Telephone exchange
In North America, the term wire center may be used to denote a central office location, indicating a facility that provides a telephone with a dial tone.[1] Telecommunication carriers also define rate centers for business and billing purposes, which in large cities, might encompass clusters of central offices to specify geographic locations for distance measurement calculations.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 or key telephone system managed by a receptionist, catering to the telecommunication needs of the enterprise.Exchanges made telephony an available and comfortable technology for everyday use and it gave the impetus for the creation of a new industrial sector.[9] The world's first state-administered telephone exchange opened on November 12, 1877 in Friedrichsberg close to Berlin under the direction of Heinrich von Stephan.Each operator sat in front of a vertical panel containing banks of ¼-inch tip-ring-sleeve (3-conductor) jacks, each of which was the local termination of a subscriber's telephone line.When a calling party lifted the receiver, the local loop current lit a signal lamp near the jack.With manual service, the customer lifts the receiver off-hook and asks the operator to connect the call to a requested number.[19] 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.The switchboard responded by interrupting the circuit, which dropped a metal tab above the subscriber's line jack and sounded a buzzer.Using the faster pulsing rate made trunk utilization more efficient because the switch spent half as long listening to digits.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.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 pure digital carrier which did not have DC continuity.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.This was effected by plugging a tool into the subscriber's office equipment on Crossbar systems or line group in step-by-step switches.Equipment testing, phone numbers reassignments, circuit lockouts and similar tasks were accomplished by data entry on a terminal.These used a crossbar switching matrix with a fully computerized control system and provided a wide range of advanced services.[citation needed] The first digital switch to be placed into a public network in England was the Empress Exchange in London which was designed by the General Post Office research labs.A major part, in terms of expense, maintenance, and logistics of the telephone system is outside plant, which is the wiring outside the central office.A typical central office may have tens of thousands of pairs of wires that appear on terminal blocks called the main distribution frame (MDF).In a typical telephone company, a large database tracks information about each subscriber pair and the status of each jumper.To reduce the expense of outside plant, some companies use "pair gain" devices to provide telephone service to subscribers.Switching, transmission and billing equipment may be slaved to very high accuracy 10 MHz standards which synchronize time events to very close intervals.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.To prevent frustration with unsensed failures, all the connections between layers in the switch are allocated using first-in-first-out lists (queues).As the capacity of individual switches and the optical fibre which interconnects them increases, potential disruption caused by destruction of one local office will only be magnified.Multiple fibre connections can be used to provide redundancy to voice and data connections between switching centres, but careful network design is required to avoid situations where a main fibre and its backup both go through the same damaged central office as a potential common mode failure.
telephone operatorbroadbandtelecommunicationspublic switched telephone networktelephone callsBell Systeminside plantdial tonenationwide numbering systemnumbering plan areaprivate branch exchangekey telephone systemTivadar Puskásschemainvention of the telephoneHungarianThomas EdisonBell Telephone CompanyBostonBerlinHeinrich von StephanGeorge W. CoyNew Haven, ConnecticutBridgeportCharles GliddenLondonManchesterInternational BellAntwerpmultiplexswitchboardplug boardsswitchboard operatorstip-ring-sleevesubscribertelephone linecord circuitcalling partyringdownAlmon Brown StrowgerKansas City, Missouristepping switchtelephone dialother exchange typescrossbarconcentratoroff-hookon-hooktelecommunicationcommon carrierswitching centerClass 5 telephone switchlocal loopsTelephone switchboardoperatorcommon-batteryrotary dialspanel switch
2L-4N or 2L-5NindicatormagnetoDry cellWoodstock, Maineparty linesringing signalAlmon Strowgertelephone calltelephoneautomatic number identificationtoll-free 800-numbersrelaysStrowger switchRotary systemcrossbar switchSignaling (telecommunications)Signalling System 7Panel Call Indicatorquaternarydial pulsesrotary dialMulti-frequency (MF)telephone numberE and M signalingfalsingfrequency-division multiplexmicrowave radio relaysT-carrierpermanent signalElectronic switching systemsstored program controlreed relayWestern Electric1ESS switchBritish GPO/BTCommon Channel Interoffice Signaling, (CCIS)Bell LabsGeneral Post OfficeStrowgerAlcatelBrittanyEricssonAXE telephone exchangecrossbar switchesAlcatel-Lucent1000-S12all-IP networksITT System 12Nokia Siemens NetworksSiemensNortelGenbandDMS100MarconiSystem XBT GroupDMS-100Francesampling ratedigital-to-analog converterlocal loopremote concentratortandem switchesdigital loop carrieroutside plantmain distribution framepair gain