  With the continued growth of Internet traffic, and the scarcity of the
optical spectrum, there is a continuous need to optimize the usage of this
resource. In the process of provisioning elastic optical networks using the
flexible frequency grid paradigm, telecommunication operators must deal with a
combinatorial optimization problem that is NP-complete namely the Routing and
Spectrum Allocation(RSA) problem. Following on our previous study, where we
used Integer Linear Programming, and proposed a Column Generation algorithm
based on a Lightpath decomposition, which proved to be the most efficient so
far, we now consider the traditional Configuration decomposition that has been
studied in other works in the past. In the process, we created an new
mathematical model using two variable sets instead of a single variable set.
Equally important,we independently rediscovered the Nested Column Generation
technique, and we used it to propose an algorithm that led to a considerable
improvement on the previous algorithms that use the same Configuration
decomposition. When compared to the latest such existing study, our algorithm
achieved an accuracy gap of 1% as opposed to 14.3% for the previous study, and
a running time two orders of magnitude faster on average.
