《计算机网络与因特网》douglars ecorner著 第六版 英文答案

tgtSolutionofSelectedExercisesfromtheEndofChapter

Exercises

Chapter1-IntroductionAndOverview

1.1Listtenindustriesthatdependoncomputernetworking.

1.2ProvideabriefhistoryoftheInternetdescribingwhenandhowitwasstarted.

1.3ListthelayersintheTCP/IPmodel,andgiveabriefexplanationofeach.

Answer:TheTCP/IPconsistsof5layers.Thelayersandtheirshortexplanationisasfollows:

• Layer1:Physical:ProtocolsinthePhysicallayerspecifydetailsabouttheunderlyingtransmissionmediumandtheassociatedhardware.

• Layer2:NetworkInterfaceProtocolsintheNetworkInterfacelayerspecifydetailsaboutcommunicationbetweenhigherlayersofprotocolsandtheunderlyingnetwork.

• Layer3:Internet:ProtocolsintheInternetlayerformthefundamentalbasisforthe

Internet.Layer3protocolsspecifycommunicationbetweentwocomputersacrosstheInternet

•Layer4:Transport:ProtocolsintheTransportlayerprovideforcommunicationfromanapplicationprogramononecomputertoanapplicationprogramonanother.

• Layer5:Application:ProtocolsinthetoplayeroftheTCP/IPstackspecifyhowa

pairoapplicationsinteractwhentheycommunicate.

1.4Whatisaprotocolsuite,andwhatistheadvantageofasuite?

Answer:protocolsaredesignedincomplete,cooperativesetscalledsuitesorfamilies,insteadofcreatingeachprotocolinisolation.Eachprotocolinasuitehandlesoneaspectofcommunication;together,theprotocolsinasuitecoverallaspectsofcommunication.Theentiresuiteisdesignedtoallowtheprotocolstoworktogetherefficiently.

1.5DescribetheTCP/IPlayeringmodel,andexplainhowitwasderived.

1.6ProvidereasonsforInternetgrowthinrecentyears.

1.7Whatisinteroperability,andwhyisitespeciallyimportantintheInternet?

1.8Accordingtothetext,isitpossibletodevelopInternetapplicationswithoutunderstandingthearchitectureoftheInternetandthetechnologies?Supportyouranswer.

Answer:Yes,possibletowritecodethatcommunicatesoveranetwork,withoutunderstandingthehardwareandsoftwaretechnologiesthatareusedtotransferdatafromoneapplicationtoanother.However,knowledgeoftheunderlyingnetworksystemallowsaprogrammertowritebettercode.

1.9Towhataspectsofnetworkingdoesdatacommunicationsrefer?

Answer:Datacommunicationsreferstothestudyoflow-levelmechanismsandtechnologiesusedtosendinformationacrossaphysicalcommunicationmedium,suchasawire,radiowave,orlightbeam.

1.10Whatisacommunicationprotocol?Conceptually,whattwoaspectsofcommunicationdoesaprotocolspecify?

Answer:Acommunicationprotocolrefertoaspecificationfornetworkcommunication.Majoraspectsofaprotocolaresyntax(format)andsemantics(meaning)oftheprotocol.

1.11Listmajorstandardizationorganizationsthatcreatestandardsfordatacommunicationsandcomputernetworking.

Answer:Variousnationalandinternationalorganizationsareinvolvedinstandardizationofcommunicationsandnetworkingservices.Tolistfew:

• InternationalOrganizationforStandardization(ISO)

• InternationalTelecommunicationsUnion,TelecommunicationStandardization

Sector(ITU-T)

• InstituteofElectricalandElectronicsEngineers(IEEE)

• InternetEngineeringTaskForce(IETF)

1.12GiveabriefexplainofthelayersintheISOOpenSystemInterconnectionmodel.

Answer:ISOOSImodelconsistsof7layers,namely:

• Layer7:Application

• Layer6:Presentation

• Layer5:Session

• Layer4:Transport

• Layer3:Network

• Layer2:DataLink

• Layer1:Physical

Readerisexpectedtofindoutshortexplanationofeachlayer.

1.13Whatispacket-switching,andwhyispacketswitchingrelevanttotheInternet?

Answer:Packetswitchingdividesdataintosmallblocks,calledpackets,andincludesanidentificationoftheintendedrecipientineachpacket.Packetswitchingchangednetworkinginafundamentalway,andprovidedthebasisforthemodernInternet.Packetswitchingallowsmultiplesenderstotransmitdataoverasharednetwork.

1.14Explainhowheadersareaddedandremovedasdatapassesthroughalayeredmodel.

Answer:eachlayeronthesendingcomputerprependsextrainformationontothepacket;thecorrespondingprotocollayeronthereceivingcomputerremovesandusestheextrainformation.

Chapter-2InternetTrends

2.1DescribetheevolutioninaudiothathasoccurredintheInternet.

Answer:Thetrendinaudioprocessinghasbeenasfollows:

AlertSoundsÆHumanVoiceÆAudiClipsÆHigh-FidelityAudio

2.2AssumethatonehundredmillionnewcomputersareaddedtotheInterneteachyear.Ifcomputersareaddedatauniformrate,howmuchtimeelapsesbetweentwosuccessiveadditions?

2.3DescribeInternetapplicationsthatyouuseregularlythatwerenotavailabletoyourparentswhentheywereyourage.

2.4Whywassharingofcomputationalresourcesimportantinthe1960s?

2.5WhatshiftinInternetuseoccurredwhentheWorldWideWebfirstappeared?

2.6TheplotinFigure2.1showsthatInternetgrowthdidnotstartuntilafter1995.Whyisthefiguremisleading?

Answer:ResearcherscomputersworkingfortheAdvancedResearchProjectsAgency(ARPA)ofUSDepartmentofDefense(DoD)neededpowerfulandcomputerswereincrediblyexpensive.TheARPAbudgetwasinsufficienttofundmanycomputers.TheARPAplannedtointerconnectallcomputerswithadatanetworkanddevisesoftwarethatwouldallowaresearchertousewhichevercomputerwasbestsuitedtoperformagiventask.

2.7ListthestepsinthetransitioningraphicspresentationfromtheearlyInternettothecurrentInternet.

Answer:Thetrendinthegraphicpresentationhasbeenasfollows:TextÆGraphicImagesÆVideoClipsÆFull-MotionVideo

2.8WhatInternettechnologyisthetelephonesystemusing?

Answer:SomecurrenttelephonesystemsusesVoice-OverIPtechnology.Readerisencouragedtorefertolaterchaptersfordetails.

2.9ExtendtheplotinFigure2.2,andestimatehowmanycomputerswillbeconnectedtotheInternetby2020.

2.10WhatimpactisInternettechnologyhavingonthecabletelevisionindustry?

2.11ListfournewInternetapplications,andtellthegroupsforwhicheachisimportant.

2.12WhyistheswitchfromwiredInternetaccesstowirelessInternetaccesssignificant?

Chapter3-InternetApplicationsAndNetworkProgramming

3.2WhatarethetwobasiccommunicationparadigmsusedintheInternet?

Answer:Therearevariousapproaches,butaccordingtotextbook,wecanspecifythemasStreamParadigmandMessageParadigm.

3.12GivesixcharacteristicsofInternetstreamcommunication.

Answer:CharacteristicsofInternetstreamcommunicationcanbelistedasfollows:

• Connection-oriented

• 1-to-1communication

• Sequenceofindividualbytes

• Arbitrarylengthtransfer

• Usedformultimediaapplications

• BuiltonUDPprotocol

3.4GivesixcharacteristicsofInternetmessagecommunication.

Answer:CharacteristicsofInternetmessagecommunicationcanbelistedasfollows:

• Connectionless

• Many-to-manycommunication

• Sequenceofindividualmessages

• Eachmessagelimitedto64Kbytes

• Usedbymostapplications

• BuiltonTCPprotocol

3.13Ifasenderusesthestreamparadigmandalwayssends1024bytesatatime,whatsizeblockscantheInternetdelivertoareceiver?

Answer:streamparadigmdoesnotprovideanyguaranteesforblocksizes,soalldependsonindividualtransfer.

3.6Ifasenderwantstohavecopiesofeachdatablockbeingsenttothreerecipients,whichparadigmshouldthesenderchoose?

Answer:Themessageparadigmissuitableforsuchtransfer,sinceitallowsmany-to-manycommunication

3.21WhatarethethreesurprisingaspectsoftheInternet’smessagedeliverysemantics?

Answer:TheInternet’smessagedeliveryhasthefollowingundesirablecharacteristics:

• Messagescanbelost

• Messagescanbeduplicated

• Messagescanbedeliveredout-of-order

3.19Givethegeneralalgorithmthataconnection-orientedsystemuses.

Answer:Theinteractionbetweentwoconnection-orientedpartiescanbesummarizedas:

• Setting-upconnectionbetweentwoparties

• Exchangeinformation

• Terminatetheconnection

3.25WhentwoapplicationscommunicateovertheInternet,whichoneistheserver?

Answer:Theapplicationthatwaitsforsomeotherapplicationstocontactiscalledserver,andtheapplicationthatcontactotheroneiscalledclient.

3.14Compareandcontrastaclientandserverapplicationbysummarizingcharacteristicsofeach.

3.5Whatisthedifferencebetweenaserverandaserver-classcomputer?

Answer:Theservertermreferstoaprogramthatwaitspassivelyforcommunication,

andnottothecomputeronwhichitexecutes.However,whenacomputerisdedicatedtorunningoneormoreserverprograms,thecomputeritselfissometimescalledaserver.HardwarevendorscontributetotheconfusionbecausetheyclassifycomputersthathavefastCPUs,largememories,andpowerfuloperatingsystemsasservermachines.

3.20Candataflowfromaclienttoaserver?Explain.

Answer:Yes,datamayflowinbothdirections(clientÆserverandserverÆclient).

3.15Listthepossiblecombinationsofclientsandserversagivencomputercanrun.

3.7Canallcomputersrunmultipleserviceseffectively?Whyorwhynot?

3.22Whattwoidentifiersareusedtospecifyaparticularserver?

Answer:Aparticularserverisidentifiedbythefollowingidentifiers:

• Anidentifierforthecomputeronwhichaserverruns(IPAddress)

• Anidentifierforaparticularserviceonthecomputer(PortNumber)

3.8Listthestepsaclientusestocontactaserverafterauserspecifiesadomainnamefortheserver.

Answer:Thestepsthataclientusestocontactaservercanbesummarizedasfollows:

• Startafterserverisalreadyrunning

• Obtainservernamefromuser

• UseDNStotranslatenametoIPaddress

• SpecifythattheserviceusesportN

Contactserverandinteract

Chapter4-TraditionalInternetApplications

4.1Whatdoesabrowsercache,andwhyiscachingused?

Answer:AbrowsersavestheLast-Modifieddateinformationalongwiththecachedcopy.Cachingprovidesanimportantoptimizationforwebaccessbecauseuserstendtovisitthesamewebsitesrepeatedly.

4.2WhatarethecharacteristicsofSMTP?

Answer:ThefollowingcanbegivenasmajorcharacteristicsofSMTP

• Followsastreamparadigm

• Usestextualcontrolmessages

• Onlytransferstextmessages

• Allowsasendertospecifyrecipients’namesandcheckeachname

• Sendsonecopyofagivenmessage

4.3Trueorfalse:amulti-nationalcompanycanchoosetodivideitsdomainnamehierarchyinsuchawaythatthecompanyhasadomainnameserverinEurope,oneinAsia,andoneinNorthAmerica.

Answer:OneofthemainfeaturesoftheDomainNameSystem(DNS)isautonomy.TheDNSisdesignedtoalloweachorganizationtoassignnamestocomputersortochangethosenameswithoutinformingacentralauthority.Eachorganizationisfreetochoosethedetailsofitsservers.Alargeorganizationthatrunsitsownservercanchoosetoplaceallnamesfortheorganizationinasinglephysicalserver,orcanchoosetodivideitsnamesamongmultipleservers.

4.4HowdoesXMLallowanapplicationtospecifyfieldssuchasanameandaddress?

Answer:XMLdoesnotassignanymeaningtotags,tagnamescanbecreatedasneeded.Inparticular,tagnamescanbeselectedtomakedataeasytoparseoraccess.Iftwocompaniesagreetoexchangecorporatetelephonedirectories,theycandefineanXMLformatthathasdataitemssuchasanemployee’sname,phonenumber,andoffice.

4.5Whatdetailsdoesanapplicationprotocolspecify?

Answer:Anapplicationprotocolmayspecifythefollowingdetails:

• Thesyntaxandsemanticsofmessagesthatcanbeexchanged

• Whethertheclientorserverinitiatesinteraction

• Actionstobetakenifanerrorarises

• Howthetwosidesknowwhentoterminatecommunication

4.6DoestheIDNAstandardrequirechangesinDNSservers?inDNSclients?Explain.

Answer:No,insteadofmodifyingtheunderlyingDNS,IDNAusesASCIItostoreallnames.Thatis,whengivenadomainnamethatcontainsanon-ASCIIcharacter,IDNAtranslatesthenameintoasequenceofASCIIcharacters,andstorestheresultintheDNS.Fortheclients(thelatestversionsofthewidely-usedbrowsers,FirefoxandInternetExplorer)canacceptanddisplaynon-ASCIIdomainnamesbecausetheyeachimplementIDNA.

4.7WhatarethefourHTTPrequesttypes,andwheniseachused?

Answer:ThefourcommonHTTPrequesttypesandtheirusagecanbegivenasfollows:

• GET:Requestsadocument;serverrespondsbysendingstatusinformationfollowedbyacopyofthedocument

• HEAD:Requestsstatusinformation;serverrespondsbysendingstatusinformation,butdoesnotsendacopyofthedocument

• POST:Sendsdatatoaserver;theserverappendsthedatatoaspecifieditem

• PUTSendsdatatoaserver;theserverusesthedatatocompletelyreplacethespecifieditem

4.8Whatarethetwomainemailaccessprotocols?

Answer:Twomajoremailaccessprotocolsare:

• PostOfficeProtocol(POP)

• InternetMailAccessProtocol(IMAP)

4.9Whereisanemailaccessprotocolused?

Answer:Anemailaccessprotocolisusedforuserstoaccesstheirmailboxes,eithertosendortoreceiveemailmessages.

4.10Whyisaprotocolforastandardizedservicedocumentedindependentofanimplementation?

4.11SearchthewebtofindoutaboutiterativeDNSlookup.Underwhatcircumstancesisiterativelookupused?

4.12CanabrowserusetransferprotocolsotherthanHTTP?Explain.

4.13Describethestepsabrowsertakestodeterminewhethertouseanitemfromitscache.

Answer:AbrowsersavestheLast-Modifieddateinformationalongwiththecachedcopy.Beforeitusesadocumentfromthelocalcache,abrowsermakesaHEADrequesttotheserverandcomparestheLast-Modifieddateoftheserver’scopytotheLast-Modifieddateonthecachedcopy.Ifthecachedversionisstale,thebrowserdownloadsthenewversion.Algorithm4.1summarizescaching.

4.14HowdoesanFTPserverknowtheportnumbertouseforadataconnection?

Answer:Beforemakingarequesttotheserver,aclientallocatesaprotocolportonitslocaloperatingsystemandsendstheportnumbertotheserver.Theclientbindstotheporttoawaitaconnection,andthentransmitsaPORTcommandoverthecontrolconnectiontoinformtheserverabouttheportnumberbeingused.

4.15Whatarethetwokeyaspectsofapplicationprotocols,andwhatdoeseachinclude?

Answer:Application-layerprotocolsspecifytwoaspectsofinteractionaregivenbelow:

• DataRepresentation:Syntaxofdataitemsthatareexchanged,specificform

usedduringtransfer,translationofintegers,characters,andfilesbetweencomputers

• DataTransfer:Interactionbetweenclientandserver,messagesyntaxandsemantics,validandinvalidexchangeerrorhandling,terminationofinteraction

4.16WhatistheoverallpurposeoftheDomainNameSystem?

Answer:TheoverallpurposeofDomainNameSystem(DNS)toprovideaservicethatmapshuman-readablesymbolicnamestocomputeraddresses.

4.17WhywasMIMEinvented?

Answer:Multi-purposeInternetMailExtensions(MIME)standardallowsthetransferofnon-textdatainamessage.MIMEspecifieshowabinaryfilecanbeencodedintoprintablecharacters,includedinamessage,anddecodedbythereceiver.

4.18Giveexamplesofwebprotocolsthatillustrateeachofthetwoaspectsofanapplicationprotocol.

4.19CanSMTPtransferanemailmessagethatcontainsaperiodonalinebyitself?Whyorwhynot?

4.20Trueorfalse:awebservermusthaveadomainnamethatbeginswithwww.Explain.

Answer:False.Anarbitrarycomputercanrunawebserver,evenifthecomputer’sdomainnamedoesnotcontainwww.Furthermore,acomputerthathasadomainnamebeginningwithwwwisnotrequiredtorunawebserver.Usingthefirstlabelinadomainnametodenoteaservice(e.g.,www)ismerelyaconventiontohelphumans.

4.21Trueorfalse:aDNSservercanreturnadifferentIPaddressforagivenname,dependingonwhetherthelookupspecifiesemailorwebservice.Explain.

Answer:Yes,acorporationmaydecidetousethenameforbothwebandemailservices.WiththeDNS,itispossibleforthecorporationtodividetheworkloadbetweenseparatecomputersbymappingtypeAlookupstoonecomputerandtypeMXlookupstoanother.

4.22SummarizethecharacteristicsofHTML.

Answer:HTMLhasthefollowinggeneralcharacteristics:

• Usesatextualrepresentation

• Describespagesthatcontainmultimedia

• Followsadeclarativeratherthanproceduralparadigm

• Providesmarkupspecificationsinsteadofformatting

• Permitsahyperlinktobeembeddedinanarbitraryobject

• Allowsadocumenttoincludemetadata

4.23Whendoesadomainnameserversendarequesttoanauthoritativeserverandwhendoesitanswertherequestwithoutsendingtotheauthoritativeserver?

4.24WhatarethefourpartsofaURL,andwhatpunctuationisusedtoseparatetheparts?

Answer:TheURLintofourcomponents:aprotocol,acomputername,adocumentname,andparameters.Thecomputernameandprotocolportareusedtoformaconnectiontotheserveronwhichthepageresides.Andthedocumentnameandparametersareusedtorequestaspecificpage.

4.25HowdoesabrowserknowwhetheranHTTPrequestissyntacticallyincorrectorwhetherthereferenceditemdoesnotexist?

4.26WhenauserrequestsanFTPdirectorylisting,howmanyTCPconnectionsareformed?Explain.

Answer:FTPusestwotypesofconnectionstoperformitsfunctionality,namely

• Acontrolconnectionisreservedforcommands.Eachtimetheserverneedstodownloadoruploadafile,theserveropensanewconnection.

• Adataconnectionisusedtotransferfiles.

4.27Listthethreetypesofprotocolsusedwithemail,anddescribeeach.

Answer:Threemajortypeofprotocolsbeingusedwithemailare:

• Transfer:Aprotocolusedtomoveacopyofanemailmessagefromonecomputertoanother

• Access:Aprotocolthatallowsausertoaccesstheirmailboxandtovieworsendemailmessages

• RepresentationAprotocolthatspecifiestheformatofanemailmessagewhenstoredondisk

4.28Trueorfalse:whenauserrunsanFTPapplication,theapplicationactsasbothaclientandserver.Explainyouranswer.

Answer:True,AnFTPmayinverttheclient-serverrelationshipfordataconnections.Thatis,whenopeningadataconnection,theclientactslikeaserver(i.e.,waitsforthedataconnection)andtheserveractslikeaclient(i.e.,initiatesthedataconnection).

4.29AssumingISOhasassignedNcountrycodes,howmanytop-leveldomainsexist?

4.30Accordingtotheoriginalemailparadigm,couldauserreceiveemailiftheuser’scomputerdidnotrunanemailserver?Explain.

PARTII

DataCommunications

Chapter5-OverviewofDataCommunications

5.1Whataretheconceptualpiecesofadatacommunicationssystem?

Acommunicationcanbeasimpleoracomplexonedependingonthetechnology,theneedandtheenvironmentbeingdeployed.Acomplexcommunicationsysteminvolvingmanypartiesanddifferenttypesofservicesmayinvolvethefollowingconceptualpieces:

• InformationSources

• SourceEncoderandSourceDecoder

• EncryptorandDecryptor

• ChannelEncoderandChannelDecoder

• MultiplexorandDemultiplexor

• ModulatorandDemodulator

• PhysicalChannelandTransmission

5.2Whichpieceofadatacommunicationssystempreventstransmissionerrorsfromcorruptingdata?

Thechannelencoderanddecoderpartsofacommunicationsystemareresponsibleforprotectingagainstpossibleerrorsduringtransmission.

5.3Whatthreedisciplinesareinvolvedindatacommunications?

Datacommunicationinvolvesconceptsfromphysics,mathematics,andelectricalengineering.

5.4Whatarethemotivationsfordatacommunications?

Thefollowingideasprovidemotivationfordatacommunicationsandhelpdefinethescope:

• Thesourcesofinformationcanbeofarbitrarytypes

• Transmissionusesaphysicalsystem

• Multiplesourcesofinformationcansharetheunderlyingmedium

5.5Whichpieceofadatacommunicationssystemhandlesanaloginput?

Theinputtoacommunicationsystemisfirsthandledbysourceencoder,whichreceivesinformationandtransformsintoaformatthatissuitableforfurtherprocessingfortransmission.

Chapter6-InformationSourcesandSignals

6.1Whenshownagraphofasinewave,whatisthequickestwaytodeterminewhetherthephaseiszero?

6.2Givethreeexamplesofinformationsourcesotherthancomputers.

6.3Whyaresinewavesfundamentaltodatacommunications?

Sinewavesareespeciallyimportantininformationsourcesbecausenaturalphenomenaproducesinewaves.

6.4Whyisananalogsignalusedtoapproximateadigitalsignal?

Fromanengineeringperspective,Fourier’sresultisimpracticalfordigitalsignalsbecauseaccuraterepresentationofadigitalsignalrequiresaninfinitesetofsinewavesinsteadanapproximateconversionofasignalfromdigitaltoanalogcanbeused.Soengineersbuildequipmenttogenerateanalogwavesthatcloselyapproximatethedigitalsignal

6.5Describethedifferencebetweenlossyandlosslesscompressions,andtellwheneachmightbeused.

• Lossycompressionisgenerallyusedwithdatathatahumanconsumes,suchasanimage,asegmentofvideo,oranaudiofile.Thekeyideaisthatthecompressiononlyneedstopreservedetailstothelevelofhumanperception.Thatis,achangeisacceptableifhumanscannotdetectthechange.

Losslesscompressionpreservestheoriginaldatawithoutanychange.Thus,losslesscompressioncanbeused

fordocumentsorinanysituationwheredatamustbepreservedexactly.

6.6Ifthemaximumfrequencyaudibletoahumanearis20,000Hz,atwhatratemusttheanalogsignalfromamicrophonebesampledwhenconvertingittodigital?

Thesamplingrate=2×fmax,sothesignalshouldbesampledat2x20,000=40,000Hz

6.7WhatdoesFourieranalysisofacompositewaveproduce?

WithFourieranalysis,itispossibletodecomposeacompositesignalintoitsconstituentparts,asetofsinefunctions,eachwithafrequency,amplitude,andphase.

6.8Whatistheanalogbandwidthofasignal?

Analogbandwidthofsignalcanbedefinedastobethedifferencebetweenthehighestandlowestfrequenciesoftheconstituentparts(i.e.,thehighestandlowestfrequenciesobtainedbyFourieranalysis)

6.9Nameacommonhouseholddevicethatemitsanaperiodicsignal.

6.10Onafrequencydomaingraph,whatdoesthey-axisrepresent?

Afrequencydomaingraphshowsasetofsimplesinewavesthatconstituteacompositefunction.They-axisgivestheamplitude,andthex-axisgivesthefrequency.

6.11Whydosomecodingtechniquesusemultiplesignalelementstorepresentasinglebit?

6.12Isbandwidtheasiertocomputefromatimedomainorfrequencydomainrepresentation?Why?

Itiseasiertocomputebandwidthfromafrequency-domainrepresentation

6.13WhattimeelapsesbetweensamplesforthePCMencodingusedinthetelephonesystem?

IncommercialPCMusedtelephonesystem8000samples/sandeachsampleisrepresentedby8bits.Therefore,datagenerationrateis64,000bits/s.Sotimeelapsedbetweensamplescanbecalculatedas1/rate=

1/64,000s.

6.14Supposeanengineerincreasesthenumberofpossiblesignallevelsfromtwotofour.Howmanymorebitscanbesentinthesameamountoftime?Explain.

Thenumberoflevelsthatcanberepresentedbynbitsisgivenby2n.Soifnumberoflevelschangesfrom2Æ

4,itmeansnumberofbitsgoesfrom1Æ2

6.15Stateanddescribethefourfundamentalcharacteristicsofasinewave.

Therearefourimportantcharacteristicsofsignalsthatrelatetosinewaves:

• Frequency:thenumberofoscillationsperunittime(usuallyseconds)

• Amplitude:thedifferencebetweenthemaximumandminimumsignalheights

• Phase:howfarthestartofthesinewaveisshiftedfromareferencetime

• Wavelength:thelengthofacycleasasignalpropagatesacrossamedium

6.16Whatisthebandwidthofadigitalsignal?Explain.

Accordingtothedefinitionofanalogbandwidth,adigitalsignalhasinfinitebandwidthbecauseFourieranalysisofadigitalsignalproducesaninfinitesetofsinewaveswithfrequenciesthatgrowtoinfinity.

6.17WhataspectofasignaldoestheManchesterEncodingusetorepresentabit?

ManchesterEncodingusesbittimingandtransitions.Itspecifiesthata1correspondstoatransitionfrom0voltstoapositivevoltagelevel.Correspondingly,a0correspondstoatransitionfromapositivevoltageleveltozero.Furthermore,thetransitionsoccurinthe“middle”ofthetimeslotallocatedtoabit.

6.18Whenconvertingananalogsignaltodigital,whatstepfollowssampling?

Aftersampling,thesamplesarematchedtoquantizationlevels.

6.19Whenisawaveclassifiedassimple?

Asignalisclassifiedassimplewhenconsistofasinglesinewavethatcannotbedecomposedfurther.

6.20Whatisthedefinitionofbaud?

Baudisdefinedasthenumberoftimesthatasignalcanchangepersecond

6.21Whatisasynchronizationerror?

Ifareceivermissestheexact/precisestart/stoppointofasignal,amismatchininterpretationproduceerrors.Sucherrorsarecalledsynchronizationerror.

6.22WhatisthechiefadvantageofaDifferentialManchesterEncoding?

Themostimportantpropertyofdifferentialencodingisthattheencodingworkscorrectlyevenifthetwowirescarryingthesignalareaccidentallyreversed.

.

Chapter7-TransmissionMedia

7.1Listthethreeformsofopticalfiber,andgivethegeneralpropertiesofeach.

Therearethreeformsofopticalfibersthatprovideachoicebetweenperformanceandcost:

• Multimode-StepIndex:Theleastexpensive,andisusedwhenperformanceisunimportant.Theboundarybetweenthefiberandthecladdingisabruptwhichcauseslighttoreflectfrequently.Therefore,dispersionishigh.

• Multimode-GradedIndex:Slightlymoreexpensivethanthestepindexfiber.Ithastheadvantageofmakingthedensityofthefiberincreaseneartheedge/Itreducesreflectionandlowersdispersion.

• SingleMode:Themostexpensive,andprovidestheleastdispersion.Thefiberhasasmallerdiameterandotherpropertiesthathelpreducereflection.Itisusedforlongdistancesandhigherbitrates.

7.2Ifatelephonesystemcanbecreatedwithasignal-to-noiseratioof40dBandananalogbandwidthof3000Hz,howmanybitspersecondcouldbetransmitted?

Firstweshouldconvert40dBtoareal