Multiplexer / Demultiplexer
A multiplexer transforms several digital streams into one stream. This is often referred to as 'Muxing.'
Multiplexer is a device that has multiple inputs and a single line output. The select lines determine which input is connected to the output, and also to increase the amount of data that can be sent over a network within certain time. It is also called a data selector.
Generally, a demultiplexer is a device which transforms one multiplexed data stream to several streams which it consists of. It is used to send a signal to one of the many devices. Satellite modem doesn't have so many outputs, so a demultiplexer here performs a drop operation, allowing to choose channels that will be transferred to output.
A demultiplexer achieves this goal by maintaining frame synchronization.
The main difference between a multiplexer and a de-multiplexer is that a multiplexer takes two or more signals and encodes them on a wire, whereas a de-multiplexer does reverse to what the multiplexer does.