In its passage from the MUX to DEMUX, the signal passes through one or more Optical Add-Drop Multiplexer (OADM) . The function of an OADM is to selectively drop one or more wavelengths by rerouting its data content to another fiber. The OADM may just allow the remaining traffic to pass or add a different data set at a wavelength equal to that of a dropped data. This helps to create a virtual point-to-point circuit.
An OADM is generally a device such as a Bragg grating which could be used to selectively reflect a wavelength that is to be dropped while allowing the others to be transmitted. OADMs are passive components of the network. They are manufactured to operate either at fixed wavelengths or at dynamically selectable wavelengths. In case of fixed wavelengths, the wavelengths to be dropped or added are pre-selected.