effectively the required protection capacities. The major problem of this method of protection resides in finding the optimal set of p-cycles which protects the network for a given working capacity distribution. The p-cycle design can be formulated as a non-joint or joint optimization problem. In the first approach, after the working paths are routed (e.g. using shortest paths) the optimal set of p-cycles is calculated using available capacity [1][2][3][4]. In the second approach, the routing of the working paths (for the demands) and their p-cycles are computed simultaneously minimizing the total capacity [10][6][7].

Several solutions have been proposed in the literature to solve this optimization problem. These solutions can be classified into two categories: exact solution and heuristic solution. The first category uses generally the Integer Linear Programming (ILP) to find the optimal solution. However, ILP becomes unsuitable as the size of the network increases. Because the number of p-cycles possible in a graph grows exponentially as the size of the network grows (number of nodes and edges). The second category of solution is heuristic which is itself divided into two sub-classes: a heuristic approach based on an ILP formulation and a pure heuristic approach. In the first sub-class of solution, the limit set of p-cycle candidates is generated, and then the good sub-set of p-cycles which protects the network is selected using ILP formulation. The second sub-class (pure heuristic) tries to find a good

1 In the ring protection, the traffic on the failed link is rerouted around the ring on the protection fibers between the nodes adjacent to the failure.

1 solution without using ILP formulation. The objective of this solution is to reduce the time required to compute the good set of p-cycles that protects the network.

In this paper, we focus on the heuristic algorithms, which first generate a set of p-cycle candidates, and then a good sub-set of

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