Defragmentation in WDM Optical Networks
As lightpath services are added to a network over time, spectrum becomes fragmented, i.e., available wavelengths on different links do not match and thus no end-to-end paths can be established. While an offline provisioning where all traffic is known can minimize this effect to some extent, a dynamic or even incremental scenario is naturally more prone to creating a fragmented spectrum with wasted capacity.
There are multiple flavours of network defragmentation, depending on the specific deployment and technological constraints. Most commonly, lightpath reconfiguration is considered, wherein existing demands’ spectrum and paths are rearranged to accommodate a known set of additional demands. Several techniques can be employed with this goal: traditionally, a “make-before-break” technique is considered, where the rearranged demand is setup before the original one is torn down in order to prevent traffic disruption, by, e.g., duplicating the existing traffic.