In any communication system that is either digital or analog, we need a communication channel for transmission. This channel can be a wired or a wireless link. It is not practical to allocate individual channels for each user. Therefore a group of signals are combined together and sent over a common channel. For this we use multiplexers. A multiplexer is a device that allows digital information from several sources to be routed onto a single line for transmission to a single destination. A demultiplexer does the reverse operation of multiplexing. It takes digital information from a single line and distributes it to a given number of output lines.
Part of the book: Multiplexing