Sonet Devices
STS Mux or DMux either multiplexes signals from multiple sources into an STS or demultiplexes an STS into different destination signals.
Regenerator is a repeater that takes a received optical signal and regenerates it. This devices function at the data link layer.
Add/drop multiplexer is able to add signals from different sources into a given path or remove a desired signal from a path and redirect it without demultiplexing the entire signal.