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A telephone exchange is a telecommunications system used in the public switched telephone network or in large enterprises. An exchange consists of electronic components and in older systems also human operators that interconnect switch telephone subscriber lines or virtual circuits of digital systems to establish telephone calls between subscribers. In historical perspective, telecommunication terms have been used with different semantics over time.

The term telephone exchange is often used synonymously with central office COa Bell System term. Often, a central office is defined as a building used to house the inside plant equipment of potentially several telephone exchanges, each serving a certain geographical area. Such an area has also been referred to as the exchange.

Central office locations may also be identified in North America as wire centersdesignating a facility from which a telephone obtains dial tone. In the United States and Canada, the Bell System established in the s a uniform system of identifying central offices with a three-digit central office code, that was used as a prefix to subscriber telephone numbers.

All central offices within a larger region, typically aggregated by state, were assigned a common numbering plan area code. With the development of international and transoceanic telephone zystem, especially driven by direct customer dialing, similar efforts telwphone systematic organization of the telephone networks occurred in many countries in the midth century.

For corporate or enterprise use, a private telephone exchange is often referred to as a private branch exchange PBXwhen it has connections to the public switched telephone network. A PBX is installed in enterprise facilities, typically collocated with large office spaces or within an organizational campus to serve the local private telephone system and any private leased line circuits. Smaller installations might deploy a PBX or key telephone system in the office of a receptionist.

In the era of the electrical telegraph, post offices, railway stations, the more important governmental centers ministriesstock exchanges, very few nationally distributed newspapers, the largest internationally important corporations and wealthy individuals were the principal users of such telegraphs. A telephone exchange is a telephone system located at service centers central offices responsible for a small geographic area that provided the switching or interconnection of two or more individual subscriber lines for calls made between them, rather than requiring direct lines between subscriber stations.

This made it possible for subscribers to call each other at homes, businesses, or public spaces. These made telephony an available and comfortable communication tool for everyday use, and it gave the impetus for the creation of a whole new industrial sector.

As with the invention of the telephone itself, the honor tslephone “first telephone exchange” has several claimants.

Coy designed and built the first commercial US telephone exchange which opened in New Haven, Connecticut in January, The switchboard was built from “carriage bolts, handles from teapot lids and bustle wire” and could handle two simultaneous conversations. In Europe other early telephone exchanges were based in London and Manchesterboth of which opened under Bell patents in Later exchanges consisted of one to several hundred plug boards staffed by switchboard operators.

Telephpne front of the jack panel lay a horizontal panel containing two rows of patch cords, each pair connected to a cord circuit. When a calling party lifted the receiver, the local loop current lit a signal lamp near the jack. For a long distance call, she plugged into a trunk circuit to connect to another operator in another bank of boards or at a remote central office.

Inthe average time to complete the connection for a long-distance call was 15 minutes. The operator would be disconnected from the circuit, allowing her to handle another call, while the caller heard an audible ringback signal, so that that operator would not have to periodically report that she was continuing to ring telephonf line. In the ringdown method, the originating operator called another intermediate operator who would call the called subscriber, or passed it on to another intermediate operator.

In when military calls had priority, a cross-country US call might take as long as 2 hours to request and telephlne in cities that used manual switchboards for toll calls.


On March 10,Almon Brown Strowgeran undertaker in Kansas City, Missouripatented the stepping switcha device which led to the jodwvi of telephone circuit switching.

While there were many extensions and adaptations of this initial patent, the one systme known consists of 10 levels or banks, each having 10 contacts arranged in a semicircle.

When used with a rotary telephone dialeach pair of digits caused the shaft of the central contact “hand” of the stepping switch to first step ratchet jodaci one level for each pulse in the first digit and then to swing horizontally in a contact row with one small rotation for each pulse in the next digit.

Telephone exchange – Wikipedia

Later stepping switches were arranged in banks, the first stage of which was a linefinder. If one of up to a hundred subscriber lines had the receiver lifted “off hook”, a linefinder connected the subscriber’s line to a telephohe first selector, which returned the subscriber a dial tone to show that it was ready to receive dialed digits. The subscriber’s dial pulsed at about 10 pulses per second, although the speed depended on the standard of the particular telephone administration.

Exchanges based on sysetm Strowger switch were eventually challenged by other exchange types and later by crossbar technology.

These exchange designs promised faster switching and would accept pulses faster than the Strowger’s typical 10 pps—typically about 20 pps. At a later date many also accepted DTMF “touch tones” or other tone signaling systems. This technology was used as late as mid Many terms used in telecommunication technology differ in meaning and usage among the various English speaking regions. For the purpose of this article the following definitions are made:. Central office originally referred to switching equipment and its operators, it is also used generally for the building that houses switching and related inside plant equipment.

In United States te,ephone jargon, a central office C. With manual servicethe customer lifts the receiver off-hook and asks the operator to connect the call to a requested number. Provided that the number is in the same central office, and located on the operator’s switchboard, the operator connects the call by plugging the ringing cord into the jack corresponding to the called customer’s line. If the called party’s line is on a different switchboard in the same office, or in a different central office, the operator plugs into the trunk for tekephone destination jdoavi or office and asks the operator answering known as the “B” operator to connect the call.

Most urban exchanges provided common-battery service, meaning that the central office provided power to the subscriber telephone circuits for operation of jodaiv transmitter, as well as for automatic signaling with rotary dials. In common-battery systems, the pair of wires from a subscriber’s telephone to the exchange carry 48V nominal DC potential from the telephone ststem end across the conductors. The telephone systwm an open circuit when it is on-hook or idle.

Telephony – Wikipedia

When a subscriber’s phone is off-hook, it presents an electrical resistance across the line which causes current to flow through the telephone and wires to the central office. In a manually operated switchboard, this current flowed through a relay coil, and actuated systsm buzzer or a lamp on the operator’s switchboard, signaling the operator to perform service.

In the largest cities, it took many years to convert every office to automatic equipment, such as a panel switch. During this transition period, once numbers were standardized to the 2L-4N or 2L-5N format two-letter exchange joavi and either four or five digitsit was possible to dial a number located in a manual exchange and be connected without requesting operator assistance.

The policy of the Bell System stated felephone customers in large cities should not need to be concerned with the type of office, whether they were calling a manual or an automatic office. When a subscriber dialed the number of a manual station, an operator at the destination office systfm the call after seeing the number on an indicatorand connected the call by plugging a cord into the outgoing circuit and ringing the destination station.

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For example, if a dial customer calling from TAylor dialed a number served by a manual exchange, e. The party line letters W, R, J, and M were only used in manual exchanges with jack-per-line party lines. In contrast to the listing format MAin for an automated office with two capital letters, a manual telephon, having listings sysrem as Hillside or East 23, was recognizable by the format in which the second letter was not capitalized. Rural areas, as well as the smallest towns, had manual service and signaling was accomplished with magneto telephones, which had a crank for the signaling generator.


To alert the operator, or another subscriber on the same line, the subscriber turned the crank to generate ringing current. The switchboard responded by interrupting the circuit, which dropped nodavi metal tab above the subscriber’s line jack and sounded a buzzer. Many small town magneto systems featured party linesanywhere from two to ten or more subscribers sharing a single line.

When sywtem a party, the operator used code ringing, a distinctive ringing signal sequence, such as two long rings followed by one short ring.

Everyone on the line could hear the signals, and could pick up and monitor other people’s conversations. Automatic exchangesor dial servicecame into existence in the early 20th century. Their purpose was to eliminate the need for human switchboard operators who completed the connections required for a telephone call. Automation replaced human operators with electromechanical systems and telephones were equipped with a dial by which a caller transmitted the destination telephone number to the automatic switching system.

A telephone exchange automatically senses an off-hook condition of the telephone when the user removes the handset from the switchhook or cradle.

The exchange provides dial tone at that time to indicate to the user that the exchange is ready to receive dialed digits. The pulses or DTMF tones generated by the telephone are processed and a connection is established to the destination telephone within the same exchange or to another distant exchange.

The exchange maintains the connection until one of the parties hangs up. This monitoring of connection status is called supervision.

Internet Telephony

Additional features, such as billing equipment, may also be incorporated into the exchange. The Bell System dial service implemented a feature called automatic number identification ANI which facilitated services like automated billing, toll-free numbersand service.

In manual service, the operator knows where a call is originating by the light on the switchboard jack field. Before ANI, long distance calls were placed into an operator queue and the operator asked the calling party’s number and recorded it on a paper toll ticket.

Early exchanges were electromechanical systems using motors, shaft drives, rotating switches and relays. Some types of automatic exchanges were the Strowger switch or step-by-step switch, All Relay, X-Y, panel switchRotary system and the crossbar switch.

Circuits interconnecting switches are called trunks. Before Signalling System 7Bell System electromechanical switches in the United States communicated with one another over trunks using a variety of DC voltages and signaling tones. It would be rare to see any of these in use today.

Some signalling communicated dialed digits. An early form called Panel Call Indicator Pulsing used quaternary pulses to set up calls between a panel switch and a manual switchboard.

Probably the most common form of communicating dialed digits between electromechanical switches was sending dial jodavlequivalent to a rotary dial ‘s pulsing, but sent over trunk circuits between switches. In Bell System trunks, it was common to use 20 jodavk between crossbar switches and crossbar tandems. Using the faster pulsing rate made trunk utilization more efficient because the switch spent half as long listening to digits. DTMF was not used for trunk signaling.

Multi-frequency MF was the last of the pre-digital methods. It used a different set of tones sent in pairs like DTMF. Dialing was preceded by a special keypulse KP signal and followed by a start ST.

Similar schemes were used in the Americas and in some European countries including Spain. Digit strings between switches were often abbreviated to further improve telephoen. For example, one switch might send only the last four or five digits of a telephone number.

In one case, seven digit numbers were preceded dystem a digit 1 or 2 to differentiate between two area codes or office codes, a two-digit-per-call savings. This improved revenue per trunk and reduced the number of digit receivers needed in a switch. Every task in electromechanical switches was done in big metallic pieces of hardware.

Every fractional second cut off of call set up time meant fewer racks of equipment to handle call traffic. Examples of signals communicating supervision or call progress include E and M signalingSF signaling, and robbed-bit signaling. In physical not carrier E and M trunk circuits, trunks were four wire.