The OADM (Optical Add/Drop Multiplexing) modules are passive devices that optically multiplex and demultiplex a specific band of four ITU wavelengths. The OADM modules supported by the Cisco ONS 15530 each add and drop a specified band of four channels at a node and pass the other bands through.In the transmit direction, the OADM modules multiplex signals transmitted by the transponder line cards and 10-Gbps ITU trunk cards over optical cross connections and provide the interfaces to connect the multiplexed signal to the DWDM (dense wavelength-division multiplexing) trunk side. In the receive direction, the OADM modules demultiplex the signals from the trunk side before passing them over optical cross connections to the transponder line cards and 10-Gbps ITU trunk cards.