What is a multiplexer and how does it works?

There are many different types of multiplexers. However, the basic function of all of them is to combine multiple signals into a single 'channel' for transmission. At the other end of the channel, a demultiplexor is responsible for splitting the combined signals back into the original, separate signals. Different types of muliplexors include Time Domain (TDM), Frequency Doman (FDM), Statistical (SM), and Space Domain (SDM). In a TDM, for example, every signal is given a specific duration time slice. This means that the bandwidth of the channel between the mulitplexor and demultiplexor (mux and demux for short) must be greater than the sum of the bandwidth of all of the input signals. So, 10 1MHz signals might be multiplexed into on 11MHz signal for transmission across a microwave or fiber link. Much signal transmission equipment is available with the mux functionality built in, freeing the designer from having to worry about it as a separate condition. For example, a fiber transmitter may have several high speed data ports. It senses the bandwidth of each signal, then automatically adjusts its own output bandwidth accordingly. Finally, muxes can be connected to other muxes in a hierarchical configuration. The phone company used such a configuration for long distance traffic, using local loops into groups, into super-groups and so on until satellite bandwidth was reached.