The 1 x 16 colorless flex spectrum (CFS) add-drop module is a passive unit comprising one 1 x 16 splitter and one 16 x 1 combiner, plus 17 photodiodes. This module is double-slot height (full height) in the mechanical frame chassis. Its primary function is to provide optical multiplexing and demultiplexing for up to 16 optical signals. Because it is based on optical splitter and combiner technology, only transceivers employing coherent detection can be directly connected to the 16 client ports of the unit. Integrated photodiodes provide connectivity check and monitoring functions. Virtual photodiodes are implemented on the drop ports by subtracting the insertion losses from the photodiode reading on the COM-RX port. Power values, as well as the manufacturing data stored in the flash memory, are provided to Cisco Transport Controller through the USB connection.