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 NPhard
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.