Contents I. Introduction
Wavelength Division Multiplexing (WDM) networks are being deployed to meet the high bandwidth needs of Internet traffic, especially in backbone networks for nationwide or global coverage. The advent of Dense WDM (DWDM) in optical networks has significantly increased the bandwidth availability on a single fiber. End-users are connected via all-optical channels referred to as lightpaths that require no processing or buffering at intermediate nodes and potentially no intermediate electric/optic conversion. A lightpath is an optical path established between two nodes, created by the allocation of a wavelength throughout the path. The lightpath provides a circuit-switched interconnection between two nodes which may be located far from each other in the physical topology. In WDM routing networks, the number of wavelengths available on fiber links limits the number of end-to-end connections. Moreover, a WDM network imposes additional constraints on the wavelength assignment. If switching/routing nodes are not equipped with a wavelength converter facility, then a lightpath must operate on the same wavelength across all fiber links that it traverses, the Routing and Wavelength Assignment (RWA) is said to satisfy the wavelength continuity constraint. However, if each node is equipped with a wavelength converter, the wavelength continuity constraint disappears and the routing problem is the same as in normal circuit-switched networks where the only limiting factor is the number of available channels on each link. This continuity constraint leads to inefficient utilization of wavelength channels and results in higher blocking probability [1]. For example, a request may have to be rejected even though a route is available because of the non availability of the same wavelength on all the links along the route.
