an equipment complex for switching telegraph channels.
The temporary connection of terminal points in a telegraph network during communications is established at telegraph stations. The connection may be established manually by a telegraph operator at manual telegraph stations equipped with telegraph switchboards, or it may be established automatically. By the mid-1970’s, nearly all manual telegraph stations had been replaced by automatic stations. Depending on the type of switching devices used, automatic telegraph stations are classified as two-motion selector stations, which use electromechanical selectors, or crossbar stations, in which switching is handled by crossbar switches. Electronic telegraph stations are currently in the development stage. They primarily use semiconductor devices for switching.
Telegraph stations include stations for teletypewriter exchange services, direct-circuit stations, and composite stations, which combine the above two services with data transmission. Small-capacity stations do not exceed the switching potential of one electromechanical selector or crossbar switch; they are usually designed for ten to 20 terminal points and not more than six channels. Large-capacity stations are designed for more than 20 terminal points.
The connections established by telegraph stations are classified as local—between terminal points connected to the same station—and intercity—between terminal points connected to different telegraph stations. If the telegraph station is originating a message, then the intercity connection it establishes is an outgoing connection; if the station is a terminal receiving station, the connection is incoming; and if it is an intermediate station, the connection is a built-up, or indirect, connection. These connections are made by terminal and relay stations.
The equipment in a telegraph station generally includes automatic switching devices, for example, selectors and crossbar switches; relay panels containing telegraph relays, which are designed for the control of switching processes and for the conversion of telegraph signals, for example, single polarity to double polarity during transmission and the reverse during reception; station telegraphs, required mainly for the handling of service calls during the monitoring and tuning of channels; various monitoring and measurement instruments; emergency and operational signaling devices; and a ±60-volt or ±20-volt DC power supply for outside plant and office equipment.
The automatic switching devices and relay panels are mounted on racks; the number of racks is determined by the type and amount of traffic. Switching devices connect the station input to the output when address selection impulses from a transmitting terminal station reach the control devices. A direct-circuit station also has equipment for retransmitting telegrams, switchboards for special correspondence and low-level service, and circuitry switchboards.
Teletypewriter exchange stations have auxiliary equipment for metering message costs and a telegraph switchboard and equipment used for transmitting particularly important messages when a subscriber’s line at the terminal station being called is busy. The switching equipment installed in composite stations is standardized as much as possible; it includes crossbar switches, high-speed relays, such as mercury-wetted hermetic contacts, and electronic switching devices. Composite stations have equipment for low-speed data transmission over telegraph channels (up to 200 bits per second) for serving subscribers to data-transmission services.
Telegraph stations may or may not be equipped with registers. In stations without registers, including certain two-motion selector stations, each selector has its own control system, and switching occurs simultaneously with the reception of address selection impulses.
In stations equipped with registers, markers and registers are used to control the switching process. The register accepts and stores data on the number of the called terminal station and then transfers the information to a marker, which controls the process of establishing a connection. The use of registers makes it possible to introduce a standardized six-unit address numbering system in the network, regardless of the capacity and structure of the telegraph station. Stations equipped with registers can establish connections along bypass routes when main routes are busy or out of service. By the mid-1970’s, all new stations were equipped with registers, and existing stations without registers were being outfitted with such equipment.
The switching of telegraph channels for both low-level and trunk communications is carried out by means of a sequential search and selection of the required line at the telegraph station. This process is performed by groups of switching devices equipped with control systems, called selection stages. Stations without registers have a preselection stage (sometimes called a finder switch), several group stages, and a line stage. The preselection stage selects a subscriber’s line from a transmitting terminal station and connects the line to station switching devices. The group stages distribute call traffic with determined routes among other selection stages. For example, when a local connection is established, the subscriber’s line from a transmitting terminal station is connected via the first group selector to the line stage, and when an outgoing connection is made, the second group selector is used. The line selection stage completes the route connection.
Stations equipped with registers have a register finder stage and a subscriber selection stage, and they may also be equipped with one or more group stages. The register finder stage is used to connect the register to the switching devices in the station. If there is no group stage, the subscriber selection stage combines the functions of the preselection and line selection stages. In stations equipped with group selection stages, the subscriber selection stage performs the functions of the preselection stage during the establishment of outgoing connections and of the line stage during the establishment of incoming connections.
The most common types of telegraph stations in the USSR telegraph network are the ATK-20 crossbar station, installed in district communications centers; the ATA-57 and ATA-K two-motion selector stations, installed in small and large oblast communications centers, respectively; the APS-Sh-I two-motion selector station and the ASP-K crossbar station, installed in republic communications centers and in the communications centers of selected cities and oblast centers; and the AT-PS-PD and Nikola Tesla composite telegraph stations, installed in republic communications centers. The Nikola Tesla station is manufactured in the Socialist Federal Republic of Yugoslavia.
The USSR is currently replacing two-motion selector stations with crossbar stations. Further improvement and development of telegraph stations is designed to meet the demands of the USSR’s Integrated Automatic Communications System.
REFERENCESKogan, V. S., E. N. Kravchenko, and Iu. M. Griaznov. Avtomaticheskie telegrafnye kommutatsionnye stantsii. Moscow, 1970.
Kogan, V. S., and E. N. Kravchenko. Proektirovanie telegrafnykh avtomaticheskikh kommutatsionnykh stantsii. Moscow, 1973.
Kogan, V. S. Telegrafiia i osnovy peredachi dannykh. Moscow, 1974.
S. T. MALINOVSKII