Transport signal multiplexer de multiplexer. It either signal from multiple sources into a STS singles or de multiplexes and STS signal into different signals .
Regenerator it is a repeater that takes a received optical signal and regenerates it functions in the data link layer. A SONET regenerator replace some of the existing overhead information with new information.
Add drop multiplexer can add signals coming from different sources into a given path or remove a desired signals from a path redirect it without de multiplexing the entire signal .