ABSTRACT
Wavelength-division multiplexing (WDM) divides the tremendous bandwidth of a fiber into many nonoverlapping wavelengths (WDM channels)
based on which multichannel lightwave networks can be built. A wavelength-routed (wide-area) all-optical WDM network consists of optical routing nodes interconnected by optical links. Signals on different wavelengths are coupled into the link using wavelength multiplexers. An optical routing node is capable of routing each wavelength on an incoming link to any outgoing link without having optoelectronic conversion.1,2
In a wavelength-routed WDM network, end users communicate with one another via lightpaths. A lightpath is an all-optical channel that may span multiple fiber links to provide a circuit-switched interconnection between two nodes. The end nodes of a lightpath access the lightpath with transmitters and receivers that are tuned to the wavelength on which the lightpath operates, while the intermediate nodes route the lightpath using their active switches. In the absence of wavelength converters, a lightpath must occupy the same wavelength on all the fiber links that it traverses. This property is known as the wavelength-continuity constraint. Two or more lightpaths traversing the same fiber link must be on different wavelengths so that they do not interfere with one another.1,3,4
To establish a circuit-oriented connection between two end nodes in a wavelength-routed WDM network, a lightpath needs to be set up between them. However, due to the limited number of wavelengths available in a fiber link and the limited number of transceivers equipped at each node (transceivers are very expensive), it is not possible to establish a lightpath between every pair of nodes.1,3 Given a set of lightpaths that need to be established and a constraint on the number of wavelengths, determining the routes over which these lightpaths should be established and the wavelengths that should be assigned to them is called the routing and wavelength assignment (RWA) problem. Lightpaths that cannot be established due to constraints on routes and wavelengths are blocked. Therefore, minimizing the blocking probability is one objective of RWA algorithms.1
