Multi-step migration of optical connections to minimize disruption time in flex-grid networks

Migration of optical connections in flex-grid networks has become increasingly necessary as the arrivals and departures of optical connection requests are more dynamic. To mitigate disruptions of existing connections during migration, former research applies the resource dependency digraph (RDD) model to allow for sequential migration of optical connections. In this paper, we argue that, in this RDD model, each connection has a quota of one migration step only and that this one-step-quota constraint can be safely lifted to accomplish even lower disruption time due to the greater degree of migration freedom. We consider the whole migration process to be composed of multiple lightpath assignment states, each of which is to be determined individually. We show that this consideration can drastically reduce disruption time and hence the ensuing service level agreement violation penalties, even when it is already minimized under the one-step-quota constraint. For that, we propose two mathematical models. The first model, namely, multi-step optical connection migration (MOCM), adopts a layer-based abstraction in which each lightpath assignment state is represented as a layer, and the transition between two consecutive layers constitutes a migration step. In the second model, namely, MOCM2, to improve the solution efficiency, we readopt the concept of RDD and integrate multiple RDDs into MOCM to further enable even larger migration freedom. This way, the transition between two consecutive layers may involve multiple so-called meta-migration steps. As a use case, the proposed approaches are applied to perform connection migration for spectrum defragmentation. The results show that our multi-step migration scheme notably reduces disruption time as compared with the one-step-quota RDD-based approaches.