The Cisco ONS 15216 blue filter is a passive unit comprising a 200 GHz 9 x 1 wavelength unidirectional multiplexer and a 200 GHz 1 x 9 wavelength unidirectional demultiplexer operating in the 1531 to 1543 nm frequency band. The upgrade filter connects via its front-panel common connector ports to the base unit's upgrade ports via user-supplied optical patch cords.