通信工程论文.docx
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通信工程论文.docx
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通信工程论文
Telecommunicationisthetransmissionofsignalsoveradistanceforthepurposeofcommunication.Inearliertimes,thismayhaveinvolvedtheuseofsmokesignals,drums,semaphore,flagsorheliograph.Inmoderntimes,telecommunicationtypicallyinvolvestheuseofelectronicdevicessuchastelephones,television,radioorcomputers.EarlyinventorsinthefieldoftelecommunicationincludeAlexanderGrahamBell,GuglielmoMarconiandJohnLogieBaird.Telecommunicationisanimportantpartoftheworldeconomyandthetelecommunicationindustry'srevenuewasestimatedtobe$3.85trillionin2008.[1]
Contents
[hide]
∙1History
o1.1Earlytelecommunications
o1.2Telegraphandtelephone
o1.3Radioandtelevision
o1.4ComputernetworksandtheInternet
∙2Keyconcepts
o2.1Basicelements
o2.2Analogueordigital
o2.3Networks
o2.4Channels
o2.5Modulation
∙3Societyandtelecommunication
o3.1Economicimpact
▪3.1.1Microeconomics
▪3.1.2Macroeconomics
o3.2Socialimpact
o3.3Otherimpacts
∙4Telecommunicationandgovernment
∙5Modernoperation
o5.1Telephone
o5.2Radioandtelevision
o5.3TheInternet
o5.4Localareanetworks
∙6Telecommunicationbyregion
∙7Seealso
∙8References
∙9Furtherreading
∙10Externallinks
[edit]History
Formoredetailsonthistopic,seeHistoryoftelecommunication.
[edit]Earlytelecommunications
AreplicaofoneofChappe'ssemaphoretowersinNalbach
IntheMiddleAges,chainsofbeaconswerecommonlyusedonhilltopsasameansofrelayingasignal.Beaconchainssufferedthedrawbackthattheycouldonlypassasinglebitofinformation,sothemeaningofthemessagesuchas"theenemyhasbeensighted"hadtobeagreeduponinadvance.OnenotableinstanceoftheirusewasduringtheSpanishArmada,whenabeaconchainrelayedasignalfromPlymouthtoLondonsignallingthearrivalofSpanishships.[2]
In1792,ClaudeChappe,aFrenchengineer,builtthefirstfixedvisualtelegraphysystem(orsemaphoreline)betweenLilleandParis.[3]Howeversemaphoresufferedfromtheneedforskilledoperatorsandexpensivetowersatintervalsoftentothirtykilometres(sixtonineteenmiles).Asaresultofcompetitionfromtheelectricaltelegraph,thelastcommerciallinewasabandonedin1880.[4]
[edit]Telegraphandtelephone
ThefirstcommercialelectricaltelegraphwasconstructedbySirCharlesWheatstoneandSirWilliamFothergillCookeandopenedon9April1839.BothWheatstoneandCookeviewedtheirdeviceas"animprovementtothe[existing]electromagnetictelegraph"notasanewdevice.[5]
SamuelMorseindependentlydevelopedaversionoftheelectricaltelegraphthatheunsuccessfullydemonstratedon2September1837.HiscodewasanimportantadvanceoverWheatstone'ssignalingmethod.Thefirsttransatlantictelegraphcablewassuccessfullycompletedon27July1866,allowingtransatlantictelecommunicationforthefirsttime.[6]
TheconventionaltelephonenowusedworldwidewasfirstpatentedbyAlexanderGrahamBellinMarch1876.[7]ThatfirstpatentbyBellwasthemasterpatentofthetelephone,fromwhichallotherpatentsforelectrictelephonedevicesandfeaturesflowed.Creditfortheinventionoftheelectrictelephonehasbeenfrequentlydisputed,andnewcontroversiesovertheissuehavearisenfromtime-to-time.Aswithothergreatinventionssuchasradio,television,lightbulb,andcomputer,therewereseveralinventorswhodidpioneeringexperimentalworkonvoicetransmissionoverawireandimprovedoneachother'sideas.
Thefirstcommercialtelephoneservicesweresetupin1878and1879onbothsidesoftheAtlanticinthecitiesofNewHavenandLondon.[8][9]
[edit]Radioandtelevision
In1832,JamesLindsaygaveaclassroomdemonstrationofwirelesstelegraphytohisstudents.By1854,hewasabletodemonstrateatransmissionacrosstheFirthofTayfromDundee,ScotlandtoWoodhaven,adistanceoftwomiles(3 km),usingwaterasthetransmissionmedium.[10]InDecember1901,GuglielmoMarconiestablishedwirelesscommunicationbetweenSt.John's,Newfoundland(Canada)andPoldhu,Cornwall(England),earninghimthe1909NobelPrizeinphysics(whichhesharedwithKarlBraun).[11]Howeversmall-scaleradiocommunicationhadalreadybeendemonstratedin1893byNikolaTeslainapresentationtotheNationalElectricLightAssociation.[12]
On25March1925,JohnLogieBairdwasabletodemonstratethetransmissionofmovingpicturesattheLondondepartmentstoreSelfridges.Baird'sdevicereliedupontheNipkowdiskandthusbecameknownasthemechanicaltelevision.ItformedthebasisofexperimentalbroadcastsdonebytheBritishBroadcastingCorporationbeginning30September1929.[13]However,formostofthetwentiethcenturytelevisionsdependeduponthecathoderaytubeinventedbyKarlBraun.ThefirstversionofsuchatelevisiontoshowpromisewasproducedbyPhiloFarnsworthanddemonstratedtohisfamilyon7September1927.[14]
[edit]ComputernetworksandtheInternet
On11September1940,GeorgeStibitzwasabletotransmitproblemsusingteletypetohisComplexNumberCalculatorinNewYorkandreceivethecomputedresultsbackatDartmouthCollegeinNewHampshire.[15]Thisconfigurationofacentralizedcomputerormainframewithremotedumbterminalsremainedpopularthroughoutthe1950s.However,itwasnotuntilthe1960sthatresearchersstartedtoinvestigatepacketswitching—atechnologythatwouldallowchunksofdatatobesenttodifferentcomputerswithoutfirstpassingthroughacentralizedmainframe.Afour-nodenetworkemergedon5December1969;thisnetworkwouldbecomeARPANET,whichby1981wouldconsistof213nodes.[16]
ARPANET'sdevelopmentcentredaroundtheRequestforCommentprocessandon7April1969,RFC1waspublished.ThisprocessisimportantbecauseARPANETwouldeventuallymergewithothernetworkstoformtheInternetandmanyoftheprotocolstheInternetreliesupontodaywerespecifiedthroughtheRequestforCommentprocess.InSeptember1981,RFC791introducedtheInternetProtocolv4(IPv4)andRFC793introducedtheTransmissionControlProtocol(TCP)—thuscreatingtheTCP/IPprotocolthatmuchoftheInternetreliesupontoday.
However,notallimportantdevelopmentsweremadethroughtheRequestforCommentprocess.Twopopularlinkprotocolsforlocalareanetworks(LANs)alsoappearedinthe1970s.ApatentforthetokenringprotocolwasfiledbyOlofSoderblomon29October1974andapaperontheEthernetprotocolwaspublishedbyRobertMetcalfeandDavidBoggsintheJuly1976issueofCommunicationsoftheACM.[17][18]
[edit]Keyconcepts
Etymology
ThewordtelecommunicationwasadaptedfromtheFrenchwordtélécommunication.ItisacompoundoftheGreekprefixtele-(τηλε-),meaning'faroff',andtheLatincommunicare,meaning'toshare'.[19]TheFrenchwordtélécommunicationwascoinedin1904byFrenchengineerandnovelistÉdouardEstaunié.[20]
Anumberofkeyconceptsreoccurthroughouttheliteratureonmoderntelecommunicationsystems.Someoftheseconceptsarediscussedbelow.
[edit]Basicelements
Abasictelecommunicationsystemconsistsofthreeelements:
∙atransmitterthattakesinformationandconvertsittoasignal;
∙atransmissionmediumthatcarriesthesignal;and,
∙areceiverthatreceivesthesignalandconvertsitbackintousableinformation.
Forexample,inaradiobroadcastthebroadcasttoweristhetransmitter,freespaceisthetransmissionmediumandtheradioisthereceiver.Oftentelecommunicationsystemsaretwo-waywithasingledeviceactingasbothatransmitterandreceiverortransceiver.Forexample,amobilephoneisatransceiver.[21]
Telecommunicationoveratelephonelineiscalledpoint-to-pointcommunicationbecauseitisbetweenonetransmitterandonereceiver.Telecommunicationthroughradiobroadcastsiscalledbroadcastcommunicationbecauseitisbetweenonepowerfultransmitterandnumerousreceivers.[21]
[edit]Analogueordigital
Signalscanbeeitheranalogueordigital.Inananaloguesignal,thesignalisvariedcontinuouslywithrespecttotheinformation.Inadigitalsignal,theinformationisencodedasasetofdiscretevalues(forexampleonesandzeros).Duringtransmissiontheinformationcontainedinanaloguesignalswillbedegradedbynoise.Conversely,unlessthenoiseexceedsacertainthreshold,theinformationcontainedindigitalsignalswillremainintact.Noiseresistancerepresentsakeyadvantageofdigitalsignalsoveranaloguesignals.[22]
[edit]Networks
Anetworkisacollectionoftransmitters,receiversandtransceiversthatcommunicatewitheachother.Digitalnetworksconsistofoneormoreroutersthatworktogethertotransmitinformationtothecorrectuser.Ananaloguenetworkconsistsofoneormoreswitchesthatestablishaconnectionbetweentwoormoreusers.Forbothtypesofnetwork,repeatersmaybenecessarytoamplifyorrecreatethesignalwhenitisbeingtransmittedoverlongdistances.Thisistocombatattenuationthatcanrenderthesignalindistinguishablefromnoise.[23]
[edit]Channels
Achannelisadivisioninatransmissionmediumsothatitcanbeusedtosendmultiplestreamsofinformation.Forexample,aradiostationmaybroadcastat96.1 MHzwhileanotherradiostationmaybroadcastat94.5 MHz.Inthiscase,themediumhasbeendividedbyfrequencyandeachchannelhasreceivedaseparatefrequencytobroadcaston.Alternatively,onecouldallocateeachchannelarecurringsegmentoftimeoverwhichtobroadcast—thisisknownastime-divisionmultiplexingandisusedinopticfibrecommunication.[24][23]
[edit]Modulation
Theshapingofasignaltoconveyinformationisknownasmodulation.Modula
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