Telephone Central Office

telephone central office

[′tel·ə‚fōn ′sen·trəl ′ȯf·əs]

Telephone Central Office


an installation housing the equipment for switching communications channels in a telephone network. Central offices connect and disconnect channels so as to allow telephone conversations between subscribers. The switching operations performed at the stations thus serve to direct the flow of telephone traffic. In essence switching centers, telephone central offices are usually located in separate buildings.

General characteristics. The switching operations at telephone central offices are performed either manually or automatically. Manual offices have telephone switchboards and operators. Automatic offices are classified according to the type of switching equipment employed. Mechanical offices and two-motion selector offices use electromechanical selectors actuated by, respectively, mechanical means and electromagnets. Crossbar offices are automatic offices using crossbar switching systems. In quasielectronic offices, the switching is done by high-speed electromagnetic switches, for example, relays having hermetic contacts. Electronic offices, as yet in the development stage, will have semiconductor switching devices.

The automatic central offices employed in various types of telephone networks differ markedly from one another both in construction and in operating algorithms. Differences may also appear within a single network. For example, certain urban networks have district central offices in addition to separate tandem offices for incoming and outgoing calls.

Manual central offices were used exclusively during the early years of the telephone. Automation began in the 20th century, and automatic offices have undergone continual development owing to improvements in switching technology. Automatic switching shortens the time required to set up connections, improves the quality of service, and lowers operating costs. With automatic switching, networks can be constructed to handle any amount of traffic volume. This type of switching makes it economically

Figure 1. Simplified structural diagrams of automatic central offices in an urban telephone network: (a) two-motion selector, (b) crossbar office, and (c) quasi-electronic office; (P) preselection, (GS) group selection, (FS) final selection, (SS) subscriber selection, (BSL) bank of subscriber lines, (BCL) bank of connecting lines, (ACO) automatic central office, (SL) subscriber lines, and (CL) connecting lines

advisable to decentralize equipment; equipment from one central office can sometimes be housed in separate buildings, thus forming substations and concentration offices. It also becomes possible to divide a telephone network into districts. By the mid-1970’s, the great majority of telephone networks relied on automatic central offices, the exception being long-distance networks, which use both manual and automatic offices.

Equipment and operating principles of an automatic central office. An automatic central office comprises a switching system, control apparatus, leading-in lines for connecting telephone lines to the switching system, an electric power supply installation, and auxiliary equipment for such purposes as ventilating and heating.

The switching system and control apparatus are usually located in a switch room. The control apparatus sets up connecting paths through the switching system between the proper terminals of the automatic office. The connecting path is chosen on the basis of information (the called subscriber’s number) received from the telephone set of the calling subscriber. The switching system is made up of groups of switching devices that have a fixed number of inputs and outputs and are constructed in the form of plates, panels, units, and stands. In most automatic offices, connections between leading-in lines and outgoing junctions are set up in stepwise fashion by hunting for and selecting each segment of the path. In a two-motion selector office, for example (Figure l,a), the switching system consists of a preselection stage, two or more group stages, and one line selection stage. The preselector locates the line of the calling subscriber in order to connect this line via calling equipment to the switching device used in the next stage. The group selectors distribute calls among existing routes, that is, to another subscriber of the office, to another office in the same network, or to an office in another network. In the line selection stage, the final selector locates the line of the called subscriber, checks the condition of the line, and, if free, establishes the connection. The number of group-selection stages in such an automatic office depends on the size of the network; each stage increases the size (number of subscribers) by a factor of ten and accounts for one more digit in the telephone number. Automatic central offices whose functions are limited to routing traffic usually perform either one or two group-selection stages; this is the principle seen in tandem offices for incoming and outgoing calls. In crossbar central offices (Figure l,b), a uniselector is used in place of preselection and final selection. In offices with quasi-electronic systems (Figure l,c), the connection between leading-in lines and outgoing junctions is made, as a rule, in one step. The switching systems of these offices do not work in stepwise fashion; instead they are made up of two groups of switching apparatus—units for subscribers’ lines and units for connecting lines. The single-step principle of establishing connections makes possible a reduction in the amount of switching equipment needed in comparison with systems working in stepwise fashion. At the same time, the number of connecting lines in such a system is reduced.

Direct and indirect methods of establishing connections are used in automatic central offices. With the direct method, the switching apparatus simultaneously performs the functions of choosing a connecting path and setting up the connection. This method is employed in mechanical and two-motion selector offices. With the indirect method, the switching apparatus performs only the function of setting up the connection; the path is chosen by the control apparatus. The indirect method is used in cross-bar and quasi-electronic central offices.

The control algorithm for the process of establishing connections is realized through the control apparatus. Depending on the method of controlling the switching operations, automatic central offices are classified as either register types or nonregister types. In nonregister offices, the connecting path is chosen simultaneously with the reception of the pulse trains of the telephone number dialing. In this case, each selector of a two-motion selector office has its own control unit, but in some crossbar and quasi-electronic offices, receivers for voice-frequency dialing are used on the input of every selection stage or on a group of inputs. In offices of the register type (crossbar and quasi-electronic), control of the switching process is separated in time from processes of receiving and storing information about the number of the called subscriber. Thus, in crossbar offices there are control devices called markers and registers. The registers receive and store all or part of the information about the number of the called subscriber and then pass the information on to the markers at the various selection stages. The information is encoded by the registers at the time of its reception and, if necessary, converted into a form suitable for interaction with other control apparatus. The markers select connecting paths and control the process of setting up the connections. Nonregister offices are characterized by a strict dependence of the number of routes in the selection stages on the decimal numbering system; this dependence prevents these offices from meeting the flexibility and durability requirements imposed on telephone networks. Offices of the register type make possible a choice in the selection stages regarding the most economical number of routes and the extent of accessibility on each route. They also permit the establishment of connections between subscribers’ sets through different numbers of selection stages and the arrangement of alternate routes.

The leading-in lines in automatic central offices are arranged in distributing frames and cable joint rooms. The distributing frame, which comprises a subscribers’ frame and a frame for connecting lines, concentrates the leading-in lines and is equipped to protect the fixed apparatus from effects coming over the lines. In the cable joint room, the main (subscribers’) communication cables and the trunk cables of large capacity are split into cables of smaller capacity suitable for connection to the distribution frame.

The office’s electric power installation comprises, in general, a set of rectifying devices (main current source), a storage battery (standby source), automatic battery-switching apparatus, input and distribution panels for the alternating current circuits, and a stationary or movable diesel power plant (reserve source of alternating current).

Prospects for development. The principal goal of development work at present is the complete automation of switching processes. Thus far, this work has produced control devices having programmed control, which, in contrast to devices with rigidly predetermined functions, possess great flexibility in operation and make possible the automation of monitoring of performance and summary of traffic statistics. Development work has also resulted in digital data service, in the ability to carry out switching and multiplexing operations with a single device, and in the creation of control centers for telephone networks. The switching processes are being automated in such a way as to meet the requirements of the Integrated Automatic Communications System, which will use universal switching apparatus to transmit all types of information (telephone, telegraph, data) over the same communications channels.


Avtomaticheskaia kommutatsiia i telefoniia, part 2. Moscow, 1969.
Gorodskie telefonnye stantsii. Moscow, 1974.
Livshits, B. S., and N. P. Mamontova. Razvitie sistem avtomaticheskoi kommutatsii kanalov. Moscow, 1976.


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