Add-drop multiplexer

An add-drop multiplexer (ADM) is an important element of optical fiber networks. A multiplexer  combines, or multiplexes, several lower-bandwidth streams of data into a single beam of light. An add-drop multiplexer is a multiplexer that has the capability to add one or more lower-bandwidth signals to an existing high-bandwidth data stream, and at the same time can extract or drop 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.
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