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2024/12/2ComputerNetworks-1-Part21/44COMPUTERNETWORKSChapter01Introduction(2)ShoubaoYangComputerScienceDepartmen

tTel:3601540E-mail:syang@ustc.edu.cnhttp://202.38.64.11/~syang2024/12/2ComputerNetworks-1-Part22/441.4ReferenceMod

els1.4.1OSIReferenceModel▪1983,InternationalStandardsOrganization(ISO)proposedthe“ISOOSI(OpenSystemsI

nterconnection)ReferenceModel”,OSI-rm▪Theprinciplesoflayering➢Alayershouldbecreatedwhereadifferentab

stractionisneeded➢Eachlayershouldperformawell-definedfunction➢Thefunctionofeachlayershouldbechosentowardsdefiningint

ernationallystandardizedprotocols➢Thelayerboundariesshouldbechosentominimizetheinformationflowacrosstheinterfaces➢Thenu

mberoflayersshouldbelargeenoughthatdistinctfunctionsandsmallenoughthatthearchitecturedoesnotbecomeunwieldy2024/12/2ComputerNetworks-1-Part23/44TheOSI

Model2024/12/2ComputerNetworks-1-Part24/44ThePhysicalLayer▪Essence:Describesthetransmissionofrawbitsintermsofmech

anicalandelectricalissues:➢Example:Connecttwocomputersbymeansofawire:Setting-3Vto-12Vonthewirecorrespondstoabinary1;+3Vto+12Visabinary0Thewire

isnottobelongerthan15metersYoumaychangethevoltageatmost20,000timespersecond(Question:what’sthetransferrate?)2024/12/2Comp

uterNetworks-1-Part25/44TheDataLinkLayer▪Datalinklayeristotransformarawtransmissionfacilityintoalinethatappearsfreeofundetected

transmissionerrorstothenetworklayer▪Observation:Weneedtoatleastdetectbittransmissionerrors,sendbitsinframe

sthataddredundancytodetectsomethingwentwrongExamples:Addaparitybittoevery7transmittedbits:1saystherewereoddnumberof1’s;0saystherewereanevennumberof1’

sAddachecksum(cyclicredundancycheck)thatshouldmatchthebitsbeforeitAlso:Providethemechanismssothatfastsendersdon’toverwhelmslowreceivers(flowcont

rol)2024/12/2ComputerNetworks-1-Part26/44OSIDataLinkLayer▪Observation:Wealsoneedtospecifyhowanumberofcomp

uterscanshareacommonchannel(e.g.wire),thatismediumaccesscontrolsublayer(MAC):1.Specifieshowoneoutofseveralcompeti

ngsenders,iseventuallyallowedexclusiveaccesstothewire2.Commonapproach1:listentoeachother;retreatwhenyouhearsomeoneelse,andtr

yagainlater3.Commonapproach2:waityourturnbypassingatokenbetweenallstationsWell-knownprotocols:Ethernet,t

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

stlyneededinsubnets.Networklayercontrolssubnetoperations.1.Howdowefindoutwhichcomputers/routersareinthenetwor

k?2.HowdowecalculatethebestroutefromAtoB?3.Whathappenswhenacomputer/routergoesdown?4.Shouldmulticasting/br

oadcastingbesupported?5.Whathappensifarouterbecomesoverloadedandstartsdroppingpackets?6.Canwedetectandavoid“hotspots?”2024/12/2ComputerNetworks-1-Par

t28/44TheTransportLayer▪Observation:Thetransportlayeristoacceptdatafromabove,splititupintosmallerunitsifneedbe,passthesetonetworklayer,andensurethat

thepiecesallarrivecorrectlyattheotherend.Generallyoffersconnection-orientedaswellasconnectionlessservices,andv

aryingdegreesofreliability.Thislayerprovidestheactualnetworkinterfacetoapplications1.Oftenprovidesnetworkinterfacethroughso

ckets(UNIX,Windows)2.Allowstosetupaconnectiontoanotherapplication,andsubsequentlydeliverdatareliably,andintheor

derthatitwassent3.Oftenalsosupportforsecureconnections4.Alsosupportfordatagrams:unreliablemessagepassingonaper-messagebasis2024/12/2ComputerNe

tworks-1-Part29/44OSISessionandPresentationLayers▪Session:Thedumbestone(andill-defined)tellshowapplicationscansetup“long-lasting”commun

ications,offersvariousservices,includingdialogcontrol,tokenmanagement,synchronization.▪Presentation:Describeseverythingthatisneededtoexch

angedatainaplatform-independentway,concernedwiththesyntaxandsemanticsoftheinformationtransmitted.▪Example:thin

kofbyteorderingindifferentcomputers,orpassing“binary”datathroughe-mail2024/12/2ComputerNetworks-1-Part210/44TheApp

licationLayerEssence:Containstherest▪Traditional:Nameservices(DNS),security,e-mail(SMTP),News(NNTP),Web(HTTP)▪Modern:Alltypesofmiddlewareproto

colstosupportdistributedsystems:➢NewtransferprotocolsforobjectsystemslikeJava(RMI),CORBA(IIOP),DCOM(pr

opriety)➢Specialprotocolstohandlereplication,faulttolerance,caching,datapersistence,etc.▪High-levelprotocols:Specialapplication-le

velprotocolsfore-commerce,banking,EDI,etc.2024/12/2ComputerNetworks-1-Part211/44DataTransmissionintheOSIModel2024/12/2ComputerNetwor

ks-1-Part212/441.4.2TheTCP/IPReferenceModel▪TCP/IPisaresultofprotocolresearchanddevelopmentconductedontheexperimentalpacketswitchednetwork,ARPANET,f

undedbytheDefenseAdvancedResearchProjectsAgency(DARPA),andisgenerallyreferredtoastheTCP/IPprotocolsuite.▪Thispr

otocolsuiteconsistsofalargecollectionofprotocolsthathavebeenissuedasInternetstandardsbytheInternetActivities

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

pendentlayers,frombottomtotop:host-to-network(physicallayer+networkaccesslayer),internetlayer,transportlaye

r(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/IPdid

notmakeacleardistinctionbetweenservices,interfaces,andprotocols.Thatmakesitmuchhardertore-implementcertainlayers.▪TCP/IPp

rotocolsuiteissuccessfulbecause(1)itwastherewhenneeded(OSIimplementationswereterrible),(2)freelydistrib

utedwiththeUNIXoperatingsystem.2024/12/2ComputerNetworks-1-Part216/441.4.2TheTCP/IPReferenceModel(4)▪TheInte

rnetLayer➢Apacket-switchingnetworkbasedonaconnectionlessinternetworklayer,permitshoststoinjectpacketsintoany

networkandhavethemtravelindependentlytothedestination.➢Theinternetlayerdefinesanofficialpacketformat

andprotocolcalledIP(InternetProtocol),deliversIPpacketswheretheyaresupposedtogo.▪TheTransportLayer➢Allowspeerentitieso

nthesourceanddestinationhoststocarryonanconversation.TCP(TransmissionControlProtocol)isareliableconnection-ori

entedprotocol,whileUDP(UserDatagramProtocol)isanunreliableconnectionlessprotocol.2024/12/2ComputerNe

tworks-1-Part217/441.4.2TheTCP/IPReferenceModel(5)▪TheApplicationLayer➢Containsallthehigher-levelprotocols:Tel

net,FTP,SMTP,DNS,HTTP,…▪TheHost-NetworkLayer➢TheTCP/IPreferencemodeldoesnotreallysaymuchaboutwhathappens

here,excepttopointoutthatthehosthastoconnecttothenetworkusingsomeprotocolsoitcansendIPpacketstoit.Thisprotocolisnotdefinedandvariesfr

omhosttohostandnetworktonetwork.2024/12/2ComputerNetworks-1-Part218/441.4.3ComparingOSIandTCP/IPModels▪ConceptscentraltotheOSImo

del➢Services➢Interfaces➢Protocols▪TheTCP/IPmodeldidnotoriginallyclearlydistinguishbetweenservice,interface,andprotocol.▪TCP/IP’sprotocolscamefirst,

andthemodelwasreallyjustadescriptionoftheexistingprotocols.▪Differentnumbersoflayers▪Anotherdifferen

ceisintheareaofconnectionlessversusconnection-orientedcommunication.2024/12/2ComputerNetworks-1-Part219/441.4.

4ACritiqueoftheOSIModelandProtocols▪WhyOSIdidnottakeovertheworld➢Badtiming➢Badtechnology➢Badimplementatio

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

tocolnotdistinguished➢Notageneralmodel➢Host-to-network“layer”notreallyalayer➢Nomentionofphysicalanddatalinklayers➢Minorprotocolsdeep

lyentrenched,hardtoreplace▪Thehybridreferencemodeltobeusedinthisbook.2024/12/2ComputerNetworks-1-Part221/441.5ExampleNetworks▪NovellNetware▪TheA

RPAnet2024/12/2ComputerNetworks-1-Part222/441.5.1TheInternet(a)Structureofthetelephonesystem.(b)Baran’sproposeddistributedswitchingsys

tem.TheARPANET2024/12/2ComputerNetworks-1-Part223/44TheARPANET(2)TheoriginalARPANETdesign.2024/12/2ComputerNetworks-1-Part224/44Th

eARPANET(3)GrowthoftheARPANET(a)December1969.(b)July1970.(c)March1971.(d)April1972.(e)September1972.2024/12/2ComputerNetworks-1-Par

t225/44NSFNETTheNSFNETbackbonein1988.2024/12/2ComputerNetworks-1-Part226/44InternetUsage▪Traditionalappli

cations(1970–1990)➢E-mail➢News➢Remotelogin➢Filetransfer2024/12/2ComputerNetworks-1-Part227/44ArchitectureoftheInternetFig.1.29OverviewoftheInte

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

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

ndsublayersandtheirfunctions.2024/12/2ComputerNetworks-1-Part231/441.5.3EthernetFig.1.34ArchitectureoftheoriginalEt

hernet.2024/12/2ComputerNetworks-1-Part232/441.5.4WirelessLANs:802.11(a)Wirelessnetworkingwithabasestation.(b)Adhocnetworking.2024/12/2Computer

Networks-1-Part233/44WirelessLANs(2)Therangeofasingleradiomaynotcovertheentiresystem.2024/12/2Computer

Networks-1-Part234/44WirelessLANs(3)Amulticell802.11network.2024/12/2ComputerNetworks-1-Part235/441.6NetworkStandardization▪Who’sWhointheTelecom

municationsWorld▪Who’sWhointheInternationalStandardsWorld▪Who’sWhointheInternetStandardsWorld2024/12/2ComputerNetworks-1-Part236/44ITU▪Mainse

ctors•Radiocommunications•TelecommunicationsStandardization•Development▪ClassesofMembers•Nationalgovernments•Sectormembers•

Associatemembers•Regulatoryagencies2024/12/2ComputerNetworks-1-Part237/44IEEE802StandardsThe802workinggroups.Theimportantonesaremarkedwi

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

refixes.2024/12/2ComputerNetworks-1-Part239/44CarrierDataCommunicationServices▪Observation:Aslongascarriersrulet

heworld(i.e.theyhavethecables),andcompetitionincreases,carrierswillincreaseandimprovetheirdatacommunicationservicesfort

hepublic:▪SMDS:SwitchedMultimegabitDataServiceprimarilyintendedtoconnectanumberofLANsthroughlong-haulnetworks(ownedbythecarrier)2024/12/2ComputerN

etworks-1-Part240/44▪X.25:TheOSInetworkprotocol(alsocoveringdatalinkandphysicallayer),intendedtoofferadatanetworkontopof

an(existing)cableinfrastructure.Prettyold.▪Framerelay:Extremelysimplefacilitythatallowsacustomertohireasinglehigh-bandwidthlink.Thereishardlyan

ysupportforerrordetection,routing,flowcontrol,etc.CarrierDataCommunicationServices2024/12/2ComputerNetworks-1-Part241/44BroadbandISDN▪Ob

servation:Thecarriersaregettingtoomanynetworkstomaintain,andaremissingoneimportanttype(i.e.thoseownedbycable-TVs)

▪Solution:Let’sinventacompletelynewnetworkthatshouldintegratedataandtelephony,andshouldbeabletoaccommodatealargerangeofbandwidthrequirements.20

24/12/2ComputerNetworks-1-Part242/44BroadbandISDN(2)▪Problem:Howdoyouimplementanetworkthatcanhandle622Mbps?➢Us

esmallfixed-sizedcellsinsteadofrelativelylargepacketsandframes(thatcanalsovaryinlength):thisallowyoutobuildlow-latencyswitches➢Dotheleastyoucan(i.e

.,don’tprovidetoomanyservices):lettheapplicationshandleitall2024/12/2ComputerNetworks-1-Part243/44ATMforB-ISDN▪Solutio

n:AsynchronousTransferMode(ATM),bywhichcellsof53bytes(!)aresentalongvirtualcircuitsfromsendertodestination

.▪Observation:ThismodeldoesnotfitintoOSI’slayeredapproach:B-ISDNassumesthere’saseparatesignalingnetworktosetupaconnection2024/12/2Compute

rNetworks-1-Part244/44Assignments▪Chapter01Pleaseseeseparatesheet.