In transparent wavelength routed optical networks, the signal quality degrades due to physical layer impairments. Certain physical effects make routing decisions made for one lightpath affect and be affected by the decisions for other lightpaths. To safely establish a lightpath for a new connection two main approaches can be used. The most common approach is to select a lightpath that has acceptable transmission quality under the worst case interference assumption. This ensures that the selected lightpath will not become infeasible due to the possible establishment of future interfering connections, but it sacrifices candidate path space for a quick and stable lightpath selection process. The second approach is to consider the actual current network utilization and account for the interference among lightpaths so as to perform a cross layer optimization between the network and physical layers. In the latter approach, the algorithm has to evaluate and check if the establishment of the new lightpath turns infeasible some of the already established connections. The question that arises is whether the performance benefits obtained through the second approach are worth the added complexity introduced by the cross-layer optimization required.
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