Add/drop contention-aware RWA with directionless ROADMs: The offline lightpath restoration case

Directionless reconfigurable add/drop multiplexers (ROADMs) permit changing the direction of an added/dropped lightpath without manual intervention. If the ROADM is also colorless, its wavelength can be automatically reconfigured. In broadcast-and-select directionless ROADM architectures, the number of lightpaths that can be added/dropped using the same wavelength is limited by the so-called add/drop contention factor C. This is a source of light-path blocking that reduces the network capacity. A previous work investigated this aspect for the case of unprotected and 1+1 protected lightpaths. In this paper, we address the case in which lightpath restoration is the fault recovery technique in the network. We hypothesize that, using appropriate network planning, the add/drop contention effects can be mitigated. For this, we propose and investigate the add/drop contention-aware routing and wavelength assignment with lightpath restoration offline planning problem, considering colored ROADMs. To solve the problem, an effective heuristic is presented. Extensive results are reported for four reference topologies. In our tests, the reduction in the network capacity caused by add/drop contention is eliminated in practice for factors C = 2, or even C = 1 in some topologies. These results contribute to the dimensioning of directionless ROADMs.