The base filter assembly, the Cisco ONS 15216 red filter, is a passive unit comprised of a 200 GHz 9 x 1 wavelength unidirectional multiplexer (mux) and a 200 GHz 1 x 9 wavelength unidirectional demultiplexer (demux) operating in the 1547 to 1561 nm frequency band. The mux and demux systems connect to independent fibers in the carrier's fiber facilities through the common ports on the chassis faceplate. The base unit integrates two expansion ports, providing the ability to upgrade, in-service, from 9 mux/demux wavelengths to 18 wavelengths. Also, the base unit incorporates two monitor ports, allowing the user unobtrusive access to the muxed and demuxed signals for monitoring or analysis.