communication in which any type of information (speech, alphanumeric, visual, or other type) is transmitted by electric signals propagated over wires or by radio signals. Depending on the means used to transmit or carry the signals, it may be classified as wire or radio communication. Wire communication is often used in many systems in combination with different forms of radio communication, for example, with radio-relay communication and satellite communication. According to the classification of the International Telecommunication Union, electrical communication also includes the transmission of information by optical (seeOPTICAL COMMUNICATION) and other electromagnetic systems.
Depending on the nature of the information being transmitted, electrical communication may be subdivided into the following basic types: telephone communication, which provides for telephone conversations between people; telegraph communication, designed to transmit alphanumeric messages, or telegrams; facsimile communication, which transmits graphical information, such as fixed images of texts, tables, drawings, diagrams, charts, and photographs; data transmission, that is, the transmission of information presented in a formalized manner (as symbols or continuous functions) for processing by a computer or the transmission of previously processed information; and video-telephone communication (seeVIDEO TELEPHONE), used for the simultaneous transmission of audio and visual information. The technical means of electrical communication are also used in wire broadcasting, radio broadcasting, and television broadcasting.
The establishment of electrical communication between a source of information (the sender) and a receiver of the information (the recipient) requires terminal equipment, such as transmitters and receivers, and a communications channel formed by one or more transmission systems connected in sequence. When there are many transmitting and receiving terminals and provision must be made for all possible paired connections between terminals, a system of switching devices is used, consisting of one or more switching stations and centers.
Terminal equipment. The transmitting terminal equipment converts a signal in its original form (speech sounds, symbols in the text of a telegram, symbols recorded in coded form on a perforated tape or any other information carrier, images of objects, and the like) into an electric signal. In telephone communication and radio broadcasting, a microphone is used for the electro-acoustic conversion. In telegraph communications the coded combinations of symbols in telegram texts are converted into a series of electric pulses—either directly (when start-stop telegraphs are used) or by recording the symbols beforehand on perforated tape (when a telegraph transmitter is used). In facsimile communication the variable intensity of the luminous flux reflected from an original is converted into electric pulses by the facsimile apparatus. Information about the varying brightness of any object whose image is to be transmitted by means of television communication is converted into a video signal by a television camera.
The receiving terminal equipment is used to present the electric signals being received in some form that the recipient of the information is able to use. Many types of terminal equipment have both transmitting and receiving devices, primarily when there is a two-way (usually duplex) exchange of information. For example, a telephone set usually has a microphone and an earphone combined in a single structural unit, or handset. In radio and television broadcasting the transmitting terminal equipment is separated from the receiving equipment, and the signals from one set of transmitting equipment are received by numerous radio and television receivers.
Communications channels and multichannel transmission systems. A communications channel for electrical communication comprises technical devices and a physical medium in which the electric signals are propagated from a transmitter to a receiver. The technical devices (modulators, demodulators, amplifiers, coding devices, decoders, and so on) are located at terminal and intermediate points of the communications lines (cables, radio relays, and the like). An information transmission system includes channel equipment and other devices that together form a set of communications channels in a single communications line (see alsoLINE MULTIPLEXING).
The communications channels used in electrical communication include analogue and digital types. Analogue channels are used to transmit continuous signals, such as voltages and currents derived from the electroacoustic conversion of speech and musical sounds or from the scanning of images. The feasibility of transmitting continuous signals from one or another source through a given channel depends primarily on such channel characteristics as the frequency bandwidth and the maximum permissible power of the signals being transmitted. Since any channel is subject to various types of noise (seeNOISE; INTERFERENCE, RADIO; and NOISE IMMUNITY), transmission is also dependent on the minimum power of the electric signal that exceeds the noise level by some given factor. The ratio of the maximum power of the signals transmitted by a channel to the minimum power is called the dynamic range.
Digital channels are used to transmit pulse signals. They are usually characterized by the rate of transmission of the information (measured in bits per sec) and by the accuracy of transmission. Digital channels can also be used to transmit analogue signals, and analogue channels can be used to transmit pulse signals; each type of communication requires that the signals be converted: analogue signals are converted into pulse signals by analogue-to-digital converters, and pulse signals are converted to analogue signals by digital-to-analogue converters. Figure 1 shows the possible ways of combining sources of analogue and digital signals with analogue and digital communications channels.
The transmission systems used in electrical communication usually provide for the simultaneous and independent transmission of messages from many senders to the same number of recipients. In such multichannel communication systems, from several tens to several thousands of individual channels are multiplexed in a common communications line. As of 1978 the most common multichannel systems used frequency-division multiplexing for analogue channels. In such systems each communications channel is assigned a certain part of the frequency band occupied by the group transmission channel (seeMULTICHANNEL COMMUNICATION: Figure 1). The spectrum of a signal is shifted to the section assigned to it in the frequency band of the group channel (the frequency conversion of the signal) by means of amplitude or frequency modulation (see alsoMODULATION OF OSCILLATIONS) of groups of sinusoidal carrier currents.
With amplitude modulation (AM) the amplitude of the harmonic current oscillations at a carrier frequency is varied in correspondence with the information being transmitted. As a result, oscillations are produced at the output of the modulator, whose spectrum contains two side bands in addition to the carrier. Since each of the side bands contains complete information about the original (modulated) signal, only one is fed to the communications line; the other side band and the carrier are suppressed by means of band-pass filters or other devices (seeMODULATION, SINGLE-SIDEBAND and TRANSMISSION, SINGLE-SIDEBAND). With frequency modulation (FM) the carrier frequency is varied to correspond with the information being transmitted. FM systems have greater noise immunity than AM systems, but this advantage is realized only when the frequency deviation is fairly wide, which requires a wide frequency band. Consequently, FM radio systems are used mainly at meter and shorter wavelengths, where every individual channel has available a frequency bandwidth 10 to 15 times greater than those of AM systems operating at longer wavelengths. A combination of AM and FM is often used in radio-relay
links, for which an intermediate spectrum is created by means of AM and is then shifted to a line frequency range by means of FM.
The transmission of different kinds of information requires channels having specific bandwidths. A characteristic feature of modern transmission systems is the provision of channels for different kinds of electrical communication within a single system. The standard for this purpose is a telephone channel, or voice-frequency (VF) channel, which occupies a frequency band from 300 to 3400 hertz. In order to simplify the characteristics of the filters that extract the channels, VF channels are separated from one another by guard bands so that the total occupied bandwidth is 4 kilohertz (kHz). In addition to the transmission of speech signals, VF channels are also used for facsimile communication, low-frequency data transmission (from 600 to 9,600 bits per sec), and several other types of electrical communication.
Because of their high percentage of use in electrical communication networks, VF channels are used as a basis for creating both wide-band (>4 kHz) and narrow-band (<4 kHz) channels. For example, a channel having twice (sometimes, three times) the width of a VF channel is used in radio broadcasting; the highspeed tranmission of data between computers and the transmission of images of newspaper pages requires channels that are 12, 60, and even 300 times wider; and the signals used for television broadcasting are transmitted over channels with a bandwidth 1600 times greater than that of a VF channel (approximately 6 megahertz). The secondary multiplexing of VF channels produces telegraph channels with bandwidths of 80, 160, and 320 Hz and transmission rates, respectively, of 50, 100, and 200 bits per sec. Radiorelay communication links can provide 300, 720, and 1,920 VF channels in each pair of high-frequency trunk lines, and communications links that use artificial earth satellites can provide from 400 to 1,000 or more VF channels in each pair of trunk lines. The wire communications lines used in transmission systems with frequency division multiplexing of channels provide the following numbers of VF channels: balanced cable pairs provide 60 (for two pairs of wires), and coaxial cables provide 1,920, 3,600, and 10,800 (per pair of coaxial lines). The development of systems having still larger numbers of channels is possible.
In wire transmission systems with frequency-division multiplexing, the communication range can be increased by decreasing the effects of noise, which builds up as a signal progresses along the line. This can be achieved through the use of amplifiers, which are operated in common for all the signals being transmitted in each communications circuit and which are inserted at a specified distance from one another. The separation, a function of the number of channels, is 1.5 km for high-capacity wire systems (10,800 channels) and 18 km for low-capacity systems (60 channels). In radio-relay communications systems the relay stations are constructed at distances of 50 km from each other.
In addition to transmission systems with frequency-division multiplexing of channels, during the 1970’s systems were introduced in which the channels were divided by time on the basis of pulsecode modulation (PCM), delta modulation, and other methods. In PCM each of the analogue signals being transmitted is converted into a train of pulses that form specific code groups (seeCODE and CODING). In order to accomplish this, narrow pulses are cut out of the signal (Figure 2,a) at specified time intervals that are equal to one-half the period corresponding to the signal’s maximum frequency variation. A number that describes the height of each excised pulse is transmitted by an eight-digit code during a time no longer than the duration (width) of the pulse (Figure 2,b). During the time intervals between transmissions of the code groups for a given message, the line is free and can be used to transmit the code groups of other messages. At the receiving end of the line the code combinations are converted back into pulse trains of various heights (Figure 2,c), from which the original analogue signal (Figure 2,d) can be reconstructed with a specified accuracy.
In delta modulation an analogue signal is first converted into a step function (Figure 3,a) for which the number of steps in a period corresponding to the maximum frequency of the signal’s variation is between 8 and 16, depending on the system. The pulse train transmitted over the line expresses whether the step function is increasing or decreasing by means of a change in the sign of the derivative of the signal: the increasing sections of the analogue function (characterized by a positive derivative) are expressed by positive pulses, and the decreasing sections (with a negative derivative) are expressed by negative pulses (Figure 3,b). Pulses created from other signals are located in the intervals between pulses. During reception the pulses for each signal are isolated and integrated so that the original analogue signal is reconstructed with a specified degree of accuracy (Figure 3,c).
PCM and delta-modulation channels (without terminal analogue-to-digital
converters), which are digital channels, are often used directly to transmit digital signals. A major advantage of systems using timedivision multiplexing of channels is that noise is not cumulative along a line; distortion of the signal shapes in the course of transmission is eliminated by regenerative repeaters located at specific intervals, like the amplifiers in systems with frequency-division multiplexing. However, systems with timedivision multiplexing exhibit quantization distortion, or noise, that arises when an analogue signal is converted into a sequence of coded numbers that only describe the signal with an accuracy of one digital increment. Quantization noise, unlike ordinary noise, is not cumulative with the progress of the signal along a line.
By the mid-1970’s PCM systems were devised for 30, 120, and 480 channels; systems for several thousand channels are in the development stage. The exploitation of transmission systems with time-division multiplexing of channels is being stimulated by the extensive use of computer components and subassemblies in such systems, which in the final analysis, reduces the cost of such systems for both wire and radio communication. Pulse transmission systems that use wave-guide and light-guide communications lines are now being developed and show much promise; the number of VF channels may be as high as 105 in a wave-guide section approximately 60 mm in diameter or in a pair of glass light-guide fibers 30–70 micrometers in diameter.
Switching systems. The systems of switching devices used in electrical communication may be of two types: centers and stations for channel switching, which with a finite number of channels make it possible to effect a temporary direct connection through a communications channel between any source and any recipient (after conversations have been completed, the connection is broken and the free channel is used to establish another connection); and centers and exchanges for message switching, which are used in those types of electrical communication where temporary delay or storage of the messages is permissible. A delay may be necessary when the immediate transmission to a called subscriber is not possible because a free channel is not available at the moment or the called subscriber’s telephone unit is busy.
Centers and stations for switching channels that are used in the most popular forms of electrical communication—telephone and telegraph systems—are the telephone central offices and telegraph stations, together with telephone and telegraph communications centers located at various points in telephone and telegraph networks. They are classified according to the functions they perform and their positions in the network. For example, a telephone network may have rural, urban, and long-distance automatic central offices as well as a variety of switching centers, such as automatic switching centers and centers for incoming and outgoing traffic. The centers interconnect various automatic central offices. Every modern channel-switching station or center contains a set of control equipment, constructed with electromechanical or electronic devices, and switching devices that in response to control signals connect or disconnect the appropriate channels (Figure 4). As of 1978 the most common channel-switching systems had control equipment based on electromechanical relays and switching equipment based on crossbar switches. Such stations and centers are called crossbar systems.
Systems that switch messages are used chiefly in telegraph communication and data transmission. In addition to control and switching equipment they have devices for storing the signals being transmitted. During the passage of signals from a sender to a recipient in message-switching systems, various technological operations are performed on the stored messages, such as changing the order of delivery to subscribers (depending on message priorities), receiving one type of message over a channel with some given transmission rate and transmitting it over another type of channel with a different rate, and various other operations corresponding to the prescribed operating algorithm. In some cases it is possible to combine messageand channel-switching centers in order to provide the most favorable conditions for the transmission of messages and the best use of electrical communication networks.
A characteristic trend in the development of modern switching stations and centers is the use of fast-acting, miniature sealed contacts, such as hermetic contacts, in switching equipment to effect connections and specialized computers to control the connection processes. Such stations and centers may be termed quasi-electronic. The introduction of computers makes it possible to offer additional services to subscribers, for example, the use of a shorter series of numbers (fewer symbols) for the most frequently called subscribers, the ability to put a telephone set on “hold” if the number of the called subscriber is busy, and the switching of a connection from one set to another.
With the introduction of transmission systems that have time-division multiplexing of channels, it has become possible to shift to purely electronic switching stations and centers (without mechanical contacts), where digital channels are switched directly, without converting the digital signals to analogue form. As a result, the transmission and switching processes can be integrated—a prerequisite for the creation of an integrated communication system in which all types of information are transmitted and switched by common methods.
In the USSR electrical communication is developing within the framework of the Integrated Automatic Communications System, which is being introduced according to a prescribed plan. The system consists of a complex of technical communication facilities that interact by using a common primary network of channels together with switching stations and centers and terminal equipment to create a variety of secondary systems that provide all types of electrical communication organization.
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G. B. DAVYDOV