|Multiplexing and Demultiplexing|
A communication channel, or link, between two points is established whenever a cable is connected or a radio transmitter and receiver are set up between the two points. When there is only one link, only one function—whether it involves signal transmission or control operations—can be performed at a time. For two-way communication, a half duplex process is set up: Both ends of the communication link can send and receive but not at the same time.
Transmitting two or more signals simultaneously can be accomplished by running multiple cables or setting up one transmitter/receiver pair for each channel, but this is an expensive approach. In fact, a single cable or radio link can handle multiple signals simultaneously by using a technique known as multiplexing, which permits hundreds or even thousands of signals to be combined and transmitted over a single medium. Multiplexing has made simultaneous communication more practical and economically feasible, helped conserve spectrum space, and allowed new, sophisticated applications to be implemented.
10-1 Multiplexing Principles