Multicast Traffic Grooming in WDM Networks Ahmed E. Kamal and Raza Ul-Mustafa Department of Electrical and Computer Engineering, Iowa State University, Ames, IA, U.S.A. ABSTRACT This paper considers the problem of grooming multicast traffic in WDM networks, with arbitrary mesh topologies. The problem is different from grooming of unicast traffic, since traffic can be delivered to destinations through other destinations in the same set, or through branching points. The paper presents an optimal Integer Linear Programming (ILP) formulation in order to minimize the cost of the network in terms of the number of SONET Add/Drop Multiplexers (ADM). The formulation also minimizes the number of wavelength channels used in the network, and does not allow bifurcation of traffic. Since the ILP formulation is able to solve limited size problems, the paper also introduces a heuristic approach to solve the problem. Keywords: WDM networks; traffic grooming; ADMs; mesh topology; nonuniform traffic; multicast traffic; optimization; Integer Linear Programming (ILP); heuristics. 1. INTRODUCTION During the last decade, Wavelength Division Multiplexing (WDM) Networks 1 have emerged as an attractive architecture for backbone networks. WDM networks provide high bandwidth, on the order of tens of Gigabits per second per channel. However, recently two observations are driving the research community to explore the traffic grooming problem in WDM networks. First, the bandwidth requirements of most of the current applications are just a fraction of the bandwidth offered by a single wavelength in WDM networks. Second, the dominant cost factor in WDM networks is not the number of wavelengths but rather the network components, specifically, higher layer equipment, such as SONET Add/Drop multiplexers (ADMs), or MPLS or IP router ports ∗ . Therefore the cost effectiveness of WDM networks depends on the amount of the optical passthrough provided by the network to the given traffic, thus reducing the number and cost of the higher layer equipment. However, the amount of the optical passthrough depends on the traffic arrangement on the optical layer. Traffic grooming is therefore defined as an intelligent allocation of the traffic demands, between different network nodes, onto an available set of wavelengths in such a way that reduces the overall cost of the network. In general the traffic grooming problem is regarded to be even harder than the combined virtual topology design and Routing and Wavelength Assignment (RWA) problem. 2 To make the problem somewhat less difficult, many relaxations have been considered in the literature. For example, most of the studies allow the traffic between each source-destination set to be (vertically) split over multiple wavelengths - a condition known as bifurcation. Due to bifurcation different components of the same traffic demand may traverse different links. This provision provides flexibility in traffic allocation, which may lead to a reduction in the number of wavelengths as well as the number of ADMs. Although allowing bifurcation simplifies the problem, it is unrealistic since it increases the complexity and the cost of traffic reassembly, and may also introduce jitter at the application layer. Many applications, especially real-time applications, require that their traffic be kept intact, i.e., without demultiplexing at the source, independent switching at intermediate nodes, and multiplexing at the destination. In this paper we have considered the case in which the traffic between each pair of nodes is not allowed to be bifurcated, i.e., traffic grooming without bifurcation. Please note that a non-bifurcated assignment does not necessarily mean that a specific traffic demand cannot traverse different wavelengths, rather in practice it may traverse many wavelengths before reaching the destination (a condition we refer to as horizontal splitting). However, the whole traffic demand needs to be intact on each wavelength it traverses. Further author information: (Send correspondence to Ahmed E. Kamal) Ahmed E. Kamal: E-mail: kamal@iastate.edu, Telephone: 1 515 294 3580 Raza Ul-Mustafa: E-mail: raza@iastate.edu, Telephone: 1 515 294 2295 * Without loss of generality, in this paper we will assume that the Synchronous Optical Networks (SONET) is being used as a higher layer in WDM networks.