Designing an Optical Island in the Core Network: From Routing to Spectrum Allocation

We consider a network design problem arising in the development of an all-optical future generation Internet network called a flex-grid. An optical island is a set of core nodes that can be fully interconnected by transparent wavelength routes. We present a mathematical model for finding an optimal optical island, show that it is an NP-hard problem, and present a decomposition for solving it. In a first phase, we choose network links and route the traffic over the resulting network. In the second phase, we allocate the light-paths associated with the traffic requests to individual fibres and spectrum segments on the fibres. This so-called routing and spectrum assignment (RSA) problem is a generalisation of the well-known routing and wavelength assignment problem (RWA) of conventional optical networks. Flex-grid optical networks allow us to bundle higher capacity connection requests by allocating channels in a number of contiguous frequency slots, providing increased throughput, as long as the connection length is below technological limits. We solve the first part of the decomposition with a large neighborhood search, and the second with a CP model using a single GEOST global constraint. Results for Ireland and Italy show that solutions of high quality can be found by this decomposition.