What is Add Drop Multiplexer?

This is the most common element to built up SDH rings.Dependent on the port configuration it gives access to all containers embedded in the STM-1 signals. These containers can be dropped and inserted in the SDH hierarchy.

Each tributary can also be inserted into each container of the STM-1 streams.

An ADM155 for example is a multiplexer, which handles 126 x 2Mbps tributaries
within 2 independent STM-1 datastreams (doublering).

Also the exchange of the containers between the STM-1 signals is possible.

The ADMs are controlled via TMN commands in embedded channels or the via
the Q-interface.

It has a built in switching unit, which supports Automatic Protection Switching
(APS) , if one line has interruptet.