A New Route Control Model

ed as a single hop. YANNUZZI LAYOUT 5/22/08 2:53 PM Page 106 Authorized licensed use limited to: UNIVERSITAT POLITÈCNICA DE CATALUNYA. Downloaded on May 4, 2009 at 08:36 from IEEE Xplore. Restrictions apply. IEEE Communications Magazine • June 2008 107 model a cost is associated with each candidate (path, wavelength) pair. For simplicity, we shall only outline here the motivations for its use, and succinctly describe its computation and advertisement; for a detailed formulation we refer the reader to [8]. The goal is that the cost reflects the current load in the availability of wavelengths in an interdomain path, allowing an IDRA to compare routes more accurately than directly using the ENAWs between the source and destination. To this end, the cost computed and advertised between the IDRAs increases when the ENAW along the different segments of an interdomain path decreases. Likewise, the cost increases when the length of an interdomain path increases, where the length is computed as the number of hops H from the source OXC to the destination OXC, considering each intradomain subpath as a single hop. The source IDRA will choose the (path, wavelength) pair with the minimum cost. It is worth highlighting that different paths offering the same ENAW will frequently have different costs (loads). For instance, in Fig. 2b OXC S can reach OXC D through both RCD T and RCD T′. The ENAW of type λ1 through RCD T is ES,D(λ1) = 1, and this is also the case for λ2 through RCD T′ (i.e., ES,D(λ2) = 1). By computing the costs, the IDRA in domain S can discriminate between these two paths and select the one through RCD T, given that λ1 is less loaded than λ2 (notice that H = 5 for both paths). It is worth highlighting that in the cases where the difference in the availability of wavelengths between two candidate paths is not as obvious as in Fig. 2b, our evaluations confirm that the proposed approach offers a reasonable trade-off between the length H and the wavelength load on the lightpaths chosen. In summary, the PSI advertised by the IDRAs consists of a set of candidate (loose) paths, together with their ENAWs and costs.