Add-drop multiplexer

An add-drop multiplexer (ADM) is a main element of optical fiber networks. It combines, or multiplexes, several lower-speed streams of data into a single beam of light. ADMs can be used both in long-haul core-networks and in shorter distance metro networks, although in recent years (2002-04) most ADM sales have been in metro networks due to the high cost required to scale the technology to high data rates (wavelength division multiplexing, better known as DWDM, is more popular in the core).

The main filtering technology used in add-drop multiplexers is the Fabry-Perot Interferometer, in the form of small, solid etalons.

A recent shift in ADM technology has introduced so called multi-service SONET/SDH (also known as a multi-service provisioning platform, or MSPP) equipment which has all the capabilities of legacy ADMs, but can also include cross-connect functionality to manage multiple fiber rings in a single chassis. These new devices can replace multiple legacy ADMs and also allow connections directly from Ethernet LANs to a service provider's optical backbone.

In the end of 2003, sales of multiservice ADMs exceeded those of legacy ADMs for the first time, as the change to next-generation SONET/SDH networks accelerated.

An emerging variety of ADMs that is becoming popular as the carriers continue to invest in metro optical networks are ROADMs, or Reconfigurable Add Drop Multiplexers.