1. Introduction
THERE is a rising concern about the energy consumption figures of communication networks [1]. For this reason every network segment, i.e., from the access to the core, has been the target of optimization studies aimed at reducing power consumption. One important result that was found for core networks is the possibility to considerably reduce power consumption by relying on optical transmission technologies [2]. In fact, transmission in the optical layer allows for a transparent flow of data from source to destination without the need for any opto-electrical-opto (0-E-O) conversion, with an evident gain in terms of energy consumption. For this reason, in order to make further improvements, energy efficiency in the optical layer has attracted a lot of attention and a wide range of topics are addressed in the literature. Energy-efficient strategies for network design [3] as well as for static [4] and for dynamic [5], [6] provisioning have been proposed to minimize the power necessary to support the traffic demands. Similar studies addressed also the issue of the power consumed by the provisioned protection resources [7]–0.