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2024/12/2ComputerNetworks-1-Part21/44COMPUTERNETWORKSChapter01Introduction(2)ShoubaoYangComputerScienceDepartmentTel:3601540E-mail:syang@ustc.

edu.cnhttp://202.38.64.11/~syang2024/12/2ComputerNetworks-1-Part22/441.4ReferenceModels1.4.1OSIReferenceModel▪1983,InternationalS

tandardsOrganization(ISO)proposedthe“ISOOSI(OpenSystemsInterconnection)ReferenceModel”,OSI-rm▪Theprinciplesofla

yering➢Alayershouldbecreatedwhereadifferentabstractionisneeded➢Eachlayershouldperformawell-definedfunction➢Thefunctionofeachlaye

rshouldbechosentowardsdefininginternationallystandardizedprotocols➢Thelayerboundariesshouldbechosentomini

mizetheinformationflowacrosstheinterfaces➢Thenumberoflayersshouldbelargeenoughthatdistinctfunctionsandsm

allenoughthatthearchitecturedoesnotbecomeunwieldy2024/12/2ComputerNetworks-1-Part23/44TheOSIModel2024/12/2ComputerNetworks-1-Part24/44ThePhysicalL

ayer▪Essence:Describesthetransmissionofrawbitsintermsofmechanicalandelectricalissues:➢Example:Connecttwocomputersbymeansofawire:Setting

-3Vto-12Vonthewirecorrespondstoabinary1;+3Vto+12Visabinary0Thewireisnottobelongerthan15metersYoumayc

hangethevoltageatmost20,000timespersecond(Question:what’sthetransferrate?)2024/12/2ComputerNetworks-1-Part25/44TheDataLinkLayer▪Datalink

layeristotransformarawtransmissionfacilityintoalinethatappearsfreeofundetectedtransmissionerrorstothenetworklayer▪Observ

ation:Weneedtoatleastdetectbittransmissionerrors,sendbitsinframesthataddredundancytodetectsomethingwentwrongExamples:Addaparitybitto

every7transmittedbits:1saystherewereoddnumberof1’s;0saystherewereanevennumberof1’sAddachecksum(cyclicredundancycheck)

thatshouldmatchthebitsbeforeitAlso:Providethemechanismssothatfastsendersdon’toverwhelmslowreceivers(flowcontrol)20

24/12/2ComputerNetworks-1-Part26/44OSIDataLinkLayer▪Observation:Wealsoneedtospecifyhowanumberofcomputerscanshareacommonchannel(e

.g.wire),thatismediumaccesscontrolsublayer(MAC):1.Specifieshowoneoutofseveralcompetingsenders,iseventuallyallowedexclusi

veaccesstothewire2.Commonapproach1:listentoeachother;retreatwhenyouhearsomeoneelse,andtryagainlater3.Commonapproach2:waityourt

urnbypassingatokenbetweenallstationsWell-knownprotocols:Ethernet,tokenring,tokenbus,FDDI2024/12/2ComputerNetwor

ks-1-Part27/44TheNetworkLayer▪Essence:Describeshowrouting(andcongestion)istobedone.Mostlyneededinsubnets.Networklayercontrolssubnetoperations.

1.Howdowefindoutwhichcomputers/routersareinthenetwork?2.HowdowecalculatethebestroutefromAtoB?3.Whathappenswhenacomputer/route

rgoesdown?4.Shouldmulticasting/broadcastingbesupported?5.Whathappensifarouterbecomesoverloadedandstartsdroppingpackets?6.Canwe

detectandavoid“hotspots?”2024/12/2ComputerNetworks-1-Part28/44TheTransportLayer▪Observation:Thetransportlayeristoacceptdatafromabove,spl

ititupintosmallerunitsifneedbe,passthesetonetworklayer,andensurethatthepiecesallarrivecorrectlyattheotherend.G

enerallyoffersconnection-orientedaswellasconnectionlessservices,andvaryingdegreesofreliability.Thislayerprovidestheact

ualnetworkinterfacetoapplications1.Oftenprovidesnetworkinterfacethroughsockets(UNIX,Windows)2.Allowstos

etupaconnectiontoanotherapplication,andsubsequentlydeliverdatareliably,andintheorderthatitwassent3.Of

tenalsosupportforsecureconnections4.Alsosupportfordatagrams:unreliablemessagepassingonaper-messagebasis2024/12/2ComputerNet

works-1-Part29/44OSISessionandPresentationLayers▪Session:Thedumbestone(andill-defined)tellshowapplication

scansetup“long-lasting”communications,offersvariousservices,includingdialogcontrol,tokenmanagement,synchronization.▪Presentation:Describesev

erythingthatisneededtoexchangedatainaplatform-independentway,concernedwiththesyntaxandsemanticsofthei

nformationtransmitted.▪Example:thinkofbyteorderingindifferentcomputers,orpassing“binary”datathroughe-mai

l2024/12/2ComputerNetworks-1-Part210/44TheApplicationLayerEssence:Containstherest▪Traditional:Nameservices(DNS),

security,e-mail(SMTP),News(NNTP),Web(HTTP)▪Modern:Alltypesofmiddlewareprotocolstosupportdistributedsystems:➢Newtransferprotocolsforobject

systemslikeJava(RMI),CORBA(IIOP),DCOM(propriety)➢Specialprotocolstohandlereplication,faulttolerance,caching,datapersistence,etc.▪High-levelproto

cols:Specialapplication-levelprotocolsfore-commerce,banking,EDI,etc.2024/12/2ComputerNetworks-1-Part211/44DataTransmissionintheOSIModel202

4/12/2ComputerNetworks-1-Part212/441.4.2TheTCP/IPReferenceModel▪TCP/IPisaresultofprotocolresearchanddevelopmentconductedontheexperimentalpac

ketswitchednetwork,ARPANET,fundedbytheDefenseAdvancedResearchProjectsAgency(DARPA),andisgenerallyreferredtoastheTCP/IPprotocolsuite.▪Th

isprotocolsuiteconsistsofalargecollectionofprotocolsthathavebeenissuedasInternetstandardsbytheInternetActivitiesBoard(IAB).▪Thereisnooffici

alTCP/IPprotocolmodel.However,basedontheprotocolstandards,wecanorganizethecommunicationtaskforTCP/IPintofourrelativelyindependentlayers,frombottomtot

op:host-to-network(physicallayer+networkaccesslayer),internetlayer,transportlayer(host-to-host),applicationlayer.2024/12/2ComputerNetw

orks-1-Part213/441.4.2TheTCP/IPReferenceModel(2)2024/12/2ComputerNetworks-1-Part214/442024/12/2Comput

erNetworks-1-Part215/441.4.2TheTCP/IPReferenceModel(3)▪ThebadthingisthatTCP/IPdidnotmakeacleardistinctionbetweens

ervices,interfaces,andprotocols.Thatmakesitmuchhardertore-implementcertainlayers.▪TCP/IPprotocolsuiteiss

uccessfulbecause(1)itwastherewhenneeded(OSIimplementationswereterrible),(2)freelydistributedwiththeUNIXoperatingsystem.2024

/12/2ComputerNetworks-1-Part216/441.4.2TheTCP/IPReferenceModel(4)▪TheInternetLayer➢Apacket-switchingnetwork

basedonaconnectionlessinternetworklayer,permitshoststoinjectpacketsintoanynetworkandhavethemtravelindependentlytothedestination

.➢TheinternetlayerdefinesanofficialpacketformatandprotocolcalledIP(InternetProtocol),deliversIPpacke

tswheretheyaresupposedtogo.▪TheTransportLayer➢Allowspeerentitiesonthesourceanddestinationhoststocarryonan

conversation.TCP(TransmissionControlProtocol)isareliableconnection-orientedprotocol,whileUDP(UserDatagramProtocol)isan

unreliableconnectionlessprotocol.2024/12/2ComputerNetworks-1-Part217/441.4.2TheTCP/IPReferenceModel(5)▪TheApplicationLaye

r➢Containsallthehigher-levelprotocols:Telnet,FTP,SMTP,DNS,HTTP,…▪TheHost-NetworkLayer➢TheTCP/IPreferencemodeldoesno

treallysaymuchaboutwhathappenshere,excepttopointoutthatthehosthastoconnecttothenetworkusingsomeprotoc

olsoitcansendIPpacketstoit.Thisprotocolisnotdefinedandvariesfromhosttohostandnetworktonetwork.2024/12/2

ComputerNetworks-1-Part218/441.4.3ComparingOSIandTCP/IPModels▪ConceptscentraltotheOSImodel➢Services➢Interfaces➢

Protocols▪TheTCP/IPmodeldidnotoriginallyclearlydistinguishbetweenservice,interface,andprotocol.▪TCP/IP’sprotocolscamefirst,andthemod

elwasreallyjustadescriptionoftheexistingprotocols.▪Differentnumbersoflayers▪Anotherdifferenceisintheareaofconnectio

nlessversusconnection-orientedcommunication.2024/12/2ComputerNetworks-1-Part219/441.4.4ACritiqueoftheOSIMo

delandProtocols▪WhyOSIdidnottakeovertheworld➢Badtiming➢Badtechnology➢Badimplementations➢Badpolitics202

4/12/2ComputerNetworks-1-Part220/441.4.5ACritiqueoftheTCP/IPReferenceModel▪Problems➢Service,interface,an

dprotocolnotdistinguished➢Notageneralmodel➢Host-to-network“layer”notreallyalayer➢Nomentionofphysicalanddatalinklayers➢Minorproto

colsdeeplyentrenched,hardtoreplace▪Thehybridreferencemodeltobeusedinthisbook.2024/12/2ComputerNetworks-1-Part221/441.5ExampleNetworks▪Novel

lNetware▪TheARPAnet2024/12/2ComputerNetworks-1-Part222/441.5.1TheInternet(a)Structureofthetelephonesystem.(b)Baran’spropose

ddistributedswitchingsystem.TheARPANET2024/12/2ComputerNetworks-1-Part223/44TheARPANET(2)TheoriginalARPANETdesign.2024/12/2ComputerNetwo

rks-1-Part224/44TheARPANET(3)GrowthoftheARPANET(a)December1969.(b)July1970.(c)March1971.(d)April1972.(e)September1972.2024/12/2ComputerNe

tworks-1-Part225/44NSFNETTheNSFNETbackbonein1988.2024/12/2ComputerNetworks-1-Part226/44InternetUsage▪Traditionalapplic

ations(1970–1990)➢E-mail➢News➢Remotelogin➢Filetransfer2024/12/2ComputerNetworks-1-Part227/44Architectu

reoftheInternetFig.1.29OverviewoftheInternet.2024/12/2ComputerNetworks-1-Part228/44ATMVirtualCircuitsFig.1.30

Avirtualcircuit.Fig.1.31AnATMcell.2024/12/2ComputerNetworks-1-Part229/44TheATMReferenceModelTheATMrefere

ncemodel.2024/12/2ComputerNetworks-1-Part230/44TheATMReferenceModel(2)TheATMlayersandsublayersandtheirfunction

s.2024/12/2ComputerNetworks-1-Part231/441.5.3EthernetFig.1.34ArchitectureoftheoriginalEthernet.2024/12/2ComputerNetworks-1-P

art232/441.5.4WirelessLANs:802.11(a)Wirelessnetworkingwithabasestation.(b)Adhocnetworking.2024/12/2ComputerNet

works-1-Part233/44WirelessLANs(2)Therangeofasingleradiomaynotcovertheentiresystem.2024/12/2ComputerNetworks-1-Part234/4

4WirelessLANs(3)Amulticell802.11network.2024/12/2ComputerNetworks-1-Part235/441.6NetworkStandardization▪Who’sWhointheTelecommunicati

onsWorld▪Who’sWhointheInternationalStandardsWorld▪Who’sWhointheInternetStandardsWorld2024/12/2ComputerNetworks-1-Part236/44ITU▪Mainsectors•Radio

communications•TelecommunicationsStandardization•Development▪ClassesofMembers•Nationalgovernments•Sectormembers•Associatemembers•Regulatoryagenci

es2024/12/2ComputerNetworks-1-Part237/44IEEE802StandardsThe802workinggroups.Theimportantonesaremarkedwith*.Theonesmarkedwitharehibernating.Theon

emarkedwith†gaveup.2024/12/2ComputerNetworks-1-Part238/441.7MetricUnitsFig.1-39.Theprincipalmetricprefixes.2024/12/2ComputerNetworks-1-Par

t239/44CarrierDataCommunicationServices▪Observation:Aslongascarriersruletheworld(i.e.theyhavethecables),andcompetitionincreas

es,carrierswillincreaseandimprovetheirdatacommunicationservicesforthepublic:▪SMDS:SwitchedMultimegabitDataServicepri

marilyintendedtoconnectanumberofLANsthroughlong-haulnetworks(ownedbythecarrier)2024/12/2ComputerNetworks-1-Part240/44▪X.25:T

heOSInetworkprotocol(alsocoveringdatalinkandphysicallayer),intendedtoofferadatanetworkontopofan(existing)cableinfrastructure.Prettyold.▪Framere

lay:Extremelysimplefacilitythatallowsacustomertohireasinglehigh-bandwidthlink.Thereishardlyanysupportforerrordetection,rout

ing,flowcontrol,etc.CarrierDataCommunicationServices2024/12/2ComputerNetworks-1-Part241/44BroadbandISDN▪Observation:Thecarriersaregettingtoomany

networkstomaintain,andaremissingoneimportanttype(i.e.thoseownedbycable-TVs)▪Solution:Let’sinventacompletelynewnetwor

kthatshouldintegratedataandtelephony,andshouldbeabletoaccommodatealargerangeofbandwidthrequirements.2024/12/2Compute

rNetworks-1-Part242/44BroadbandISDN(2)▪Problem:Howdoyouimplementanetworkthatcanhandle622Mbps?➢Usesmallfixed-sizedcellsinsteadof

relativelylargepacketsandframes(thatcanalsovaryinlength):thisallowyoutobuildlow-latencyswitches➢Dotheleastyoucan(i.e.,don’

tprovidetoomanyservices):lettheapplicationshandleitall2024/12/2ComputerNetworks-1-Part243/44ATMforB-ISDN▪Solu

tion:AsynchronousTransferMode(ATM),bywhichcellsof53bytes(!)aresentalongvirtualcircuitsfromsendertodestination.▪Observation:Thismodeldoesnotf

itintoOSI’slayeredapproach:B-ISDNassumesthere’saseparatesignalingnetworktosetupaconnection2024/12/2ComputerNetworks-1-Part244/44Assignments▪Chap

ter01Pleaseseeseparatesheet.