An '''add-drop multiplexer''' ('''ADM''') is an important element of aan [[optical fiber]] network. A [[multiplexer]] combines, or [[Multiplexing|multiplexes]], several lower-[[bandwidth]] streams of data into a single beam of [[light]]. An ''add-drop'' multiplexer also 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.<ref>{{cite web
| title = Electrical Engineering Glossary Definition for ADM
