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A channel is essentially a communication circuit between two or more devices. You can think of a channel as a pipe for sending data between two systems. In a computer system, a channel provides an input/output interface between the processor and some peripheral device. In telecommunication, a channel may take one of the forms described below and pictured in Figure C-10. Keep in mind that a channel may be one of many channels that run over the same physical circuit. These channels are called virtual channels or virtual circuits.

• One channel carried on a physical wire or wireless medium between two systems. The twisted-pair copper wire in the local loop between your phone and the local central office is a dedicated channel that forms its own circuit.


• Time-division multiplexed channels, in which signals from several sources such as telephones and computers are merged into a single stream of data and separated by intervals of time.


• Frequency-division multiplexed channels, in which signals from many sources are transmitted over a single wire or wireless medium by modulating each signal on carriers at different frequencies.


• Packet or cell switching, in which packets from many sources are multiplexed and transmitted from endpoint to endpoint over a switched network.