In the case of optical add/drop multiplexers( OADM), configurability was a key development goal early on, and this was pursued in the various development phases. Hence the various OADM versions such as BOADM, COADM, FOADM, ROADM and TOADM.
The filter-based add/drop multiplexers, FOADM, used thin-film filters to select wavelengths, which could add or remove individual wavelength channels from the many wavelengths of DWDM systems. Since DWDM channel spacing is only 100 GHz, they worked with a filter bandwidth of 50 GHz and optical frequency interleavers. The configurable FOADM multiplexers had the designation COADM, C for configurable.