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Telephone exchange

Telephone exchange

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Encyclopedia
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls. A central office is the physical building used to house inside plant
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 and all the equipment extending inward therefrom, such as PABX or central office equipment, MDF heat coil...

 equipment including telephone switches, which make telephone call
Telephone call
A telephone call is a connection over a telephone network between the calling party and the called party.-Information transmission:A telephone call may carry ordinary voice transmission using a telephone, data transmission when the calling party and called party are using modems, or facsimile...

s "work" in the sense of making connections and relaying the speech information.

The term exchange area can be used to refer to an area served by a particular switch, but is typically known as a wire center in the US telecommunications industry. The exchange code or Central Office Code refers to the first three digits of the local number (NXX). It is sometimes confused with the area code (NPA). In the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, local exchange areas together make up a legal entity called local access and transport area
Local access and transport area
Local access and transport area is a term used in U.S. telecommunications regulation. It represents a geographical area of the United States under the terms of the that precipitated the breakup of the original AT&T into the "Baby Bells" or created since that time for wireline...

s (LATA
Local access and transport area
Local access and transport area is a term used in U.S. telecommunications regulation. It represents a geographical area of the United States under the terms of the that precipitated the breakup of the original AT&T into the "Baby Bells" or created since that time for wireline...

) under the Modification of Final Judgment
Modification of Final Judgment
In United States telecommunication law, Modification of Final Judgment is the August 1982 agreement approved by the court settling United States v. AT&T, a landmark antitrust suit, originally filed on January, 14, 1949 and modifying the previous Final Judgment of January 24, 1956...

 (MFJ).

Historic perspective


Prior to the telephone, electrical switches were used to switch telegraph lines. One of the first people to build a telephone exchange was Hungarian
Hungary
Hungary , officially the Republic of Hungary , is a landlocked country in Central Europe. It is situated in the Carpathian Basin and is bordered by Slovakia to the north, Ukraine and Romania to the east, Serbia and Croatia to the south, Slovenia to the southwest and Austria to the west. The...

 Tivadar Puskás
Tivadar Puskás
Tivadar Puskás was a Hungarian inventor, telephone pioneer, and inventor of the telephone exchange He was also the founder of Telefon Hírmondó.-Biography:...

 in 1877 while he was working for Thomas Edison
Thomas Edison
Thomas Alva Edison was an American inventor and businessman. He developed many devices that greatly influenced life around the world, including the phonograph, the motion picture camera, and a long-lasting, practical electric light bulb. In addition, he created the world’s first industrial...

.
George W. Coy designed and built the first commercial telephone exchange which opened in New Haven, Connecticut
New Haven, Connecticut
New Haven is the second-largest city in Connecticut and the sixth-largest in New England. According to the 2010 Census, New Haven's population increased by 5.0% between 2000 and 2010, a rate higher than that of the State of Connecticut, and higher than that of the state's five largest cities, and...

 in January, 1878. The switchboard was built from "carriage bolts, handles from teapot lids and bustle wire" and could handle two simultaneous conversations .. In Europe the earliest telephone exchanges were based in London
London
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...

 and Manchester
Manchester
Manchester is a city and metropolitan borough in Greater Manchester, England. According to the Office for National Statistics, the 2010 mid-year population estimate for Manchester was 498,800. Manchester lies within one of the UK's largest metropolitan areas, the metropolitan county of Greater...

, both of which opened under Bell patents in 1878. The first in Germany was opened in Berlin 1881. Belgium had its first International Bell
International Bell Telephone Company
The International Bell Telephone Company of Brussels, Belgium, was created in 1879 by the National Bell Telephone Company of Boston, Massachusetts, United States, initially to sell imported telephones and switchboards in Continental Europe....

 exchange (in Antwerp) a year later.

Later exchanges consisted of one to several hundred plug boards
Telephone switchboard
A switchboard was a device used to connect a group of telephones manually to one another or to an outside connection, within and between telephone exchanges or private branch exchanges . The user was typically known as an operator...

 staffed by telephone operator
Telephone operator
A telephone operator is either* a person who provides assistance to a telephone caller, usually in the placing of operator assisted telephone calls such as calls from a pay phone, collect calls , calls which are billed to a credit card, station-to-station and person-to-person calls, and certain...

s. Each operator sat in front of a vertical panel containing banks of ¼-inch tip-ring-sleeve
TRS connector
A TRS connector is a common family of connector typically used for analog signals including audio. It is cylindrical in shape, typically with three contacts, although sometimes with two or four . It is also called an audio jack, phone jack, phone plug, and jack plug...

 (3-conductor) jacks, each of which was the local termination of a subscriber's telephone line
Telephone line
A telephone line or telephone circuit is a single-user circuit on a telephone communication system...

. In front of the jack panel lay a horizontal panel containing two rows of patch cords, each pair connected to a cord circuit
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 switchboard position equipment. The cords may be...

. When a calling party
Calling party
The calling party is a person who initiates a telephone call over the public switched telephone network, usually by dialing a telephone number....

 lifted the receiver, a signal lamp near the jack would light. The operator would plug one of the cords (the "answering cord") into the subscriber's jack and switch her headset into the circuit to ask, "Number, please?" Depending upon the answer, the operator might plug the other cord of the pair (the "ringing cord") into the called party's local jack and start the ringing cycle, or plug into a trunk circuit to start what might be a long distance call handled by subsequent operators in another bank of boards or in another building miles away. In 1918, the average time to complete the connection for a long-distance call was 15 minutes. In the ringdown
Ringdown
-Operator signaling: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...

 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
Kansas City, Missouri
Kansas City, Missouri is the largest city in the U.S. state of Missouri and is the anchor city of the Kansas City Metropolitan Area, the second largest metropolitan area in Missouri. It encompasses in parts of Jackson, Clay, Cass, and Platte counties...

, patented the stepping switch
Stepping switch
In electrical controls, a stepping switch, also known as a stepping relay, is an electromechanical device which allows an input connection to be connected to one of a number of possible output connections, under the control of a series of electrical pulses. It can step on one axis , or on two axes...

, 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, 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 switch
Stepping switch
In electrical controls, a stepping switch, also known as a stepping relay, is an electromechanical device which allows an input connection to be connected to one of a number of possible output connections, under the control of a series of electrical pulses. It can step on one axis , or on two axes...

es were arranged in banks, the first stage of which was a linefinder. If one of up to a hundred subscriber lines had the receiver lifted "off hook", a linefinder connected the subscriber's line to a free first selector, which returned the subscriber a dial tone
Dial tone
A dial tone is a telephony signal used to indicate that the telephone exchange is working, has recognized an off-hook, and is ready to accept a call. The tone stops when the first numeral is dialed...

 to show that it was ready to receive dialed digits. The subscriber's dial pulsed at about 10 pulses per second, although the speed depended on the standard of the particular telephone administration.

Exchanges based on the Strowger switch were eventually challenged by other exchange types
Panel switch
The panel switching system was an early type of automatic telephone exchange, first put into urban service by the Bell System in the 1920s and removed during the 1970s...

 and later by crossbar
Crossbar switch
In electronics, a crossbar switch is a switch connecting multiple inputs to multiple outputs in a matrix manner....

 technology. These exchange designs promised faster switching and would accept pulses faster than the Strowger's typical 10 pps—typically about 20 pps. At a later date many also accepted DTMF "touch tones" or other tone signaling systems.

A transitional technology (from pulse to DTMF) had DTMF link finders which converted DTMF to pulse, to feed to older Strowger, panel, or crossbar switches. This technology was used as late as mid 2002.

Technologies


This article will use the following terms:
  • manual service for a condition where a human operator routes calls inside an exchange and a dial is not used
  • dial service for an exchange where calls are routed by a switch interpreting dialed digits
  • telephone exchange for the building housing the switching equipment
  • telephone switch for the switching equipment
  • concentrator for a device that concentrates traffic, be it remote or co-located with the switch
  • off-hook
    Off-hook
    In telephony, the term off-hook has the following meanings:# The condition that exists when a telephone or other user instrument is in use, i.e., during dialing or communicating. Note: off-hook originally referred to the condition that prevailed when telephones had a separate earpiece , which hung...

    for a tip condition or to describe a circuit that is in use (i.e., when a phone call is in progress)
  • on-hook
    On-hook
    In telephony, the term on-hook has the following meanings:# The condition that exists when a telephone or other user instrument is not in use, i.e., when idle waiting for a call. Note: on-hook originally referred to the storage of an idle telephone receiver, i.e., separate earpiece, on a switchhook...

    for an idle circuit (i.e., no phone call is in progress)
  • wire center for the area served by a particular switch or central office


Many of the terms in this article have conflicting UK and US usages.
  • central office originally referred to switching equipment and its operators. Now it is used generally for the building housing switching and related inside plant
    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 and all the equipment extending inward therefrom, such as PABX or central office equipment, MDF heat coil...

     equipment.
  • telephone exchange means an exchange building in the UK, and is also the UK name for a telephone switch, and also has a legal meaning in U.S. telecoms.
  • telephone switch is the U.S. term, but is in increasing use in technical UK telecoms usage, to make the CO/switch/concentrator distinction clear.

Manual service exchanges



With manual service, the customer lifts the receiver off-hook
Off-hook
In telephony, the term off-hook has the following meanings:# The condition that exists when a telephone or other user instrument is in use, i.e., during dialing or communicating. Note: off-hook originally referred to the condition that prevailed when telephones had a separate earpiece , which hung...

 and asks the operator
Telephone operator
A telephone operator is either* a person who provides assistance to a telephone caller, usually in the placing of operator assisted telephone calls such as calls from a pay phone, collect calls , calls which are billed to a credit card, station-to-station and person-to-person calls, and certain...

 to connect the call to a requested number. Provided that the number is in the same central office, the operator connects the call by plugging into the jack on the switchboard
Telephone switchboard
A switchboard was a device used to connect a group of telephones manually to one another or to an outside connection, within and between telephone exchanges or private branch exchanges . The user was typically known as an operator...

 corresponding to the called customer's line. If the call is to another central office, the operator plugs into the trunk for the other office and asks the operator answering (known as the "inward" operator) to connect the call.

Most urban exchanges were common-battery, meaning that the central office provided power for the telephone circuits, as is the case today. In common-battery systems, the pair of wires from a subscriber's telephone to the switch (or manual exchange) carry -48VDC (nominal) from the telephone company end, across the conductors. The telephone presents an open circuit when it is on-hook
On-hook
In telephony, the term on-hook has the following meanings:# The condition that exists when a telephone or other user instrument is not in use, i.e., when idle waiting for a call. Note: on-hook originally referred to the storage of an idle telephone receiver, i.e., separate earpiece, on a switchhook...

 or idle. When the subscriber goes off-hook, the telephone puts a DC resistance short across the line. In manual service, this current flowing through the off-hook telephone flows through a relay coil actuating a buzzer and lamp on the operator's switchboard. The buzzer and lamp would tell an operator the subscriber was off-hook (requesting service).

In the largest U.S. cities, it took many years to convert every office to automatic equipment, such as panel switch
Panel switch
The panel switching system was an early type of automatic telephone exchange, first put into urban service by the Bell System in the 1920s and removed during the 1970s...

es. During this transition period, it was possible to dial a manual number and be connected without requesting an operator's assistance. This was because the policy of the Bell System
Bell System
The Bell System was the American Bell Telephone Company and then, subsequently, AT&T led system which provided telephone services to much of the United States and Canada from 1877 to 1984, at various times as a monopoly. In 1984, the company was broken up into separate companies, by a U.S...

 was that customers should not need to know whether they were calling a manual or automated office. If a subscriber dialed a manual number, an inward operator would answer the call, see the called number on a display device, and manually connect the call. For instance, if a customer calling from TAylor 4725 dialed a manual number, ADams 1233, the call would go through, from the subscriber's perspective, exactly as a call to LEnnox 5813, in an automated exchange.

In contrast to the common-battery system, smaller towns with manual service often had magneto
Magneto
A magneto is a type of electrical generator.Magneto may also refer to:* Magneto , permanent magnetic alternating current rotary generator* ignition magneto, magnetos on internal combustion engines...

, or crank, phones. Using a magneto set, the subscriber turned a crank to generate ringing current, to gain the operator's attention. The switchboard would respond by dropping a metal tab above the subscriber's line jack and sounding a buzzer. Dry cell
Dry Cell
-Dry Cell's formation:Part of the band formed in 1998 when guitarist Danny Hartwell and drummer Brandon Brown met at the Ratt Show on the Sunset Strip. They later met up with then-vocalist Judd Gruenbaum. The original name of the band was "Beyond Control"....

 batteries (normally two large "No 6" cells) in the subscriber's telephone provided the DC power for conversation. Magneto systems were in use in one American small town, Bryant Pond, Woodstock, Maine
Woodstock, Maine
Woodstock is a town in Oxford County, Maine, United States. The population was 1,307 at the 2000 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.-Geography:...

 as late as 1983. In general, this type of system had a poorer call quality compared to common-battery systems.

Many small town magneto systems featured party lines
Party line (telephony)
In twentieth-century telephone systems, a party line is an arrangement in which two or more customers are connected directly to the same local loop. Prior to World War II in the United States, party lines were the primary way residential subscribers acquired local telephone service...

, anywhere from two to ten or more subscribers sharing a single line. When calling a party, the operator would use a distinctive ringing signal sequence, such as two long rings followed by one short. Everyone on the line could hear the rings, and of course could pick up and listen in if they wanted. On rural lines which were not connected to a central office (thus not connected to the outside world), subscribers would crank the correct sequence of rings to reach their party.

Early automatic exchanges



Automatic exchanges, or dial service, came into existence in the early 1900s. Their purpose was to eliminate the need for human telephone operator
Telephone operator
A telephone operator is either* a person who provides assistance to a telephone caller, usually in the placing of operator assisted telephone calls such as calls from a pay phone, collect calls , calls which are billed to a credit card, station-to-station and person-to-person calls, and certain...

s. Before the exchanges became automated, operators had to complete the connections required for a telephone call
Telephone call
A telephone call is a connection over a telephone network between the calling party and the called party.-Information transmission:A telephone call may carry ordinary voice transmission using a telephone, data transmission when the calling party and called party are using modems, or facsimile...

. Almost everywhere, operators have been replaced by computerized exchanges. A telephone switch is the brains of an automatic exchange. It is a device for routing
Routing
Routing is the process of selecting paths in a network along which to send network traffic. Routing is performed for many kinds of networks, including the telephone network , electronic data networks , and transportation networks...

 calls from one telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...

 to another, generally as part of the public switched telephone network
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...

.

The local exchange automatically senses an off hook (tip) telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...

 condition, provides dial tone
Dial tone
A dial tone is a telephony signal used to indicate that the telephone exchange is working, has recognized an off-hook, and is ready to accept a call. The tone stops when the first numeral is dialed...

 to that phone, receives the pulses or DTMF tones generated by the phone, and then completes a connection to the called phone within the same exchange or to another distant exchange.

The exchange then maintains the connection until a party hangs up, and the connection is disconnected. This tracking of a connection's status is called supervision. Additional features, such as billing equipment, may also be incorporated into the exchange.

In Bell System dial service, a feature called automatic number identification
Automatic number identification
Automatic number identification is a feature of telephony intelligent network services that permits subscribers to display or capture the billing telephone number of a calling party. In the United States it is part of Inward Wide Area Telephone Service . ANI service was created by AT&T for...

(ANI) was implemented. ANI allowed services like automated billing, toll-free 800-numbers, and 9-1-1
9-1-1
9-1-1 is the emergency telephone number for the North American Numbering Plan .It is one of eight N11 codes.The use of this number is for emergency circumstances only, and to use it for any other purpose can be a crime.-History:In the earliest days of telephone technology, prior to the...

 service. In manual service, the operator knows where a call is originating by the light on the switchboard's jack field. In early dial service, ANI did not exist. Long distance calls would go to an operator queue and the operator would ask the calling party's number, then write it on a paper toll ticket. See also Automatic Message Accounting
Automatic Message Accounting
Automatic message accounting provides detail billing for telephone calls. When direct distance dialing was introduced in the US, message registers no longer sufficed for dialed telephone calls...

.

Early exchanges used motors, shaft drives, rotating switches and relays. In a sense, switches were relay-logic computers. Some types of automatic exchanges were Strowger
Strowger switch
The Strowger switch, also known as Step-by-Step or SXS, is an early electromechanical telephone switching system invented by Almon Brown Strowger...

 (also known as Step-By-Step), All Relay, X-Y, Panel
Panel switch
The panel switching system was an early type of automatic telephone exchange, first put into urban service by the Bell System in the 1920s and removed during the 1970s...

 and crossbar
Crossbar switch
In electronics, a crossbar switch is a switch connecting multiple inputs to multiple outputs in a matrix manner....

. These are referred to collectively as electromechanical switches.

Electromechanical signaling



Circuits connecting two switches are called trunks. Before Signalling System 7
Signalling System 7
Signalling System No. 7 is a set of telephony signaling protocols which are used to set up most of the world's public switched telephone network telephone calls. The main purpose is to set up and tear down telephone calls...

, Bell System
Bell System
The Bell System was the American Bell Telephone Company and then, subsequently, AT&T led system which provided telephone services to much of the United States and Canada from 1877 to 1984, at various times as a monopoly. In 1984, the company was broken up into separate companies, by a U.S...

 electromechanical switches in the United States communicated with one another over trunks using a variety of DC voltages and signaling tones. It would be rare to see any of these in use today.

Some signalling communicated dialed digits. An early form called Panel Call Indicator Pulsing used quaternary
Quaternary numeral system
Quaternary is the base- numeral system. It uses the digits 0, 1, 2 and 3 to represent any real number.It shares with all fixed-radix numeral systems many properties, such as the ability to represent any real number with a canonical representation and the characteristics of the representations of...

 pulses to set up calls between a panel switch
Panel switch
The panel switching system was an early type of automatic telephone exchange, first put into urban service by the Bell System in the 1920s and removed during the 1970s...

 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
Rotary dial
The rotary dial is a device mounted on or in a telephone or switchboard that is designed to send electrical pulses, known as pulse dialing, corresponding to the number dialed. The early form of the rotary dial used lugs on a finger plate instead of holes. Almon Brown Strowger filed the first patent...

'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)
Multi-frequency
In telephony, multi-frequency signaling is an outdated, in-band signaling technique. Numbers were represented in a two-out-of-five code for transmission from a multi-frequency sender, to be received by a multi-frequency receiver in a distant telephone exchange...

 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
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....

 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
Telephone number
A telephone number or phone number is a sequence of digits used to call from one telephone line to another in a public switched telephone network. When telephone numbers were invented, they were short — as few as one, two or three digits — and were given orally to a switchboard operator...

. 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
E&M 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....

, 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). 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 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
Falsing
In telecommunications, falsing describes a decoder assuming that it is detecting a valid input when one is not present. This is also known as a false decode...

). Some systems used tone frequencies over 3,000 Hz, particularly on SSB frequency division multiplex microwave radio relays. On T-carrier
T-carrier
In telecommunications, T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and South Korea....

 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.

Sounds


A characteristic of electromechanical switching equipment is that the maintenance staff could hear the mechanical clattering of Strowgers, panel switches or crossbar relays. Most Bell System central offices were housed in reinforced concrete buildings with concrete ceilings and floors. In rural areas, some smaller switching facilities, such as Community Dial Offices (CDOs)
Community Dial Office
A "Community Dial Office" was a small Class 5 telephone exchange in a rural area. CDOs could be step by step, all relay or crossbar. They provided capacity for 1000 or fewer customers. Late in the 20th century they were replaced by remote concentrators connected to the host office, which may be...

, were sometimes housed in prefabricated metal buildings. These facilities almost always had concrete floors. The hard surfaces reflected sounds.

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
Wire spring relay
A wire spring relay is a type of relay, primarily manufactured by the Western Electric Company for use by the Bell System in electromechanical telephone exchanges. It was licenced for use around the world, and was commonplace in Japan...


markers
Marker (telecommunications)
A marker is a type of special purpose control system that was used in electromechanical telephone central office switches. Central office switches are the large devices that telephone companies use to make the connections that support telephone calls...

 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 relay
Wire spring relay
A wire spring relay is a type of relay, primarily manufactured by the Western Electric Company for use by the Bell System in electromechanical telephone exchanges. It was licenced for use around the world, and was commonplace in Japan...

s that made reorder
Reorder tone
The reorder tone, also known as the fast busy tone, is the congestion tone or all trunks busy tone of a public switched telephone network. It varies from country to country; in the USA it is a dual-frequency tone of 480 Hz and 620 Hz at a cadence of 0.25 seconds on, 0.25 off; that is two beeps per...

 (120 ipm) and busy (60 ipm) signals. In Bell System installations, there were typically alarm bells, gongs, or chimes. These would annunciate alarms calling attention to a failed switch element. Another noisemaker: a trouble reporting card system was connected to switch common control elements. These trouble reporting systems would puncture cardboard cards
Punched card
A punched card, punch card, IBM card, or Hollerith card is a piece of stiff paper that contains digital information represented by the presence or absence of holes in predefined positions...

 with a code that logged the nature of a failure. Remreed
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...

 technology in Stored Program Control exchange
Stored Program Control exchange
Stored Program Control exchange is the technical name used for telephone exchanges controlled by a computer program stored in the memory of the system. Early exchanges such as Strowger, panel, rotary, and crossbar switches were electromechanical and had no software control...

s finally quieted the environment.

Maintenance tasks


The maintenance of electromechanical systems was partly DC electricity and partly mechanical adjustments. Unlike modern switches, a circuit connecting a dialed call through an electromechanical switch actually had DC continuity. The talking path was a physical, metallic one.

In all systems, subscribers were not supposed to notice changes in quality of service because of failures or maintenance work. A variety of tools referred to as make-busys were plugged into electromechanical switch elements during repairs or failures. A make-busy would identify the part being worked on as in-use, causing the switching logic to route around it. A similar tool was called a TD tool. Subscribers who got behind in payments would have their service temporarily denied (TDed). This was effected by plugging a tool into the subscriber's office equipment (Crossbar) or line group (step). The subscriber could receive calls but could not dial out.

Strowger-based, step-by-step offices in the Bell System were under continual maintenance. They required constant cleaning. Indicator lights on equipment bays in step offices 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
Automatic number identification is a feature of telephony intelligent network services that permits subscribers to display or capture the billing telephone number of a calling party. In the United States it is part of Inward Wide Area Telephone Service . ANI service was created by AT&T for...

had been retrofitted to nearly all step-by-step and crossbar switches in the Bell System.

Electronic switches


The first Electronic Switching System
Electronic switching system
In telecommunications, an electronic switching system is:* A telephone exchange based on the principles of time-division multiplexing of digitized analog signals. An electronic switching system digitizes analog signals from subscriber loops, and interconnects them by assigning the digitized...

s were not entirely digital. The Western Electric
Western Electric
Western Electric Company was an American electrical engineering company, the manufacturing arm of AT&T from 1881 to 1995. It was the scene of a number of technological innovations and also some seminal developments in industrial management...

 1ESS switch
1ESS switch
The Number One Electronic Switching System, the first large-scale Stored Program Control telephone exchange or Electronic Switching System in the Bell System, was introduced in Succasunna, New Jersey, in May 1965. The switching fabric was composed of reed matrixes controlled by wire spring relays...

 had reed relay
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...

 metallic paths which were stored-program-controlled
Stored Program Control exchange
Stored Program Control exchange is the technical name used for telephone exchanges controlled by a computer program stored in the memory of the system. Early exchanges such as Strowger, panel, rotary, and crossbar switches were electromechanical and had no software control...

. Equipment testing, changes to phone numbers, circuit lockouts and similar tasks were accomplished by typing on a terminal. Northern Telecom SP1, Ericsson AKE, Philips PRX
PRX (telephony)
The PRX205 is a processor controlled reed relay telephone exchange developed by Philips Telecommunicatie Industrie BV in Hilversum during the late 1960s and early 1970s. The first public switch was installed in Overvecht in Utrecht in 1972. About half of all sales were in the Netherlands...

/A, ITT Metaconta, British Telecom TXE
TXE
TXE, which stands for Telephone eXchange Electronic, was the designation given to a family of telephone exchanges developed by the British General Post Office , now BT, designed to replace the ageing Strowger systems....

 series and several other designs were similar. 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)
Common Channel Signaling
In telephony, Common Channel Signaling , in the US also Common Channel Interoffice Signaling , is the transmission of signaling information on a separate channel from the data, and, more specifically, where that signaling channel controls multiple data channels.For example, in the public switched...

. This data link was based on CCITT 6, a predecessor to SS7.

Digital switches


Digital switches work by connecting two or more digital circuits together, according to a dialed telephone number
Telephone number
A telephone number or phone number is a sequence of digits used to call from one telephone line to another in a public switched telephone network. When telephone numbers were invented, they were short — as few as one, two or three digits — and were given orally to a switchboard operator...

. Calls are set up between switches using the Signalling System 7
Signalling System 7
Signalling System No. 7 is a set of telephony signaling protocols which are used to set up most of the world's public switched telephone network telephone calls. The main purpose is to set up and tear down telephone calls...

 protocol, or one of its variants. In U.S. and military telecommunication
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...

, a digital switch is a switch that performs time division switching of digitized signals. This was first done in a few small and little used systems. The first product using a digital switch system was made by Amtelco
Amtelco
Amtelco is a manufacturer of telecommunications equipment and telephone answering service and call center systems, founded in 1976.-History:Amtelco was founded in 1976 by Bill Curtin in the wake of the 1968 U.S...

. Prominent examples include Nortel DMS-100
DMS-100
The DMS-100 Switch is a line of Digital Multiplex System telephone exchange switches manufactured by Nortel Networks.The purpose of the DMS-100 Switch is to provide local service and connections to the PSTN public telephone network. It is designed to deliver services over subscribers' telephone...

, Lucent 5ESS switch
5ESS Switch
The 5ESS Switch is a Class 5 telephone electronic switching system sold by Alcatel-Lucent. This digital central office telephone circuit switching system is used by many telecommunications service providers.-History:...

, Siemens EWSD
EWSD
EWSD is one of the most widely installed telephone exchange systems in the world. EWSD can work as a local or tandem switch or combined local/tandem, and for landline or mobile phones...

 and Ericsson AXE telephone exchange
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 Televerket.. The first system was deployed in 1976...

. With few exceptions, most switches built since the 1980s are digital. This article describes digital switches, including algorithms and equipment.


Digital switches encode the speech going on, in 8000 time slices per second. At each time slice, a digital PCM representation of the tone is made. The digits are then sent to the receiving end of the line, where the reverse process occurs, to produce the sound for the receiving phone. In other words, when you use a telephone, you are generally having your voice "encoded" and then reconstructed for the person on the other end. Your voice is delayed in the process by a small fraction of one second — it is not "live", it is reconstructed — delayed only minutely. (See below for more info.)

Individual local loop
Local loop
In telephony, the local loop is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the carrier or telecommunications service provider's network...

 telephone lines are connected to a remote concentrator
Remote concentrator
In modern telephony a remote concentrator, Remote Concentrator Unit , or Remote Line Concentrator is the lowest level in the telephone switch hierarchy.Subscribers' analogue telephone/PSTN lines are terminated on concentrators...

. 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
V5 interface
V5 is a family of telephone network protocols defined by ETSI which allow communications between the telephone exchange, also known in the specifications as the local exchange , and the local loop...

 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 (or a series of them) in a central exchange building are referred to as "carrier-level" switches or tandems.

Some telephone exchange buildings in small towns now 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 and number plan information. Unlike a digital loop carrier
Digital loop carrier
A digital loop carrier 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 remote switch can route calls between local phones itself, without using trunks to the parent switch.

Telephone switches are usually owned and operated by a telephone service provider
Telephone company
A telephone company is a service provider of telecommunications services such as telephony and data communications access. Many were at one time nationalized or state-regulated monopolies...

 or carrier and located in their premises, but sometimes individual businesses or private commercial buildings will house their own switch, called a PBX, or Private branch exchange.



The switch's place in the system


Telephone switches are a small part of a large network. The majority of work and expense of the phone system is the wiring outside the central office, or the outside plant
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 , and any associated hardware located between a demarcation point in a switching facility and a demarcation...

. In the middle 20th century, each subscriber telephone number required an individual pair of wires from the switch to the subscriber's phone. A typical central office may have tens-of-thousands of pairs of wires that appear on terminal blocks called the main distribution frame
Main distribution frame
In telephony, a main distribution frame is a signal distribution frame for connecting equipment to cables and subscriber carrier equipment . The MDF is a termination point within the local telephone exchange where exchange equipment and terminations of local loops are connected by jumper wires...

or 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
Pair gain
In telephony, pair gain is a method of transmitting 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...

" 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 ISDN or DSL
Digital Subscriber Line
Digital subscriber line is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop. In telecommunications marketing, the term DSL is widely understood to mean Asymmetric Digital Subscriber Line ,...

. Pair gain or digital loop carrier
Digital loop carrier
A digital loop carrier 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...

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 proprietary product.

DLCs can be configured as universal (UDLCs) or integrated (IDLCs). Universal DLCs 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. Several standards cover DLCs, including Telcordia Technologies
Telcordia Technologies
Telcordia Technologies, formerly Bell Communications Research, Inc. or Bellcore, is a telecommunications research and development company based in the United States created as part of the 1982 Modification of Final Judgment that broke up American Telephone & Telegraph...

 Generic Requirements documents GR-8-CORE and GR-303-CORE.

Switches are used in both local central offices and in long distance centers. There are two major types in the Public switched telephone network
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...

 (PSTN):
  1. Class 4 telephone switch
    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....

    es designed for toll or switch-to-switch connections.
  2. Class 5 telephone switches
    Class 5 telephone switches
    A Class 5 telephone switch is a telephone switch or telephone exchange in the Public Switched Telephone Network located at the local telephone company's central office, directly serving subscribers. Class 5 switch services include basic dial-tone, calling features, and additional digital and data...

     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
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...

 receiver.

Switch design


Long distance switches may use a slower, more efficient switch-allocation algorithm than local central offices
Class 5 telephone switches
A Class 5 telephone switch is a telephone switch or telephone exchange in the Public Switched Telephone Network located at the local telephone company's central office, directly serving subscribers. Class 5 switch services include basic dial-tone, calling features, and additional digital and data...

, 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 (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 8000 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
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, it can always make the connection...

 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 most 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 callees.

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
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, it can always make the connection...

, or a crossover switch
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 output path...

.

Fully connected mesh network


One way is to have enough switching fabric to assure that the pairwise allocation will always succeed by building a fully connected mesh network
Network topology
Network topology is the layout pattern of interconnections of the various elements of a computer or biological network....

. This is the method usually used in central office switches, which have low utilization of their resources.

Clos's nonblocking switch algorithm



The scarce resources in a telephone switch are the connections between layers of subswitches. The control logic has to allocate these connections, and most switches do so in a way that is fault tolerant. See nonblocking minimal spanning switch for a discussion of the Charles Clos algorithm, used in many telephone switches, and a very important algorithm to the telephone industry.

Fault tolerance


Composite switches are inherently fault-tolerant. If a subswitch fails, the controlling computer can sense it during a periodic test. The computer marks all the connections to the subswitch as "in use". This prevents new calls, and does not interrupt old calls that remain working. As calls in progress end, the subswitch becomes unused, and new calls avoid the subswitch because it's already "in use." Some time later, a technician can replace the circuit board. When the next test succeeds, the connections to the repaired subsystem are marked "not in use," and 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
FIFO
FIFO is an acronym for First In, First Out, an abstraction related to ways of organizing and manipulation of data relative to time and prioritization...

 (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.

Internet exchanges


The telephone exchange concept has been adapted for use in Internet exchanges
Internet Exchange Point
An Internet exchange point is a physical infrastructure through which Internet service providers exchange Internet traffic between their networks . IXPs reduce the portion of an ISP's traffic which must be delivered via their upstream transit providers, thereby reducing the average per-bit...

. Voice over IP
Voice over IP
Voice over Internet Protocol is a family of technologies, methodologies, communication protocols, and transmission techniques for the delivery of voice communications and multimedia sessions over Internet Protocol networks, such as the Internet...

 (VoIP) traffic may pass through both kinds of exchanges, depending on what kind of service the caller and the called subscriber are using.

See also

  • History of telecommunication
    History of telecommunication
    The history of telecommunication began with the use of smoke signals and drums in Africa, the Americas and parts of Asia. In the 1790s, the first fixed semaphore systems emerged in Europe; however it was not until the 1830s that electrical telecommunication systems started to appear...

  • List of telephone switches
  • Pair gain system
  • Full Availability, Limited Availability and Gradings
  • Softswitch
    Softswitch
    A softswitch is a central device in a telecommunications network which connects telephone calls from one phone line to another, typically via the internet, entirely by means of software running on a general-purpose computer system...

  • Stored Program Control exchange
    Stored Program Control exchange
    Stored Program Control exchange is the technical name used for telephone exchanges controlled by a computer program stored in the memory of the system. Early exchanges such as Strowger, panel, rotary, and crossbar switches were electromechanical and had no software control...

  • Strowger switch
    Strowger switch
    The Strowger switch, also known as Step-by-Step or SXS, is an early electromechanical telephone switching system invented by Almon Brown Strowger...

  • Telephone number
    Telephone number
    A telephone number or phone number is a sequence of digits used to call from one telephone line to another in a public switched telephone network. When telephone numbers were invented, they were short — as few as one, two or three digits — and were given orally to a switchboard operator...

  • DSLAM
    Digital subscriber line access multiplexer
    A digital subscriber line access multiplexer is a network device, located in the telephone exchanges of the telecommunications operators. It connects multiple customer digital subscriber line interfaces to a high-speed digital communications channel using multiplexing techniques...

  • DSL
    Digital Subscriber Line
    Digital subscriber line is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop. In telecommunications marketing, the term DSL is widely understood to mean Asymmetric Digital Subscriber Line ,...

  • ISDN
    Integrated Services Digital Network
    Integrated Services Digital Network is a set of communications standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network...

  • PDH
    Plesiochronous Digital Hierarchy
    The Plesiochronous Digital Hierarchy 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...

     Plesiochronous Digital Hierarchy
  • PBX Private Branch Exchange or business-level switch
  • Telephone exchange names
    Telephone exchange names
    During the early years of telephone service, communities that required more than 10,000 telephone numbers, whether dial service was available or not, utilized exchange names to distinguish identical numerics for different customers....

  • First telephone exchange in UK - Faraday building
    Faraday building
    The Faraday Building was the GPO's first telephone exchange in London.It started life as the Central telephone exchange at the Savings Bank building in Queen Victoria Street, opening for business on 1 March 1902 with just 200 subscribers...



In US telecommunication
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...

 jargon, a central office (C.O.) is a common carrier
Common carrier
A common carrier in common-law countries is a person or company that transports goods or people for any person or company and that is responsible for any possible loss of the goods during transport...

 switching center Class 5 telephone switches
Class 5 telephone switches
A Class 5 telephone switch is a telephone switch or telephone exchange in the Public Switched Telephone Network located at the local telephone company's central office, directly serving subscribers. Class 5 switch services include basic dial-tone, calling features, and additional digital and data...

 in which trunks and local loop
Local loop
In telephony, the local loop is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the carrier or telecommunications service provider's network...

s are terminated and switched.

Note: In the DOD, "common carrier" is called "commercial carrier." Synonyms exchange, local central office, local exchange, local office, switching center (except in DOD Defense Switched Network
Defense Switched Network
The Defense Switched Network is a primary information transfer network for the Defense Information Systems Network . The DSN provides the worldwide non-secure voice, secure voice, data, facsimile, and video teleconferencing services for DOD Command and Control elements, their supporting...

 (formerly AUTOVON
Autovon
AUTOVON, short for Automatic Voice Network, was an American military phone system built in 1963 to survive nuclear attacks. AUTOVON was first established in the United States, using the Army's SCAN system. Around the mid-1970s AUTOVON expanded to the United Kingdom, Asia, the Middle East, and Panama...

) usage), switching exchange, telephone exchange. Deprecated synonym switch
Switch
In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....

.

External links