Impact of Partial-Contentional and Directional Switching Designs on Switching Capacity of WDM-ROADM

The value of colorless, directionless, and contentionless (CDC-)ROADM (reconfigurable optical add/drop multiplexer) nodes is strongly contested in the optical networking community. In this work, we compare known ROADM node designs incorporating different switching elements and account for their total nodal switching state support (in consideration of both channel routing and add/drop). This allows us to quantify the impact of directional/contentional accessibility constraints to add/drop transceivers. By considering the network node entity as a permutation network among its ingress/egress ports for all wavelength channels, which covers both through routing and add/drop assignments, we tabulate the node's switching capacity, or total allowable connection states, per different ROADM architecture, hardware constraints, and finite number of add/drop transceivers. We further introduce the impact of idle wavelength channels on fiber links, as well as bidirectional routing assignments. Our switching capacity enumerations demonstrate that CDC-ROADM outperforms other designs, but parallel contentional aggregation hardware (partially contentional) and directional transceivers (permanently assigned to port directions) offer competitive performance under certain scenarios (at lower and higher number of deployed transceivers, or a combination of both). These findings suggest that design alternatives to the “difficult to implement” CDC-ROADM exist, with nearly equivalent switching capacity, and additional system considerations must be taken into account for ROADM design selection such as hardware availability, cost, impact of traffic churn, and disaster recovery with over-provisioned add/drop transceivers.