计算机专业英语讲义

计

算

机

专

业

英

语

讲

义

素全

室吴

教研

计算机

Computernetworks

Theworld'sleadingintroductiontonetworking-fullyupdatedfortomorrow'skeytechnologies.

1.Physicallayer(e.g.,copper,fiber,wireless,satellites,andInternetovercable)

2.Datalinklayer(e.g.,protocolprinciples,protocolverification,HDLC,andPPP)

3.MACSublayer(e.g.,gigabitEthernet,802.11,broadbandwireless,andswitching)

4.Networklayer(e.g.,routingalgorithms,congestioncontrol,QoS,IPv4,andIPv6)

5.Transportlayer(e.g.,socketprogramming,UDP,TCP,RTP,andnetworkperformance)

6.Applicationlayer(e.g.,e-mail,theWeb,PHP,wirelessWeb,MP3,andstreamingaudio)

7.Networksecurity(e.g.,AES,RSA,quantumcryptography,IPsec,andWebsecurity)

Ch1Introduction

Themergingofcomputersandcommunicationshashadaprofoundinfluenceonthewaycomputer

systemsareorganized.Theconceptofthe''computercenter"asaroomwithalargecomputerto

whichusersbringtheirworkforprocessingisnowtotallyobsolete.Theoldmodelofasinglecomputer

servingalloftheorganization'scomputationalneedshasbeenreplacedbyoneinwhichalarge

numberofseparatebutinterconnectedcomputersdothejob.Thesesystemsarecalledcomputer

networks.Thedesignandorganizationofthesenetworksarethesubjectsofthisbook..

Throughoutthebookwew川usetheterm"computernetwork"tomeanacollectionofautonomous

computersinterconnectedbyasingletechnology.Twocomputersaresaidtobeinterconnectedifthey

areabletoexchangeinformation.Theconnectionneednotbeviaacopperwire;fiberoptics,

microwaves,infrared,andcommunicationsatellitescanalsobeused.Networkscomeinmanysizes,

shapesandforms,aswewillseelater.Althoughitmaysoundstrangetosomepeople,neitherthe

InternetnortheWorldWideWebisacomputernetwork.Bytheendofthisbook,itshouldbeclearwhy.

Thequickansweris:theInternetisnotasinglenetworkbutanetworkofnetworksandtheWebisa

distributedsystemthatrunsontopoftheInternet.

Thereisconsiderableconfusionintheliteraturebetweenacomputernetworkandadistributedsystem.

Thekeydistinctionisthatinadistributedsystem,acollectionofindependentcomputersappearstoits

usersasasinglecoherentsystem.Usually,ithasasinglemodelorparadigmthatitpresentstothe

users.Oftenalayerofsoftwareontopoftheoperatingsystem,calledmiddleware,isresponsiblefor

implementingthismodel.Awell-knownexampleofadistributedsystemistheWorldWideWeb,in

whicheverythinglookslikeadocument(Webpage).

Ineffect,adistributedsystemisasoftwaresystembuiltontopofanetwork.Thesoftwaregivesita

highdegreeofcohesivenessandtransparency.Thus,thedistinctionbetweenanetworkanda

distributedsystemlieswiththesoftware(especiallytheoperatingsystem),ratherthanwiththe

hardware.

Nevertheless,thereisconsiderableoverlapbetweenthetwosubjects.Forexample,bothdistributed

systemsandcomputernetworksneedtomovefilesaround.Thedifferenceliesinwhoinvokesthe

南通职业大学电子系计算机教研室吴素全编i

movement,thesystemortheuser.Althoughthisbookprimarilyfocusesonnetworks,manyofthe

topicsarealsoimportantindistributedsystems.

1.1UsesofComputerNetworks

1.1.1BusinessApplications

Putinslightlymoregeneralform,theissuehereisresourcesharing,andthegoalistomakeall

programs,equipment,andespeciallydataavailabletoanyoneonthenetworkwithoutregardtothe

physicallocationoftheresourceandtheuser.

However,probablyevenmoreimportantthansharingphysicalresourcessuchasprinters,scanners,

andCDburners,issharinginformation.Everylargeandmedium-sizedcompanyandmanysmall

companiesarevitallydependentoncomputerizedinformation.Mostcompanieshavecustomer

records,inventories,accountsreceivable,financialstatements,taxinformation,andmuchmoreonline.

Ifallofitscomputerswentdown,abankcouldnotlastmorethanfiveminutes.

Inthesimplestofterms,onecanimagineacompany'sinformationsystemasconsistingofoneor

moredatabasesandsomenumberofemployeeswhoneedtoaccessthemremotely.Inthismodel,

thedataarestoredonpowerfulcomputerscalledservers.Oftenthesearecentrallyhousedand

maintainedbyasystemadministrator.Incontrast,theemployeeshavesimplermachines,called

clients,ontheirdesks,withwhichtheyaccessremotedata.Theclientandservermachinesare

connectedbyanetwork,asillustratedinFig.11whichiscalledtheclient-servermodel.

Figure1-1.Anetworkwithtwoclientsandoneserver.

Asecondgoalofsettingupacomputernetworkhassomethingtodowithpeopleratherthan

informationorevencomputers.Virtuallyeverycompanythathastwoormorecomputersnowhas

e-mail(electronicmail),whichemployeesgenerallyuseforagreatdealofdailycommunication.Yet

anotherformofcomputer-assistedcommunicationisvideoconferencing.Athirdgoalforincreasingly

manycompaniesisdoingbusinesselectronicallywithothercompanies,especiallysuppliersand

customers.Afourthgoalthatisstartingtobecomemoreimportantisdoingbusinesswithconsumers

overtheInternet.

1.1.2HomeApplications

SomeofthemorepopularusesoftheInternetforhomeusersareasfollows:

1.Accesstoremoteinformation.

2.Person-to-personcommunication.

3.Interactiveentertainment.

4.Electroniccommerce.

Accesstoremoteinformationcomesinmanyforms.Informationavailableincludesthearts,business,

cooking,government,health,history,hobbies,recreation,science,sports,travel,andmanyothers.

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Instantmessagingallowstwopeopletotypemessagesateachotherinrealtime.Amultiperson

versionofthisideaisthechatroom,inwhichagroupofpeoplecantypemessagesforalltosee.

Worldwidenewsgroups,withdiscussionsoneveryconceivabletopic,arealreadycommonplace

amongaselectgroupofpeople,andthisphenomenonwillgrowtoincludethepopulationatlarge.

Ourthirdcategoryisentertainment,whichisahugeandgrowingindustry.TheIdlerapplicationhere

(theonethatmaydrivealltherest)isvideoondemand.Adecadeorsohence,itmaybepossibleto

selectanymovieortelevisionprogramevermade,inanycountry,andhaveitdisplayedonyour

screeninstantly.Ontheotherhand,maybethekillerapplicationwillnotbevideoondemand.Maybeit

willbegameplaying.

Ourfourthcategoryiselectroniccommerceinthebroadestsenseoftheterm.Homeshoppingis

alreadypopularandenablesuserstoinspecttheon-linecatalogsofthousandsofcompanies.

Anotherareainwhiche-commerceisalreadyhappeningisaccesstofinancialinstitutions.Many

peoplealreadypaytheirbills,managetheirbankaccounts,andhandletheirinvestmentselectronically.

Thiswillsurelygrowasnetworksbecomemoresecure.

Oneareathatvirtuallynobodyforesawiselectronicfleamarkets(e-flea?).

1.1.3MobileUsers

Mobilecomputers,suchasnotebookcomputersandpersonaldigitalassistants(PDAs),areoneofthe

fastest-growingsegmentsofthecomputerindustry.

Whywouldanyonewantone?Acommonreasonistheportableoffice.Peopleontheroadoftenwant

tousetheirportableelectronicequipmenttosendandreceivetelephonecalls,faxes,andelectronic

mail,surftheWeb,accessremotefiles,andlogontoremotemachines.

1.2NetworkHardware

Thereisnogenerallyacceptedtaxonomyintowhichallcomputernetworksfit,buttwodimensions

standoutasimportant:transmissiontechnologyandscale.

Broadlyspeaking,therearetwotypesoftransmissiontechnologythatareinwidespreaduse.Theyare

asfollows:

1.Broadcastlinks.

2.Point-to-pointlinks.

Broadcastnetworkshaveasinglecommunicationchannelthatissharedbyallthemachinesonthe

network.Shortmessages,calledpacketsincertaincontexts,sentbyanymachinearereceivedbyall

theothers.Anaddressfieldwithinthepacketspecifiestheintendedrecipient.

Broadcastsystemsgenerallyalsoallowthepossibilityofaddressingapackettoalldestinationsby

usingaspecialcodeintheaddressfield.Thismodeofoperationiscalledbroadcasting.Some

broadcastsystemsalsosupporttransmissiontoasubsetofthemachines,somethingknownas

multicasting.

Incontrast,point-to-pointnetworksconsistofmanyconnectionsbetweenindividualpairsofmachines.

Asageneralrule(althoughtherearemanyexceptions),smaller,geographicallylocalizednetworks

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tendtousebroadcasting,whereaslargernetworksusuallyarepoint-to-point.Point-to-point

transmissionwithonesenderandonereceiverissometimescalledunicasting.

Analternativecriterionforclassifyingnetworksistheirscale.InFig.1-6weclassifymultipleprocessor

systemsbytheirphysicalsize.Atthetoparethepersonalareanetworks,networksthataremeantfor

oneperson.

Figure1-6.Classificationofinterconnectedprocessorsbyscale.

Localareanetworks,generallycalledLANs,areprivately-ownednetworkswithinasinglebuildingor

campusofuptoafewkilometersinsize.Theyarewidelyusedtoconnectpersonalcomputersand

workstationsincompanyofficesandfactoriestoshareresourcesandexchangeinformation.

1.2.2MetropolitanAreaNetworks

Ametropolitanareanetwork,orMAN,coversacity.Thebest-knownexampleofaMANisthecable

televisionnetworkavailableinmanycities.

1.2.3WideAreaNetworks

Awideareanetwork,orWAN,spansalargegeographicalarea,oftenacountryorcontinent.It

containsacollectionofmachinesintendedforrunninguserprograms.Wewillfollowtraditionalusage

andcallthesemachineshosts.Thehostsareconnectedbyacommunicationsubnet,orjustsubnetfor

short.Thehostsareownedbythecustomers,whereasthecommunicationsubnetistypicallyowned

andoperatedbyatelephonecompanyorInternetserviceprovider.Thejobofthesubnetistocarry

messagesfromhosttohost.

1.2.4WirelessNetworks

Toafirstapproximation,wirelessnetworkscanbedividedintothreemaincategories:

1.Systeminterconnection.

2.WirelessLANs.

3.WirelessWANs.

1.2.5HomeNetworks

Homenetworkingisonthehorizon.Thefundamentalideaisthatinthefuturemosthomeswillbeset

upfornetworking.Everydeviceinthehomewillbecapableofcommunicatingwitheveryotherdevice,

andallofthemwillbeaccessibleovertheInternet.

Manydevicesarecapableofbeingnetworked.Someofthemoreobviouscategories(withexamples)

areasfollows:

1.Computers(desktopPC,notebookPC,PDA,sharedperipherals).

2.Entertainment(TV,DVD,VCR,camcorder,camera,stereo,MP3).

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3.Telecommunications(telephone,mobiletelephone,intercom,fax).

4.Appliances(microwave,refrigerator,clock,furnace,airco,lights).

5.Telemetry(utilitymeter,smoke/burglaralarm,thermostat,babycam).

1.2.6Internetworks

Manynetworksexistintheworld,oftenwithdifferenthardwareandsoftware.Peopleconnectedtoone

networkoftenwanttocommunicatewithpeopleattachedtoadifferentone.Thefulfillmentofthis

desirerequiresthatdifferent,andfrequentlyincompatiblenetworks,beconnected,sometimesby

meansofmachinescalledgatewaystomaketheconnectionandprovidethenecessarytranslation,

bothintermsofhardwareandsoftware.Acollectionofinterconnectednetworksiscalledan

internetworkorinternet.Thesetermswillbeusedinagenericsense,incontrasttotheworldwide

Internet,whichwewillalwayscapitalize.

1.3NetworkSoftware

1.3.1ProtocolHierarchies

Toreducetheirdesigncomplexity,mostnetworksareorganizedasastackoflayersorlevels,each

onebuiltupontheonebelowit.Thenumberoflayers,thenameofeachlayer,thecontentsofeach

layer,andthefunctionofeachlayerdifferfromnetworktonetwork.Thepurposeofeachlayeristo

offercertainservicestothehigherlayers,shieldingthoselayersfromthedetailsofhowtheoffered

servicesareactuallyimplemented.Inasense,eachlayerisakindofvirtualmachine,offeringcertain

servicestothelayeraboveit.

Layernononemachinecarriesonaconversationwithlayernonanothermachine.Therulesand

conventionsusedinthisconversationarecollectivelyknownasthelayernprotocol.Basically,a

protocolisanagreementbetweenthecommunicatingpartiesonhowcommunicationistoproceed.

Afive-layernetworkisillustratedinFig.1・13.Theentitiescomprisingthecorrespondinglayerson

differentmachinesarecalledpeers.Thepeersmaybeprocesses,hardwaredevices,orevenhuman

beings.Inotherwords,itisthepeersthatcommunicatebyusingtheprotocol.

Figure1-13.Layers,protocols,andinterfaces.

Inreality,nodataaredirectlytransferredfromlayernononemachinetolayernonanothermachine.

Instead,eachlayerpassesdataandcontrolinformationtothelayerimmediatelybelowit,untilthe

lowestlayerisreached.Belowlayer1isthephysicalmediumthroughwhichactualcommunication

occurs.InFig.1・13,virtualcommunicationisshownbydottedlinesandphysicalcommunicationby

solidlines.

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Betweeneachpairofadjacentlayersisaninterface.Theinterfacedefineswhichprimitiveoperations

andservicesthelowerlayermakesavailabletotheupperone.

Asetoflayersandprotocolsiscalledanetworkarchitecture.Alistofprotocolsusedbyacertain

system,oneprotocolperlayer,iscalledaprotocolstack.Thesubjectsofnetworkarchitectures,

protocolstacks,andtheprotocolsthemselvesaretheprincipaltopicsofthisbook.

1.3.2DesignIssuesfortheLayers

Someofthekeydesignissuesthatoccurincomputernetworksarepresentinseverallayers.Below,

wewillbrieflymentionsomeofthemoreimportantones.

Everylayerneedsamechanismforidentifyingsendersandreceivers.Asaconsequenceofhaving

multipledestinations,someformofaddressingisneededinordertospecifyaspecificdestination.

Errorcontrolisanimportantissuebecausephysicalcommunicationcircuitsarenotperfect..

Anissuethatoccursateverylevelishowtokeepafastsenderfromswampingaslowreceiverwith

data.Thissubjectiscalledflowcontrol.

Anotherproblemthatmustbesolvedatseverallevelsistheinabilityofallprocessestoaccept

arbitrarilylongmessages.Thispropertyleadstomechanismsfordisassembling,transmitting,and

thenreassemblingmessages.

1.3.3Connection-OrientedandConnectionlessServices

Layerscanoffertwodifferenttypesofservicetothelayersabovethem:connection-orientedand

connectionless.

Connection-orientedserviceismodeledafterthetelephonesystem.Similarly,tousea

connection-orientednetworkservice,theserviceuserfirstestablishesaconnection,usesthe

connection,andthenreleasestheconnection.Theessentialaspectofaconnectionisthatitactslikea

tube:thesenderpushesobjects(bits)inatoneend,andthereceivertakesthemoutattheotherend.

Inmostcasestheorderispreservedsothatthebitsarriveintheordertheyweresent.

Incontrast,connectionlessserviceismodeledafterthepostalsystem.Eachmessage(letter)carries

thefulldestinationaddress,andeachoneisroutedthroughthesystemindependentofalltheothers.

1.3.4ServicePrimitives

Aserviceisformallyspecifiedbyasetofprimitives(operations)availabletoauserprocesstoaccess

theservice.Theseprimitivestelltheservicetoperformsomeactionorreportonanactiontakenbya

peerentity.

1.3.5TheRelationshipofServicestoProtocols

Servicesandprotocolsaredistinctconcepts,althoughtheyarefrequentlyconfused.Theservice

defineswhatoperationsthelayerispreparedtoperformonbehalfofitsusers,butitsaysnothingatall

abouthowtheseoperationsareimplemented.Aservicerelatestoaninterfacebetweentwolayers,

withthelowerlayerbeingtheserviceproviderandtheupperlayerbeingtheserviceuser.

Aprotocol,incontrast,isasetofrulesgoverningtheformatandmeaningofthepackets,ormessages

thatareexchangedbythepeerentitieswithinalayer.Entitiesuseprotocolstoimplementtheirservice

definitions.

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Inotherwords,servicesrelatetotheinterfacesbetweenlayers,asillustratedinFig.1-19.Incontrast,

protocolsrelatetothepacketssentbetweenpeerentitiesondifferentmachines.

Figure1-19.Therelationshipbetweenaserviceandaprotocol.

Layerk♦1Layerk♦1

Layerk-1Layerk-1

1.4ReferenceModels

Nowthatwehavediscussedlayerednetworksintheabstract,itistimetolookatsomeexamples.In

thenexttwosectionswewilldiscusstwoimportantnetworkarchitectures,theOSIreferencemodel

andtheTCP/IPreferencemodel.AlthoughtheprotocolsassociatedwiththeOSImodelarerarely

usedanymore,themodelitselfisactuallyquitegeneralandstillvalid,andthefeaturesdiscussedat

eachlayerarestillveryimportant.TheTCP/IPmodelhastheoppositeproperties:themodelitselfis

notofmuchusebuttheprotocolsarewidelyused.Forthisreasonwewilllookatbothofthemindetail.

Also,sometimesyoucanlearnmorefromfailuresthanfromsuccesses.

1.4.1TheOSIReferenceModel

TheOSImodel(minusthephysicalmedium)isshowninFig.1・20.Thismodelisbasedonaproposal

developedbytheInternationalStandardsOrganization(ISO)asafirststeptowardinternational

standardizationoftheprotocolsusedinthevariouslayers(DayandZimmermann,1983).Itwas

revisedin1995(Day,1995).ThemodeliscalledtheISOOSI(OpenSystemsInterconnection)

ReferenceModelbecauseitdealswithconnectingopensystems—thatis,systemsthatareopenfor

communicationwithothersystems.WewilljustcallittheOSImodelforshort.

Figure1-20.TheOSIreferencemodel.

Thephysicallayerisconcernedwithtransmittingrawbitsoveracommunicationchannel.Thedesign

issueshavetodowithmakingsurethatwhenonesidesendsa1bit,itisreceivedbytheothersideas

a1bit,notasa0bit.

TheDataLinkLayer

Themaintaskofthedatalinklayeristotransformarawtransmissionfacilityintoalinethatappears

freeofundetectedtransmissionerrorstothenetworklayer.Itaccomplishesthistaskbyhavingthe

南通职业大学电子系计算机教研室吴索全编7

senderbreakuptheinputdataintodataframes(typicallyafewhundredorafewthousandbytes)and

transmittheframessequentially.

TheNetworkLayer

Thenetworklayercontrolstheoperationofthesubnet.Akeydesignissueisdetermininghowpackets

areroutedfromsourcetodestination.

TheTransportLayer

Thebasicfunctionofthetransportlayeristoacceptdatafromabove,splititupintosmallerunitsif

needbe,passthesetothenetworklayer,andensurethatthepiecesallarrivecorrectlyattheother

end.

TheSessionLayer

Thesessionlayerallowsusersondifferentmachinestoestablishsessionsbetweenthem.Sessions

offervariousservices,includingdialogcontrol(keepingtrackofwhoseturnitistotransmit),token

management(preventingtwopartiesfromattemptingthesamecriticaloperationatthesametime),

andsynchronization(checkpointinglongtransmissionstoallowthemtocontinuefromwherethey

wereafteracrash).

ThePresentationLayer

Unlikelowerlayers,whicharemostlyconcernedwithmovingbitsaround,thepresentationlayeris

concernedwiththesyntaxandsemanticsoftheinformationtransmitted.

TheApplicationLayer

Theapplicationlayercontainsavarietyofprotocolsthatarecommonlyneededbyusers.

1.4.2TheTCP/IPReferenceModel

TheInternetLayer

Thislayer,calledtheinternetlayer,isthelinchpinthatholdsthewholearchitecturetogether.Itsjobis

topermithoststoinjectpacketsintoanynetworkandhavethemtravelindependentlytothe

destination(potentiallyonadifferentnetwork).

TheTransportLayer

ThelayerabovetheinternetlayerintheTCP/IPmodelisnowusuallycalledthetransportlayer.Itis

designedtoallowpeerentitiesonthesourceanddestinationhoststocarryonaconversation.Two

end-to-endtransportprotocolshavebeendefinedhere.Thefirstone,TCP(TransmissionControl

Protocol),isareliableconnection-orientedprotocolthatallowsabytestreamoriginatingonone

machinetobedeliveredwithouterroronanyothermachineintheinternet.

Thesecondprotocolinthislayer,UDP(UserDatagramProtocol),isanunreliable,connectionless

protocolforapplicationsthatdonotwantTCP'ssequencingorflowcontrolandwishtoprovidetheir

own.TherelationofIP,TCP,andUDPisshowninFig.1・22.Sincethemodelwasdeveloped,IPhas

beenimplementedonmanyothernetworks.

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Figure1-22.ProtocolsandnetworksintheTCP/IPmodelinitially.

Ontopofthetransportlayeristheapplicationlayer.Itcontainsallthehigher-levelprotocols.Theearly

onesincludedvirtualterminal(TELNET),filetransfer(FTP),andelectronicmail(SMTP),asshownin

Fig.1-22.

TheHost-to-NetworkLayer

Belowtheinternetlayerisagreatvoid.TheTCP/IPreferencemodeldoesnotreallysaymuchabout

whathappenshere,excepttopointoutthatthehosthastoconnecttothenetworkusingsome

protocolsoitcansendIPpacketstoit.Thisprotocolisnotdefinedandvariesfromhosttohostand

networktonetwork.

Keywords:

1.computernetworks计算机网络interconnected相互连接的WorldWideWeb万维网

2.distributedsystem分布式系统middleware中间件

3.resourcesharing资源共享

4.server服务器client客户client-servermodel客户-服务器模型

5.communicationmedium通信媒介electronicmail电子邮件electroniccommerce电子商务

6.instantmessaging即时消息chatroom聊天室

7.peer-to-peercommunication对等通信e-flea电子跳蚤市场

8.PDA(personaldigitalassistant)个人数字助理fixedwireless固定无线mobilewireless移动无线

9.WAP(wirelessapplicationprotocol)无线应用协议m-commerce移动商务

10.broadcasting广播multicasting多播unicasting单播personalareanetwork个人区域网络

11.LAN(localareanetwork)局域网MAN(metropolitanareanetwork)城域网WAN(widearea

network)广域网host主机subnet子网router路由器bluetooth蓝牙gateway网关

12.layer/level层次peer对等体networkarchitecture网络体系结构protocolstack协议栈

13.addressing编址机制errorcontrol错误控制flowcontrol流控制(de)multiplexing多路(解)复用

14.connection-orientedservice面向连接的服务connectionlessservice面向无连接的服务

15.primitive原语sessionlayer会话层presentationlayer表示层

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Ch2ThePhysicalLayer

wewillcoverthreekindsoftransmissionmedia:guided(copperwireandfiberoptics),wireless

(terrestrialradio),andsatellite.Thismaterialwillprovidebackgroundinformationonthekey

transmissiontechnologiesusedinmodernnetworks.

2.2GuidedTransmissionMedia

Thepurposeofthephysicallayeristotransportarawbitstreamfromonemachinetoanother.Various

physicalmediacanbeusedfortheactualtransmission.Eachonehasitsownnicheintermsof

bandwidth,delay,cost,andeaseofinstallationandmaintenance.Mediaareroughlygroupedinto

guidedmedia,suchascopperwireandfiberoptics,andunguidedmedia,suchasradioandlasers

throughtheair.Wewilllookatalloftheseinthefollowingsections.

2.2.1MagneticMedia

Oneofthemostcommonwaystotransportdatafromonecomputertoanotheristowritethemonto

magnetictapeorremovablemedia(e.g.,recordableDVDs),physicallytransportthetapeordisksto

thedestinationmachine,andreadthembackinagain.Althoughthismethodisnotassophisticatedas

usingageosynchronouscommunicationsatellite,itisoftenmorecosteffective,especiallyfor

applicationsinwhichhighbandwidthorcostperbittransportedisthekeyfactor.

2.2.2TwistedPair

Atwistedpairconsistsoftwoinsulatedcopperwires,typicallyabout1mmthick.Thewiresaretwisted

togetherinahelicalform,justlikeaDNAmolecule.Twistingisdonebecausetwoparallelwires

constituteafineantenna.Whenthewiresaretwisted,thewavesfromdifferenttwistscancelout,so

thewireradiateslesseffectively.Twistedpairscanbeusedfortransmittingeitheranalogordigital

signals.

Twistedpaircablingcomesinseveralvarieties,twoofwhichareimportantforcomputernetworks.

Category3twistedpairsconsistoftwoinsulatedwiresgentlytwistedtogether.Foursuchpairsare

typicallygroupedinaplasticsheathtoprotectthewiresandkeepthem