通信工程论文.docx

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