通信网(第二版)2

CommunicationNetworks(2ndEdition)Chapter2SolutionsSolutionstoChapter2(Note:solutiontoProblem62tobeadded)1.ExplainhowthenotionoflayeringandinternetworkingmaketherapidgrowthofapplicationssuchastheWorldWideWebpossible.Solution:Internetworkingallowsmanycomponentnetworkseachwithdifferentunderlyingtechnologyandoperationtoworktogetherandformonelargenetwork.AsnewnetworktechnologiesareintroducedtheycanbereadilyincorporatedintotheInternet.Thisprovidestheubiquitousconnectivityforapplicationslike(a)WhatuniversalsetofcommunicationservicesisprovidedbyTCP/IP?Solution:TheTCP/IPprotocolstackprovidestwobasictypesofcommunicationsservicesthroughitstwotransportlayerprotocols:TCPprovidesreliableconnection-orientedtransferofabytestream;UDPprovidesforbest-effortconnectionlesstransferofindividualmessages.TCP/IPprovideswithgloballyuniquelogicaladdressingthatenablesmachinesconnectedtotheInternettoaccessthesetwoservices.TheIPaddressingschemeisveryscalablebecauseofitshierarchicalstructure.NetworkinterfacenNetworkinterface2Networkinterface1IPUDPTCPRTPDNSSMTPHTTP2.(b)Howisindependencefromunderlyingnetworktechnologiesachieved?Solution:ThetwobasiccommunicationsservicesprovidedbyTCPandUDParebuiltontheconnectionlesspackettransferserviceprovidedbytheInternetProtocol(IP).ManynetworkinterfacesaredefinedtoLeon-Garcia/Widjaja1CommunicationNetworks(2ndEdition)Chapter2SolutionssupportIP.ThesalientpartoftheabovefigureisthatallofthehigherlayerprotocolsaccessthenetworkinterfacesthroughIP.Thisiswhatprovidestheabilitytooperateovermultiplenetworks.2.(c)Whateconomiesofscaleresultfrom(a)and(b)?Solution:OnceanetworkinterfaceforIPisdefinedforagivennetworktechnology,thenhostsconnectedusingthegivennetworktechnologycanconnecttotheInternet.ThisallowsthereachoftheInternettogrowrapidly,leveragingmultiplecoexistingnetworkstechnologies.ThusinvestmentinnewnetworktechnologiesextendsthereachoftheInternet.3.Whatdifferencedoesitmaketothenetworklayeriftheunderlyingdatalinklayerprovidesaconnection-orientedserviceversusaconnectionlessservice?Solution:Ifthedatalinklayerprovidesaconnection-orientedservicetothenetworklayer,thenthenetworklayermustprecedealltransferofinformationwithaconnectionsetupprocedure.Iftheconnection-orientedserviceincludesassurancesthatframesofinformationaretransferredcorrectlyandinsequencebythedatalinklayer,thenetworklayercanthenassumethatthepacketsitsendstoitsneighbortraverseanerror-freepipe.Ontheotherhand,ifthedatalinklayerisconnectionless,theneachframeissentindependentlythroughthedatalink,probablyinunconfirmedmanner(withoutacknowledgmentsorretransmissions).Inthiscasethenetworklayercannotmakeassumptionsaboutthesequencingorcorrectnessofthepacketsitexchangeswithitsneighbors.TheEthernetlocalareanetworkprovidesanexampleofconnectionlesstransferofdatalinkframes.ThetransferofframesusingType2serviceinLogicalLinkControl(discussedinChapter6)providesaconnection-orienteddatalinkcontrolexample.4.Supposetransmissionchannelsbecomevirtuallyerror-free.Isthedatalinklayerstillneeded?Solution:Thedatalinklayerisstillneededforframingthedataandforflowcontroloverthetransmissionchannel.InamultipleaccessmediumsuchasaLAN,thedatalinklayerisrequiredtocoordinateaccesstothesharedmediumamongthemultipleusers.5.Whyisthetransportlayernotpresentinsidethenetwork?Solution:Someofthefunctionsprovidedbythetransportlayercanbeprovidedinsidethenetworks,butotherfunctionscannot.Forexample,thetransportlayerprovidesfunctionsattheend-systemtocompensateforthelimitationsandimpairmentsofthenetworklayer,inordertomeetrequirements(e.g.QoS)oftheupperlayer.ForexampleinTCP/IP,IPprovidesonlybesteffortservice.Toprovidethereliableservicerequiredbysomeapplications-thatis,tocompensatefortheshortcomingsofbesteffortservice-TCPestablishesconnectionsandimplementserrorcontrolonanend-to-endbasis.Onecanimaginethataserviceprovidercouldincorporatethiserrorcontrolfunctionattheedgeofitsnetwork.Ontheotherhand,oneofthemainpurposesofthetransportlayeristoallowmultipleprocessesintheendsystemstoshareanetworkservice.Thiscannotbeachievedinsidethenetwork.Leon-Garcia/Widjaja2CommunicationNetworks(2ndEdition)Chapter2Solutions6.WhichOSIlayerisresponsibleforthefollowing?(a)Determiningthebestpathtoroutepackets.Thenetworklayerisconcernedwiththeselectionofpathsacrossthenetwork.(b)Providingend-to-endcommunicationswithreliableservice.Thetransportlayerisconcernedwithprovidingreliableserviceonanend-to-endbasisacrossthenetwork.(c)Providingnode-to-nodecommunicationswithreliableservice.Thedatalinklayerprovidesforthereliabletransferofinformationbetweenadjacentnodesinanetwork.7.Shouldconnectionestablishmentbeaconfirmedserviceoranunconfirmedservice?Whataboutdatatransferinaconnection-orientedservice?Connectionrelease?Solution:Ingeneral,theestablishmentofaconnectionneedstobeconfirmedbeforeinformationtransfercancommenceacro