telephone exchange - wikipedia, the free encyclopedia

8/12/2019 Telephone Exchange - Wikipedia, The Free Encyclopedia

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7/14/2014 Telephone exchange - Wikipedia, the free encyclopedia

http://en.wikipedia.org/wiki/Telephone_exchange

A telephone operator manually connectingcalls with cord pairs at a telephoneswitchboard.

Telephone exchangeFrom Wikipedia, the free encyclopedia

A telephone exchangeis a telecommunications system used in thepublic switched telephone network or in large enterprises. Anexchange consists of electronic components and in older systemsalso human operators that interconnect (switch) telephone

subscriber lines or virtual circuits of digital systems to establishtelephone calls between subscribers.

In the public telecommunication networks a telephone exchange islocated in a central office (CO), typically a building used to housethe inside plant equipment of potentially several telephoneexchanges, each serving a certain geographical exchange area.Central office locations are often identified in North America as awire centers, designating a facility from which a telephone obtains

dial tone.[1]For business and billing purposes, telephony carriers

also define rate centers, which in larger cities may be clusters of central offices, to define specified geographicallocations for determining distance measurements.

In the United States and Canada, the Bell System established in the 1940s a uniform system of identifying eachtelephone exchange with a three-digit exchange code, or central office code, that was used as a prefix to subscribetelephone numbers. All exchanges within a larger region, typically aggregated by state, were assigned a commonarea code. With the development of international and transoceanic telephone trunks, especially driven by directcustomer dialing, similar efforts of systematic organization of the telephone networks occurred in many countries inthe mid-20th century.

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

Contents

1 Historic perspective2 Technologies

2.1 Manual service exchanges

2.2 Early automatic exchanges

2.2.1 Electromechanical signaling

2.2.2 Sounds

2.2.3 Maintenance tasks

2.2.4 Electronic switches
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http://en.wikipedia.org/wiki/Telephone_exchange 2

Tivadar Pusks, inventor of the telephoneexchange.

2.3 Digital switches

2.4 The switch's place in the system

3 Switch design

4 Switch control algorithms

4.1 Fully connected mesh network

4.2 Clos's nonblocking switch algorithm

5 Fault tolerance

6 Fire and disaster recovery

7 Internet exchanges

8 See also

9 References

10 External links

Historic perspective

In the era of the electrical telegraph, post offices, railway stations,the more important governmental centers (ministries), stockexchanges, very few nationally distributed newspapers, the largestinternationally important corporations and wealthy individuals were

the principal users of such telegraphs.[2]Despite the fact thattelephone devices existed before the invention of the telephoneexchange, their success and economical operation would have been

impossible on the same schema and structure of the contemporarytelegraph, as prior to the invention of the telephone exchangeswitchboard, early telephones were hardwired to andcommunicated with only a single other telephone (such as from anindividual's home to the person's business).

A telephone exchange is a telephone system located at servicecenters (central offices) responsible for a small geographic area that

provided the switching or interconnection of two or more individualsubscriber lines for calls made between them, rather than requiring

direct lines between subscriber stations. This made it possible forsubscribers to call each other at homes, businesses, or publicspaces. These made telephony an available and comfortablecommunication tool for everyday use, and it gave the impetus for thecreation of a whole new industrial sector.

One of the first people to build a telephone exchange was Hungarian Tivadar Pusks in 1877 while he was workin

for Thomas Edison.[3][4][5][6]The first experimental telephone exchange was based on the ideas of Pusks, and it

was built by the Bell Telephone Company in Boston in 1877. [7]The worlds first commercial telephone exchange

opened on November 12 1877 in Friedrichsberg close to Berlin.[8]George W. Coy designed and built the first
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Exchange in Miskolc, Hungary

commercial US telephone exchange which opened in New Haven, Connecticut in January, 1878. The switchboardwas built from "carriage bolts, handles from teapot lids and bustle wire" and could handle two simultaneous

conversations.[9]Charles Glidden is also credited with establishing an exchange in Lowell, MA. with 50 subscriberin 1878.

In Europe other early telephone exchanges were based in London and Manchester, both of which opened under

Bell patents in 1879.[10]Belgium had its first International Bell exchange (in Antwerp) a year later.

In 1887 Pusks introduced the multiplex switchboard, that had anepochal significance in the further development of telephone

exchange.[11]

Later exchanges consisted of one to several hundred plug boardsstaffed by switchboard operators. Each operator sat in front of avertical panel containing banks of -inch tip-ring-sleeve (3-conductor) jacks, each of which was the local termination of asubscriber's telephone line. In front of the jack panel lay a horizontal

panel containing two rows of patch cords, each pair connected to acord circuit. When a calling party lifted the receiver, a signal lamp

near the jack would light.[12]

The operator would plug one of the cords (the "answering cord") into the subscriber's jack and switch her headsetinto the circuit to ask, "Number, please?" Depending upon the answer, the operator might plug the other cord of th

pair (the "ringing cord") into the called party's local jack and start the ringing cycle, or plug into a trunk circuit tostart 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.[12]

In the ringdown method, the originating operator called another intermediate operator who would call the called

subscriber, or passed it on to another intermediate operator.[13]This chain of intermediate operators couldcomplete the call only if intermediate trunk lines were available between all the centers at the same time. In 1943when military calls had priority, a cross-country US call might take as long as 2 hours to request and schedule incities that used manual switchboards for toll calls.

On March 10, 1891, Almon Brown Strowger, an undertaker in Kansas City, Missouri, patented the steppingswitch, a device which led to the automation of telephone circuit switching. While there were many extensions andadaptations 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 centralcontact "hand" of the stepping switch to first step (ratchet) up one level for each pulse in the first digit and then toswing horizontally in a contact row with one small rotation for each pulse in the next digit.

Later stepping switches were arranged in banks, the first stage of which was a linefinder. If one of up to a hundresubscriber lines had the receiver lifted "off hook", a linefinder connected the subscriber's line to a free first selectorwhich returned the subscriber a dial tone to show that it was ready to receive dialed digits. The subscriber's dial

pulsed at about 10 pulses per second, although the speed depended on the standard of the particular telephoneadministration.
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Exchanges based on the Strowger switch were eventually challenged by other exchange types and later by crossbtechnology. These exchange designs promised faster switching and would accept pulses faster than the Strowger'stypical 10 ppstypically about 20 pps. At a later date many also accepted DTMF "touch tones" or other tonesignaling systems.

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

Technologies

Many terms used in telecommunication technology differ in meaning and usage in various regions of the worldamong English speaking regions. For the purpose of this article the following definitions are made:

Manual serviceis a condition in which a human operator routes calls inside an exchange without the use of

dial.

Dial serviceis when an exchange routes calls by a switch interpreting dialed digits.

A telephone switchis the switching equipment of an exchange.A concentratoris a device that concentrates traffic, be it remote or co-located with the switch.

An off-hookcondition is a tip condition or describes a circuit that is in use, e.g., when a phone call is in

progress.

An on-hookcondition represents an idle circuit, i.e. no phone call is in progress.

A wire centeris the area served by a particular switch or central office.

Central officeoriginally referred to switching equipment and its operators, it is also used generally for the buildingthat houses switching and related inside plant equipment. In United States telecommunication jargon, a central offic

(C.O.) is a common carrier switching center Class 5 telephone switch in which trunks and local loops areterminated and switched.[14]In the UK, a telephone exchangemeans an exchange building, and is also the namefor a telephone switch.

Manual service exchanges

With manual service, the customer lifts the receiver off-hook and asks the operator to connect the call to arequested number. Provided that the number is in the same central office, and located on the operator'sswitchboard, the operator connects the call by plugging the ringing cord into the jack on the switchboard

corresponding to the called customer's line. If the called party's line is on a different switchboard in the same officeor in a different central office, the operator plugs into the trunk for the destination switchboard or office and asksthe operator answering (known as the "B" operator) to connect the call.

Most urban exchanges provided common-battery service, meaning that the central office provided power to thesubscriber telephone circuits for operation of the transmitter, as well as for automatic signaling with rotary dials. Incommon-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 i

on-hook or idle.[15]
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1924 PBX switchboard

Montreal telephone exchange (c1895)

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

perform service.[15]

In the largest cities, it took many years to convert every office to automatic equipment, such as a panel switch.During this transition period, once numbers were standardized to the 2L-5N format (two-letter exchange name anfive digits), it was possible to dial a number on a manual exchange and be connected without requesting operatorassistance. The policy of the Bell System stated that customers in large cities should not need to be concernedabout whether they were calling a manual or automated office.

If a subscriber dialed a manual number, an operator at the destination office would answer, see the number on anindicator, and connect the call by plugging into the correct circuit and ringing the call. For instance, if a customercalling from TAylor 4725 dialed a manual number, ADams 1383, the callwould go through, from the subscriber's perspective, exactly as would acall to LEnnox 5813-W, in an automated exchange.

In contrast to the format MAin

1234, indicating an automatedoffice, or a manual office withoperator indicators forincoming dialed calls, a numbersuch as Hillside 834 or East 23would be clearly recognized asa manual number, based on thesecond letter not beingcapitalized.

The smallest towns withmanual service often had

magneto, or crank, phones. Using this type of service, the subscriberturned a crank to generate ringing current, to gain the operator'sattention. The switchboard would respond by interrupting the circuit,which dropped a metal tab above the subscriber's line jack and soundeda buzzer. Dry cell batteries (normally two large No. 6 cells) in thesubscriber's telephone provided the DC power for conversation. Magneto systems were in use in one Americansmall town, Bryant Pond, Woodstock, Maine as late as 1983. In general, dry cell battery systems had a poorer caquality compared to common-battery systems.

Many small town magneto systems featured party lines, anywhere from two to ten or more subscribers sharing asingle line. When calling a party, the operator would use a distinctive ringing signal sequence, such as two long ringfollowed by one short. Everyone on the line could hear the rings, and of course could pick up and listen in if theywanted. 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
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A rural telephone exchange building inAustralia.

Automatic exchanges, or dial service, came into existence in the early 1900s. Their purpose was to eliminate thneed for human switchboard operators who completed theconnections required for a telephone call. Automation replacedhuman operators with electromechanical systems and telephoneswere equipped with a dial by which a caller transmitted thedestination telephone number to the automatic switching system.

A telephone exchange automatically senses an off-hook condition of

the telephone when the user removes the handset from theswitchhook or cradle. The exchange provides dial tone at that timeto indicate to the user that the exchange is ready to receive dialeddigits. The pulses or DTMF tones generated by the telephone are

processed and a connection is established to the destinationtelephone within the same exchange or to another distant exchange.

The exchange maintains the connection until one of the parties hangsup. This monitoring of connection status is calledsupervision.Additional features, such as billing equipment, mayalso be incorporated into the exchange.

The Bell System dial service implemented a feature called automatic number identification (ANI) which facilitatedservices like automated billing, toll-free 800-numbers, and 9-1-1 service. In manual service, the operator knowswhere a call is originating by the light on the switchboard jack field. Before ANI, long distance calls were placedinto an operator queue and the operator asked the calling party's number and record it on a paper toll ticket.

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

Electromechanical signaling

Circuits interconnecting switches are called trunks. Before Signalling System 7, Bell System electromechanicalswitches in the United States communicated with one another over trunks using a variety of DC voltages andsignaling 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 quaternarypulses to set up calls between a panel switch and a manual switchboard. Probably the most common form ofcommunicating dialed digits between electromechanical switches was sending dial pulses, equivalent to a rotarydial'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 crossbartandems. This was twice the rate of Western Electric/Bell System telephone dials. Using the faster pulsing ratemade trunk utilization more efficient because the switch spent half as long listening to digits. DTMF was not used ftrunk signaling.

Multi-frequency (MF) was the last of the pre-digital methods. It used a different set of tones sent in pairs likeDTMF. Dialing was preceded by a special keypulse(KP) signal and followed by astart(ST). Variations of theBell System MF tone scheme became a CCITT standard. Similar schemes were used in the Americas and in someEuropean countries including Spain. Digit strings between switches were often abbreviated to further improveutilization.
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Step-by-step call

Subscribers hear a different-soundingdialtone in a step-by-step call.

Problems playing this file? See media help.

For example, one switch might send only the last four or five digits of a telephone number. In one case, seven digitnumbers were preceded by a digit 1 or 2 to differentiate between two area codes or office codes, (a two-digit-percall savings). This improved revenue per trunk and reduced the number of digit receivers needed in a switch. Evertask in electromechanical switches was done in big metallic pieces of hardware. Every fractional second cut off ofcall 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, SF signaling, androbbed-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 wernamed 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 bydialing calls on the same trunk at the same time. By changing the state of these leads from ground to -48 volts, theswitches stepped through a handshake protocol. Using DC voltage changes, the local switch would send a signal tget ready for a call and the remote switch would reply with an acknowledgment to go ahead with dial pulsing. Thiswas 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, causeda second set of clunks when the called party answered.

A second common form of signaling for supervision was called single-frequencyor SF signaling. The mostcommon form of this used a steady 2,600 Hz tone to identify a trunk as idle. Trunk circuitry hearing a 2,600 Hztone for a certain duration would go idle. (The duration requirement reduced falsing.) Some systems used tonefrequencies over 3,000 Hz, particularly on SSB frequency division multiplex microwave radio relays.

On T-carrier digital transmission systems, bits within the T-1 data stream were used to transmit supervision. Bycareful design, the appropriated bits did not change voice quality appreciably. Robbed bitswere translated tochanges in contact states (opens and closures) by electronics in the channel bank hardware. This allowed directcurrent E and M signaling, or dial pulses, to be sent between electromechanical switches over a digital carrier whicdid not have DC continuity.

Sounds

A characteristic of electromechanical switching equipment isthat the maintenance staff could hear the mechanicalclattering 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 ascommunity dial offices (CDOs), were housed in

prefabricated metal buildings. These facilities almost always had concrete floors. The hard surfaces reflectedsounds.

During heavy use periods, it could be difficult to converse in a central office switch room due to the clatter of callsbeing processed in a large switch. For example, on Mother's Day in the US, or on a Friday evening around 5pm,the metallic rattling could make raised voices necessary. For wire spring relay markers these noises resembled hailfalling on a metallic roof.

0:00 MENU
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On a pre-dawn Sunday morning, call processing might slow to the extent that one might be able to hear individualcalls being dialed and set up. There were also noises from whining power inverters and whirring ringing generatorsSome systems had a continual, rhythmic "clack-clack-clack" from wire spring relays that made reorder (120 ipm)and busy (60 ipm) signals.

Bell System installations typically had alarm bells, gongs, or chimes to announce alarms calling attention to a failedswitch element. A trouble reporting card system was connected to switch common control elements. These troublreporting systems punctured cardboard cards with a code that logged the nature of a failure. Reed relay technolog

in stored program control exchange finally quieted the environment.

Maintenance tasks

The maintenance of electromechanical systems involved electricity in form of direct current (DC), alternating ringcurrent (AC), and adjustments of many mechanical parts. Unlike modern switches, a circuit connecting a dialed cathrough an electromechanical switch had DC continuity within the local exchange area via metallic conductors.

In all systems, it was avoided for subscribers to notice changes in quality of service because of failures ormaintenance work. A variety of tools referred to as make-busys were plugged into electromechanical switchelements during repairs or failures. A make-busy would identify the part being worked on as in-use, causing theswitching logic to route around it. A similar tool was called a TD tool.Subscribers who got behind in paymentswould have their service temporarily denied (TDed). This was effected by plugging a tool into the subscriber's officequipment (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 constancleaning. Indicator lights on equipment bays in step offices alerted staff to conditions such as blown fuses (usuallywhite lamps) or a permanent signal (stuck off-hook condition, usually green indicators). Step offices were moresusceptible to single-point failures than newer technologies.

Crossbar offices used more shared, common control circuits. For example, a digit receiver (part of an elementcalled an Originating Register) would be connected to a call just long enough to collect the subscriber's dialeddigits. Crossbar architecture was more flexible than step offices. Later crossbar systems had punch-card-basedtrouble 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 systems gradually evolved in stages from electromechanical hybrids with stored programcontrol to the fully digital systems. Early systems used reed relay-switched metallic paths under digital control.

Equipment testing, phone numbers reassignments, circuit lockouts and similar tasks were accomplished by dataentry on a terminal.

Examples of these systems included the Western Electric 1ESS switch, Northern Telecom SP1, Ericsson AKE,Philips PRX/A, ITT Metaconta, British GPO/BT TXE series and several other designs were similar. Ericsson alsodeveloped a fully computerized version of their ARF crossbar exchange called ARE. These used a crossbarswitching matrix with a fully computerized control system and provided a wide range of advanced services. Localversions 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 withdigital technology.
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A typical satellite PBX with front coverremoved.

These systems could use the old electromechanical signaling methods inherited from crossbar and step-by-stepswitches. They also introduced a new form of data communications: two 1ESS exchanges could communicate witone another using a data link called Common Channel Interoffice Signaling, (CCIS). This data link was based onCCITT 6, a predecessor to SS7. In European systems R2 signalling was normally used.

Digital switches

Digital switches work by connecting two or more digital circuits,according to a dialed telephone number or other instruction. Callsare set up between switches. In modern networks, this is usuallycontrolled using the Signalling System 7 (SS7) protocol, or one ofits variants. Many networks around the world are now transitioningto voice over IP technologies which use Internet-based protocolssuch as the Session Initiation Protocol (SIP). These may havesuperseded TDM and SS7 based technologies in some networks.

The concepts of digital switching were developed by various labs inthe United States and in Europe from the 1930s onwards. The first

prototype digital switch was developed by Bell Labs as part of theESSEX project while the first true digital exchange to be combinedwith digital transmission systems was designed by LCT (LaboratoireCentral de Telecommunications) in Paris. The first digital switch to be placed into a public network was theEmpress Exchange in London, England which was designed by the General Post Office research labs. This was atandem switch that connected three Strowger exchanges in the London area. The first commercial roll-out of a fullydigital local switching system was Alcatel's E10 system which began serving customers in Brittany in NorthwesternFrance in 1972.

Prominent examples of digital switches include:

Ericsson's AXE telephone exchange is the most widely used digital switching platform in the world and can

be found throughout Europe and in most countries around the world. It is also very popular in mobile

applications. This highly modular system was developed in Sweden in the 1970s as a replacement for the

very popular range of Ericsson crossbar switches ARF, ARM, ARK and ARE used by many European

networks from the 1950s onwards.

Alcatel-Lucent inherited three of the world's most iconic digital switching systems : Alcatel E10, 1000-S12,

and the Western Electric 5ESS.

Alcatel developed the E10 system in France during the late 1960s and 1970s. This widely used family of

digital switches was one of the earliest TDM switches to be widely used in public networks. Subscribers

were first connected to E10A switches in France in 1972. This system is used in France, Ireland, China, an

many other countries. It has been through many revisions and current versions are even integrated into All IP

networks.

Alcatel also acquired ITT System 12 which when it bought ITT's European operations. The S12 system an

E10 systems were merged into a single platform in the 1990s. The S12 system is used in Germany, Italy,
http://en.wikipedia.org/wiki/ITT_System_12http://en.wikipedia.org/wiki/Internet_protocolhttp://en.wikipedia.org/wiki/5ESS_switchhttp://en.wikipedia.org/wiki/Western_Electrichttp://en.wikipedia.org/wiki/ITT_System_12http://en.wikipedia.org/wiki/Alcatelhttp://en.wikipedia.org/wiki/Alcatel-Lucenthttp://en.wikipedia.org/w/index.php?title=Crossbar_switche&action=edit&redlink=1http://en.wikipedia.org/wiki/AXE_telephone_exchangehttp://en.wikipedia.org/wiki/Ericssonhttp://en.wikipedia.org/wiki/Brittanyhttp://en.wikipedia.org/wiki/Alcatelhttp://en.wikipedia.org/w/index.php?title=Stroweger_switch&action=edit&redlink=1http://en.wikipedia.org/wiki/General_Post_Officehttp://en.wikipedia.org/wiki/Londonhttp://en.wikipedia.org/wiki/Bell_Labshttp://en.wikipedia.org/wiki/Session_Initiation_Protocolhttp://en.wikipedia.org/wiki/Voice_over_IPhttp://en.wikipedia.org/wiki/Signalling_System_7http://en.wikipedia.org/wiki/Telephone_numberhttp://en.wikipedia.org/wiki/Signaling_System_7http://en.wikipedia.org/wiki/Common_Channel_Signalinghttp://en.wikipedia.org/wiki/File:Pbx-satellite-telephone-exchange-0a.jpg


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Map of the Wire Center locations in the US

Map of the Central Office locations in the US

Telephone switches are a small component of a large network. The majority of work and expense of the phonesystem is the wiring outside the central office, or the outside plant. In the middle 20th century, each subscribertelephone 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 pairsof wires that appear on terminal blocks called the maindistribution 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 otherforeign voltages. In a typical telephone company, a largedatabase tracks information about each subscriber pair andthe status of each jumper. Before computerization of BellSystem records in the 1980s, this information washandwritten in pencil in accounting ledger books.

To reduce the expense of outside plant, some companiesuse "pair gain" devices to provide telephone service tosubscribers. These devices are used to provide service

where existing copper facilities have been exhausted or bysiting in a neighborhood, can reduce the length of copper

pairs, enabling digital services such as Integrated ServicesDigital Network (ISDN) or Digital Subscriber Line (DSL).

Pair gain or digital loop carriers (DLCs) are located outsidethe central office, usually in a large neighborhood distantfrom the CO. DLCs are often referred to as SubscriberLoop Carriers (SLCs), after a Lucent proprietary product.

DLCs can be configured as universal (UDLCs) orintegrated (IDLCs). Universal DLCs have two terminals, acentral office terminal (COT) and a remote terminal (RT),that function similarly. Both terminals interface with analogsignals, convert to digital signals, and transport to the otherside 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 telephoneswitch. This reduces the total amount of equipment required.

Switches are used in both local central offices and in long distance centers. There are two major types in the Publiswitched telephone network (PSTN), the Class 4 telephone switches designed for toll or switch-to-switchconnections, and the Class 5 telephone switches or subscriber switches, which manage connections from subscribtelephones. 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 beslaved 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 Systemreceiver.
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Switch design

Long distance switches may use a slower, more efficient switch-allocation algorithm than local central offices,because they have near 100% utilization of their input and output channels. Central offices have more than 90% otheir 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 slo(say 1 or 2) on a physical wire pair (A or B). In order to connect two voice channels (say A1 and B2) together, thtelephone switch interchanges the information between A1 and B2. It switches both the time slot and physicalconnection. To do this, it exchanges data between the time slots and connections 8000 times per second, undercontrol of digital logic that cycles through electronic lists of the current connections. Using both types of switchingmakes a modern switch far smaller than either a space or time switch could be by itself.

The structure of a switch is an odd number of layers of smaller, simpler subswitches. Each layer is interconnectedby a web of wires that goes from each subswitch, to a set of the next layer of subswitches. In most designs, aphysical (space) switching layer alternates with a time switching layer. The layers are symmetric, because in atelephone 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 differentorder, 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 spanningswitch, or a crossover switch.

Switch control algorithms

Fully connected mesh network

One way is to have enough switching fabric to assure that the pairwise allocation will always succeed by building afully connected mesh network. This is the method usually used in central office switches, which have low utilizationof their resources.

Clos's nonblocking switch algorithm

The scarce resources in a telephone switch are the connections between layers of subswitches. The control logichas 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 veryimportant algorithm to the telephone industry.

Fault tolerance

Composite switches are inherently fault-tolerant. If a subswitch fails, the controlling computer can sense it during aperiodic test. The computer marks all the connections to the subswitch as "in use". This prevents new calls, anddoes not interrupt old calls that remain working. As calls in progress end, the subswitch becomes unused, and new
http://en.wikipedia.org/w/index.php?title=Charles_Clos&action=edit&redlink=1http://en.wikipedia.org/wiki/Fault_toleranthttp://en.wikipedia.org/wiki/Network_topologyhttp://en.wikipedia.org/wiki/Switching_fabrichttp://en.wikipedia.org/wiki/Crossover_switchhttp://en.wikipedia.org/wiki/Nonblocking_minimal_spanning_switchhttp://en.wikipedia.org/wiki/Nonblocking_minimal_spanning_switchhttp://en.wikipedia.org/wiki/Time_division_multiplexinghttp://en.wikipedia.org/wiki/Multiplexershttp://en.wikipedia.org/wiki/Class_5_telephone_switches


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Second Avenue exchange, NYC

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 switchreturns 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. A last-in-first-out (stack) allocation of connectionsmight cause a continuing string of very frustrating failures.

Fire and disaster recovery

In July 1951, during massive flooding in Kansas and Missouri, a manualswitchboard in Manhattan, Kansas was abandoned as water levels rosein the central office; operators regained access to the town's four trunklines from a local filling station on higher ground to send emergency

messages and radiotelephone was used to bypass damaged facilities.[16]

On February 27, 1975 a fire at New York Telephone's building at 204Second Avenue (at East 13th Street) in Manhattan destroyed the maindistribution frame and damaged much of the underground cabling,disconnecting 170000 subscribers. This office connects many circuits toBrooklyn which were disrupted. Equipment was redirected from otherBell System operating companies in multiple US states to establish

temporary service and rebuild the destroyed exchange.[17]

In 1978, a central office fire in Mebane, North Carolina knocked out

every one of the small community's 3900 phones.[18]

In May 1988, a central office fire in the Chicago suburb of Hinsdale,Illinois knocked out 35000 local subscribers, broke the link between theFAA and air traffic control at Chicago O'Hare International Airport (then the world's busiest) and disrupted theMidwest's ability to communicate with the rest of the country. The office had a fire alarm but no automatic firesuppression equipment. The facility was unattended and monitored remotely by an Illinois Bell technician inSpringfield; it took an hour to notify firefighters of the blaze as the distant technician's attempts to call the fire

department in Hinsdale did not get through. The fire had already knocked out the phone lines.[19]

In 1991, all twenty-eight exchanges serving Kuwait were out of service in the wake of a 1990 invasion by Iraq;

equipment had been looted and central offices destroyed. Service was initially restored via satellite. [20]

On September 11, 2001 a terrorist attack destroyed a central office in the World Trade Center in New York Cityand heavily damaged an adjacent exchange. The Verizon Building at 140 West Street was restored by 3500

workers at a cost of $1.2 billion,[21]after 200000 voice lines and three million data circuits had been knocked out

of operation.[22]

The central exchange, due to the system's design, is almost always a single point of failure for local calls. As thecapacity of individual switches and the optical fibre which interconnects them increases, potential disruption caused

by destruction of one local office will only be magnified. Multiple fibre connections can be used to provide
http://en.wikipedia.org/wiki/Optical_fibrehttp://en.wikipedia.org/wiki/Single_point_of_failurehttp://en.wikipedia.org/wiki/US_dollarhttp://en.wikipedia.org/wiki/Verizon_Buildinghttp://en.wikipedia.org/wiki/New_York_Cityhttp://en.wikipedia.org/wiki/World_Trade_Centerhttp://en.wikipedia.org/wiki/Terrorist_attacks_of_September_11,_2001http://en.wikipedia.org/wiki/Communications_satellitehttp://en.wikipedia.org/wiki/Iraqhttp://en.wikipedia.org/wiki/Invasion_of_Kuwaithttp://en.wikipedia.org/wiki/Kuwaithttp://en.wikipedia.org/wiki/Springfield,_Illinoishttp://en.wikipedia.org/wiki/Illinois_Bellhttp://en.wikipedia.org/wiki/Automatic_fire_suppressionhttp://en.wikipedia.org/wiki/O%27Hare_International_Airporthttp://en.wikipedia.org/wiki/Air_traffic_controlhttp://en.wikipedia.org/wiki/Federal_Aviation_Administrationhttp://en.wikipedia.org/wiki/Hinsdale,_Illinoishttp://en.wikipedia.org/wiki/Chicagohttp://en.wikipedia.org/wiki/Mebane,_North_Carolinahttp://en.wikipedia.org/wiki/Brooklynhttp://en.wikipedia.org/wiki/New_York_Telephone#Service_crisishttp://en.wikipedia.org/wiki/Main_distribution_framehttp://en.wikipedia.org/wiki/Manhattanhttp://en.wikipedia.org/wiki/Second_Avenue_(Manhattan)http://en.wikipedia.org/wiki/New_York_Telephonehttp://en.wikipedia.org/wiki/Radiotelephonehttp://en.wikipedia.org/wiki/Filling_stationhttp://en.wikipedia.org/wiki/Manhattan,_Kansashttp://en.wikipedia.org/wiki/Missourihttp://en.wikipedia.org/wiki/Kansashttp://en.wikipedia.org/wiki/Great_Flood_of_1951http://en.wikipedia.org/wiki/LIFO_(computing)http://en.wikipedia.org/wiki/FIFOhttp://en.wikipedia.org/wiki/File:East_13th_Telephone_Building,_Manhattan.jpg


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redundancyto voice and data connections between switching centres, but careful network design is required toavoid situations where a main fibre and its backup both go through the same damaged central office as a potential

common mode failure.[19]

Internet exchanges

The telephone exchange concept has been adapted for use in Internet exchange points. Voice over IP (VoIP)

traffic maypass through both kinds of exchanges, depending on what kind of service the caller and the calledsubscriberare using.

See also

History of telecommunication

Listof telephone switches

Pair gain system

Full Availability, Limited Availability and GradingsSoftswitch

Plesiochronous Digital Hierarchy

Telephone exchange names

Faraday Building - First telephone exchange in UK

References

1. ^"General Definitions" (http://www.verizonenterprise.com/external/service_guide/reg/g_general_definitions.htm)Verizon service. Verizon Enterprise Solutions.

2. ^Private Telegraphs (http://news.google.com/newspapers?

nid=1301&dat=18780419&id=ceVhAAAAIBAJ&sjid=dpEDAAAAIBAJ&pg=6902,636781), The Sydney Morning

Herald, credited to The Times, April 19, 1878, p. 6.

3. ^TIVADAR PUSKS (1844 - 1893) (http://wayback.archive.org/web/20110204071321/http://hungarian-

history.hu/mszh/epuskas.htm)

4. ^"SZTNH" (http://www.mszh.hu/English/feltalalok/puskas.html). Mszh.hu. Retrieved 2012-07-01.

5. ^"Pusks, Tivadar" (http://www.omikk.bme.hu/archivum/angol/htm/puskas_t.htm). Omikk.bme.hu. Retrieved

2012-07-01.6. ^"Welcome hunreal.com - BlueHost.com"

(http://wayback.archive.org/web/20120316155715/http://www.hunreal.com/known-hungarians/tivadar-puskas/).

Hunreal.com. Archived from the original (http://www.hunreal.com/known-hungarians/tivadar-puskas/) on 2012-

03-16. Retrieved 2012-07-01.

7. ^Frank Lewis Dyer: Edison His Life And Inventions. (page: 71)

8. ^120 Year Telephone anniversary (http://cdrecord.berlios.de/private/tel.html)

9. ^See National Park Service "first switchboard" page

(http://wayback.archive.org/web/20130302120900/http://www.cr.nps.gov/nhl/DOE_dedesignations/Telephone.h
http://wayback.archive.org/web/20130302120900/http://www.cr.nps.gov/nhl/DOE_dedesignations/Telephone.htmhttp://wayback.archive.org/web/20130302120900/http://www.cr.nps.gov/nhl/DOE_dedesignations/Telephone.htmhttp://cdrecord.berlios.de/private/tel.htmlhttp://www.hunreal.com/known-hungarians/tivadar-puskas/http://wayback.archive.org/web/20120316155715/http://www.hunreal.com/known-hungarians/tivadar-puskas/http://www.omikk.bme.hu/archivum/angol/htm/puskas_t.htmhttp://www.mszh.hu/English/feltalalok/puskas.htmlhttp://wayback.archive.org/web/20110204071321/http://hungarian-history.hu/mszh/epuskas.htmhttp://en.wikipedia.org/wiki/The_Timeshttp://en.wikipedia.org/wiki/The_Sydney_Morning_Heraldhttp://news.google.com/newspapers?nid=1301&dat=18780419&id=ceVhAAAAIBAJ&sjid=dpEDAAAAIBAJ&pg=6902,636781http://www.verizonenterprise.com/external/service_guide/reg/g_general_definitions.htmhttp://en.wikipedia.org/wiki/Faraday_Buildinghttp://en.wikipedia.org/wiki/Telephone_exchange_nameshttp://en.wikipedia.org/wiki/Plesiochronous_Digital_Hierarchyhttp://en.wikipedia.org/wiki/Softswitchhttp://en.wikipedia.org/wiki/Full_Availability,_Limited_Availability_and_Gradingshttp://en.wikipedia.org/wiki/Pair_gain_systemhttp://en.wikipedia.org/wiki/List_of_telephone_switcheshttp://en.wikipedia.org/wiki/History_of_telecommunicationhttp://en.wikipedia.org/wiki/Voice_over_IPhttp://en.wikipedia.org/wiki/Internet_exchange_pointhttp://en.wikipedia.org/wiki/Common_mode_failurehttp://wayback.archive.org/web/20130302120900/http://www.cr.nps.gov/nhl/DOE_dedesignations/Telephone.htm


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).

10. ^"Early Manchester telephone exchanges"

(http://www.mosi.org.uk/media/33871608/early%20manchester%20telephone%20exchanges.pdf). mosi.org.uk.

Retrieved 2013-07-30.

11. ^Francis S. Wagner: Hungarian Contributions to World Civilization - Page 68

12. ^ abCalvert, J. B. (2003-09-07). "Basic Telephones" (http://www.du.edu/~jcalvert/tech/phones.htm). Retrieved

2007-09-13.

13. ^Calvert, J. B. (2003-09-07). "Basic Telephones, The Switchboard (ringdown is near bottom)"

(http://www.du.edu/~jcalvert/tech/phones.htm). Retrieved 2006-09-13.

14. ^Source: from Federal Standard 1037C.

15. ^ abConnected to a switch, an off-hook condition operates a relay to connect the line to a dial tone generator and

a device to collect dialed digits.

16. ^AT&T Tech Channel (2011-06-17). "AT&T Archives : Flood Waters" (http://techchannel.att.com/play-

video.cfm/2011/6/17/AT&T-Archives-Flood-Waters). AT&T. Retrieved 2013-07-30.

17. ^"Miracle on Second Avenue: Reconnecting 170,000 Phone Customers in NYC After a Major Fire"

(http://techchannel.att.com/play-video.cfm/2011/3/9/AT&T-Archives-Miracle-on-Second-Avenue) (archive video0:22:40 including modern introduction). AT&T.

18. ^AT&T Tech Channel (2012-07-13). "AT&T Archives : The Town That Lost Its Voice"

(http://techchannel.att.com/play-video.cfm/2012/7/13/AT&T-Archives-The-Town-That-Lost-Its-Voice). AT&T.


19. ^ abAndrew Pollack (1988-05-26). "Phone System Feared Vulnerable To Wider Disruptions of Service"

(http://www.billhamel.net/uverse/Phone_System_Vulnerable_CO_fire_1988_Hinsdale.pdf). New York Times.


20. ^"Operation Desert Switch" (http://www.youtube.com/watch?v=8bNGi4SQuHo) (archive video, 0:17:04). AT&

1991.

21. ^Wall Street a year on: Annus horribilis(http://www.economist.com/node/1317930). The Economist. 2002-09-

05. Retrieved 2013-07-30.

22. ^"Bond trading resumes, stocks remain on hold" (http://news.google.com/newspapers?

id=9aE_AAAAIBAJ&sjid=81UMAAAAIBAJ&pg=6523,7832918). The Mount Airy News. Sep 14, 2001.

Ronayne, John P. (1986).Introduction to Digital Communications Switching(1st edition ed.).

Indianapolis: Howard W. Sams & Co., Inc. ISBN 0-672-22498-4.

External links

Hundreds of Telephone Central Office Pictures (http://www.flickr.com/groups/telephoneexchanges/)

Telephone Central Office History and Pictures (http://www.thecentraloffice.com/)

Telephone Central Office Building Pictures (historical preservation) (http://www.co-buildings.com/)

History of Central Offices (http://www.centralofficeonline.com/)

Clive Feather's guide to the BT network (http://www.davros.org/phones/btnetwork.html)
http://wayback.archive.org/web/20130302120900/http://www.cr.nps.gov/nhl/DOE_dedesignations/Telephone.htmhttp://wayback.archive.org/web/20130302120900/http://www.cr.nps.gov/nhl/DOE_dedesignations/Telephone.htmhttp://www.davros.org/phones/btnetwork.htmlhttp://www.centralofficeonline.com/http://www.co-buildings.com/http://www.thecentraloffice.com/http://www.flickr.com/groups/telephoneexchanges/http://en.wikipedia.org/wiki/Special:BookSources/0-672-22498-4http://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://news.google.com/newspapers?id=9aE_AAAAIBAJ&sjid=81UMAAAAIBAJ&pg=6523,7832918http://www.economist.com/node/1317930http://en.wikipedia.org/wiki/AT%26Thttp://www.youtube.com/watch?v=8bNGi4SQuHohttp://www.billhamel.net/uverse/Phone_System_Vulnerable_CO_fire_1988_Hinsdale.pdfhttp://en.wikipedia.org/wiki/AT%26Thttp://techchannel.att.com/play-video.cfm/2012/7/13/AT&T-Archives-The-Town-That-Lost-Its-Voicehttp://en.wikipedia.org/wiki/AT%26Thttp://techchannel.att.com/play-video.cfm/2011/3/9/AT&T-Archives-Miracle-on-Second-Avenuehttp://en.wikipedia.org/wiki/AT%26Thttp://techchannel.att.com/play-video.cfm/2011/6/17/AT&T-Archives-Flood-Watershttp://en.wikipedia.org/wiki/Federal_Standard_1037Chttp://www.du.edu/~jcalvert/tech/phones.htmhttp://www.du.edu/~jcalvert/tech/phones.htmhttp://www.mosi.org.uk/media/33871608/early%20manchester%20telephone%20exchanges.pdfhttp://wayback.archive.org/web/20130302120900/http://www.cr.nps.gov/nhl/DOE_dedesignations/Telephone.htm


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Basic Telephones Technology (http://www.du.edu/~jcalvert/tech/phones.htm)

Roger W. Haworth's guide to London (UK) Director Exchange Names

(http://www.rhaworth.myby.co.uk/phreak/tenp_01.htm)

National Park Service's page about the first telephone exchange

(http://wayback.archive.org/web/20130302120900/http://www.cr.nps.gov/nhl/DOE_dedesignations/Teleph

one.htm)

patent 252,576 for the first telephone switchboard in 1881 (http://www.google.com/patents?

id=xL9WAAAAEBAJ&dq=252576)

A Telecom Exchange Tour in NZ (http://www.adslgeek.com/dslforum/index.php?topic=207.0)

Picture collection Telephon and Exchange (http://www.bayern-online.com/v2261/kategorie.cfm?

DID=203&CATID=1471)

Retrieved from "http://en.wikipedia.org/w/index.php?title=Telephone_exchange&oldid=611562656"

Categories: Telephone exchanges Hungarian inventions

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http://www.wikimediafoundation.org/http://wikimediafoundation.org/wiki/Privacy_policyhttp://wikimediafoundation.org/wiki/Terms_of_Usehttp://en.wikipedia.org/wiki/Wikipedia:Text_of_Creative_Commons_Attribution-ShareAlike_3.0_Unported_Licensehttp://en.wikipedia.org/wiki/Help:Categoryhttp://en.wikipedia.org/w/index.php?title=Telephone_exchange&oldid=611562656http://www.bayern-online.com/v2261/kategorie.cfm?DID=203&CATID=1471http://www.adslgeek.com/dslforum/index.php?topic=207.0http://www.google.com/patents?id=xL9WAAAAEBAJ&dq=252576http://wayback.archive.org/web/20130302120900/http://www.cr.nps.gov/nhl/DOE_dedesignations/Telephone.htmhttp://www.rhaworth.myby.co.uk/phreak/tenp_01.htmhttp://www.du.edu/~jcalvert/tech/phones.htmhttp://en.wikipedia.org/wiki/Category:Hungarian_inventionshttp://en.wikipedia.org/wiki/Category:Telephone_exchanges

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