ROADMs

Reconfigurable Optical Add-Drop Multiplexers (ROADMs) are a class of optical networking equipment used in wavelength-division multiplexing (WDM) networks to dynamically route and provision lightpaths. Unlike fixed optical cross-connects, ROADMs can switch individual wavelength channels in the optical domain, enabling flexible management of bandwidth without requiring optical-electrical-optical (OEO) conversion at every node.

Most ROADMs incorporate a wavelength-selective switch (WSS) or an arrayed waveguide grating (AWG) based demux/mux and tunable filtering to add, drop, or pass through wavelengths. Modern ROADMs support features such as colorless, directionless, and contentionless (CDC) operation, allowing any wavelength to be added/dropped on any port and any direction without preplanning. They are typically deployed in WDM networks at central offices or metro edge nodes and are controlled by a management plane that provisions lightpaths across the network.

ROADM architectures vary: centralized ROADMs with a few switching planes; distributed multi-degree ROADMs; and two-stage ROADM configurations designed for scalability. They may be fixed-grid (e.g., 50 GHz ITU grid) or elastic/flexible-grid to accommodate variable channel spacing and higher spectral efficiency. Transponders can be directly connected or remotely controlled through the ROADM.

Control and standards: ROADMs are typically controlled by GMPLS or SDN-based control planes, enabling automatic lightpath provisioning, restoration, and bandwidth optimization. Standards and interoperability are supported by ITU-T, IEEE, and industry fora such as the OIF.

Applications and benefits: ROADMs are widely used in long-haul, metro and data center interconnect networks to reduce manual cross-connects, increase flexibility, improve service turn-up times, and support on-demand bandwidth changes. Limitations include higher cost, power consumption, complexity, and potential signal degradation if not properly managed due to routing and switching in the optical layer.