Multiplexers, concentrators, and selectors, also known as datacom handlers, increase the number of input/output devices that can use a communication channel. They allow multiple I/O devices to share one channel. It is advantageous to increase the number of I/O devices because these devices operate at much lower speeds (100 to 150 bits per second) than communication channels (300 to 56,000 bits per second for voice-grade channels). Thus, a channel is not used to full capacity by a single I/O device.
Multiplexing can promote more economical use of a communication channel; it acts as a communication interface, combining the input streams from several devices into a single stream that can be sent over a single channel to the computer system. This allows a single communication channel (typically voice-grade) to substitute for many slower sub voice channels that might otherwise have been operating at less than full capacity. Once the computer system has completed processing, the data is sent to the multiplexer, which then routes the data to the appropriate device.
A concentrator differs from a multiplexer in that it allows data to be transmitted from only one terminal at a time over a communication channel. The concentrator polls the terminals one at a time to see if they have any messages to send. When a communication channel is free, the first terminal ready to send or receive data will get control of the channel and continue to control it for the length of the transaction. The use of a concentrator relies on the assumption that not all terminals will be ready to send or receive data at a given time. The figure below shows examples of communication systems with and without multiplexers and concentrators.
A selector is similar to a concentrator in that it allows data to be transmitted from only one input/output device at a time. Selectors are normally used with high-speed devices such as a magnetic-tape or magnetic-disk unit.
Last Updated Jan.7/99