Dimensioning the Add/Drop Contention Factor of Directionless ROADMs

Reconfigurable Optical Add/Drop Multiplexers (ROADM) are the optical switching equipment of transparent optical networks. Directionless ROADMs permit the network carriers to change the direction of an added and/or dropped lightpath without the need of a technician on-site intervention in the lightpath end nodes. Colorless ROADMs provide the same versatility for changing the lightpath transmission wavelength. Cost-effective directionless ROADM architectures (colorless or colored) can be built if the maximum number of lightpaths that can be added/dropped using the same wavelength is limited. We name this limit as the node add/drop contention factor, and denote it as C. In this paper we investigate the network lightpath blocking performance as a function of this add/drop contention factor of the nodes. The scenarios considered are the static planning of a network (i) with unprotected traffic, (ii) with traffic 1+1 protected for single-link failures, and (iii) with traffic 1+1 protected for single-link or single-node failures. Since for these scenarios, the wavelength of an existing lightpath does not have to be dynamically reconfigured, the work in this paper applies to both colorless and colored nodes. An ILP model and an effective heuristic are presented to solve the so-called Add/Drop Contention Aware RWA (ADCA-RWA) planning of the network. Extensive results are reported. In all the cases, an add/drop factor C = 2 is sufficient to provide the same performance as contentionless nodes (C = ∞). Furthermore, in all the tests a factor C = 1 was also sufficient, or produced a minor lightpath blocking performance degradation (below 0.5% in the unprotected cases, and below 2.5% in the 1 + 1 protected cases).

I. Introduction

Transparent optical networks based on wavelength division multiplexing (WDM) are an enabling technology for high-speed backbone networks [1], [2]. In transparent networks, traffic is carried over all-optical connections, called lightpaths. A lightpath originates at an E/O transmitter in the ingress node, where it is said to be added. It occupies a wavelength channel in each traversed link, and terminates at an O/E receiver in the egress node, where it is said to be dropped. The lightpaths are optically switched at the intermediate nodes, and with respect to those nodes, are commonly referred as express lightpaths.