Telephone

TouchTonesinglelinebusinesstelephonewithmessagewaitinglamp

CopyoftheoriginalphoneofGrahamBellattheMuséedesArtsetMétiersinParis

1896Telephone(Sweden)

Thetelephoneisatelecommunicationsdevicewhichisusedtotransmitandreceivesound(mostcommonlyspeech).Mosttelephonesoperatethroughtransmissionofelectricsignalsoveracomplextelephonenetworkwhichallowsalmostanyphoneusertocommunicatewithalmostanyone.

Basicprinciple

Thetelephonehandlestwotypesofinformation:signalsandvoice,atdifferenttimesonthesametwistedpairofwires.Thesignalingequipmentconsistsofabelltoalerttheuserofincomingcalls,andadialtoenterthephonenumberforoutgoingcalls.Acallingpartywishingtospeaktoanothertelephonewillpickupthehandset,thusoperatingtheswitchhook,whichputsthetelephoneintoactivestateoroffhookwitharesistanceshortacrossthewires,causingcurrenttoflow.ThetelephoneexchangedetectstheDCcurrent,attachesadigitreceiver,andsendsdialtonetoindicatereadiness.Theuserpushesthenumberbuttons,whichareconnectedtoatonegeneratorinsidethedial,whichgeneratesDTMFtones.Theexchangeconnectsthelinetothedesiredlineandalertsthatline.

Whenaphoneisinactive(onhook),itsbell,beeper,flasherorotheralertingdeviceisconnectedacrossthelinethroughacapacitor.Theinactivephonedoesnotshorttheline,thustheexchangeknowsitisonhookandonlythebelliselectricallyconnected.Whensomeonecallsthisphone,thetelephoneexchangeappliesahighvoltagepulsatingsignal,whichcausesthesoundmechanismtoring,beeporotherwisealertthecalledparty.Whenthatuserpicksupthehandset,theswitchhookdisconnectsthebell,connectsthevoicepartsofthetelephone,andputsaresistanceshortontheline,confirmingthatthephonehasbeenansweredandisactive.Bothlinesbeingoffhook,thesignalingjobiscomplete.Thepartiesareconnectedtogetherandmayconverseusingthevoicepartsoftheirtelephones.

Thevoicepartsofthetelephoneareinthehandset,andconsistofatransmitter(oftencalledmicrophone)andareceiver.Thetransmitter,poweredfromtheline,putsoutanelectriccurrentwhichvariesinresponsetotheacousticpressurewavesproducedbythevoice.Theresultingvariationsinelectriccurrentaretransmittedalongthetelephonelinetotheotherphone,wheretheyarefedintothecoilofthereceiver,whichisaminiatureloudspeaker.Thevaryingelectriccurrentinthecoilcausesittomovebackandforth,reproducingtheacousticpressurewavesofthetransmitter.

Whenaparty"hangsup"(putsthehandsetonthecradle),DCcurrentceasestoflowinthatline,thussignalingtotheexchangeswitchtodisconnectthetelephonecall.

History

Creditforinventingtheelectrictelephoneremainsindispute.AntonioMeucci,JohannPhilippReis,AlexanderGrahamBell,andElishaGray,amongothers,haveallbeencreditedwiththeinvention.

Theearlyhistoryofthetelephoneisaconfusingmorassofclaimandcounterclaim,whichwasnotclarifiedbythehugemassoflawsuitswhichhopedtoresolvethepatentclaimsofindividuals.TheBellandEdisonpatents,however,wereforensicallyvictoriousandcommerciallydecisive.

Earlydevelopment

Thefollowingisabriefsummaryofthedevelopmentofthetelephone:

28December1871—AntonioMeuccifilesapatentcaveatinthe.PatentOfficetitled"SoundTelegraph",describingcommunicationofvoicebetweentwopeoplebywire.

1874—Meucci,afterhavingrenewedthecaveatfortwoyears,failstofindthemoneytorenewit.Thecaveatlapses.

6April1875—Bell's.Patent161,739"TransmittersandReceiversforElectricTelegraphs"isgranted.Thisusesmultiplevibratingsteelreedsinmake-breakcircuits.

11February1876—Grayinventsaliquidtransmitterforusewithatelephonebutdoesnotbuildone.

14February1876—ElishaGrayfilesapatentcaveatfortransmittingthehumanvoicethroughatelegraphiccircuit.

14February1876—AlexanderBellappliesforthepatent"ImprovementsinTelegraphy",forelectromagnetictelephonesusingundulatingcurrents.

19February1876—Grayisnotifiedbythe.PatentOfficeofaninterferencebetweenhiscaveatandBell'spatentapplication.Graydecidestoabandonhiscaveat.

7March1876—Bell's.patent174,465"ImprovementinTelegraphy"isgranted,covering"themethodof,andapparatusfor,transmittingvocalorothersoundstelegraphically…bycausingelectricalundulations,similarinformtothevibrationsoftheairaccompanyingthesaidvocalorothersound."

30January1877—Bell'sU.S.patent186,787isgrantedforanelectromagnetictelephoneusingpermanentmagnets,irondiaphragms,andacallbell.

27April1877—Edisonfilesforapatentonacarbon(graphite)transmitter.Thepatent474,230wasgranted3May1892,aftera15yeardelaybecauseoflitigation.Edisonwasgrantedpatent222,390foracarbongranulestransmitterin1879.

Earlycommercialinstruments

Earlytelephonesweretechnicallydiverse.Someusedaliquidtransmitter,somehadametaldiaphragmthatinducedcurrentinanelectromagnetwoundaroundapermanentmagnet,andsomewere"dynamic"-theirdiaphragmvibratedacoilofwireinthefieldofapermanentmagnetorthecoilvibratedthediaphragm.Thisdynamickindsurvivedinsmallnumbersthroughthe20thcenturyinmilitaryandmaritimeapplicationswhereitsabilitytocreateitsownelectricalpowerwascrucial.Most,however,usedtheEdison/Berlinercarbontransmitter,whichwasmuchlouderthantheotherkinds,eventhoughitrequiredaninductioncoil,actuallyactingasanimpedancematchingtransformertomakeitcompatibletotheimpedanceoftheline.TheEdisonpatentskepttheBellmonopolyviableintothe20thcentury,bywhichtimethenetworkwasmoreimportantthantheinstrument.

Earlytelephoneswerelocallypowered,usingadynamictransmitterorelsepoweringthetransmitterwithalocalbattery.Oneofthejobsofoutsideplantpersonnelwastovisiteachtelephoneperiodicallytoinspectthebattery.Duringthe20thcentury,"commonbattery"operationcametodominate,poweredby"talkbattery"fromthetelephoneexchangeoverthesamewiresthatcarriedthevoicesignals.Lateinthecentury,wirelesshandsetsbroughtarevivaloflocalbatterypower.

Earlytelephoneshadonewireforbothtransmittingandreceivingofaudio,withgroundreturnasusedintelegraphs.Theearliestdynamictelephonesalsohadonlyoneopeningforsound,andtheuseralternatelylistenedandspoke(rather,shouted)intothesamehole.Sometimestheinstrumentswereoperatedinpairsateachend,makingconversationmoreconvenientbutweremoreexpensive.

Atfirst,thebenefitsofanexchangewerenotexploited.Telephonesinsteadwereleasedinpairstothesubscriber,whohadtoarrangetelegraphcontractorstoconstructalinebetweenthem,forexamplebetweenhishomeandhisshop.Userswhowantedtheabilitytospeaktoseveraldifferentlocationswouldneedtoobtainandsetupthreeorfourpairsoftelephones.WesternUnion,alreadyusingtelegraphexchanges,quicklyextendedtheprincipletoitstelephonesinNewYorkCityandSanFrancisco,andBellwasnotslowinappreciatingthepotential.

Signallingbeganinanappropriatelyprimitivemanner.Theuseralertedtheotherend,ortheexchangeoperator,bywhistlingintothetransmitter.Exchangeoperationsoonresultedintelephonesbeingequippedwithabell,firstoperatedoverasecondwireandlaterwiththesamewireusingacondenser.TelephonesconnectedtotheearliestStrowgerautomaticexchangeshadsevenwires,onefortheknifeswitch,oneforeachtelegraphkey,oneforthebell,oneforthepushbuttonandtwoforspeaking.

Ruralandothertelephonesthatwerenotonacommonbatteryexchangehada"magneto"orhandcrankedgeneratortoproduceahighvoltagealternatingsignaltoringthebellsofothertelephonesonthelineandtoalerttheoperator.

Inthe1890sanewsmallerstyleoftelephonewasintroduced,packagedinthreeparts.Thetransmitterstoodonastand,knownasa"candlestick"foritsshape.Whennotinuse,thereceiverhungonahookwithaswitchinit,knownasa"switchhook."Previoustelephonesrequiredtheusertooperateaseparateswitchtoconnecteitherthevoiceorthebell.Withthenewkind,theuserwaslesslikelytoleavethephone"offthehook".Inphonesconnectedtomagnetoexchanges,thebell,inductioncoil,batteryandmagnetowereinaseparate"bellbox."Inphonesconnectedtocommonbatteryexchanges,thebellboxwasinstalledunderadesk,orotheroutofthewayplace,sinceitdidnotneedabatteryormagneto.

Cradledesignswerealsousedatthistime,havingahandlewiththereceiverandtransmitterattached,separatefromthecradlebasethathousedthemagnetocrankandotherparts.Theywerelargerthanthe"candlestick"andmorepopular.

DisadvantagesofsinglewireoperationsuchascrosstalkandhumfromnearbyACpowerwireshadalreadyledtotheuseoftwistedpairsand,forlongdistancetelephones,four-wirecircuits.Usersatthebeginningofthe20thcenturydidnotplacelongdistancecallsfromtheirowntelephonesbutmadeanappointmenttouseaspecialsoundproofedlongdistancetelephoneboothfurnishedwiththelatesttechnology.

Whatturnedouttobethemostpopularandlongestlastingphysicalstyleoftelephonewasintroducedintheearly20thcentury,includingBell'sModel102.Acarbongranuletransmitterandelectromagneticreceiverwereunitedinasinglemoldedplastichandle,whichwhennotinusesatinacradleinthebaseunit.ThecircuitdiagramoftheModel102showsthedirectconnectionofthereceivertotheline,whilethetransmitterwasinductioncoupled,withenergysuppliedbyalocalbattery.Thecouplingtransformer,battery,andringerwereinaseparateenclosure.Thedialswitchinthebaseinterruptedthelinecurrentbyrepeatedlybutverybrieflydisconnectingtheline1-10timesforeachdigit,andthehookswitch(inthecenterofthecircuitdiagram)permanentlydisconnectedthelineandthetransmitterbatterywhilethehandsetwasonthecradle.

Afterthe1930s,thebasealsoenclosedthebellandinductioncoil,obviatingtheoldseparatebellbox.Powerwassuppliedtoeachsubscriberlinebycentralofficebatteriesinsteadofalocalbattery,whichrequiredperiodicservice.Forthenexthalfcentury,thenetworkbehindthetelephonebecameprogressivelylargerandmuchmoreefficient,butafterthedialwasaddedtheinstrumentitselfchangedlittleuntiltouchtonereplacedthedialinthe1960s.

Digitaltelephony

ThePublicSwitchedTelephoneNetwork(PSTN)hasgraduallyevolvedtowardsdigitaltelephonywhichhasimprovedthecapacityandqualityofthenetwork.End-to-endanalogtelephonenetworkswerefirstmodifiedintheearly1960sbyupgradingtransmissionnetworkswithT1carriersystems.LatertechnologiessuchasSONETandfiberoptictransmissionmethodsfurtheradvanceddigitaltransmission.Althoughanalogcarriersystemsexisted,digitaltransmissionmadeitpossibletosignificantlyincreasethenumberofchannelsmultiplexedonasingletransmissionmedium.Whiletodaytheendinstrumentremainsanalog,theanalogsignalsreachingtheaggregationpoint(ServingAreaInterface(SAI)orthecentraloffice(CO))aretypicallyconvertedtodigitalsignals.Digitalloopcarriers(DLC)areoftenused,placingthedigitalnetworkeverclosertothecustomerpremises,relegatingtheanaloglocallooptolegacystatus.

IPtelephony

AWiFi-basedVoIPphone

InternetProtocol(IP)telephony(alsoknownasInternettelephony)isaservicebasedonVoiceoverIP(VoIP),adisruptivetechnologythatisrapidlygaininggroundagainsttraditionaltelephonenetworktechnologies.InJapanandSouthKoreaupto10%ofsubscribers,asofJanuary2005,haveswitchedtothisdigitaltelephoneservice.AJanuary2005NewsweekarticlesuggestedthatInternettelephonymaybe"thenextbigthing."

Asof2006manyVoIPcompaniesofferservicetoconsumersandbusinesses.

IPtelephonyusesabroadbandInternetconnectiontotransmitconversationsasdatapackets.InadditiontoreplacingPOTS(plainoldtelephoneservice),IPtelephonyisalsocompetingwithmobilephonenetworksbyofferingfreeorlowercostconnectionsviaWiFihotspots.VoIPisalsousedonprivatewirelessnetworkswhichmayormaynothaveaconnectiontotheoutsidetelephonenetwork.

Usage

Bytheendof2006,therewereatotalofn