Telephone Communication

the transmission of speech over a distance either by electric signals propagated along conductors or by radio signals; a type of telecommunication. Telephone communication permits conversations to be carried on between people (subscribers) separated by almost any distance. The sound waves of a subscriber speaking into the transmitter of a telephone set are converted into electric signals; these signals are transmitted over communications channels and then reconverted into sound waves in the receiver of the listener’s telephone set, thus reproducing speech. The switching of communications channels required to set up temporary connections between subscribers is carried out at central offices either manually, semi-automatically, or automatically.

Telephone communication is classified so as to correspond to the functional divisions of telephone networks for general use; the categories are local (urban and rural), long-distance, and international. Communication is also possible within a department or industry (seeTRAFFIC-CONTROL COMMUNICATIONS and SELECTIVE TELEPHONE COMMUNICATIONS) and in situations where one or both parties are in motion. Parties in automobiles or aircraft or on board ship can converse with each other by means of radio communication (seeRADIOTELEPHONE COMMUNICATIONS; RADIO STATION, LOW-LEVEL; and RADIO TRANSMITTER-RECEIVER).

Telephone communication is one of the most widespread and expeditious forms of communication; it provides for the exchange of information in all areas of human endeavor, including industry, agriculture, government, science, culture, public health, and personal services.

Brief historical survey. The invention of the telephone (1876, A. G. Bell) and the development of the first telephone switchboard (1878, New Haven, Conn.) marked the beginning of telephone communication. In Russia, the first city telephone exchanges were set up in 1882 in St. Petersburg, Moscow, Odessa, and Riga. Subsequent developments in telephone communication were reflected in improvements in equipment, a growth in the number of subscribers, a greater range of communication, and a higher degree of automation. In 1889, A. B. Strowger (United States) developed the rotary stepping switch (step-by-step, or two-motion selector, switch). In 1893, M. F. Freiden-berg, working with S. M. Berdichevskii-Apostolov, built a model of an automatic switching system with rotary stepping switches, and in 1895, Freidenberg patented the concept and design of an automatic switching system incorporating preselectors. An automatic switching system was first put into operation in 1896 (Augusta, USA). During the 1940’s, crossbar switching systems were developed, followed by quasi-electronic systems in the 1960’s and the first models of electronic systems in the 1970’s.

In 1902, in order to increase the range of telephone communication, a method was developed of artificially increasing the inductance of the telephone cable to diminish the attenuation of the signal (seePUPIN SYSTEM and KRARUP LOADING). In the 1920’s, repeaters were first used on telephone lines to amplify the signal, devices proposed by the Russian engineer V. I. Kovalenkov (1915). The development of the equipment for telephone communication and the extension of telephone systems were accompanied by an increase in the cost of an outside plant, an increase that spurred the development of multichannel communication systems. As far back as 1880, the Russian inventor G. G. Ignat’ev had proposed one of the methods for simultaneous telegraphy and telephony. In the 1920’s, M. V. Shuleikin did theoretical work on high-frequency communication. The transition from telephony using voice-frequency currents (up to 3400 hertz [Hz]) to high-frequency telephone communication (more than 16 kilohertz [kHz]) was practically complete by 1950. The invention of highly selective electric filters and modulators has made possible the creation of multichannel communication systems using frequency division of multiplexing on cable, radio relay, and satellite communication links designed to handle many channels (10,000 and more). Beginning in the 1960’s, line multiplexing has also been possible through time-division multiplexing.

A striking indication of the development of telephone communication is the growth in the number of telephone sets. In 1890 there were 233,000 sets in the world, in 1928 approximately 30 million, in 1958 approximately 118 million, and in 1974 more than 330 million. In Russia in 1885, there were 1,704 telephone subscribers; by 1917 the number had increased to 223,000. The USSR had more than 1 million subscribers in 1940 and approximately 4 million in 1965; telephone sets in the USSR numbered approximately 16 million in 1974.

Quality of telephone communication; arrangement of connections. The quality of telephone communication is reflected in factors that characterize mainly the quality of speech transmission and the quality of telephone service.

The quality of speech transmission (intelligibility, naturalness, loudness) depends mainly on the technical characteristics of the telephone set, the central office, and the telephone channels. There are three requirements for high quality. The first is that all the harmonic components of the human voice (formants) in the frequency range from 300 to 3400 Hz pass through the electric circuits of the telephone system. The second is that the weakening (attenuation) of the electric signals during passage along the channels of the telephone system from one telephone set to another be limited, on the average, to ~30 decibels. The third requirement is that the noise level arising from crosstalk interference and internal interference from, for example, sparking contacts, be at least 35 decibels lower than the level of the telephone signal currents. To meet these requirements, telephone communication systems make use of high-quality telephone sets and of multichannel transmission systems that can create standard voice-frequency channels, the attenuation and frequency characteristics of which are practically independent of the length of the communications line. Crossbar and quasi-electronic automatic switching systems are also used to meet the requirements for high quality. These systems set up connections with the aid of reliable, low-noise (creating little interference) contacts.

The quality of the service is dependent on the system by which subscriber connections are organized. It is determined by statistical indexes obtained from an analysis of the time distribution of the intensity of telephone traffic on the basis of queuing theory.

With automatic telephone communication, a subscriber, by either dialing or pushing buttons, sends the number of the subscriber being called to the switching machine. As a result of the sequential action of the signals sent to the control equipment at the central office, an electric circuit is set up that connects the telephone set of the calling subscriber with the central office serving the subscriber being called. The called number is checked by this office for a busy condition and, if not busy, ringing is applied. The connection is completed when the called party picks up the handset of his telephone.

Since the number of subscribers making telephone calls at one time is always substantially less than the total number of subscribers, the number of channels in a telephone system, as well as the number of internal connecting paths at the central office, is considerably less than the number of subscribers served by the central office (usually by a factor of 7–10 in local telephone systems and of 200–250 in intercity systems). Owing to this design, a desired connection may be blocked during a period of high telephone traffic because the necessary channels and the internal connecting paths at the central office are busy. The quality of automatic telephone service can be read in the percentage of blocked, or refused, calls during the hours of heaviest traffic. For example, in the USSR, the permissible percentage is 0.2–0.4 for local calls and 2 for intercity calls. If a telephone system is designed with the duration of telephone conversations in mind and the average duration of the conversations does not exceed the computed value, then overloading (avalanching) during the hours of heaviest traffic is unlikely, and the telephone service is considered to be of high quality.

In setting up the connections for long-distance and international calls, automatic methods are used in conjunction with manual and semiautomatic methods. With the manual method, the connection is made by an operator working in an exchange equipped with switchboards. With the semiautomatic method, the connection is made at long-distance offices with the assistance of an operator; the operator, having received a request for a call, dials the number of the called party, and the connection is then completed automatically. Manual and semiautomatic methods of setting up connections make possible different types of service. With one type, for example, requests for calls are submitted to one operator, and the calls are then put through by a second operator after an interval of time that depends on the order in which the requests are made. A second type of service provides for operator-assisted long-distance calls. Here, if a call cannot be placed, the operator will hold open the caller’s line until the necessary channel is free. The quality of manual and semiautomatic telephone service is usually determined by the probability of a blocked call and the average waiting period for making a connection.

The cost of long-distance calls depends on the distance of the call and the length of the conversation and is calculated in accordance with an established system of rates. In the USSR, electronic billing equipment has been installed at long-distance offices having automatic switching systems. This equipment automatically records the caller’s number and compiles detailed billing information, including the number of the rate zone, the per-minute rate for the conversation, and the duration and total cost of the conversation. The equipment also prints the billing information on a special form. In long-distance offices operated manually, registers are used for billing purposes. In many countries, the charge for local calls depends on usage, but in the USSR (1976), there is a flat rate for local service.

Present state and prospects for development in telephone communication. Modern telephone communication is characterized by a high degree of automation and by the versatility of equipment. In many countries (Federal Republic of Germany, Belgium, the Netherlands), telephone communication is completely automatic, while in others (Polish People’s Republic, German Democratic Republic, Czechoslovak Socialist Republic, United States, Sweden, Italy), local telephone communication is completely automatic, but the degree of automation for intercity service varies between 70 and 99 percent. In the USSR, local service is 92 percent automatic, and intercity service 34 percent (including semiautomatic connections). In central offices with automatic switching systems, the crossbar system is the most common. Offices with quasi-electronic and electronic switching systems hold the greatest promise; here, switching functions and the distribution of telephone traffic will be subject to programmed control. Telephone systems in which the central offices have automatic switching equipment under programmed control make possible the introduction of a new range of services. These services include abbreviated dialing for frequently called numbers, putting a telephone set on “hold” if the number of the called party is busy, notifying a subscriber during a conversation of an incoming call, transferring a connection to another telephone set, setting up interconnections between more than two telephone sets for group conversations (conference communications), and providing priority access to connections for a limited number of subscribers.

The signals in telephone communication are transmitted through aerial wires, cables, radio relay systems, and satellites. Multichannel telephone systems often include a combination of the above and are therefore quite complex. For example, on certain intercity cables (seeTRUNK CABLES), the number of repeaters may reach several thousand. A combined type of electrical communication—video-telephone service (seeVIDEOTELEPHONE)—is possible using high-frequency cables and radio relay systems. For long-range communication, such as intercontinental communication, artificial earth satellites (seeSPACE COMMUNICATIONS) are becoming increasingly common.

Telephone communication in the USSR has developed from the technological base of the state-run telephone system. The technology has also been designed with the requirements of the Integrated Automatic Communications System in mind. In order to provide automatic telephone communication for the country’s subscribers, a unified numbering system has been gradually introduced. For example, in making an intercity call, a caller will first dial the one-digit intercity access code, followed by a three-digit area code, a two-digit number for the group of 100,000 subscribers to which the called party belongs, and the called party’s five-digit number within that group. Local calls are made with five-, six-, or seven-digit numbers. In theory, seven-digit numbers permit the formation of areas encompassing as many as 10 million subscribers. However, since the numbers “8” (intercity access code) and “0” (information, special services) are not used, areas usually encompass only 8 million subscribers.

Production of telephone equipment constitutes one of the most rapidly developing branches of industry. During the years 1965–75, the average rate of growth in the number of telephone sets in the world (and, correspondingly, in the number of telephone calls) was 7.5 percent a year. Telephone communication systems are generally state-run, but in several capitalist countries, for example, the United States, systems are operated by private companies.