電子信息類專業(yè)英語(第三版)李白萍章-(15)課件

Unit15NetworkPassageAComputerNetworks(1)PassageBComputerNetworks(2)PassageCNetworkUnit15NetworkPassageAComput

PassageAComputerNetworks(1)

Thefieldofcommunicationsisexperiencingrapidanddramaticchange.Traditionally,telecommunicationshasbeenthetransmissionofinformationoverlongdistancesbymeansoftelephone,teletype,radio,ortelevision.Datacommunicationshasbeentheexchangeofdatabetweencomputer-relateddevices.Today,thesedistinctionshaveblurredasthetwohavebeenconvergingintonetworks-agroupofcomputersandotherdevicesconnectedbycommunicationfacilities-inwhichinformationisprocessedandcommunicatedintheformofsound,data,andimagesovercommonmedia.

PassageAComputerNetworks

Whatdoesthischangemeanforsociety?Justasduringthelate1800sandearly1900sglobalsocietieswereshapedbysuchtechnologiesasships,electricity,andtheinternalcombustionengine,sointhelate1900sandearly2000stheywillbeshapedbyconvergingcomputerandcommunicationtechnologies.Theincreasedcapabilitiesofthenetworksallowproviderstoofferawidervarietyofcommunicationservices(e.g.,video,facsimile,data,andmultimediaservices)thanthetraditionalvoiceservices.Howweobtainnewsandentertainment,conductbusinessandeducation,andaccomplishscientificandtechnicalresearchwillallbeaffected.［1］

Whatdoesthischangemean

Networkscanbeanalog,inwhichinformationisrepresentedbycontinuoussmoothlyvaryingsignals,ordigital,inwhichinformationisrepresentedbydiscreteon/offsignals.[2]

Digitalsignalsareanewertechnologyfoundincomputersystems,localareanetworks,compactdisks,andhighdefinitiontelevision(HDTV).Digitalsignalsarediscreteon/offsignalsthatcanbeeasilystored,compressed,edited,andmanipulated.Thetelephoneindustryisintheprocessofconvertingtoanall-digitalnetworkcalledIntegratedServicesDigitalNetwork(ISDN)andsimilarupgradesareproposedforcellulartelephonenetworks.

Networkscanbeanalog,in

Therearethreemajorcategoriesofnetworksdistinguishedprimarilybygeography:wideareanetworks,metropolitanareanetworks,andlocal-areanetworks.

Therearethreemajorcateg

1.WideareaNetworks

Wideareanetworks(WANs)providecommunicationsacrossacountryortheentireworld.Thetelephonenetworkisthemostpervasiveexample.Itisoftenviewedasasingleentity,eventhoughitismadeupofavarietyofseparatecompanies.

1.WideareaNetworks

W

Whereasthephonesystemcreatesavoiceconnectionbetweenyourphoneandanyotherphonesintheworld,theInternet-aworldwidenetworkofcomputernetworks-makesitpossibleforyoutoconnectyourcomputer(equippedwiththeappropriatehardwareandsoftware)toawidevarietyofremotecomputer.Theremotecomputersmaybethousandsofmilesaway,yetyoucansearchlibrariesofsoftware,accessnewsandotherinformation,conductbusiness,andmeetpeoplewithinterestsorhobbiessimilartoyourownaroundtheglobe.

Whereasthephonesystemcr

TheInternetisn’ttheonlyworldwidecomputernetwork.Internationalcompanieshaveprivatenetworkstolinkofficesindifferentcountriestogetherelectronicallytoform24-hour-a-daybusinessoperations.CommercialpublicnetworkslikeCompuServe,Prodigy,andAmericaOnlinearealsousedbypeoplearoundtheworld.Asthesenetworkshavegrowninpopularity,theyhavebeguntointerconnect,creatingaglobalcollectionofpeopleandcomputersthatareabletoelectronicallycommunicatewithoneanother.［3］

TheInternetisn’ttheonly

2.MetropolitanareaNetworks

MetropolitanareaNetworks(MANs)providecommunicationsacrossandamongmajormetropolitanareassuchasNewYork,Chicago,orLosAngeles.Public-safety

agenciessuchaspoliceandfiredepartments,anddispatch-orientedcompanies,likeFederalExpress,operateandmaintaintheirownprivatenetworks.TheyuseMANstoallowtheirmobile“out-of-office”workerstotalktoandpassdatabackandforthtooneanother.

2.MetropolitanareaNetwo

Amorerecentdevelopmentisthepublicvoiceanddatanetworks.Anexampleyoumaybefamiliarwithisthecellulartelephonenetwork,whichismadepossiblebydividingageographicalareaintohexagonal-shapedcells,eachwithabasestation.［4］

Eachcell,whichisapproximately10miles(16kilometers)across,hasitsownantenna.Likethepublictelephonenetwork,publicMANseliminatetheneedforindividualsandorganizationstobuildandoperatetheirownnetworks.

Amorerecentdevelopmenti

3.LocalareaNetworks

LocalareaNetworks(LANs)providecommunicationswithinspecificbuildingsorfacilities.Thistypeofnetworkcouldbeused,forexample,inacompanywherecomputersarelocatedinthesales,accounting,production,andpurchasingdepartments.Eachcomputerwouldhandletheapplicationsuniquetoitsdepartment.Whencommunicationamongthecomputersisrequired,thecomputerspassdatatooneanotheroverthenetwork.

3.LocalareaNetworks

L

Forexample,purchasinginformationmustbepassedtotheaccountingdepartmentforinclusionintheaccountspayablesystem,orproduction-schedulinginformationmustbepassedtothepurchasingdepartmentforusebythepurchasingandinventorysystems.

Local-areanetworkshavemanydifferenttopologies,orwaysinwhichcomputercanbeconnectedtothenetwork.Today,thetwomostcommontopologiesarecalledringandbusnetworks.

Forexample,purchasinginfor

(1)RingNetworks.Computersonaringnetworkarehookedtogetheroneaftertheothertoformonecontinuousring.Toaccesseachcomputer,apieceofsoftwarecalledatokenispassedaroundthering.Atokenistheelectronicequivalentofanenvelope.Itcontainsadestinationaddressandafixedamountofinformation.Themostpopulartokenmethodiscalledtokenpassing,inwhichonetokenatatimepassesfromcomputertocomputerandcarriesmessagesaroundthenetwork.

(1)RingNetworks.Computer

(2)BusNetworks.Onabusnetworkcomputersaresimplyconnectedbyacableroutedalongapathinthevicinityofeachdevicethatmustbeconnectedtoit.ThemostpopularbusnetworkiscalledEthernet.Busnetworksallowmultiplemessagestobesentsimultaneously.Thisisoftenaccomplishedby“l(fā)istening”tothenetworktoseeifanycomputeristransmitting.Ifthenetworkisfree,thecomputercansenditsdata.Iftwocomputersaccidentallysenddataatthesametime,acollisionoccurs,andafterdetectingthecollision,thecomputerswaitarandomamountoftimetotransmitagain.ThisaccesscontrolmethodgoesbythenameofCarrierSenseMultipleAccesswithCollisionDetection(CSMA/CD).

(2)BusNetworks.Onabus

Thethreemajorcategoriesofnetworksarenotmutuallyexclusive.Amobileworkerusingaportablecomputercansendamessagetoacoworkerinacrossthecountryofficebecausethenetworkscanbelinkedthroughgateways-devicesusedtoconnectdissimilarnetworks.Forexample,themobileworker’smessagemightfirstbetransmittedtoalocalcellularbasestation,switchedtothetelephonenetworkandcarriedacrossthecountrytothecoworker’soffice,thentransferredviagatewayintotheofficeLAN.

Thethreemajorcategories

Notes

［1］Howweobtainnewsandentertainment,conductbusinessandeducation,andaccomplishscientificandtechnicalresearchwillallbeaffected.

我們獲得新聞和娛樂、處理事務(wù)和進(jìn)行教育以及完成科學(xué)研究的方式將全部受到影響。

·how引導(dǎo)的是主語從句。

·主語從句中的謂語是三個(gè)并列成分。

Notes

［1］Howweobtainne

［2］Networkscanbeanalog,inwhichinformationisrepresentedbycontinuoussmoothlyvaryingsignals,ordigital,inwhichinformationisrepresentedbydiscreteon/offsignals.

網(wǎng)絡(luò)可以是模擬的,其中信息以連續(xù)平穩(wěn)變化的信號表示;也可以是數(shù)字的,其中信息以離散的有/無信號表示。

·主句有并列表語analog和ordigital。

·analog和ordigital分別有一個(gè)非限定性定語從句以inwhich引導(dǎo),在這里which顯然指的是network,所以可知analog和digital是analognetwork和digitalnetwork的省略,為避免重復(fù),句中去掉了network。

［2］Networkscanbeanalog

［3］Asthesenetworkshavegrowninpopularity,theyhavebeguntointerconnect,creatingaglobalcollectionofpeopleandcomputersthatareabletoelectronicallycommunicatewithoneanother.

由于這些網(wǎng)絡(luò)漸漸普及,它們已開始互聯(lián),創(chuàng)建了一種全球性的人機(jī)集合,使他們彼此間可以進(jìn)行電子化通信。

·as引導(dǎo)的是表示原因的狀語從句。

·主句中的主語they指的是前面提到的thesenetworks。

·creating引導(dǎo)的是分詞短語作為狀語,表示伴隨發(fā)生的情況。

·that引導(dǎo)的定語從句修飾computers。

［3］Asthesenetworkshave

［4］Anexampleyoumaybefamiliarwithisthecellulartelephonenetwork,whichismadepossiblebydividingageographicalareaintohexagonal-shapedcells,eachwithabasestation.

你可能熟知的一個(gè)例子是蜂窩式電話網(wǎng),其構(gòu)成通常是把一塊地理區(qū)域劃分為若干六邊形的單元,每個(gè)單元具有一個(gè)基站。

·youmaybefamiliarwith是example的定語從句。

·which引導(dǎo)的定語從句修飾cellulartelephonenetwork,是非限定性定語從句。

·each是eachcell的省略語。

［4］Anexampleyoumaybe

［5］Onabusnetworkcomputersaresimplyconnectedbyacableroutedalongapathinthevicinityofeachdevicethatmustbeconnectedtoit.

在總線形網(wǎng)絡(luò)上,計(jì)算機(jī)由一根電纜簡單地連接,電纜的路由是按附近每個(gè)必須連接到電纜上的設(shè)備的路徑而定的。

·routed引導(dǎo)的分詞短語是cable的定語。

·最后的it指的是cable。

·that引導(dǎo)的定語從句修飾device。

［5］Onabusnetworkcompu

Exercises

1.PleasetranslatethefollowingphrasesintoChinese.(1)IntegratedServicesDigitalNetwork(ISDN)

(2)commercialpublicnetwork

(3)communicationamongthecomputers

(4)tokepassing

(5)transmissionofinformation

(6)collisiondetection(CD)

Exercises

1.Pleasetrans

2.PleasetranslatethefollowingphrasesintoEnglish.

(1)廣域網(wǎng)(2)城域網(wǎng)

(3)局域網(wǎng)(4)環(huán)形網(wǎng)

(5)總線形網(wǎng)(6)網(wǎng)關(guān)

2.Pleasetranslatethefo

3.Matchthefollowingkeytermstotheappropriatedefinition.

(1)

Telecommunications

(2)

Datacommunications

(3)

Network

(4)

Analog

(5)

Digital

(6)

Wide-areanetwork

(7)

Metropolitan-areanetwork

(8)

Local-areanetwork

3.Matchthefollowingkey

a.Atypeofnetworkinwhichinformationisrepresentedbycontinuoussmoothlyvaryingsignals.

b.Thetransmissionofinformationoverlongdistancesbymeansoftelephone,teletype,radio,ortelevision.

c.AnetworkthatprovidescommunicationsacrossandamongmajorareassuchasNewYork,Chicago,orLosAngeles.

d.Anetworkthatprovidescommunicationswithinspecificbuildingsorfacilities.

a.Atypeofnetworkinwhi

e.Thetransferofdataorinformationbetweencomputerrelateddevices.

f.Anetworkinwhichinformationisrepresentedbydiscreteon/offsignals.

g.Anetworkthatprovidescommunicationsacrossacountryortheentireworld.

h.Agroupofcomputersandotherdevicesconnectedbycommunicationfacilities.

e.Thetransferofdataor

4.True/False.

(1)Thedistinctionbetweentelecommunicationsanddatacommunicationshasbecomemuchclearerwiththeadventofcomputernetworks.()

(2)Analogsignalsforcommunicationweredevelopedbeforecomputerswereinvented.()

(3)Usersofmetropolitan-areanetworksincludepoliceandfiredepartments.()

(4)Metropolitan-areanetworksaredesignedtoprovidecommunicationswithinspecificbuildings.()

(5)Thethreemajorcategoriesofnetworksaremutuallyexclusive.()

4.True/False.

(1)The

5.Answerthequestions.

(1)Arethethreemajorcategoriesofnetworksmutuallyexclusive?Why?

(2)WhatkindofthecomputernetworkistheInternet?Ifyouhaveusedit,doyouthinkitisconvenient?

5.Answerthequestions.

PassageBComputerNetworks(2)

Communicationnetworksenableuserstotransferinformationintheformofvoice,video,electronicmailorE-mail,andcomputerfiles.Usersrequestthecommunicationservicetheyneedbymeansofsimpleproceduresusingatelephonehandsetcellularphone,set-topTVbox,orthroughapplicationsrunningonahostcomputersuchasaPCorworkstation.

PassageBComputerNetworks(

TelephoneNetworks

Notehowthetransmissionlinesareallocatedtothephoneconversation.Thisisaccomplishedbycircuitswitching,where“circuit”referstothecapabilityoftransmittingonetelephoneconversationalongonelink.［1］

Tosetupacall,asetofcircuitshastobeconnected,joiningthetwotelephonesets.Bymodifyingtheconnections,theoperatorscanswitchthecircuits.Circuitsswitchingoccursatthebeginningofanewtelephonecall.Operatorswerelaterreplacedbymechanicalswitchesand,100yearslater,byelectronicswitches.

TelephoneNetworks

Note

Figure23.1illustratesthetelephonenetworkaround1988.Onmajordevelopmentatthisstageisthatthetransmissionofthevoicesignalsbetweenswitchesisdigital,asindicatedbytheletterD,insteadofanalog.

Anelectronicinterfaceintheswitchconvertstheanalogsignaltravelingonthelinkfromthetelephonesettotheswitchintoadigitalsignal,calledabitstream.Thesameinterfaceconvertsthedigitalsignalthattravelsbetweentheswitchesintoananalogsignalbeforesendingitfromtheswitchtothetelephone.

Figure23.1illustratesthFigure15.1Telephonenetworkaround1988.Thetransmissions

areanalog(A)ordigital(D).Theswitchesare

electronicandexchangecontrolinformationbyusing

adatanetworkcalledcommonchannelsignaling(CCS)Figure15.1Telephonenetwork

Theswitchthemselvesarecomputers,whichmakesthemveryflexible.Thisflexibilityallowsthetelephonecompanytomodifyconnectionsbysendingspecificinstructionstothecomputer.Figure15.1alsoshowsanothermajordevelopment-commonchannelsignaling(CCS).CCSisadatacommunicationnetworksthattheswitchesusetoexchangecontrolinformation.This“conversation”betweenswitchesservesthesamefunctionastheconversationthattookplacebetweenoperatorsinthemanualnetwork.ThusCCSseparatesthefunctionsofcallcontrolfromtransferofvoice.Combinedwiththeflexiblecomputerizedswitches,thisseparationoffunctionfacilitatesnewservicessuchascallwaiting,callforwarding,andcallback.

Theswitchthemselvesarec

Incurrenttelephonenetworks,thebitstreamsinthetrunks(linesconnectingswitches)andaccesslinks(linesconnectingsubscribertelephonestotheswitch)areorganizedinthedigitalsignal(DS)hierarchy.Thelinksthemselves-the“hardware”-arecalleddigitalcarriersystems.Trunkcapacityisdividedintoahierarchyoflogicalchannels.InNorthAmericathesechannels,listedinTable23.1,arecalledDS-1,DS-4andhaveratesrangingfrom1.544to274.176Mbps(megabitspersecond).ThebasicunitissetbytheDS-0channel,whichcarries64kbps(kilobitspersecond)andaccommodatesonevoicecircuit.Larger-capacitychannelsmultiplexseveralvoicechannels.TheratesinJapanandEuropearedifferent.ThemostcommonchannelsareDS-1andDS-3.

Incurrenttelephonenetwor電子信息類專業(yè)英語(第三版)李白萍章-(15)課件

Thisisthehierarchyofdigitalsignalsthattelephonenetworksuses.NotethatthebitrateofaDS-1signalisgreaterthan24timestherateofavoicesignal(64kbps)becauseoftheadditionalframingbitsrequired.

Thisisthehierarchyofdi

Observeinthetable,thattheratesarenotmultiplesofoneanother:theDS-1signalcarries24DS-0channels,butitsratesaremorethan24times64kbps.TheadditionalbitsareusedtoaccommodateDS-0channelswithratesthatdeviatefromthenominal64kbpsbecausethesignalsaregeneratedusingclocksthatarenotperfectlysynchronized.

Observeinthetable,that

ThelastmajorinnovationintelephonyistheintegrationofvoiceanddatasignalsthroughtheintroductionoftheIntegratedServicesDigitalNetwork(ISDN),illustratedinFigure15.2.TheISDNbasicaccessofferedtoacustomerconsistsoftwoBchannelsandoneDchannel(bothBandDchannelsaredigital).EachBchannelisabidirectional,orfull-duplex,channelat64kbps.OneBchannelcancarryeitheracircuit-switchedconnection,apacket-switchedtransmissionservices(describedbelow),orapermanentdigitalconnection.TheDchannelcarriesa16-kbpspacket-switchedservice.ISDNmakesavailabletosubscribersthedigitaltransmissionfacilitiesthatwerepreviouslyusedbetweentheswitchesofthenetwork,thusextendingthedigitaltransmissionallthewaytotheusers.［2］

Thelastmajorinnovationi

ApplicationsoftheISDNservicesincludecomputercommunication,high-speedfacsimiles,remotemonitoringofbuildings,videotext,andlowbitratemicrophones.WithISDN,thetelephonesystemistransformedintoanetworkthatcantransferinformationinmanyforms,ifatmodestspeeds.

ApplicationsoftheISDNservFigure15.2IntegratedServicesDigitalNetwork(ISDN).

Thebasicaccessprovidedtwobidirectional

64-kbpslinksandone16-kbpslink.Theselinks

canbeusedtotransmitvoiceordataFigure15.2IntegratedServices

Anewtechnologyfortransmissionofdataoveruntwistedortwistedpaircablefordistancesupto4000mwilldisplaceISDN.Thetechnologyusesexistingtelephonesub-scriberlinesinamannersimilartoISDN.Althoughthetelephonevoicechannelislimitedtoamannersimilartoabandwidthof3kHz,thetwistedpaircableitselfwhichconnectstothecentralofficehasabandwidthofmorethan1MHz,limitedbysignalattenuationandnoise.AsymmetricDigitalSubscriberLine(ADSL)servicenowofferedbythetelephonecompaniescanprovideupto1.5Mbpsormoredownstream(tothehome)andupto1.5Mbpsupstream(fromthehome),inadditiontoregularanalogtelephoneservice.［3］

Itisestimatedthat60%ofsubscriberloopsintheUnitedStatescansupporttheADSLtechnology.

Anewtechnologyfortrans

ComputerNetworks

Thissectiondiscussesthefollowingkeyinnovationsincomputerordatanetworks:organizationofdatainpackets,packetswitching,theInternetProtocolhierarchy,multipleaccessmethods,andserviceintegration.

ComputerNetworks

This

Aseriallinkisoftenusedtoattachacomputertoamodem.Amodem,ormodulator-demodulator,transmitsdatabyconvertingbitsintotonesthatcanbetransportedbythetelephonenetworkasiftheywerevoicesignals.Thereceivingmodemthenconvertsthesetonesbackintobits,thusenablingtwocomputerswithcompatiblemodemstocommunicateoverthetelephonenetworkasiftheyweredirectlyconnectedbyaseriallink.In1999mostmodemranataspeedof28,800bps.ModemsconformingtothenewV.90standardcantransmit56,000bpsinthedownstreamdirection.

Aseriallinkisoftenuse

Thesynchronoustransmissionstandardswereintroducedinthe1970stoincreasethetransmissionrateandtheusablelengthoftransmissionlinks.ThesestandardsareknownasSDLC(SynchronousDataLinkControl).AnumberofstandardsarebasedonSDLC,includingHDLC(High-LevelDataLinkControl),LAPB(LinkAccessProcedureB),LAPD,andLAPM.ThemainideaofSDLCistoavoidthetimewastedbyRS-232-Ccausedbygapsbetweensuccessivecharacters.Toeliminatethatlosttime,SDLCgroupsmanydatabitsintopackets.Apacketisasequenceofbitsprecededbyaspecialbitpatterncalledtheheaderandfollowedbyanotherspecialbitpatterncalledthetrailer.Thenumberofbitsinapacketmaybefixedorvariable.

Thesynchronoustransmissi

Thereceiverissynchronizedbyapreamblecontainedintheheader(H)ofthepacketandbyaself-synchronizingcodethatcontainsthetiminginformationinadditiontothedata.Moreover,SDLCusesanerror-detectioncodecalledthecyclicredundancycheck,orCRC,thatismoreefficientandmorepowerfulthanthesingleparitybitofRS-232-C.Twocomputers,then,canexchangeinformationoveratransmissionlinkusingeitherRS-232-CorSDLC.Butwhatifmanycomputersaretobeinterconnected?Inthe1960s,communicationengineersproposedthestore-and-forwardpacket-switchingmethodillustratedinFigure15.3.

Thereceiverissynchroniz

Thisfigureshowscomputersconnectedbypoint-topointlinks.TosendapackettocomputerE,computerAputsthesourceaddressAandthedestinationaddressEintothepacketheaderandsendsthepackettocomputerB.WhenBgetsthepacketfromA,itreadsthedestinationaddressanddeterminesthatitmustforwardthepackettoD.WhenDgetsthepacket,itreadsthedestinationaddressandforwardsthepackettoE.Inthisscheme,whenanodereceivesapacket,itmustfirststoreit,andthenforwardittoanothernode(ifnecessary).Hencethenamestore-and-forwardisgiventothisswitchingmethod.

ThisfigureshowscomputerscStore-and-forwardtransmissionsproceedbysendingthepacketsuccessivelyalonglinksfromthesourcetothedestination.Thepacketheaderspecifiesthesourceanddestinationaddresses(AandE,forexample)ofthepacket.Whenitreceivesapacket,acomputerchecksaroutingtabletofindoutonwhichlinkitshouldnextsendthepacketFigure15.3Store-and-forwardtransmission

Whencomputersusestore-and-forwardpacketswitching,theyuseagivenlinkonlywhentheysendapacket.Asaresult,thesamelinkscanbeusedefficientlybyalargenumberofintermittenttransmissions.Thismethodforsharingalinkamongtransmissionsiscalledstatisticalmultiplexing.Statisticalmultiplexingcontrastswithtime-divisionmultiplexing-basedcircuitswitchingwhichreservesforthedurationoftheconversationeventhoughthepartiesconnectedbythecircuitmaynottransmitcontinuously.［4］

Whencomputersusestore-a

Figure15.4illustratesapopularimplementationofmultipleaccesscalledEthernet.InanEthernetnetwork,computersareattachedtoacommoncoaxialcableviaaninter-facethattodayconsistsofasmallchipsetmountedonthemainboard.WhencomputerAwantstosendapackettocomputerE,itputsthesourceaddressAandthedestinationaddressEintothepacketheaderandtransmitsthepacketonthecable.Allthecomputersreadthepacket,butonlythecomputerwiththedestinationaddressindicatedonthepacketcopiesit.TheoriginalEthernettransmissionratewas10Mbps,now100Mbpsand1000MbpsEthernetareavailable.

Figure15.4illustratesapEthernet.Inthisnetwork,computersareattachedtoacommoncoaxialcable.ThecomputersreadeverytransmittedpacketanddiscardthosenotaddressedtothemFigure23.4Ethernet.Inthisnetwork,com

FastLANandWANnetworksaretodaybeingdeployed,usingAsynchronousTransferMode(ATM).WithATM,acomputertransmitsinformationatratesbetween

25Mbpsand2.5Gbps(gigabitspersecond)inpacketsof53bytes(1bytes=8bits).Thesefixed-sizepackets,calledcells,canbeswitchedrapidlybyATMswitches.TheheadercontainsavirtualcircuitaddressorVCI,insteadofasourceanddestinationaddress.

FastLANandWANnetworks

Withtheappropriatecontrolsoftware,networkengineerscanconnectmanyATMswitchestogethertobuildlargenetwork.Moreover,thelinksbetweenATMswitchescanbelongopticalfibers.Usingthistechnology,then,companiescanbuildaworld-widenetwork.InanATMnetwork,dataistransferredfromsourcetodestinationoverafixedroute,justlikeinatelephoneconnection.Unliketelephonenetworks,however,anATMconnectionisnotallocatedafixedbandwidth.TheATMnetworkdetermineshowmuchbandwidthtoallocatesothatinformationistransportedwithverylowlossrateordelay,asrequiredbytheapplication.Thusthistechnologyiswellsuitedforbuildinglargerintegratedservicesnetworks.

Withtheappropriatecontr

CableTelevisionNetworks

Cabletelevision,originallyknownasCommunityAntennaTelevisionorCATV,wasintroducedin1940sinareasthatcouldnotreceivetheTVsignalwithoutobstruction.Thesolutionwastoplaceanantennaontopofalargeutilitypoleandlocallysharethesignal.CATV,aswenowknowit,wascreatedwhenthesignalfromonemasterantennawasdistributedoveralargeareausingcoaxialcableandamplifiers.ThekeyinnovationsincableTVareopticalfeederlinks,digitalcompressiontechniques,andserviceintegration.

CableTelevisionNetworks

Today,CATVusesfrequency-divisionmultiplexingtotransmitupto69analogTVchannels,each4.5MHzwide.Transmissionisovercoaxialcablesarrangedasaunidirectionaltree,withwidebandamplifiersusedtocompensatefortheattenuationofthecablesignal.ThenumberofTVchannelsislimitedbythebandwidthofcoaxialcables.ThespanofaCATVnetworkislimitedbythenoisepower,whichincreasesasmoreamplifiersareaddedtocompensateforthesignalpowerlossduringpropagation.

Today,CATVusesfrequency

ThenextmajorstepinCATVistoutilizeopticalfiberstotransmittheTVsignaloverlongdistances.Fibershaveamuchlowerattenuationthancoaxialcables,sotheycantransmitsignalsoverlingerdistancesbeforeitbecomesnecessarytouseanamplifier.Inthisimplementation,thetransmissionoverthefiberisstillanalog.Thesignalisfedintothecoaxialcablenetworkatvariouspoints,wheretheopticalsignalisconvertedintoelectricalsignals.Thecostofeachopticaltransmissionlineisspreadoverafewhundredusers.Moreover,existingcoaxialcablescanbereused.

ThenextmajorstepinCAT

Thishybridfiber/coaxial(HFC)cabledistributionsystemhasalongerspanandbettersignalquality

thanacoaxialcablenetwork.Thenetworkisnowatreewhosefirstlevelisafibernetworkandwhosebottomlevelarecoaxialcables.Thisnetworkisalsocalledafiber-to-the-curb(FTTC)network,where“curb”designatesalocationinsomeneighborhoodwherethefiberisconnectedtothelocalcoaxialdistributionnetwork.

Thishybridfiber/co