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In an optical network, an Optical Add/Drop Multiplexer (OADM) is used to drop one or many wavelengths at a specific site. Wavelengths on the same fibre that are not destined to terminate at other sites pass through the OADM transparently. Traffic that is not terminated is known as ‘express traffic’.

Each wavelength can be at a different bit rate and can be dropped or added without the need to multiplex/de-multiplex.

OADM technology is used to cost-effectively access a small part of the bandwidth in the optical network domain with a minimum amount of electronics.