ABSTRACT
This chapter focuses on spectrum fragmentation management approaches that consider defragmentation. The defragmentation approaches are considered to fill-in the gaps left behind after terminating a lightpath. These approaches are typically classified into two main strands: reactive and proactive. The defragmentation approaches that cause traffic disruption are referred as nonhitless defragmentation approaches. There are three main hitless defragmentation or retuning approaches; hop-retuning, push-pull retuning, and Make-before-break. The main limitation of the make-before-break technique is that it needs the additional resources and transponders that allow hitless defragmentation. Fragmentation management approaches in elastic optical networks (EON) employ various strategies to enhance network performance and improve the utilization of spectrum resources. In, a hitless optical path shift approach using push and pull retuning is introduced in order to suppress bandwidth blocking in EONs. The chapter highlights the research works that focus on the experimental demonstrations that have been carried out for spectrum defragmentation in EONs.
