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 fe ...
system used in the
public switched telephone network (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 term. Often, a ''central office'' is defined as a building used to house the
inside plant In telecommunication, the term inside plant has the following meanings:
*All the cabling and equipment installed in a telecommunications facility, including the main distribution frame (MDF) and all the equipment extending inward therefrom, such as ...
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
The North American Numbering Plan (NANP) is a telephone numbering plan for twenty-five regions in twenty countries, primarily in North America and the Caribbean. This group is historically known as World Zone 1 and has the international calling ...
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 (PBX), when it has connections to the
public switched telephone network. 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
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 ...
in the office of a receptionist.
History
![Telephone switchboard cross-switching (Rankin Kennedy, Electrical Installations, Vol V, 1903)](https://upload.wikimedia.org/wikipedia/commons/c/c3/Telephone_switchboard_cross-switching_%28Rankin_Kennedy%2C_Electrical_Installations%2C_Vol_V%2C_1903%29.jpg)
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](_blank)
''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'' (f ...
'', 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 Te ...
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 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 List of cities in Germany by population, largest city of Germany by both area and population. Its 3.7 million inhabitants make it the European Union's List of cities in the European Union by population within ci ...
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-general, director for the German Empire who reorganized the German Postal administration, postal service. He was integral i ...
.
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 Pequonnoc ...
. The switchboard was built from "carriage bolts, handles from teapot lids and bustle wire" and could handle two simultaneous conversations.
Charles Glidden
Charles Jasper Glidden (August 29, 1857 – September 11, 1927) was an American telephone pioneer, financier and supporter of the automobile in the United States. Charles Glidden, with his wife Lucy, were the first (in 1902) to circle the wo ...
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 t ...
, both of which opened under Bell patents in 1879. Belgium had its first
International Bell exchange (in
Antwerp) 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 operators. 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'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
In telecommunication, a cord circuit is a switchboard circuit in which a plug-terminated cord is used to establish connections manually between user lines or between trunks and user lines. A number of cord circuits are furnished as part of the sw ...
.
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 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, a ...
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, an undertaker in
Kansas City, Missouri, 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 number ...
, 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 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
Dual-tone multi-frequency signaling (DTMF) is a telecommunication signaling system using the voice-frequency band over telephone lines between telephone equipment and other communications devices and switching centers. DTMF was first developed ...
"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 In the evolution of modern telecommunications systems there was a requirement to connect large numbers of low-speed access devices with large telephone company 'central office' switches over common paths. During the first generations of digital netw ...
'' is a device that concentrates traffic, be it remote or co-located with the switch.
* An ''
off-hook In telephony, on-hook and off-hook are two states of a communication circuit. On subscriber telephones the states are produced by placing the handset onto or off the hookswitch. Placing the circuit into the off-hook state is also called ''seizing th ...
'' condition represents a circuit that is in use, e.g., when a telephone call is in progress.
* An ''
on-hook In telephony, on-hook and off-hook are two states of a communication circuit. On subscriber telephones the states are produced by placing the handset onto or off the hookswitch. Placing the circuit into the off-hook state is also called ''seizing th ...
'' 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 In telecommunication, the term inside plant has the following meanings:
*All the cabling and equipment installed in a telecommunications facility, including the main distribution frame (MDF) and all the equipment extending inward therefrom, such as ...
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 fe ...
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 {{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, ...
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
Federal Standard 1037C, titled Telecommunications: Glossary of Telecommunication Terms, is a United States Federal Standard issued by the General Services Administration pursuant to the Federal Property and Administrative Services Act of 1949, a ...
.
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 In telephony, on-hook and off-hook are two states of a communication circuit. On subscriber telephones the states are produced by placing the handset onto or off the hookswitch. Placing the circuit into the off-hook state is also called ''seizing th ...
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 number ...
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 In telephony, on-hook and off-hook are two states of a communication circuit. On subscriber telephones the states are produced by placing the handset onto or off the hookswitch. Placing the circuit into the off-hook state is also called ''seizing th ...
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 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 ...
, 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.
![Telephone exchange Montreal QE3 33](https://upload.wikimedia.org/wikipedia/commons/1/1a/Telephone_exchange_Montreal_QE3_33.jpg)
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
A magneto is an electrical generator that uses permanent magnets to produce periodic pulses of alternating current. Unlike a dynamo, a magneto does not contain a commutator to produce direct current. It is categorized as a form of alternator, ...
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 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
A ringtone, ring tone or ring is the sound made by a telephone to indicate an incoming call. Originally referring to and made by the electromechanical striking of bells, the term now refers to any sound on any device alerting of a new incoming ...
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
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 ...
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 operators 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
Dual-tone multi-frequency signaling (DTMF) is a telecommunication signaling system using the voice-frequency band over telephone lines between telephone equipment and other communications devices and switching centers. DTMF was first developed ...
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 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
The rotary machine switching system, or most commonly known as the rotary system, was a type of automatic telephone exchange manufactured and used primarily in Europe from the 1910s. The system was developed and tested by AT&T's American engineerin ...
and the
crossbar switch.
Electromechanical signaling
Circuits interconnecting switches are called ''trunks''. Before
Signalling System 7
Signalling System No. 7 (SS7) is a set of telephony signaling protocols developed in 1975, which is used to set up and tear down telephone calls in most parts of the world-wide public switched telephone network (PSTN). The protocol also perform ...
,
Bell System 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 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 pulse
Pulse dialing is a signaling technology in telecommunications in which a direct current local loop circuit is interrupted according to a defined coding system for each signal transmitted, usually a digit. This lends the method the often used name ...
s, 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 number ...
'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 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
In telecommunications, falsing is when a decoder assumes that it is detecting a valid input even though one is not present. This is also known as a false decode. This article will discuss analog circuits used before digital signal processing.
Exam ...
.) Some systems used tone frequencies over 3,000 Hz, particularly on SSB
frequency-division multiplex microwave radio relays.
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 Syste ...
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, nar ...
technology in
stored program control exchange finally quieted the environment.
Maintenance tasks
![Panel_Office_Typical_OGT-colourbalance-2](https://upload.wikimedia.org/wikipedia/commons/a/a9/Panel_Office_Typical_OGT-colourbalance-2.jpg)
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
Permanent signal (PS) in American telephony terminology, or permanent loop in British usage, is a condition in which a POTS line is off-hook without connection for an extended period of time.Telcordia Technologies, GR-505-CORE Issue 2, December ...
(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, nar ...
-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 1ESS switch
The Number One Electronic Switching System (1ESS) was the first large-scale stored program control (SPC) telephone exchange or electronic switching system in the Bell System. It was manufactured by Western Electric and first placed into servi ...
, 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 development, research and scientific developm ...
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 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 Alcatel may refer to:
* Alcatel, a former French telecommunications equipment company, which became Alcatel-Lucent and is now part of Nokia
* Alcatel Mobile, a brand of mobile phones, tablets and wearables, formerly a joint venture between 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 informa ...
'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 switches 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 s ...
inherited three of the world's most iconic digital switching systems :
Alcatel Alcatel may refer to:
* Alcatel, a former French telecommunications equipment company, which became Alcatel-Lucent and is now part of Nokia
* Alcatel Mobile, a brand of mobile phones, tablets and wearables, formerly a joint venture between Alcatel ...
E10,
1000-S12, and the
Western Electric 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 med ...
networks.
:Alcatel also acquired
ITT System 12 System 1240 (later System 12) was an ambitious new Digital Telephone Exchange developed in 1977 by ITT Corporation. It reportedly cost US$1 billion. It anticipated the features of ISDN of the 1990s.
The design was done at the Advanced Technology ...
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 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
The Elektronisches Wählsystem Digital (EWSD), translated to ''Electronic Digital Switching System'' in English, is a widely installed German telephone exchange system, originally introduced in 1975 by Siemens AG, but discontinued in 2017.
EWSD ...
originally developed by
Siemens,
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 S ...
, and now Ribbon Communications
DMS100
The DMS-100 is a member of the Digital Multiplex System (DMS) product line of telephone exchange switches manufactured by Northern Telecom. Designed during the 1970s and released in 1979, it can control 100,000 telephone lines.
The purpose of t ...
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 to a discrete-time signal. A common example is the conversion of a sound wave to a sequence of "samples".
A sample is a value of the signal at a point in time and/or s ...
of 8khz). At each time slice, a digital
PCM 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 modern telephony a remote concentrator, remote concentrator unit (RCU), or remote line concentrator (RLC) is a concentrator at the lowest level in the telephone switch hierarchy.
Subscribers' analogue telephone/PSTN lines are terminated on co ...
. 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 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 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.
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 digital loop carrier (DLC) is a system which uses digital transmission to extend the range of the local loop farther than would be possible using only twisted pair copper wires. A DLC digitizes and multiplexes the individual signals carried by t ...
, 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 associ ...
, 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
In telephony, a main distribution frame (MDF or main frame) is a signal distribution frame for connecting equipment (inside plant) to cables and pair gain, subscriber carrier equipment (outside plant).
Overview
The MDF is a termination point wi ...
(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
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 ...
" 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 (DSL).
Pair gain or
digital loop carrier
A digital loop carrier (DLC) is a system which uses digital transmission to extend the range of the local loop farther than would be possible using only twisted pair copper wires. A DLC digitizes and multiplexes the individual signals carried by t ...
s (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 (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 sy ...
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
In electronics, a crossover switch or matrix switch is a switch connecting multiple inputs to multiple outputs using complex array matrices designed to switch any one input path to any one (or more) output path(s). There are blocking and non-block ...
.
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
![East Village 20191129 - 06 - Second btwn 12 & 13](https://upload.wikimedia.org/wikipedia/commons/0/0e/East_Village_20191129_-_06_-_Second_btwn_12_%26_13.jpg)
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 appl ...
for local calls. As the capacity of individual switches and the
optical fibre 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
Common and special causes are the two distinct origins of variation in a process, as defined in the statistical thinking and methods of Walter A. Shewhart and W. Edwards Deming. Briefly, "common causes", also called natural patterns, are the ...
.
See also
*
History of telecommunication
The history of telecommunication began with the use of smoke signals and drums in Africa, Asia, and the Americas. In the 1790s, the first fixed semaphore systems emerged in Europe. However, it was not until the 1830s that electrical telecommuni ...
*
List of telephone switches
*
Pair gain system
*
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 pa ...
*
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 PicturesTelephone World's Central Office Building Pictures*
ttp://www.google.com/patents?id=xL9WAAAAEBAJ&dq=252576 patent 252,576 for the first telephone switchboard in 1881A Telecom Exchange Tour in NZ
{{DEFAULTSORT:Exchange, Telephone
Telephony equipment
Telecommunications infrastructure
Exchanges
Hungarian inventions