27-03-2012, 12:07 PM
Routing and Wavelength Assignment in Optical WDM Networks
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Introduction to Optical WDM Networks
A basic property of single mode optical ber is its enormous low-loss bandwidth of several tens of
Terahertz. However, due to dispersive eects and limitations in optical device technology, single
channel transmission is limited to only a small fraction of the ber capacity. To take full advantage
of the potential of ber, the use of wavelength division multiplexing (WDM) technology has become
the option of choice. With WDM, a number of distinct wavelengths are used to implement separate
channels [1]. An optical ber can carry several channels in parallel, each on a particular wavelength.
The number of wavelengths that each ber can carry simultaneously is limited by the physical
characteristics of the ber and the state of the optical technology used to combine these wavelengths
onto the ber and isolate them o the ber.
Static Routing and Wavelength Assignment
If the trac patterns in the network are reasonably well-known in advance and any trac variations
take place over long time scales, the most eective technique for establishing optical connections
(lightpaths) between edge nodes is by formulating and solving a static RWA problem. For exam-
ple, static RWA is appropriate for provisioning a set of semipermanent connections. Since these
connections are assumed to remain in place for relatively long periods of time, it is worthwhile to
attempt to optimize the way in which network resources (e.g., physical links and wavelengths) are
assigned to each connection, even though optimization may require a considerable computational
Dynamic Routing and Wavelength Assignment
Under a dynamic trac scenario, edge nodes submit to the network requests for lightpaths to be set
up as needed. Thus, connection requests are initiated in some random fashion. Depending on the
state of the network at the time of a request, the available resources may or may not be sucient
to establish a lightpath between the corresponding source-destination edge node pair. The network
state consists of the physical path (route) and wavelength assignment for all active lightpaths. The
state evolves randomly in time as new lightpaths are admitted and existing lightpaths are released.