In fiber optics a reconfigurable optical add drop multiplexing is a form of optical add drop multiplexer that adds the ability to remotely switch traffic from a wavelength division mulr=tiplexing system at the wavelength layer. This is achieved by the use of wavelength selective switching module. This allows individual or multiple wavelength carrying data cahnnels to be added or dropped from a transport fiber without the need of converting the signal on all the WDM channel to electronic signals and back to optical signals.The main advantage of ROADM is1. The planning of entire bandwidth assignment need not be carried out during the initial deployment of a system the configuration can be done as and when required withoud affecting the traffic already passing through the ROADM2. ROADM allows for remote configuration and reconfiguration.3. In ROADM

Before the development of optical multiplexing devices such as ROADM signals routing in fiber optic network was done by converting the IR or visible beams to electrical signals and routing those signals using conventional electronics switching. The rerouted electrical signals are then converted back into visible beams.In a conventional ROADM switching is accomplished without the optical-to-electrical or electrical-to-optical conversion using three operations called add, drop and cut through. Cut through means passing the wavelength without modification.