Telephone exchange

Atelephone operatormanually connecting calls with cord pairs at a telephone switchboard

A modern central office, equipped with voice communication andbroadbanddata capabilities

Atelephone exchange,also known as atelephone switchorcentral office,is a crucial component in thepublic switched telephone network(PSTN) or large enterprise telecommunications systems. It facilitates the interconnection of telephone subscriber lines or digital system virtual circuits, enablingtelephone callsbetween subscribers.

The terminology used in telecommunications has evolved over time, withtelephone exchangeandcentral officeoften used interchangeably, the latter term originating from theBell System.A central office typically refers to a facility that houses theinside plantequipment for one or several telephone exchanges, each catering to a specific geographical region. This region is sometimes known as the exchange area. In North America, the termwire centermay be used to denote a central office location, indicating a facility that provides a telephone with adial tone.Telecommunication carriers also definerate centersfor business and billing purposes, which in large cities, might encompass clusters of central offices to specify geographic locations for distance measurement calculations.

In the 1940s, the Bell System in the United States and Canada introduced anationwide numbering systemthat identified central offices with a unique three-digit code, along with a three-digitnumbering plan areacode (NPA code or area code), making central office codes distinctive within each numbering plan area. These codes served as prefixes in subscriber telephone numbers. The mid-20th century saw similar organizational efforts in telephone networks globally, propelled by the advent of international and transoceanic telephone trunks and direct customer dialing.

For corporate or enterprise applications, a private telephone exchange is termed aprivate branch exchange(PBX), which connects to the public switched telephone network. A PBX serves an organization's telephones and any private leased line circuits, typically situated in large office spaces or organizational campuses. Smaller setups might use a PBX orkey telephone systemmanaged by a receptionist, catering to the telecommunication needs of the enterprise.

History

Tivadar Puskás

1922 diagram of 1877 Boston exchange

1903 manual switch for four subscriber lines (top) with four cross-bar talking circuits (horizontal), and one bar to connect the operator (T). The lowest cross-bar connects idle stations to ground to enable the signaling indicators (F).

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.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 sameschemaand 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 theinvention of the telephoneitself, the honour of "first telephone exchange" has several claimants. One of the first to propose a telephone exchange wasHungarianTivadar Puskásin 1877 while he was working forThomas Edison.The first experimental telephone exchange was based on the ideas of Puskás, and it was built by theBell Telephone CompanyinBostonin 1877.The world's first state-administered telephone exchange opened on November 12, 1877 in Friedrichsberg close toBerlinunder the direction ofHeinrich von Stephan.George W. Coydesigned and built the first commercial US telephone exchange which opened inNew Haven, Connecticutin January, 1878, and the first telephone booth was built in nearbyBridgeport.The switchboard was built from "carriage bolts, handles from teapot lids and bustle wire" and could handle two simultaneous conversations.Charles Gliddenis also credited with establishing an exchange in Lowell, MA. with 50 subscribers in 1878.

In Europe other early telephone exchanges were based inLondonandManchester,both of which opened under Bell patents in 1879.Belgium had its firstInternational Bellexchange (inAntwerp) a year later.

In 1887 Puskás introduced themultiplexswitchboard..

Later exchanges consisted of one to several hundredplug boardsstaffed byswitchboard operators.Each operator sat in front of a vertical panel containing banks of ¼-inchtip-ring-sleeve(3-conductor) jacks, each of which was the local termination of asubscriber'stelephone line.In front of the jack panel lay a horizontal panel containing two rows of patch cords, each pair connected to acord circuit.

When acalling partylifted 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 their 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, the operator plugged into atrunkcircuit 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 them to handle another call, while the caller heard an audible ringback signal, so that that operator would not have to periodically report that they was continuing to ring the line.

In theringdownmethod, 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 inKansas City, Missouri,patented thestepping switch,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 rotarytelephone 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 switches were arranged in banks, the first stage of which was alinefinder.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 adial toneto 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 byother exchange typesand later bycrossbartechnology. 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 acceptedDTMF"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 serviceis telephone service in which a humantelephone operatorroutes calls as instructed by a subscriber with a telephone set that does not have a dial.
• Dial serviceis when an exchange routes calls by interpreting subscriber-dialed digits.
• Atelephone switchis the switching equipment of an exchange.
• Awire centeris the area served by a particular switch or central office.
• Aconcentratoris a device that concentrates traffic, be it remote or co-located with the switch.
• Anoff-hookcondition represents a circuit that is in use, e.g., when a telephone call is in progress.
• Anon-hookcondition represents an idle circuit, i.e. no telephone call is in progress.

Acentral officeoriginally 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 relatedinside plantequipment. In United Statestelecommunicationjargon, a central office (C.O.) is acommon carrierswitching centerClass 5 telephone switchin which trunks andlocal loopsare terminated and switched. In the UK, atelephone exchangemeans an exchange building, and is also the name for a telephone switch.

Manual service exchanges

Main article:Telephone switchboard

1924 PBX switchboard

Withmanual service,the customer lifts the receiveroff-hookand asks theoperatorto 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 providedcommon-batteryservice, meaning that the central office provided power to the subscriber telephone circuits for operation of the transmitter, as well as for automatic signaling withrotary dials.In common-battery systems, the pair of wires from a subscriber's telephone to the exchange carry 48V (nominal) DC potential from the telephone company end across the conductors. The telephone presents an open circuit when it ison-hookor idle.

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 apanel switch.During this transition period, once numbers were standardized to the 2L-4N or 2L-5Nformat (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 theBell Systemstated 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 anindicator,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.

Montreal telephone exchange (c. 1895)

In contrast to the listing formatMAin 1234for 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 withmagnetotelephones, 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 cellbatteries, 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.

Many small town magneto systems featuredparty lines,anywhere from two to ten or more subscribers sharing a single line. When calling a party, the operator used code ringing, a distinctiveringing signalsequence, 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

A rural telephone exchange building in Australia

Automatic exchanges,which provideddial service,were invented byAlmon Strowgerin 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 humanswitchboard operatorswho completed the connections required for atelephone call.Automation replaced human operators with electromechanical systems, and telephones were equipped with a dial by which a caller transmitted the destination telephone number to the automatic switching system.

A telephone exchange automatically senses an off-hook condition of thetelephonewhen the user removes the handset from the switchhook or cradle. The exchange providesdial toneat that time to indicate to the user that the exchange is ready to receive dialed digits. The pulses orDTMFtones 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 calledsupervision.Additional features, such as billing equipment, may also be incorporated into the exchange.

The Bell System dial service implemented a feature calledautomatic number identification(ANI) which facilitated services like automated billing,toll-free 800-numbers,and9-1-1service. 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 andrelays.Some types of automatic exchanges were theStrowger switchor step-by-step switch, All Relay,panel switch,Rotary systemand thecrossbar switch.

Electromechanical signaling

Main article:Signalling (telecommunications)

Circuits interconnecting switches are calledtrunks.BeforeSignalling System 7,Bell Systemelectromechanical 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 calledPanel Call IndicatorPulsing usedquaternarypulses to set up calls between apanel switchand a manual switchboard. Probably the most common form of communicating dialed digits between electromechanical switches was sendingdial pulses,equivalent to arotary dial'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 specialkeypulse(KP) signal and followed by astart(ST). Variations of the Bell System MF tone scheme became aCCITTstandard. 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 atelephone number.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 includeE and M signaling,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 withE 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 calledsingle-frequencyorSF 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 reducedfalsing.) Some systems used tone frequencies over 3,000 Hz, particularly on SSBfrequency-division multiplexmicrowave radio relays.

OnT-carrierdigital 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 bitswere 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

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 cardboardcardswith a code that logged the nature of a failure.

Maintenance tasks

Manual test board in an electromechanical switching office staffed by a technician

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 asmake-busys 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 aTD 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 apermanent 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 anOriginating 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 identificationhad been retrofitted to nearly all step-by-step and crossbar switches in the Bell System.

Electronic switches

Electronic switching systemsgradually evolved in stages from electromechanical hybrids withstored program controlto the fully digital systems. Early systems usedreed relay-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 theWestern Electric1ESS switch,Northern TelecomSP1,Ericsson AXE, Automatic Electric EAX-1 & EAX-2, PhilipsPRX/A, ITT Metaconta,British GPO/BTTXEseries 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 calledCommon Channel Interoffice Signaling, (CCIS).This data link was based on CCITT 6, a predecessor toSS7. In European systems R2 signalling was normally used.

Digital switches

A typical satellite PABX with front cover removed

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 byBell Labsas 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 inLondonwhich was designed by theGeneral Post Officeresearch labs.It was a tandem switch that connected threeStrowgerexchanges. The first commercial roll-out of a fully digital local switching system wasAlcatel's E10 system which began serving customers inBrittanyin Northwestern France in 1972.

Prominent examples of digital switches include:

• Ericsson'sAXE telephone exchangeis 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 Ericssoncrossbar switchesARF, ARM, ARK and ARE used by many European networks from the 1950s onwards.
• Alcatel-Lucentinherited three of the world's most iconic digital switching systems:AlcatelE10,1000-S12,and theWestern Electric5ESS.

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 intoall-IP networks.

Alcatel also acquiredITT System 12which 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's5ESSand4ESSsystems used throughout the United States of America and in many other countries.

• Nokia Siemens NetworksEWSDoriginally developed bySiemens,BoschandDeTeWefor the German market is used throughout the world.
• NortelthenGenband,and now Ribbon CommunicationsDMS100and 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
• NECNEAX used in Japan, New Zealand and many other countries.
• MarconiSystem Xoriginally developed by GPT and Plessey is a type of digital exchange used byBT Groupin the UK public telephone network.

A digital exchange (NortelDMS-100) used by an operator to offer local and long-distance services inFrance.Each switch typically serves 10,000–100,000+ subscribers depending on the geographic area

Digital switches encode the speech going on, in 8,000 time slices per second. (Asampling rateof 8khz). At each time slice, a digitalPCMrepresentation 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), 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.

Individuallocal looptelephone lines are connected to aremote concentrator.In many cases, the concentrator is co-located in the same building as the switch. The interface between remote concentrators and telephone switches has been standardised byETSIas theV5protocol. 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 ortandem switches.

Some telephone exchange buildings in small towns house onlyremoteorsatelliteswitches, and are homed upon a "parent" switch, usually several kilometres away. The remote switch is dependent on the parent switch for routing. Unlike adigital loop carrier,a remote switch can route calls between local phones itself, without using trunks to the parent switch.

Map of wire center locations in the US

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 isoutside plant,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 themain distribution frame(MDF). A component of the MDF is protection: fuses or other devices that protect the switch from lightning, shorts with electric power lines, or other foreign voltages. In a typical telephone company, a large database tracks information about each subscriber pair and the status of each jumper. Before computerization of Bell System records in the 1980s, this information was handwritten in pencil in accounting ledger books.

To reduce the expense of outside plant, some companies use "pair gain"devices to provide telephone service to subscribers. These devices are used to provide service where existing copper facilities have been exhausted or by siting in a neighborhood, can reduce the length of copper pairs, enabling digital services such asIntegrated Services Digital Network(ISDN) ordigital subscriber line(DSL).

Pair gain ordigital loop carriers(DLCs) are located outside the central office, usually in a large neighborhood distant from the CO. DLCs are often referred to asSubscriber Loop Carriers(SLCs), after aLucentproprietary 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 anIntegrated 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 inlong distancecenters. There are two major types in thePublic switched telephone network(PSTN), theClass 4 telephone switchesdesigned for toll or switch-to-switch connections, and theClass 5 telephone switchesor 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 accuracy10 MHz standardswhich synchronize time events to very close intervals. Time-standards equipment may include Rubidium- or Caesium-based standards and aGlobal Positioning Systemreceiver.

Switch design

Long-distance switches may use a slower, more efficient switch-allocation algorithm thanlocal 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 ofspace-and time-division switching. In other words, each voice channel is represented by atime 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 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.

Thestructure of a switchis 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 alsobe 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 anonblocking minimal spanning switch,or acrossover switch.

Fault tolerance

Composite switches are inherently fault-tolerant. If a subswitch fails, the controlling computer can sense the failure during a periodic test. The computer marks all the connections to the subswitch as "in use". This prevents new calls, and does not interrupt established calls. As established calls end, the subswitch becomes unused, and can be repaired. When the next test succeeds, the switch returns to full operation.

To prevent frustration with unsensed failures, all the connections between layers in the switch are allocated usingfirst-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. Alast-in-first-out(stack) allocation of connections might cause a continuing string of very frustrating failures.

Fire and disaster recovery

Second Avenue exchange, NYC, site of the1975 New York Telephone Exchange fire.

A central exchange is almost always asingle point of failurefor local calls. As the capacity of individual switches and theoptical fibrewhich 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 potentialcommon mode failure.

See also

• History of telecommunication
• List of telephone switches
• Pair gain system
• Limited availability
• Softswitch
• Plesiochronous digital hierarchy
• Telephone exchange names
• Automatic call distributor
• Public utility building

External links

• patent 252,576 for the first telephone switchboard in 1881
• A Telecom Exchange Tour in NZ