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Appendix A. Overview of the OSI Seven-Layer Networking Reference Model

Appendix A. Overview of the OSI Seven-Layer Networking Reference Model

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It'sOnlyaModel!

TheOSInetworkingmodelisjustthatamodel.It'sareference

youcanusewhendiscussingnetworkingwithcolleagues.The

modelspecifiessevenlayers,whichyoucanthinkofas

separatemodules,eachofwhichperformsaspecificsetof

functions.Eachlayerinthemodelcommunicateswithadjacent

layersinthemodelusingastandarddefinedinterface.

Therefore,theinternalworkingsofeachlayercanbeleftupto

thevendortodevelop.Allthatmattersisthatallthelayers

worktogether,nomatterwhichvendorprovidesthem.For

example,networkadaptersandnetworkcablingfallintothe

bottomlayer,thePhysicallayer.Networkcardsaredesignedto

workwiththesoftwarethatfallsintothenexthigherlayer,the

DataLinklayer.

FigureA.1showshowthelayersofthemodelinteractwithone

another,fromtwoendsofanetworkconnection.



FigureA.1.EachlayerintheOSIreferencemodel

providesfunctionstoadjacentlayersinthe

model.



AsyoucanseeinFigureA.1,arrowsshowtheflowof

informationdownthestackfromonecomputer.Whenthe

informationreachesthePhysicallayer,thecomponentsatthat

layeracttodeliverthedatatotheremotesystem.Atthe

remotesystem,thePhysicallayerreceivestheelectrical(or

light)impulses,convertsthemintotheappropriatemessage

format,andpassestheinformationbackupthestacksothat

thedataeventuallyreachestheapplicationforwhichitwas

intended.

AlsoinFigureA.1youcanseethatarrowspointbothways

betweenthelayersononesystemandthecorrespondinglayers

ontheothersystem.Thisimpliesthatfromalogicalpointof

view,eachlayerinthemodelperformsfunctionsasifitwere

talkingdirectlytoitscorrespondinglayerattheremotesystem.

Eachlayerisunawareofwhatisgoingoninlayersunderneath

itorhowthemessageisdeliveredtothematchinglayeronthe

remotemachine.

Forexample,TCPbreaksuplargemessagesintosmaller



messagescalledsegments.Thesesegmentsarethenpassedto

lowerlayersthatencapsulatetheminIPdatagramsandthen

intoaphysicallayerprotocol,suchasEthernet.Onthe

receivingend,theTCPsoftwaredoesnothavetoknowthat

Ethernet,Token-Ring,oranyothertechnologywasusedto

deliverthemessage.Instead,theTCPsoftwareontheremote

systemreceivesthesegmentsthattheTCPsoftwareonthe

sendingsystemoriginallysent.



Encapsulation

Mostlayersattachinformationtothedatathattheypassto

lowerlayers.Thisdataiscalledheaderinformation.For

example,anIPdatagramcontainsheaderinformationsuchas

thesourceanddestinationIPaddressesandportnumbers.

WhentheIPdatagramispasseddownthestack,itcanbe

encapsulatedinanEthernetframe.AnEthernetframeaddsits

ownheaderinformation(refertoChapter13,"Ethernet:The

UniversalStandard").Here,insteadofIPaddresses,physical

MACaddressesareusedintheEthernetheader.

Onthereceivingendofthecommunication,theEthernet

headerinformationisstrippedoffbeforetheremainingdatais

passedupthestack.AttheIPleveltheIPheaderisremoved

andthedataispassedbackuptotheTCPsoftware,andsoon.

Thus,eachlayeroneachcomputeractuallysees,withfew

exceptions,onlytheheaderinformationthatwasattachedto

theoriginalmessagebythecorrespondinglayerontheother

machine.Thisishowthelayerslogicallyinteract,regardlessof

howlayersaboveorbeneathoperateinternally.

Thefollowingsectionsdescribethefunctionsthatareperformed

ateachlayer,startingfromthebottom(Physicallayer)and

workinguptothetop.Animportantpointtorememberisthat

somevendorscombinetwoormorelayersintoasingle

softwareapplication.Asmentioned,theOSISeven-Layer



ReferenceModelisjustthat,amodel.It'sawaytodiscuss

networkingtechnologiesinarationalmannerthatprofessionals

canunderstand.Itdoesnotmeanthatallnetworkingproducts

mustconformtothismodel!



PhysicalLayer

ThePhysicallayercomprisesthephysicalcomponentsthat

makeupthenetworkinghardwareofthenetwork,includingthe

networkadapter,connectors,networkmedia(copperwiresor

opticalcables),andsoon.Tosumitupinasimplesentence:

ThePhysicallayergetsthedatafromheretothere.Thislayer

coversbothelectricalandmechanicalaspectsofthenetwork.

Forexample,themethodusedtoencodedataintoelectricalor

lightsignalsonthenetworkmediaisdecidedatthislayer.



DataLinkLayer

TheDataLinklayerservesseveralfunctions,whichtheIEEE

hasdividedintotwosublayers.ThefirstistheLogicalLink

Control(LLC)andthesecondistheMediaAccessControl

(MAC).Asawhole,theDataLinklayerisresponsiblefor

transmittingdatafromoneplacetoanotheranddoingsome

minimalerrorcorrection.ThepurposeoftheLLCistoprovide

ServiceAccessPoints(SAPs)thatdevicescanusetosend

information.TheMACcomponenttakescareoftransmittingthe

dataandcorrectingerrors.

TheDataLinklayerisresponsibleforputtingtogetherthe

Ethernetframe,forexample.Thisincludesformattingthe

headerinformationintothecorrectfieldsandplacingthedata

intherightplace.Functionsoperatingatthislayeralso

determinetheorderinwhichbitsareinterpreted(thatis,bigorlittle-endian),andaddchecksuminformationusedtoensure



thattheframearrivesintactatitsdestination.

Bridgesarenetworkdevicesthatoperateatthislevelinthe

model.BridgesexaminetheMACaddressesofpacketsanduse

thatinformationtodecidewhethertoforwardapacketto

anotherport.



NetworkLayer

TheNetworklayerprovidesanimportantfunctionalitytoa

networkprotocolstack.Here,protocolsarecreatedthat

managehowpacketsaredeliveredonthenetworkorroutedto

anothernetwork.Forexample,theInternetProtocol(IP)

residesatthislayer.IPaddressingworksatthislevel.

RememberthatIPaddresseshavetwocomponents:anetwork

IDandahostID.Therefore,packetscanbedeliveredonthe

localLAN(usingthehostID)orroutedtoanothernetwork

(usingthenetworkID).Thislayerisalsoresponsiblefor

breakinglargermessagesintosmalleronesthatfitintothe

framescreatedattheDataLinklayer.Thissizeiscalledthe

MaximumTransmissionUnit(MTU).Atthereceivingend,the

Networklayerreassemblestheseintothelargeroriginal

messagebeforepassingthedatauptotheTransportlayer.The

easiestwaytorememberthislayeristorememberthatit

providesforaddressingandrouting.

Itshouldbeobviousthattraditionalroutersoperateatthis

level.Routersuseaprotocol'snetworkaddresstodetermineon

whichportapacketistobeforwarded.



TransportLayer

WhereastheNetworklayerisresponsibleforroutingdata

packets,protocolsattheTransportlayertakeonthedutyof



makingsurethatthosepacketsactuallygetdelivered,andin

thecorrectorder.Forexample,theTransmissionControl

Protocol(TCP)canbefoundatthislayer.TCPusesIP(atthe

Networklayer)andtrackswhichsegmentsgetlostinthe

networkandcauseIPtoretransmitsegmentsasnecessary.

Althoughthislayercanprovideforretransmissionsforlost

packets,itdoesnothaveto.Forexample,theUserDatagram

Protocol(UDP)isfoundatthislayer.UDPalsousesIPtogetits

messagesdelivered.However,UDPdoesveryminimal

managementofthemessagesitsends.Itdoesnot

acknowledgepacketdelivery,butitdoesrespondtoICMP

messages,suchasthosedesignedtothrottleback

transmissionswhentheyarearrivingatthereceivingendattoo

fastarate.



SessionLayer

TheSessionlayerisresponsiblefordecidingtheformatofthe

datatransmitted.Sessionprotocolexamplesaretheremote

procedurecall(usedbyNFSandotherapplications).Another

waytothinkoftheSessionlayeristhatitfunctionstoallow

processesonnetworkedcomputerstotalktoeachother.TCP

andNetBIOSarebothprotocolsthatresideattheSessionlayer.



PresentationLayer

ThePresentationlayerinterpretstheactualdatathatisbeing

exchanged.Forexample,differentsystemscanusedifferent

methodstorepresentfloating-pointnumbersorotherdata.The

orderofbitsinabyteistranslatedatthislevel.Thenecessary

conversionstakeplaceatthislayer.Itisatthislayerthat

translationsfromdifferentcharacter-encodingmethodstake

place.Forexample,whenonecomputerusesASCIIcharacters



andanothercomputerusesIBM'sEBCDICencoding,

translationsbetweenthesetwomethodsofrepresenting

charactersaremadeatthePresentationlayer.



ApplicationLayer

TheusercomesintothepictureintheApplicationlayer.Without

applicationsthatneedtousethenetwork,wenetwork

administratorswouldbeoutofajob.Examplesofnetwork

componentsthatresideatthislayerincludefirewallsor

networkedfilesystems(suchasNFS).Enduserscanrecognize

Applicationlayercomponentsasprogramsthattheyuseevery

day,suchasemailandFTP.







AppendixB.NetworkingGlossary

10BASE-2

10Mbpsnetworkingusingasmaller,moreflexiblecoaxial

cable(RG-58)than10BASE-5.Devicesonthenetwork

connecttothecableusingBNCconnectors.Alsoknownas

thinnet.



10BASE-5

10Mbpsnetworkingusingacoaxialcable(RG-8orRG-11)

largerthanoneusedby10BASE-2.Toattachanetwork

droptoconnectadevicetothecoaxialcable,thecableis

tappedbypunchingaholethroughthecable(aprocedure

oftenreferredtoasavampiretap).Alsoknownasthicknet.



10BASE-T

10MbpsnetworkingusingCAT3orCAT5twisted-pair

wiringandRJ-45connectors.Awiringconcentratorsuchas

ahuborswitchisusedtoconnectdevicesontheLAN.



100BASE-T

100MbpsnetworkingusingCAT5or5etwisted-pairwiring

andRJ-45connectors.AlsoknownasFastEthernet.A

wiringconcentratorsuchasahuborswitchisusedto



connectdevicesontheLAN.



1000BASE-T

1000MbpsnetworkingusingCAT5,5eor6twisted-pair

wiringandRJ-45connectors.AlsoknownasGigabit

Ethernet.AswitchisusedtoconnectdevicesontheLAN.



10GigabitEthernet

Ethernetoperatingat10billionbitspersecond.Definedby

IEEE802.3a.



AcceleratedGraphicsPort(AGP)

APCI-basedportthatenablesyoutoconnectagraphics

cardthatsupportsadvancedfeatures.



AccessPoint(AP)

SeeWirelessAccessPoint.



ActiveDirectory

AnLDAP-baseddirectoryanddirectoryservicethatcontains



informationsuchasuseraccounts,computeraccounts,and

linkstonetworkresources.TheActiveDirectoryisusedon

Windowsnetworksbutcaninteroperatewithsomeother

LDAPdirectories.



Activemonitor

AnodeonaToken-Ringnetworkthatinitializesand

monitorsthetrafficonthering.Theactivemonitordetects

errorconditionsandcanresetthestateofthering.There

canbeonlyoneactivemonitorontheringatanypointin

time.Iftheactivemonitorfails,anothernodeinthering

canbepromotedtothatposition.



Adhocwirelessnetwork

AwirelessnetworkthatdoesnotuseaWirelessAccess

Point(AP).Instead,eachcomputerthatispartofthead

hocnetworkcanestablishaconnectionwithothersinthe

networkwithoutusinganAPorawirednetwork.Seealso

WirelessAccessPointtocontrastthiswithanetworkthat

usesanAP.Seealsoinfrastructuremode.



AddressResolutionProtocol(ARP)

TheprotocolusedtoprobeaLANtodiscovertheMAC

addressofacomputerbasedonanIPaddress.Thearp

commandcanbeusedformanyotherpurposes,suchas

viewingandmanagingthecurrenttableofMACaddresses

storedinthelocalARPcache.



AGP

SeeAcceleratedGraphicsPort.



AmericanNationalStandardsInstitute(ANSI)

ANSIwasfoundedin1918andisthemajorstandards

organizationfortheUnitedStates.ANSIisalsoamember

ofotherstandardsorganizations,suchasISO.



AmericanStandardCodeforInformationInterchange

(ASCII)

Anassignmentofalphabeticcharacters(bothupper-and

lowercase)andcertainothersymbols(suchaspunctuation

marks)toanumericformat.FormanyyearsASCIIwasthe

standardmeansusedtorepresenttextincomputer

systems.Newersystemsexpandonthisconcepttoinclude

otherlanguagesthatuseadditionalsymbols/characters,

suchasUnicode.



Americanwiregauge(AWG)

ThewiregaugeisthestandardforsizingwiresintheU.S.

Wiresizeisbasedprimarilyonthecurrentcarryingcapacity

ofthewiresetbytheNationalElectricalCode.Asthewire

gaugeincreases,thephysicaldiameterofthewire



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