“Spectrum defragmentation in elastic optical networks”

“Spectrum defragmentation in elastic optical networks”.In elastic optical networks (EONs), dynamic setup and release (tear down) of lightpaths will lead to spreading the optical spectrum into nonaligned, isolated and small sized spectrum blocks. As introduced in previous chapters, this issue is called spectrum fragmentation. Defragmentation strategies are key counter measures to minimize the problem of spectrum fragmentation. Defragmentation, implemented by algorithms and usually running as defragmentation engine in the control plane, is a process that attempts to reduces the amount of fragmentation with the aim to reduce the blocking of new connection attempt and therefore allowing a higher utilization of network resource.Fragmentation management techniques deal with spectrum fragmentation and increase the amount of traffic that can be accepted and carried by a network. The classification of fragmentation management techniques basically focuses in two types, such as non-defragmentation and defragmentation techniques. Non-defragmentation and defragmentation approaches are not mutually exclusive and specific fragmentation management scheme can be implemented applying a synergy of both non-defragmentation and defragmentation approaches. In the Non-defragmentation techniques, preventive actions are taken to avoid fragmentation before the establishment of a lightpath, but no actions on already established connections are allowed. As far as concern defragmentation technique, a necessary action is taken about in-service lightpaths (e.g. rerouting or frequency shifting of lightpath) to reduce degree of the fragmentation of the network.The defragmentation methods can be arranged into two fundamental classes. The first one is reactive and the second one is proactive. Reactive defragmentation techniques are typically activated without waiting for the arrival of a new connection request, i.e. they take measures to leave enough resources available for future connections. Basically, the reactive algorithms are triggered when a connection request cannot be allocated due to the Spectrum Fragmentation, and the spectrum needs to be reconfigured to make enough room for the new connection request. In the reactive techniques, the defragmentation is triggered to allocate a connection request that would be blocked otherwise, usually by rerouting the established connections that will conflict with the selected route and slot-blocks for the new connection request. This necessary rerouting usually causes disruptions in the network. On the opposite side, proactive defragmentation techniques are activated without sitting tight for another lightpath ask. In this case Defragmentation is triggered to allocate a connection request that would be blocked otherwise, usually by rerouting the established connections that will conflict with the selected route and slot-blocks for the new connection request. Both reactive and proactive are again grouped into two kinds. The rerouting techniques reallocate existing lightpaths to the various range openings by changing their paths to maintain a strategic distance from the negative impact. Then method without rerouting techniques don't enable existing lightpaths to change their courses, range reallocation might be permitted. In view of activity interruption, both with and without re-routing of existing lightpaths are ordered into the non-hitless and hitless defragmentation techniques. The proactive techniques are those where a defragmentation is invoked to consolidate the spectrum (minimize the total required spectral resources for the existing connections) and minimize the rejection of future connection request. It can be an algorithm executed periodically to reconfigure established connections, or a RSA algorithm that selects the route and spectral resources of a connection request, based on how fragmented that allocation will leave the spectrum (Fragmentation Aware).