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Add-drop multiplexer
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<p>An add-drop multiplexer (ADM) is an important element of an <a href="/facts/Optical_fiber/5ruuIFbj">optical fiber</a> network. A <a href="/facts/Multiplexer/lVro0Sfz">multiplexer</a> combines, or <a href="/facts/Multiplexing/YE7hK8cw">multiplexes</a>, several lower-<a href="/facts/Bandwidth_(computing)/lY6Tb4gX">bandwidth</a> streams of data into a single beam of <a href="/facts/Light/jeofpMn4">light</a>. An <i>add-drop</i> multiplexer also has the capability to <i>add</i> one or more lower-bandwidth signals to an existing high-bandwidth data stream, and at the same time can extract or <i>drop</i> other low-bandwidth signals, removing them from the stream and redirecting them to some other network path. This is used as a local "on-ramp" and "off-ramp" to the high-speed network. </p><p>ADMs can be used both in long-haul core networks and in shorter-distance <a href="/facts/Metropolitan_area_network/NtJvkYjf">metro networks</a>, although the former are much more expensive due to the difficulty of scaling the technology to the high data rates and <a href="/facts/Wavelength_division_multiplexing/xSkNRxfu">dense wavelength division multiplexing</a> (DWDM) used for long-haul communications. The main optical filtering technology used in add-drop multiplexers is the <a href="/facts/Fabry%25E2%2580%2593P%25C3%25A9rot_etalon/n949LwQD">Fabry–Pérot etalon</a>. </p><p>Newer "multi-service <a href="/facts/SONET%2fSDH/G3LgtGS6">SONET/SDH</a>" (also known as a multi-service provisioning platform or MSPP) equipment has all the capabilities of legacy ADMs, but can also include cross-connect functionality to manage multiple fiber rings in a single chassis. These devices can replace multiple legacy ADMs and also allow connections directly from <a href="/facts/Ethernet/nMfhU8tU">Ethernet</a> <a href="/facts/LAN/edUDxJ6J">LANs</a> to a service provider's optical backbone. At 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. </p><p>An emerging variety of ADMs that is becoming popular as the carriers continue to invest in metro optical networks are <a href="/facts/Reconfigurable_optical_add-drop_multiplexer/b6srKe9s">reconfigurable optical add-drop multiplexers</a> (ROADMs). </p>