MULTIPLEXING

SONET muliplexers handle electrical and optical conversions.  There are two types of SONET multiplexers: the terminal concentrator has many-to-one and one-to-many multiplexing/demultiplexing capabilities  and the add/drop multiplexer which allows lower rate signals to be separated from incoming signals and added to outgoing signals.  However, all nonterminating signals  are passed transparently from one port to another.