The upgrade filter assembly, the Cisco ONS 15216 blue filter, is a passive unit comprising a 100-GHz 1-x-16 wavelength unidirectional multiplexer and a 100-GHz 1-x-16 wavelength unidirectional demultiplexer operating in the 1530- to 1545-nm frequency band. The front-panel common connector ports on the upgrade filter are used to connect to the base unit's upgrade ports via user-supplied optical patch cords. The filter mounts in a 19- or 23-inch rack and occupies two RU.