RWA problem in wavelength routed WDM networks

On the introductory part of the article by Varvarigos, &
Christodoulopoulos, (2011)*, they offer a very clear and straight
forward explanation on the light-path assignment problem on a
wavelength routed WDM network where they explain that that type of
routing method is the most common in a optical transport network.

In such a network data is transmitted via optical channels that may use
multiple consecutive fibers. The data will follow a light-path. The
establishment of a light-path is based on the assignment of a route
(sequence of links) and a wavelength that is free on all the links that
comprise that path. The establishment of networks path usually must
optimize one of more performance metrics. This problem is then known as the Routing and Wavelength Assignment problem (RWA).

The generic constraints of this problem are:

1. Paths that share the same link (s) cannot be assigned to the same wavelength.
2. In case there is no wavelength converter in a light-path, this light-path data flow must be transfered using the same wavelength throughout the links it passes by.

There are two traffic models where the RWA problem takes place:

1. Off-line or Static RWA: happens during the planning phase of the WDM network, when it is configure based on the predicted traffic it will handle. The set of connection is known in advance.
2. On-line or Dynamic RWA: happens during the operational phase, as light-paths requests arrive and are established one-by-one upon arrival, restrained to the current network utilization state (the light-paths that have been already set-up)

*Varvarigos, E. A. & Christodoulopoulos, K. (2011), 'Algorithmic  Aspects of Optical Network Design'' 15th Conference on Optical Networks Design and Modeling (ONDM), 2011 Bologna Italy.