As wavelength division multiplexing (WDM) technology matures and multicast applications become increasingly popular,supporting multicast at the WDM layer becomes an important and yet challenging topic. In this paper, we study constrained multicast routing in WDM networks with sparse light splitting, i.e., where some switches are incapable of splitting light (or copying data in the optical domain) due to evolutional and/or economical reasons. Specifically, we propose four WDM multicast routing algorithms, namely,Re-route-to-Source, Re-route-to-Any, Member-First, and Member-Only. Given the network topology, multicast membership information, and light splitting capability of the switches, these algorithms construct a source-based multicast"light-forest"(consisting one or more multicast trees) for each multicast session. While the first two algorithms can build on a multicast tree constructed by IP (which does not take into consideration the splitting capability of the WDM switches),the last two algorithms attempt to address the joint problem of optimal multicast routing and sparse splitting in WDM networks. The performance of these algorithms are compared in terms of the average number of wavelengths used per forest (or multicast session), average number of branches involved (bandwidth) per forest as well as average number of hops encountered (delay) from a multicast source to a multicast member. The results obtained from this research should present new and exciting opportunities for further theoretical as well as experimental work.
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