Multicast routing and wavelength assignment in WDM networks with limited drop-offs

In WDM networks with limited drop-offs, the route of a multicast connection consists of a set of light-trees. Each of the light-tree is rooted at the source node and contains no more than a limited number, say k, destination nodes due to the power loss of dropping optical signals off at destination nodes. We call such a light-tree k-drop light-tree. In this paper we study the multicast routing problem of constructing a set of k-drop light-trees that have the minimal network cost. The network cost of a set of light-trees is defined as the summation of the link cost of all the light-trees. We first prove that this problem is polynomial-time solvable for k = 2 and NP-hard for k ≥ 3. We then propose a 4-approximation algorithm for the problem for k ≥ 3. A wavelength assignment algorithm is also proposed to assign wavelengths to the light-trees of a multicast connection. In the end we give simulation results showing that k-drop multi-tree routing can significantly save not only the network cost but also wavelengths used. Moreover, when k ≥ 5 its performance is very close to the case where k is infinite (i.e., the case of using a single tree for a multicast connection).