The base filter assembly, the Cisco ONS 15216 red filter, is a passive unit comprising a 100-GHz 16-x-1 wavelength unidirectional multiplexer (mux) and a 100-GHz 1-x-16 wavelength unidirectional demultiplexer (demux) operating in the 1546- to 1561-nanometer (nm) frequency band. The mux and demux systems connect independent fibers to the carrier's fiber facilities through the common ports on the chassis faceplate. The base unit integrates two pairs of expansion ports, enabling in-service upgrades from 16 mux/demux wavelengths to 32 wavelengths. The base unit also incorporates two monitor ports, allowing the user unobtrusive access to the transmit and receive signals for monitoring or analysis. The filter mounts in a 19- or 23-inch rack and occupies two rack units (RU).