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A telephone exchange, telephone switch, or central office is a
telecommunications Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that ...
system used in the
public switched telephone network The public switched telephone network (PSTN) provides infrastructure and services for public telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local telep ...
(PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital systems to establish
telephone call A telephone call is a connection over a telephone network between the called party and the calling party. First telephone call The first telephone call was made on March 10, 1876, by Alexander Graham Bell. Bell demonstrated his ability to "ta ...
s 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'', a
Bell System The Bell System was a system of telecommunication companies, led by the Bell Telephone Company and later by the American Telephone and Telegraph Company (AT&T), that dominated the telephone services industry in North America for over one hundr ...
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 or exchange area. In North America, a central office location may also be identified as a ''wire center'', designating a facility to which a telephone is connected and obtains dial tone. For business and billing purposes, telecommunication carriers define ''rate centers'', which in larger cities may be clusters of central offices, to define specified geographical locations for determining distance measurements. In the United States and Canada, the Bell System established in the 1940s a uniform nationwide numbering system of identifying central offices with a three-digit central office code and a three-digit numbering plan area code (NPA code, or area code). Central office codes were unique in each numbering plan area. The NPA code and the central office code were used as prefixes in subscriber telephone numbers. With the development of international and transoceanic telephone trunks, especially driven by direct customer dialing, similar efforts of systematic organization of the telephone networks occurred in many countries in the mid-20th century. For corporate or enterprise use, a private telephone exchange is often referred to as a
private branch exchange A business telephone system is a multiline telephone system typically used in business environments, encompassing systems ranging in technology from the key telephone system (KTS) to the private branch exchange (PBX). A business telephone syst ...
(PBX), when it has connections to the
public switched telephone network The public switched telephone network (PSTN) provides infrastructure and services for public telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local telep ...
. A PBX is installed in enterprise facilities, typically near large office spaces or within an organizational campus to serve the organization's telephones and any private leased line circuits. Smaller installations might deploy a PBX or key telephone system in the office of a receptionist.


History

In the era of the electrical telegraph, its principal users were post offices, railway stations, the more important governmental centers (ministries), stock exchanges, very few nationally distributed newspapers, the largest internationally important corporations, and wealthy individuals.Private Telegraphs
''
The Sydney Morning Herald ''The Sydney Morning Herald'' (''SMH'') is a daily compact newspaper published in Sydney, New South Wales, Australia, and owned by Nine. Founded in 1831 as the ''Sydney Herald'', the ''Herald'' is the oldest continuously published newspaper ...
'', credited to ''
The Times ''The Times'' is a British daily national newspaper based in London. It began in 1785 under the title ''The Daily Universal Register'', adopting its current name on 1 January 1788. ''The Times'' and its sister paper '' The Sunday Times'' ( ...
'', April 19, 1878, p. 6.
Despite the fact that telephone devices existed before the invention of the telephone exchange, their success and economical operation would have been impossible on the same
schema The word schema comes from the Greek word ('), which means ''shape'', or more generally, ''plan''. The plural is ('). In English, both ''schemas'' and ''schemata'' are used as plural forms. Schema may refer to: Science and technology * SCHEMA ...
and structure of the contemporary telegraph, as prior to the invention of the telephone exchange switchboard, early telephones were hardwired to and communicated with only a single other telephone (such as from an individual's home to the person's business). A telephone exchange is a telephone system for a small geographic area that provides the switching (interconnection) of subscriber lines for calls made between them. Telephone exchanges replaced small telephone systems that connected its users with direct lines between each and every subscriber station. Exchanges made telephony an available and comfortable technology for everyday use and it gave the impetus for the creation of a new industrial sector. As with the
invention of the telephone The invention of the telephone was the culmination of work done by more than one individual, and led to an array of lawsuits relating to the patent claims of several individuals and numerous companies. Early development The concept of th ...
itself, the honour of "first telephone exchange" has several claimants. One of the first to propose a telephone exchange was Hungarian
Tivadar Puskás Tivadar Puskás de Ditró (in older English technical literature: Theodore Puskás) (17 September 1844 – 16 March 1893) was a Hungarian inventor, telephone pioneer, and inventor of the telephone exchange. He was also the founder of Telef ...
in 1877 while he was working for
Thomas Edison Thomas Alva Edison (February 11, 1847October 18, 1931) was an American inventor and businessman. He developed many devices in fields such as electric power generation, mass communication, sound recording, and motion pictures. These inventi ...
. The first experimental telephone exchange was based on the ideas of Puskás, and it was built by the
Bell Telephone Company The Bell Telephone Company, a common law joint stock company, was organized in Boston, Massachusetts, on July 9, 1877, by Alexander Graham Bell's father-in-law Gardiner Greene Hubbard, who also helped organize a sister company – the New Engl ...
in
Boston Boston (), officially the City of Boston, is the state capital and most populous city of the Commonwealth of Massachusetts, as well as the cultural and financial center of the New England region of the United States. It is the 24th- mo ...
in 1877. The world's first state-administered telephone exchange opened on November 12, 1877 in Friedrichsberg close to
Berlin Berlin ( , ) is the capital and largest city of Germany by both area and population. Its 3.7 million inhabitants make it the European Union's most populous city, according to population within city limits. One of Germany's sixteen constitu ...
under the direction of
Heinrich von Stephan Ernst Heinrich Wilhelm von Stephan (born Heinrich Stephan, January 7, 1831 – April 8, 1897) was a general post director for the German Empire who reorganized the German postal service. He was integral in the founding of the Universal Postal ...
. George W. Coy designed and built the first commercial US telephone exchange which opened in
New Haven, Connecticut New Haven is a city in the U.S. state of Connecticut. It is located on New Haven Harbor on the northern shore of Long Island Sound in New Haven County, Connecticut and is part of the New York City metropolitan area. With a population of 134 ...
in January, 1878, and the first telephone booth was built in nearby
Bridgeport Bridgeport is the most populous city and a major port in the U.S. state of Connecticut. With a population of 148,654 in 2020, it is also the fifth-most populous in New England. Located in eastern Fairfield County at the mouth of the Pequonn ...
. The switchboard was built from "carriage bolts, handles from teapot lids and bustle wire" and could handle two simultaneous conversations. Charles Glidden is also credited with establishing an exchange in Lowell, MA. with 50 subscribers in 1878. In Europe other early telephone exchanges were based in
London London is the capital and List of urban areas in the United Kingdom, largest city of England and the United Kingdom, with a population of just under 9 million. It stands on the River Thames in south-east England at the head of a estuary dow ...
and
Manchester Manchester () is a city in Greater Manchester, England. It had a population of 552,000 in 2021. It is bordered by the Cheshire Plain to the south, the Pennines to the north and east, and the neighbouring city of Salford to the west. The ...
, both of which opened under Bell patents in 1879. Belgium had its first International Bell exchange (in
Antwerp Antwerp (; nl, Antwerpen ; french: Anvers ; es, Amberes) is the largest city in Belgium by area at and the capital of Antwerp Province in the Flemish Region. With a population of 520,504,
) a year later. In 1887 Puskás introduced the
multiplex Multiplex may refer to: * Multiplex (automobile), a former American car make * Multiplex (comics), a DC comic book supervillain * Multiplex (company), a global contracting and development company * Multiplex (assay), a biological assay which measu ...
switchboard. . Later exchanges consisted of one to several hundred plug boards staffed by
switchboard operator In the early days of telephony, companies used manual telephone switchboards, and switchboard operators connected calls by inserting a pair of phone plugs into the appropriate jacks. They were gradually phased out and replaced by automated system ...
s. Each operator sat in front of a vertical panel containing banks of ¼-inch tip-ring-sleeve (3-conductor) jacks, each of which was the local termination of a
subscriber The subscription business model is a business model in which a customer must pay a recurring price at regular intervals for access to a product or service. The model was pioneered by publishers of books and periodicals in the 17th century, and ...
's
telephone line A telephone line or telephone circuit (or just line or circuit industrywide) is a single-user circuit on a telephone communication system. It is designed to reproduce speech of a quality that is understandable. It is the physical wire or ot ...
. In 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 The calling party (in some contexts called the "A-Number") is a person who (or device that) initiates a telephone call. The person who, or device that, receives a telephone call is the called party (or callee or B-party). In some countries, it ...
lifted the receiver, the local loop current lit a signal lamp near the jack. The operator responded by inserting the rear cord (''answering cord'') into the subscriber's jack and switched her headset into the circuit to ask, "Number, please?" For a local call, the operator inserted the front cord of the pair (''ringing cord'') into the called party's local jack and started the ringing cycle. For a long-distance call, she plugged into a
trunk Trunk may refer to: Biology * Trunk (anatomy), synonym for torso * Trunk (botany), a tree's central superstructure * Trunk of corpus callosum, in neuroanatomy * Elephant trunk, the proboscis of an elephant Computing * Trunk (software), in rev ...
circuit to connect to another operator in another bank of boards or at a remote central office. In 1918, the average time to complete the connection for a long-distance call was 15 minutes. Early manual switchboards required the operator to operate listening keys and ringing keys, but by the late 1910s and 1920s, advances in switchboard technology led to features which allowed the call to be automatically answered immediately as the operator inserted the answering cord, and ringing would automatically begin as soon as the operator inserted the ringing cord into the called party's jack. 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 the line. In the
ringdown In telephony, ringdown is a method of signaling an operator in which telephone ringing current is sent over the line to operate a lamp or cause the operation of a self-locking relay known as a ''drop''. Ringdown is used in manual operation, ...
method, the originating operator called another intermediate operator who would call the called subscriber, or passed it on to another intermediate operator. This chain of intermediate operators could complete the call only if intermediate trunk lines were available between all the centers at the same time. In 1943 when military calls had priority, a cross-country US call might take as long as 2 hours to request and schedule in cities that used manual switchboards for toll calls. On March 10, 1891,
Almon Brown Strowger Almon Brown Strowger (February 11, 1839 – May 26, 1902) was an American inventor who gave his name to the Strowger switch, an electromechanical telephone exchange technology that his invention and patent inspired. Early years Strowger was bo ...
, an undertaker in
Kansas City, Missouri Kansas City (abbreviated KC or KCMO) is the largest city in Missouri by population and area. As of the 2020 United States census, 2020 census, the city had a population of 508,090 in 2020, making it the List of United States cities by populat ...
, patented the
stepping switch In electrical control engineering, a stepping switch or stepping relay, also known as a uniselector, is an electromechanical device that switches an input signal path to one of several possible output paths, directed by a train of electrical puls ...
, a device which led to the automation of telephone circuit switching. While there were many extensions and adaptations of this initial patent, the one best known consists of 10 levels or banks, each having 10 contacts arranged in a semicircle. When used with a rotary
telephone dial A rotary dial is a component of a telephone or a telephone switchboard that implements a signaling technology in telecommunications known as pulse dialing. It is used when initiating a telephone call to transmit the destination telephone n ...
, each pair of digits caused the shaft of the central contact "hand" of the stepping switch to first step (ratchet) up 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. Later stepping switches were arranged in banks, the first stage of which was a '' linefinder''. If one of up to a hundred subscriber lines (two hundred lines in later linefinders) had the receiver lifted "off hook", a linefinder connected the subscriber's line to a free first selector, which returned the subscriber a dial tone to show that it was ready to receive dialled 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 the Strowger switch were eventually challenged by other exchange types and later by
crossbar Crossbar may refer to: Structures * Latch (hardware), a post barring a door * Top tube of a bicycle frame * Crossbar, the horizontal member of various sports goals * Crossbar, a horizontal member of an electricity pylon Other * In electronic ...
technology. These exchange designs promised faster switching and would accept inter-switch 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. A transitional technology (from pulse to DTMF) had converters to convert DTMF to pulse, to feed to older Strowger, panel, or crossbar switches. This technology was used as late as mid-2002.


Terminology

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: * ''Manual service'' is telephone service in which a human telephone operator routes calls as instructed by a subscriber with a telephone set that does not have a dial. * ''Dial service'' is when an exchange routes calls by interpreting subscriber-dialed digits. * A ''telephone switch'' is the switching equipment of an exchange. * A ''wire center'' is the area served by a particular switch or central office. * A '' concentrator'' is a device that concentrates traffic, be it remote or co-located with the switch. * An '' off-hook'' condition represents a circuit that is in use, e.g., when a telephone call is in progress. * An '' on-hook'' condition represents an idle circuit, i.e. no telephone call is in progress. A ''central office'' originally was a primary exchange in a city with other exchanges service parts of the area. The term became to mean any switching system including its facilities and operators. It is also used generally for the building that houses switching and related inside plant equipment. In United States
telecommunication Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that ...
jargon, a central office (C.O.) is a
common carrier A common carrier in common law countries (corresponding to a public carrier in some civil law systems,Encyclopædia Britannica CD 2000 "Civil-law public carrier" from "carriage of goods" usually called simply a ''carrier'') is a person or compan ...
switching center Class 5 telephone switch in which trunks and
local loop In telephony, the local loop (also referred to as the local tail, subscriber line, or in the aggregate as the last mile) is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the commo ...
s are terminated and switched.Source: from Federal Standard 1037C. In the UK, a ''telephone exchange'' means an exchange building, and is also the name for a telephone switch.


Manual service exchanges

With manual service, the 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 the destination switchboard 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 the transmitter, as well as for automatic signaling with
rotary dial A rotary dial is a component of a telephone or a telephone switchboard that implements a signaling technology in telecommunications known as pulse dialing. It is used when initiating a telephone call to transmit the destination telephone nu ...
s. In common-battery systems, the pair of wires from a subscriber's telephone to the exchange carry 48V (nominal) DC potential from the telephone company end across the conductors. The telephone presents an open circuit when it is on-hook or idle.Connected to a switch, an off-hook condition operates a relay to connect the line to a dial tone generator and a device to collect dialed digits. 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 a 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 The Panel Machine Switching System is a type of automatic telephone exchange for urban service that was used in the Bell System in the United States for seven decades. The first semi-mechanical types of this design were installed in 1915 in Newark, ...
. During this transition period, once numbers were standardized to the 2L-4N or 2L-5N format (two-letter exchange name and either four or five digits), it was possible to dial a number located in a manual exchange and be connected without requesting operator assistance. The policy of the
Bell System The Bell System was a system of telecommunication companies, led by the Bell Telephone Company and later by the American Telephone and Telegraph Company (AT&T), that dominated the telephone services industry in North America for over one hundr ...
stated that 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 answered the call after seeing the number on an
indicator Indicator may refer to: Biology * Environmental indicator of environmental health (pressures, conditions and responses) * Ecological indicator of ecosystem health (ecological processes) * Health indicator, which is used to describe the health o ...
, and connected the call by plugging a cord into the outgoing circuit and ringing the destination station. For example, if a dial customer calling from TAylor 4725 dialed a number served by a manual exchange, e.g., ADams 1383-W, the call was completed, from the subscriber's perspective, exactly as a call to LEnnox 5813, in an automated exchange. 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 1234'' for an automated office with two capital letters, a manual office, having listings such as Hillside 834 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 a metal tab above the subscriber's line jack and sounded a buzzer.
Dry cell upLine art drawing of a dry cell: 1. brass cap, 2. plastic seal, 3. expansion space, 4. porous cardboard, 5. zinc can, 6. carbon rod, 7. chemical mixture A dry cell is a type of electric battery, commonly used for portable electrical devices. Unl ...
batteries, normally two large N°. 6 cells in the subscriber's telephone, provided the direct current for the transmitter. Such magneto systems were in use in the US as late as 1983, as in the small town, Bryant Pond,
Woodstock, Maine Woodstock is a town in Oxford County, Maine, United States. The population was 1,352 at the 2020 census. The village of Bryant Pond, on State Route 26 in the northern part of Woodstock, is the town's urban center and largest settlement. Histor ...
. Many small town magneto systems featured party lines, anywhere from two to ten or more subscribers sharing a single line. When calling 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.


Early automatic exchanges

Automatic exchanges, which provided dial service, were invented by Almon Strowger in 1888. First used commercially in 1892, they did not gain widespread use until the first decade of the 20th century. They eliminated the need for human
switchboard operator In the early days of telephony, companies used manual telephone switchboards, and switchboard operators connected calls by inserting a pair of phone plugs into the appropriate jacks. They were gradually phased out and replaced by automated system ...
s who completed the connections required for a
telephone call A telephone call is a connection over a telephone network between the called party and the calling party. First telephone call The first telephone call was made on March 10, 1876, by Alexander Graham Bell. Bell demonstrated his ability to "ta ...
. 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 A telephone is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most efficiently the human voice, into e ...
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.'' 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 Automatic number identification (ANI) is a feature of a telecommunications network for automatically determining the origination telephone number on toll calls for billing purposes. Automatic number identification was originally created by the A ...
(ANI) which facilitated services like automated billing, toll-free 800-numbers, and
9-1-1 , usually written 911, is an emergency telephone number for the United States, Canada, Mexico, Panama, Palau, Argentina, Philippines, Jordan, as well as the North American Numbering Plan (NANP), one of eight N11 codes. Like other emergency n ...
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 The Strowger switch is the first commercially successful electromechanical stepping switch telephone exchange system. It was developed by the Strowger Automatic Telephone Exchange Company founded in 1891 by Almon Brown Strowger. Because of it ...
or step-by-step switch, All Relay,
panel switch The Panel Machine Switching System is a type of automatic telephone exchange for urban service that was used in the Bell System in the United States for seven decades. The first semi-mechanical types of this design were installed in 1915 in Newark, ...
, Rotary system and the
crossbar switch In electronics and telecommunications, a crossbar switch (cross-point switch, matrix switch) is a collection of switches arranged in a matrix configuration. A crossbar switch has multiple input and output lines that form a crossed pattern of int ...
.


Electromechanical signaling

Circuits interconnecting switches are called ''trunks''. Before Signalling System 7,
Bell System The Bell System was a system of telecommunication companies, led by the Bell Telephone Company and later by the American Telephone and Telegraph Company (AT&T), that dominated the telephone services industry in North America for over one hundr ...
electromechanical switches in the United States originally communicated with one another over trunks using a variety of DC voltages and signaling tones, replaced today by digital signals. Some signaling communicated dialed digits. An early form called
Panel Call Indicator Panel Call Indicator, or PCI, is a form of signalling used between two telephone offices. It was also originally called Relay Call Indicator (RCI). Originally designed along with the panel type telephone office, PCI was intended to allow subscr ...
Pulsing used
quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million year ...
pulses to set up calls between a
panel switch The Panel Machine Switching System is a type of automatic telephone exchange for urban service that was used in the Bell System in the United States for seven decades. The first semi-mechanical types of this design were installed in 1915 in Newark, ...
and a manual switchboard. Probably the most common form of communicating dialed digits between electromechanical switches was sending dial pulses, equivalent to a
rotary dial A rotary dial is a component of a telephone or a telephone switchboard that implements a signaling technology in telecommunications known as pulse dialing. It is used when initiating a telephone call to transmit the destination telephone nu ...
's pulsing, but sent over trunk circuits between switches. In Bell System trunks, it was common to use 20 pulse-per-second between crossbar switches and crossbar tandems. This was twice the rate of Western Electric/Bell System telephone dials. 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). Variations of the Bell System MF tone scheme became a
CCITT The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Comm ...
standard. Similar schemes were used in the Americas and in some European countries including Spain. Digit strings between switches were often abbreviated to further improve utilization. For example, one switch might send only the last four or five digits of a
telephone number A telephone number is a sequence of digits assigned to a landline telephone subscriber station connected to a telephone line or to a wireless electronic telephony device, such as a radio telephone or a mobile telephone, or to other devices f ...
. In one case, seven digit numbers were preceded by 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 signaling E and M signaling is a type of supervisory line signaling that uses DC signals on separate leads, called the "E" lead and "M" lead, traditionally used in the telecommunications industry between telephone switches. Various mnemonic names have been us ...
, SF signaling, and robbed-bit signaling. In physical (not carrier) E and M trunk circuits, trunks were four wire. Fifty trunks would require a hundred pair cable between switches, for example. Conductors in one common circuit configuration were named tip, ring, ear (E) and mouth (M). Tip and ring were the voice-carrying pair, and named after the tip and ring on the three conductor cords on the manual operator's console. In two-way trunks with E and M signaling, a handshake took place to prevent both switches from colliding by dialing calls on the same trunk at the same time. By changing the state of these leads from ground to -48 volts, the switches stepped through a handshake protocol. Using DC voltage changes, the local switch would send a signal to get ready for a call and the remote switch would reply with an acknowledgment (a wink) to go ahead with dial pulsing. This was done with relay logic and discrete electronics. These voltage changes on the trunk circuit would cause pops or clicks that were audible to the subscriber as the electrical handshaking stepped through its protocol. Another handshake, to start timing for billing purposes, caused a second set of clunks when the called party answered. A second common form of signaling for supervision was called single-frequency or ''SF signaling''. The most common form of this used a steady 2,600 Hz tone to identify a trunk as idle. Trunk circuitry hearing a 2,600 Hz tone for a certain duration would go idle. (The duration requirement reduced falsing.) Some systems used tone frequencies over 3,000 Hz, particularly on SSB
frequency-division multiplex In telecommunications, frequency-division multiplexing (FDM) is a technique by which the total bandwidth available in a communication medium is divided into a series of non-overlapping frequency bands, each of which is used to carry a separate ...
microwave radio relay Microwave transmission is the transmission of information by electromagnetic waves with wavelengths in the microwave frequency range of 300MHz to 300GHz(1 m - 1 mm wavelength) of the electromagnetic spectrum. Microwave signals are normally lim ...
s. On
T-carrier The T-carrier is a member of the series of carrier systems developed by AT&T Bell Laboratories for digital transmission of multiplexed telephone calls. The first version, the Transmission System 1 (T1), was introduced in 1962 in the Bell ...
digital transmission systems, bits within the T-1 data stream were used to transmit supervision. By careful design, the appropriated bits did not change voice quality appreciably. Robbed bits were translated to changes in contact states (opens and closures) by electronics in the channel bank hardware. This allowed direct current E and M signaling, or dial pulses, to be sent between electromechanical switches over a digital carrier which did not have DC continuity.


Noise

A characteristic of electromechanical switching equipment is that the maintenance staff could hear the mechanical clattering of Strowgers, panel switches or crossbar relays. During heavy use periods, it could be difficult to converse in a central office switch room due to the clatter of calls being processed in a large switch. For example, on Mother's Day in the US, or on a Friday evening around 5pm, the metallic rattling could make raised voices necessary. For wire spring relay markers these noises resembled hail falling on a metallic roof. On a pre-dawn Sunday morning, call processing might slow to the extent that one might be able to hear individual calls being dialed and set up. There were also noises from whining power inverters and whirring ringing generators. Some systems had a continual, rhythmic "clack-clack-clack" from wire spring relays that made reorder (120 ipm) and busy (60 ipm) signals. Bell System installations typically had alarm bells, gongs, or chimes to announce alarms calling attention to a failed switch element. A trouble reporting card system was connected to switch common control elements. These trouble reporting systems punctured cardboard cards with a code that logged the nature of a failure.
Reed relay A reed relay is a type of relay that uses an electromagnet to control one or more reed switches. The contacts are of magnetic material and the electromagnet acts directly on them without requiring an armature to move them. Sealed in a long, narro ...
technology in stored program control exchange finally quieted the environment.


Maintenance tasks

Electromechanical switching systems required sources of electricity in form of direct current (DC), as well as alternating ring current (AC), which were generated on-site with mechanical generators. In addition, telephone switches required adjustment of many mechanical parts. Unlike modern switches, a circuit connecting a dialed call through an electromechanical switch had DC continuity within the local exchange area via metallic conductors. The design and maintenance procedures of all systems involved methods to avoid that subscribers experienced undue changes in the quality of the service or that they noticed failures. A variety of tools referred to as ''make-busy''s were plugged into electromechanical switch elements upon failure and during repairs. A make-busy identified the part being worked on as in-use, causing the switching logic to route around it. A similar tool was called a ''TD tool.'' Delinquent subscribers had their service temporarily denied (TDed). This was effected by plugging a tool into the subscriber's office equipment on Crossbar systems or line group in step-by-step switches. The subscriber could receive calls but could not dial out. Strowger-based, step-by-step offices in the Bell System required continuous maintenance, such as cleaning. Indicator lights on equipment bays alerted staff to conditions such as blown fuses (usually white lamps) or a permanent signal (stuck off-hook condition, usually green indicators). Step offices were more susceptible to single-point failures than newer technologies. Crossbar offices used more shared, common control circuits. For example, a digit receiver (part of an element called an ''Originating Register'') would be connected to a call just long enough to collect the subscriber's dialed digits. Crossbar architecture was more flexible than step offices. Later crossbar systems had punch-card-based trouble reporting systems. By the 1970s, ''
automatic number identification Automatic number identification (ANI) is a feature of a telecommunications network for automatically determining the origination telephone number on toll calls for billing purposes. Automatic number identification was originally created by the A ...
'' had been retrofitted to nearly all step-by-step and crossbar switches in the Bell System.


Electronic switches

Electronic switching systems gradually evolved in stages from electromechanical hybrids with stored program control to the fully digital systems. Early systems used
reed relay A reed relay is a type of relay that uses an electromagnet to control one or more reed switches. The contacts are of magnetic material and the electromagnet acts directly on them without requiring an armature to move them. Sealed in a long, narro ...
-switched metallic paths under digital control. Equipment testing, phone numbers reassignments, circuit lockouts and similar tasks were accomplished by data entry on a terminal. Examples of these systems included the
Western Electric The Western Electric Company was an American electrical engineering and manufacturing company officially founded in 1869. A wholly owned subsidiary of American Telephone & Telegraph for most of its lifespan, it served as the primary equipment ma ...
1ESS switch, Northern Telecom SP1, Ericsson AXE, Automatic Electric EAX-1 & EAX-2, Philips PRX/A, ITT Metaconta, British GPO/BT TXE series and several other designs were similar. Ericsson also developed a fully computerized version of their ARF crossbar exchange called ARE. These used a crossbar switching matrix with a fully computerized control system and provided a wide range of advanced services. Local versions were called ARE11 while tandem versions were known as ARE13. They were used in Scandinavia, Australia, Ireland and many other countries in the late 1970s and into the 1980s when they were replaced with digital technology. These systems could use the old electromechanical signaling methods inherited from crossbar and step-by-step switches. They also introduced a new form of data communications: two 1ESS exchanges could communicate with one another using a data link called Common Channel Interoffice Signaling, (CCIS). This data link was based on CCITT 6, a predecessor to SS7. In European systems R2 signalling was normally used.


Digital switches

First concepts of digital switching and transmission were developed by various labs in the United States and in Europe starting in the 1930s. The first prototype digital switch was developed by
Bell Labs Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984), then AT&T Bell Laboratories (1984–1996) and Bell Labs Innovations (1996–2007), is an American industrial research and scientific development company owned by mul ...
as part of the ESSEX project while the first true digital exchange to be combined with digital transmission systems was designed by LCT (Laboratoire Central de Telecommunications) in Paris. The first digital switch to be placed into a public network in England was the Empress Exchange in
London London is the capital and List of urban areas in the United Kingdom, largest city of England and the United Kingdom, with a population of just under 9 million. It stands on the River Thames in south-east England at the head of a estuary dow ...
which was designed by the
General Post Office The General Post Office (GPO) was the state postal system and telecommunications carrier of the United Kingdom until 1969. Before the Acts of Union 1707, it was the postal system of the Kingdom of England, established by Charles II in 1660. ...
research labs. It was a tandem switch that connected three Strowger exchanges. The first commercial roll-out of a fully digital local switching system was Alcatel's E10 system which began serving customers in
Brittany Brittany (; french: link=no, Bretagne ; br, Breizh, or ; Gallo: ''Bertaèyn'' ) is a peninsula, historical country and cultural area in the west of modern France, covering the western part of what was known as Armorica during the period ...
in Northwestern France in 1972. Prominent examples of digital switches include: *
Ericsson (lit. "Telephone Stock Company of LM Ericsson"), commonly known as Ericsson, is a Swedish multinational networking and telecommunications company headquartered in Stockholm. The company sells infrastructure, software, and services in inform ...
's
AXE telephone exchange The AXE telephone exchange is a product line of circuit switched digital telephone exchanges manufactured by Ericsson, a Swedish telecom company. It was developed in 1974 by Ellemtel, a research and development subsidiary of Ericsson and Tele ...
is the most widely used digital switching platform in the world and can be found throughout Europe and in most countries around the world. It is also very popular in mobile applications. This highly modular system was developed in Sweden in the 1970s as a replacement for the very popular range of Ericsson
crossbar switch In electronics and telecommunications, a crossbar switch (cross-point switch, matrix switch) is a collection of switches arranged in a matrix configuration. A crossbar switch has multiple input and output lines that form a crossed pattern of int ...
es ARF, ARM, ARK and ARE used by many European networks from the 1950s onwards. *
Alcatel-Lucent Alcatel–Lucent S.A. () was a French–American global telecommunications equipment company, headquartered in Boulogne-Billancourt, France. It was formed in 2006 by the merger of France-based Alcatel and U.S.-based Lucent, the latter being a su ...
inherited three of the world's most iconic digital switching systems : Alcatel E10, 1000-S12, and the
Western Electric The Western Electric Company was an American electrical engineering and manufacturing company officially founded in 1869. A wholly owned subsidiary of American Telephone & Telegraph for most of its lifespan, it served as the primary equipment ma ...
5ESS. :Alcatel developed the E10 system in France during the late 1960s and 1970s. This widely used family of digital switches was one of the earliest TDM switches to be widely used in public networks. Subscribers were first connected to E10A switches in France in 1972. This system is used in France, Ireland, China, and many other countries. It has been through many revisions and current versions are even integrated into
All IP The next-generation network (NGN) is a body of key architectural changes in telecommunication core and access networks. The general idea behind the NGN is that one network transports all information and services (voice, data, and all sorts of me ...
networks. :Alcatel also acquired ITT System 12 which when it bought ITT's European operations. The S12 system and E10 systems were merged into a single platform in the 1990s. The S12 system is used in Germany, Italy, Australia, Belgium, China, India, and many other countries around the world. :Finally, when Alcatel and Lucent merged, the company acquired Lucent's 5ESS and
4ESS The No. 4 Electronic Switching System (4ESS) is a class 4 telephone electronic switching system that was the first digital electronic toll switch introduced by Western Electric for long-distance switching. It was introduced in Chicago in Janua ...
systems used throughout the United States of America and in many other countries. *
Nokia Siemens Networks Nokia Networks (formerly Nokia Solutions and Networks (NSN) and Nokia Siemens Networks (NSN)) is a multinational data networking and telecommunications equipment company headquartered in Espoo, Finland, and wholly owned subsidiary of Nokia Corp ...
EWSD originally developed by
Siemens Siemens AG ( ) is a German multinational conglomerate corporation and the largest industrial manufacturing company in Europe headquartered in Munich with branch offices abroad. The principal divisions of the corporation are ''Industry'', ''E ...
, Bosch and for the German market is used throughout the world. *
Nortel Nortel Networks Corporation (Nortel), formerly Northern Telecom Limited, was a Canadian multinational telecommunications and data networking equipment manufacturer headquartered in Ottawa, Ontario, Canada. It was founded in Montreal, Quebec, ...
then
Genband Ribbon Communications US LLC is a public company that makes software, IP and optical networking solutions for service providers, enterprises and critical infrastructure sectors. The company was formed in 2017, following the merger of Genband and ...
, and now Ribbon Communications DMS100 and other versions are very popular with operators all over the world. * GTD-5 EAX developed by GTE Automatic Electric, the GTD-5 was acquired by Lucent which became Alcatel-Lucent, which then became Nokia *
NEC is a Japanese multinational information technology and electronics corporation, headquartered in Minato, Tokyo. The company was known as the Nippon Electric Company, Limited, before rebranding in 1983 as NEC. It provides IT and network soluti ...
NEAX used in Japan, New Zealand and many other countries. * Marconi System X originally developed by GPT and Plessey is a type of digital exchange used by
BT Group BT Group plc ( trading as BT and formerly British Telecom) is a British multinational telecommunications holding company headquartered in London, England. It has operations in around 180 countries and is the largest provider of fixed-line, bro ...
in the UK public telephone network. Digital switches encode the speech going on, in 8,000 time slices per second. (A
sampling rate In signal processing, sampling is the reduction of a continuous-time signal In mathematical dynamics, discrete time and continuous time are two alternative frameworks within which variables that evolve over time are modeled. Discrete time ...
of 8khz). At each time slice, a digital
PCM Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals. It is the standard form of digital audio in computers, compact discs, digital telephony and other digital audio applications. In a PCM stream, the am ...
representation of the sound is made. The digital PCM signals are then sent to the receiving end of the line, where the reverse process occurs using a DAC (
digital-to-analog converter In electronics, a digital-to-analog converter (DAC, D/A, D2A, or D-to-A) is a system that converts a digital signal into an analog signal. An analog-to-digital converter (ADC) performs the reverse function. There are several DAC archit ...
), to produce the sound for the receiving phone. In other words, when someone uses a telephone, the speaker's voice is "encoded" using PCM for switching then reconstructed for the person on the other end. The speaker's voice is delayed in the process by a small fraction of one second — it is not "live", it is reconstructed — delayed only minutely. Individual
local loop In telephony, the local loop (also referred to as the local tail, subscriber line, or in the aggregate as the last mile) is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the commo ...
telephone lines are connected to a remote concentrator. In many cases, the concentrator is co-located in the same building as the switch. The interface between remote concentrators and telephone switches has been standardised by
ETSI The European Telecommunications Standards Institute (ETSI) is an independent, not-for-profit, standardization organization in the field of information and communications. ETSI supports the development and testing of global technical standard ...
as the V5 protocol. Concentrators are used because most telephones are idle most of the day, hence the traffic from hundreds or thousands of them may be concentrated into only tens or hundreds of shared connections. Some telephone switches do not have concentrators directly connected to them, but rather are used to connect calls between other telephone switches. These complex machines are referred to as "carrier-level" switches or tandem switches. Some telephone exchange buildings in small towns house only ''remote'' or ''satellite'' switches, and are homed upon a "parent" switch, usually several kilometres away. The remote switch is dependent on the parent switch for routing. Unlike a digital loop carrier, a remote switch can route calls between local phones itself, without using trunks to the parent switch. 300px, Map of wire center locations in the US 300px, Map of central office locations in the US


The switch's place in the network

Telephone switches are a small component of a large network. A major part, in terms of expense, maintenance, and logistics of the telephone system is
outside plant In telecommunication, the term outside plant has the following meanings: *In civilian telecommunications, outside plant refers to all of the physical cabling and supporting infrastructure (such as conduit, cabinets, tower or poles), and any asso ...
, which is the wiring outside the central office. While many subscribers were served with party-lines in the middle of the 20th century, it was the goal that each subscriber telephone station were connected to an individual pair of wires from the switching system. A typical central office may have tens of thousands of pairs of wires that appear on terminal blocks called the main distribution frame (MDF). A component of the MDF is protection: fuses or other devices that protect the switch from lightning, shorts with electric power lines, or other foreign voltages. In a typical telephone company, a large database tracks information about each subscriber pair and the status of each jumper. Before computerization of Bell System records in the 1980s, this information was handwritten in pencil in accounting ledger books. To reduce the expense of outside plant, some companies use " pair gain" devices to provide telephone service to subscribers. These devices are used to provide service where existing copper facilities have been exhausted or by siting in a neighborhood, can reduce the length of copper pairs, enabling digital services such as
Integrated Services Digital Network Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network. Work ...
(ISDN) or
digital subscriber line Digital subscriber line (DSL; originally digital subscriber loop) is a family of technologies that are used to transmit digital data over telephone lines. In telecommunications marketing, the term DSL is widely understood to mean asymmetric dig ...
(DSL). Pair gain or digital loop carriers (DLCs) are located outside the central office, usually in a large neighborhood distant from the CO. DLCs are often referred to as Subscriber Loop Carriers (SLCs), after a
Lucent Lucent Technologies, Inc. was an American multinational telecommunications equipment company headquartered in Murray Hill, New Jersey. It was established on September 30, 1996, through the divestiture of the former AT&T Technologies business u ...
proprietary product. DLCs can be configured as universal (UDLCs) or integrated (IDLCs). ''Universal DLC''s have two terminals, a central office terminal (COT) and a remote terminal (RT), that function similarly. Both terminals interface with analog signals, convert to digital signals, and transport to the other side where the reverse is performed. Sometimes, the transport is handled by separate equipment. In an ''Integrated DLC'', the COT is eliminated. Instead, the RT is connected digitally to equipment in the telephone switch. This reduces the total amount of equipment required. Switches are used in both local central offices and in long distance centers. There are two major types in the
Public switched telephone network The public switched telephone network (PSTN) provides infrastructure and services for public telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local telep ...
(PSTN), the
Class 4 telephone switch A class-4, or tandem, telephone switch is a U.S. telephone company central office telephone exchange used to interconnect local exchange carrier offices for long distance communications in the public switched telephone network. A class-4 switc ...
es designed for toll or switch-to-switch connections, and the
Class 5 telephone switches {{No footnotes, date=August 2008 A class-5 telephone switch is a telephone exchange in the public switched telephone network (PSTN) that directly serves subscribers and manages subscriber calling features. Class-5 services include basic dial-tone, ...
or subscriber switches, which manage connections from subscriber telephones. Since the 1990s, hybrid Class 4/5 switching systems that serve both functions have become common. Another element of the telephone network is time and timing. Switching, transmission and billing equipment may be slaved to very high accuracy 10 MHz standards which synchronize time events to very close intervals. Time-standards equipment may include Rubidium- or Caesium-based standards and a
Global Positioning System The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite ...
receiver.


Switch design

Long-distance switches may use a slower, more efficient switch-allocation algorithm than local central offices, because they have near 100% utilization of their input and output channels. Central offices have more than 90% of their channel capacity unused. Traditional telephone switches connected physical circuits (e.g., wire pairs) while modern telephone switches use a combination of space- and time-division switching. In other words, each voice channel is represented by a
time slot Broadcast programming is the practice of organizing or ordering (scheduling) of broadcast media shows, typically radio and television, in a daily, weekly, monthly, quarterly or season-long schedule. Modern broadcasters use broadcast automation ...
(say 1 or 2) on a physical wire pair (A or B). In order to connect two voice channels (say A1 and B2) together, the telephone switch interchanges the information between A1 and B2. It switches both the time slot and physical connection. To do this, it exchanges data between the time slots and connections 8,000 times per second, under control of digital logic that cycles through electronic lists of the current connections. Using both types of switching makes a modern switch far smaller than either a space or time switch could be by itself. The structure of a switch is an odd number of layers of smaller, simpler subswitches. Each layer is interconnected by a web of wires that goes from each subswitch, to a set of the next layer of subswitches. In some designs, a physical (space) switching layer alternates with a time switching layer. The layers are symmetric, because in a telephone system callers can also be called. Other designs use time-switching only, throughout the switch. A time-division subswitch reads a complete cycle of time slots into a memory, and then writes it out in a different order, also under control of a cyclic computer memory. This causes some delay in the signal. A space-division subswitch switches electrical paths, often using some variant of a
nonblocking minimal spanning switch A nonblocking minimal spanning switch is a device that can connect N inputs to N outputs in any combination. The most familiar use of switches of this type is in a telephone exchange. The term "non-blocking" means that if it is not defective, ...
, or a crossover switch.


Fault tolerance

Composite switches are inherently fault-tolerant. If a subswitch fails, the controlling computer can sense the failure during a periodic test. The computer marks all the connections to the subswitch as "in use". This prevents new calls, and does not interrupt established calls. As established calls end, the subswitch becomes unused, and can be repaired. When the next test succeeds, the switch returns to full operation. To prevent frustration with unsensed failures, all the connections between layers in the switch are allocated using first-in-first-out lists (queues). As a result, if a connection is faulty or noisy and the customer hangs up and redials, they will get a different set of connections and subswitches. A last-in-first-out (stack) allocation of connections might cause a continuing string of very frustrating failures.


Fire and disaster recovery

A central exchange is almost always a
single point of failure A single point of failure (SPOF) is a part of a system that, if it fails, will stop the entire system from working. SPOFs are undesirable in any system with a goal of high availability or reliability, be it a business practice, software ap ...
for local calls. As the capacity of individual switches and the
optical fibre An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means t ...
which interconnects them increases, potential disruption caused by destruction of one local office will only be magnified. Multiple fibre connections can be used to provide redundancy to voice and data connections between switching centres, but careful network design is required to avoid situations where a main fibre and its backup both go through the same damaged central office as a potential common mode failure.


See also

* History of telecommunication *
List of telephone switches This list of telephone switches is a compilation of telephone switches used in the public switched telephone network (PSTN) or in large enterprises. American Digital Switching * Centura 2000 Alcatel This lists Alcatel switches before the merger ...
*
Pair gain system In telephony, pair gain is the transmitting of multiple POTS signals over the twisted pairs traditionally used for a single traditional subscriber line in telephone systems. Pair gain has the effect of creating additional subscriber lines. This i ...
*
Limited availability When customers of a public switched telephone network make telephone calls, they utilize a telecommunications network called a switched-circuit network. In a switched-circuit network, devices known as switches are used to connect the calling par ...
*
Softswitch A softswitch (''software switch'') is a call-switching node in a telecommunications network, based not on the specialized switching hardware of the traditional telephone exchange, but implemented in software running on a general-purpose computing ...
*
Plesiochronous digital hierarchy The plesiochronous digital hierarchy (PDH) is a technology used in telecommunications networks to transport large quantities of data over digital transport equipment such as fibre optic and microwave radio systems. The term '' plesiochronous'' i ...
*
Telephone exchange names A telephone exchange name or central office name was a distinguishing and memorable name assigned to a central office. It identified the switching system to which a telephone was connected, and facilitated the connection of telephone calls betwee ...
*
Automatic call distributor An automated call distribution system, commonly known as automatic call distributor (ACD), is a telephony device that answers and distributes incoming calls to a specific group of terminals or agents within an organization. ACDs direct calls based ...


References


External links


Telephone Central Office History and Pictures

Telephone World's Central Office Building Pictures




* ttp://www.google.com/patents?id=xL9WAAAAEBAJ&dq=252576 patent 252,576 for the first telephone switchboard in 1881
A Telecom Exchange Tour in NZ
{{DEFAULTSORT:Exchange, Telephone Telephony equipment Telecommunications infrastructure Exchanges Hungarian inventions