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2024/12/2ComputerNetworks-1-Part21/44COMPUTERNETWORKSChapter01Introduction(2)ShoubaoYangComputerScienceDepartmentTel:36

01540E-mail:syang@ustc.edu.cnhttp://202.38.64.11/~syang2024/12/2ComputerNetworks-1-Part22/441.4ReferenceModels1.4.1OSIReferenceMo

del▪1983,InternationalStandardsOrganization(ISO)proposedthe“ISOOSI(OpenSystemsInterconnection)ReferenceMo

del”,OSI-rm▪Theprinciplesoflayering➢Alayershouldbecreatedwhereadifferentabstractionisneeded➢Eachlayershouldperforma

well-definedfunction➢Thefunctionofeachlayershouldbechosentowardsdefininginternationallystandardizedprotocols➢Thelayerboundariesshouldbechosentominim

izetheinformationflowacrosstheinterfaces➢Thenumberoflayersshouldbelargeenoughthatdistinctfunctionsandsmallenoughthatthearchitecturedoesn

otbecomeunwieldy2024/12/2ComputerNetworks-1-Part23/44TheOSIModel2024/12/2ComputerNetworks-1-Part24/44ThePhysic

alLayer▪Essence:Describesthetransmissionofrawbitsintermsofmechanicalandelectricalissues:➢Example:Conn

ecttwocomputersbymeansofawire:Setting-3Vto-12Vonthewirecorrespondstoabinary1;+3Vto+12Visabinary0Thewireisnottobelonge

rthan15metersYoumaychangethevoltageatmost20,000timespersecond(Question:what’sthetransferrate?)2024/12

/2ComputerNetworks-1-Part25/44TheDataLinkLayer▪Datalinklayeristotransformarawtransmissionfacilityintoalinethatappea

rsfreeofundetectedtransmissionerrorstothenetworklayer▪Observation:Weneedtoatleastdetectbittransmissionerrors,send

bitsinframesthataddredundancytodetectsomethingwentwrongExamples:Addaparitybittoevery7transmittedbits:1saystherewere

oddnumberof1’s;0saystherewereanevennumberof1’sAddachecksum(cyclicredundancycheck)thatshouldmatchthebitsbeforeitAlso:Providethemechanismssoth

atfastsendersdon’toverwhelmslowreceivers(flowcontrol)2024/12/2ComputerNetworks-1-Part26/44OSIDataLinkLayer▪Observation

:Wealsoneedtospecifyhowanumberofcomputerscanshareacommonchannel(e.g.wire),thatismediumaccesscontrolsublayer(MAC):1.Specifieshowoneoutofsev

eralcompetingsenders,iseventuallyallowedexclusiveaccesstothewire2.Commonapproach1:listentoeachother;retreatwheny

ouhearsomeoneelse,andtryagainlater3.Commonapproach2:waityourturnbypassingatokenbetweenallstationsWell-knownprotocols:Ethernet,tokenring,tokenbus

,FDDI2024/12/2ComputerNetworks-1-Part27/44TheNetworkLayer▪Essence:Describeshowrouting(andcongestion)istobedone.Mostlyneed

edinsubnets.Networklayercontrolssubnetoperations.1.Howdowefindoutwhichcomputers/routersareinthenetwork?2.Howdowec

alculatethebestroutefromAtoB?3.Whathappenswhenacomputer/routergoesdown?4.Shouldmulticasting/broadcastingbesupported?5.Whathappensifarouterbeco

mesoverloadedandstartsdroppingpackets?6.Canwedetectandavoid“hotspots?”2024/12/2ComputerNetworks-1-Part28/44

TheTransportLayer▪Observation:Thetransportlayeristoacceptdatafromabove,splititupintosmallerunitsifneedbe,passthesetonetworklayer,anden

surethatthepiecesallarrivecorrectlyattheotherend.Generallyoffersconnection-orientedaswellasconnectionlessservices,andvaryingd

egreesofreliability.Thislayerprovidestheactualnetworkinterfacetoapplications1.Oftenprovidesnetworkinterfacethroughsockets(UNIX,Windows)2.Allowst

osetupaconnectiontoanotherapplication,andsubsequentlydeliverdatareliably,andintheorderthatitwassent3.Oftenalsosupportforsecureconnectio

ns4.Alsosupportfordatagrams:unreliablemessagepassingonaper-messagebasis2024/12/2ComputerNetworks-1-Part29/44OSISe

ssionandPresentationLayers▪Session:Thedumbestone(andill-defined)tellshowapplicationscansetup“long-lasting”communications,offersvariousservices,incl

udingdialogcontrol,tokenmanagement,synchronization.▪Presentation:Describeseverythingthatisneededtoexchangedatainaplatform-independentway,concer

nedwiththesyntaxandsemanticsoftheinformationtransmitted.▪Example:thinkofbyteorderingindifferentcomputers,orpassing“binary”datathroughe-mail2024/12/2

ComputerNetworks-1-Part210/44TheApplicationLayerEssence:Containstherest▪Traditional:Nameservices(DNS),security,e-mail(SMTP),News(NNTP),

Web(HTTP)▪Modern:Alltypesofmiddlewareprotocolstosupportdistributedsystems:➢NewtransferprotocolsforobjectsystemslikeJava(RMI

),CORBA(IIOP),DCOM(propriety)➢Specialprotocolstohandlereplication,faulttolerance,caching,datapersistence,etc.▪High-levelprotocols:Specialapplic

ation-levelprotocolsfore-commerce,banking,EDI,etc.2024/12/2ComputerNetworks-1-Part211/44DataTransmissionintheOSIModel2024/12/2Comp

uterNetworks-1-Part212/441.4.2TheTCP/IPReferenceModel▪TCP/IPisaresultofprotocolresearchanddevelopmentconductedontheexperimentalpacket

switchednetwork,ARPANET,fundedbytheDefenseAdvancedResearchProjectsAgency(DARPA),andisgenerallyreferredtoastheTCP/IPprotocolsuite.▪

ThisprotocolsuiteconsistsofalargecollectionofprotocolsthathavebeenissuedasInternetstandardsbytheInternetActivities

Board(IAB).▪ThereisnoofficialTCP/IPprotocolmodel.However,basedontheprotocolstandards,wecanorganizethecommunicationtaskforT

CP/IPintofourrelativelyindependentlayers,frombottomtotop:host-to-network(physicallayer+networkaccesslayer),internetlayer,transportlay

er(host-to-host),applicationlayer.2024/12/2ComputerNetworks-1-Part213/441.4.2TheTCP/IPReferenceModel(2)2024/12/2ComputerNetworks-1-Part214/442024

/12/2ComputerNetworks-1-Part215/441.4.2TheTCP/IPReferenceModel(3)▪ThebadthingisthatTCP/IPdidnotmakeacle

ardistinctionbetweenservices,interfaces,andprotocols.Thatmakesitmuchhardertore-implementcertainlayers.▪TCP/IPprotocolsuiteissuccessfulbeca

use(1)itwastherewhenneeded(OSIimplementationswereterrible),(2)freelydistributedwiththeUNIXoperatingsystem.2024/12/2ComputerNet

works-1-Part216/441.4.2TheTCP/IPReferenceModel(4)▪TheInternetLayer➢Apacket-switchingnetworkbasedonaconnectio

nlessinternetworklayer,permitshoststoinjectpacketsintoanynetworkandhavethemtravelindependentlytothedestination.➢Theinternetlayerdefinesanoff

icialpacketformatandprotocolcalledIP(InternetProtocol),deliversIPpacketswheretheyaresupposedtogo.▪TheTransportLayer➢Allowspeerentitiesonthesourc

eanddestinationhoststocarryonanconversation.TCP(TransmissionControlProtocol)isareliableconnection-orientedprotocol,whileUDP(UserDatagram

Protocol)isanunreliableconnectionlessprotocol.2024/12/2ComputerNetworks-1-Part217/441.4.2TheTCP/IPRe

ferenceModel(5)▪TheApplicationLayer➢Containsallthehigher-levelprotocols:Telnet,FTP,SMTP,DNS,HTTP,…▪TheHost-NetworkLayer➢The

TCP/IPreferencemodeldoesnotreallysaymuchaboutwhathappenshere,excepttopointoutthatthehosthastoconnecttothene

tworkusingsomeprotocolsoitcansendIPpacketstoit.Thisprotocolisnotdefinedandvariesfromhosttohostandnetworktonetwork.2024/12/2ComputerNetwor

ks-1-Part218/441.4.3ComparingOSIandTCP/IPModels▪ConceptscentraltotheOSImodel➢Services➢Interfaces➢Protocols▪TheT

CP/IPmodeldidnotoriginallyclearlydistinguishbetweenservice,interface,andprotocol.▪TCP/IP’sprotocolscamefirst,andthemodelwasreallyjustadescriptiono

ftheexistingprotocols.▪Differentnumbersoflayers▪Anotherdifferenceisintheareaofconnectionlessversusconnection-orientedcommunication.2024/12/2Compu

terNetworks-1-Part219/441.4.4ACritiqueoftheOSIModelandProtocols▪WhyOSIdidnottakeovertheworld➢Badtiming➢Badtechnology➢Badimpl

ementations➢Badpolitics2024/12/2ComputerNetworks-1-Part220/441.4.5ACritiqueoftheTCP/IPReferenceModel▪Problems➢Service,interface,andprotocoln

otdistinguished➢Notageneralmodel➢Host-to-network“layer”notreallyalayer➢Nomentionofphysicalanddatalinklayers➢Mi

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

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

itchingsystem.TheARPANET2024/12/2ComputerNetworks-1-Part223/44TheARPANET(2)TheoriginalARPANETdesign.2024/12/2ComputerNetwor

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

/12/2ComputerNetworks-1-Part225/44NSFNETTheNSFNETbackbonein1988.2024/12/2ComputerNetworks-1-Part226/44InternetUsage▪Traditionalapplications(19

70–1990)➢E-mail➢News➢Remotelogin➢Filetransfer2024/12/2ComputerNetworks-1-Part227/44ArchitectureoftheInternetFig.1.29OverviewoftheInternet.2024/1

2/2ComputerNetworks-1-Part228/44ATMVirtualCircuitsFig.1.30Avirtualcircuit.Fig.1.31AnATMcell.2024/12/2ComputerNetworks-1-Part229/44Th

eATMReferenceModelTheATMreferencemodel.2024/12/2ComputerNetworks-1-Part230/44TheATMReferenceModel(2)TheATMlayersandsublayersandtheirfun

ctions.2024/12/2ComputerNetworks-1-Part231/441.5.3EthernetFig.1.34ArchitectureoftheoriginalEthernet.2024/12/2ComputerNetworks-1-Part232/441.5.4Wir

elessLANs:802.11(a)Wirelessnetworkingwithabasestation.(b)Adhocnetworking.2024/12/2ComputerNetworks-1-Part233/44WirelessLANs(2)Therang

eofasingleradiomaynotcovertheentiresystem.2024/12/2ComputerNetworks-1-Part234/44WirelessLANs(3)Amulticell802.11network.2024/12/2Com

puterNetworks-1-Part235/441.6NetworkStandardization▪Who’sWhointheTelecommunicationsWorld▪Who’sWhointheInternational

StandardsWorld▪Who’sWhointheInternetStandardsWorld2024/12/2ComputerNetworks-1-Part236/44ITU▪Mainsectors•Radiocommunications•Telecommunicati

onsStandardization•Development▪ClassesofMembers•Nationalgovernments•Sectormembers•Associatemembers•Regulat

oryagencies2024/12/2ComputerNetworks-1-Part237/44IEEE802StandardsThe802workinggroups.Theimportantonesaremarkedwith*

.Theonesmarkedwitharehibernating.Theonemarkedwith†gaveup.2024/12/2ComputerNetworks-1-Part238/441.7MetricUnitsFig.1-39.Theprincipalmetricpre

fixes.2024/12/2ComputerNetworks-1-Part239/44CarrierDataCommunicationServices▪Observation:Aslongascarriersruletheworld(i.e.

theyhavethecables),andcompetitionincreases,carrierswillincreaseandimprovetheirdatacommunicationservicesforthepublic:▪SMDS:SwitchedM

ultimegabitDataServiceprimarilyintendedtoconnectanumberofLANsthroughlong-haulnetworks(ownedbythecarrie

r)2024/12/2ComputerNetworks-1-Part240/44▪X.25:TheOSInetworkprotocol(alsocoveringdatalinkandphysicalla

yer),intendedtoofferadatanetworkontopofan(existing)cableinfrastructure.Prettyold.▪Framerelay:Extremelysimplefacilitythatallows

acustomertohireasinglehigh-bandwidthlink.Thereishardlyanysupportforerrordetection,routing,flowcontrol,etc.CarrierDataCommunicationServices2024/12/2Co

mputerNetworks-1-Part241/44BroadbandISDN▪Observation:Thecarriersaregettingtoomanynetworkstomaintain,andaremissingoneimportanttype(i.e.thoseownedbyca

ble-TVs)▪Solution:Let’sinventacompletelynewnetworkthatshouldintegratedataandtelephony,andshouldbeabletoaccommodateala

rgerangeofbandwidthrequirements.2024/12/2ComputerNetworks-1-Part242/44BroadbandISDN(2)▪Problem:Howdoyouimplemen

tanetworkthatcanhandle622Mbps?➢Usesmallfixed-sizedcellsinsteadofrelativelylargepacketsandframes(thatcanalsovaryinlengt

h):thisallowyoutobuildlow-latencyswitches➢Dotheleastyoucan(i.e.,don’tprovidetoomanyservices):lettheapplicationshandleitall2024/12/2ComputerNetworks

-1-Part243/44ATMforB-ISDN▪Solution:AsynchronousTransferMode(ATM),bywhichcellsof53bytes(!)aresentalongvirtualcircu

itsfromsendertodestination.▪Observation:ThismodeldoesnotfitintoOSI’slayeredapproach:B-ISDNassumesthere’saseparatesignalin

gnetworktosetupaconnection2024/12/2ComputerNetworks-1-Part244/44Assignments▪Chapter01Pleaseseeseparatesheet.