Multichannel Communication

an electric communication system providing simultaneous, independent transmission of messages from several senders to an equal number of recipients. Multichannel communication is used in transmitting telephone or telegraph messages, telemetry data, remote-control signals, television and facsimile images, computer data, and signals of automatic control systems over cables or radio-relay or satellite communications lines. Multichannel communication systems combined with switching systems are the most important components of an integrated automatic communication system.

The principle of multiplexing of communication lines is the basis for the design of multichannel communication systems. The most common scheme is frequency-division multiplexing, in which each channel is assigned a certain part of the frequency band occupied by the group transmission channel. An audiofrequency channel has been accepted as standard; it provides transmission of speech (telephone) communications with an effective frequency band of 300–3400 hertz (Hz). Taking into account the protective channel spacing, each such channel is assigned a nominal frequency band of 4 kHz. The method of combining standard channels into standard group channels is used in designing multichannel communication systems with frequency-division multiplexing. The first step is the formation of a primary group channel, which consists of 12 standard channels occupying a frequency band of 60–108 kHz (see Figure 1). For this purpose, each channel is transferred to the corresponding region of the frequency band of the primary group channel by means of an individual channel frequency converter (modulator). A secondary group channel is formed from the five primary group channels, and so on.

Figure 1. Formation of primary group channel

Multichannel communication systems in actual use have 12, 60, 120, 180, 300, 600, 900, 1,920 or 10,800 standard channels. This method not only substantially simplifies the design of electric filters but also broadens the possibilities for standardization of equipment. The formation of group channels also makes possible transmission of types of information that require a frequency band wider than the bandwidth of a standard channel. For example, in transmitting audio broadcasts in a frequency band of 50–10,000 Hz, three standard channels are combined; in transmitting black-and-white or color television images the entire quaternary group channel is used (900 standard channels). To transmit messages that require a band width narrower than a standard audio-frequency channel (for example, in multiplexing standard audio-frequency channels by means of low-speed data transmission channels), the standard channel is divided into 24–48 narrow-band channels by multiplexing apparatus. As a result, the standard audio-frequency channel becomes a multiplexed communication channel. Such multiplexing is often called secondary multiplexing.

A basic advantage of multichannel communication systems with frequency-division multiplexing and single-sideband modulation is the economical use of the frequency spectrum. Among the essential disadvantages of such systems are the accumulation of interference noise originating at repeater stations and the resulting low noise immunity. Systems with time-division multiplexing and pulse-code modulation do not have the latter disadvantage. In designing multichannel communication systems of high capacity (in terms of the number of channels) there is a trend toward simultaneous use of frequency-division and time-division multiplexing methods. Multichannel communications theory and engineering are being developed with the goal of improving the noise immunity of message transmission and the efficiency of use of communications lines.