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Chapter 13. Ethernet: The Universal Standard

Chapter 13. Ethernet: The Universal Standard

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networkbyoneformofEthernetoranother.Althoughother

localareanetworking(LAN)technologies,suchasToken-Ringor

Novell'sproprietaryIPX/SPX,arestillaround,EthernetconnectedcomputersoutnumberallotherLANtechnologies

combined.You'llalsofindthat,mostlikely,Ethernetisthe

underlyingnetworkingtechnologythatconnectsservers,

printers,andotherdevicesonyournetwork.Ethernethas

becomesopervasivethateverymajormanufacturerof

networkingequipmentsellsequipmentthatisdesignedtowork

withorprovideinterconnectivitywithEthernetLANs.

Sobeforewestarttalkingaboutnetworktransportprotocols,

services,andapplications,itisimportantthatyougetagood

understandingofwhatEthernetisandhowitfunctions.It'salso

importantthatyouunderstandthatthereismorethanonetype

ofEthernet.WhatstartedoutasasimpleLANtechnologyhas

evolvedtothepointthatitisnowseriouslyconsideredawide

areanetworking(WAN)technology.Withtechnologiessuchas

PPPOE,(Point-to-PointProtocoloverEthernet)whichisa

methodforbuildingPPPsessionsandencapsulatingpackets,as

describedinRFC2516(http://www.ietf.org/rfc/rfc2516.txt),

networkprovidersofalltypesDSL-basedISPs,forexampleuse

Ethernettoprovideconnectivitytotheirnetworks.Fromthe

firstcommercialversionsthatoperatedat10Mbpstothe

newest10GigabitEthernet,you'llfindthatthere'sanEthernet

solutiontomostnetworkproblemsyouencounter.It'sonthe

desktop.It'sinthewiringcloset.It'sthebackboneofyour

network.

Inthischapter,we'llfirstlookathowEthernetgotitsstart,and

thendescribethedifferentversionsthatwerestandardizedand

marketed.AftergivingyouathoroughlessoninEthernet

technology,we'lllookattechniquesthatcanbeusedto

troubleshootEthernetnetworks.



AShortHistoryofEthernet

EthernetwasoriginallydevelopedatXeroxPARC(PaloAlto

ResearchCenter)Laboratoriesinthe1970s.RobertMetcalfe

waschargedwiththeresponsibilityofnetworkingagroupof

computersthatcouldalluseanewlaserprinterthatXeroxhad

developed.Xeroxalsohadjustdevelopedwhatwasprobably

thefirstpersonalworkstation,andhadaneedtonetworkmore

thantheusualtwoorthreecomputersthatyouwouldfindina

singlebuildingduringthattime.

TheoriginalEthernetstandardwasdevelopedoverthenextfew

years,andthisresultedinapaper,"Ethernet:Distributed

Packet-SwitchingforLocalComputerNetworks,"writtenby

MetcalfeandDavidBoggs(CommunicationsoftheACM,Vol.

19,No.5,July1976,pp.395404).Thispapergivescreditto

theALOHAprojectthathadbeendoneinHawaiiwithpacket

radiotransmissionsintothe"ether,"notingthatscientistsonce

thoughtthatelectromagneticradiosignalstraveledthrougha

substanceknownas"ether."InthisfirstEthernetexperimental

networkdescribedinthepaper,thenetworkcoveredadistance

of1kilometer,ranat3Mbps,andhad256stationsconnectedto

it.Foritstime,thiswasanaccomplishment.



Note

TheMetcalfeandBoggspaperanticipatedmany

otherinnovationsthatwouldappearinthenextfew

yearsinthearenaofnetworking.Theyrecognized

thelimitsofalocalareanetworkusingashared

medium.Theyalsoanticipatedtheuseofbridges

(repeaterswithpacketfilters)andofhigher-level

protocolsthatwouldhaveanexpandedaddress

spacethatwouldallowforadditionalfieldsthatcould

beusedforroutingpurposes.Anotherinteresting



thingtonoteisthat,similartoIBMwhenitcreated

thenow-famouspersonalcomputerthathasbecome

astandard,MetcalfeandBoggsalsochosetouse

"offtheshelf"partsforthenetworktransmission

medium:ordinaryCATVcoaxialcablesandthetaps

andconnectorsusedwiththem.Thismadeiteven

cheapertothinkaboutcreatingacommercial

versionofEthernet.Anonlinecopyofthispapercan

befoundatwww.acm.org/classics/apr96.



Later,aconsortiumofthreecompaniesDigitalEquipment

Corporation,Intel,andXeroxfurtherdevelopedtheEthernetII

standard,sometimesreferredtoinolderliteratureastheDIX

standard,basedonthefirstinitialsoftheparticipating

corporations.Thesecompaniesusedthetechnologytoadd

networkingcapabilitiestotheirproductlines.Forexample,at

onetimeDigitalEquipmentCorporationhadthelargest

commercialnetworkintheworld,usingDECnetprotocols

connectingEthernetLANs.Todaythedominantprotocolused

withEthernetisTCP/IP.

Theideaofnetworkinghundreds,andthousands,ofPCsintoa

LANwouldhavesoundedprettyoptimisticbackwhenEthernet

wasfirstdeveloped.Yet,inpartbecauseofitssimplicityand

widespreadsupportamongmanufacturers,Ethernethas

adaptedovertheyearstogrowandsurvive,runningonnewer

devicesandnetworkmedia.YoucannowrunanEthernet

networkonwiredmediasuchascoaxialcable,twisted-pair

wiring(shieldedandunshielded),andfiber-opticcabling.

Ethernetobviouslyalsofunctionswithwirelesstechnologiesas

well,asdiscussedinPartVofthisbook,"WirelessNetworking

Protocols."



VariationsonaTheme:HowManyKindsof

EthernetAreThere?

In1985,theIEEEstandard802.3"CarrierSenseMultiple

AccesswithCollisionDetection(CSMA/CD)AccessMethodand

PhysicalLayerSpecifications"waspublished.These

specificationsmadeiteasyforvendorstocreatehardware,

fromcablingtoLANcards,whichcouldinteroperate.Themany

differentEthernetstandardsyouwillneedtoknowaboutareall

identifiedbyanamethatincludes"IEEE802."followedbya

numberandpossiblyaletterortwo.

TheIEEE802LAN/MANStandardsCommitteeisresponsiblefor

creatingstandardsforlocalandwideareanetworking.This

committeewasformedinFebruaryof1980andwasoriginally

calledtheLocalNetworkStandardsCommittee.Thenamehas

beenchangedtoreflecttheevolutionarydevelopmentofsome

ofthecommittee'sstandardstoMAN(metropolitanarea

network)speeds.Chapter12,"TheIEEELAN/MANCommittee

NetworkingStandards,"containsshortdescriptionsofsomeof

themorerelevantstandardsthatweredefinedforLAN/MAN

networks.



Note

It'ssafeatthispointtosaythatweallknowLAN

standsforlocalareanetwork.It'salsocommon

knowledgethatWANstandsforwideareanetwork.

Sowhat,then,isaMAN?It'sametropolitanarea

networksmallerthanaWANbutlargerthana

LANgenerallyusedtorefertoanetworkthat

connectsagroupofsmallerrelatednetworkswithin

acityorregionalizedgeographicarea.Other

acronymsusedtodescribespecificnetworksare

PANs(personalareanetworkscreatedbyBluetooth

devices),CANs(whichstandforCampusArea



Networkscontainedonlywithinacampus

environment,suchasacollegeuniversity),and

SANs(whicharededicatedtohigh-speedstorage

areanetworks).Makesurethatyoudonotget

confusedwhenyouarediscussingnetworktypes.

YoucanlearnmoreabouttheactivitiesoftheIEEE

802LAN/MANStandardsCommitteeandthe

differentworkinggroupsthatconcentrateonspecific

networkstandardsbyvisitingthewebsiteat

http://ieee802.org/.Youcanalsodownloadmanyof

thestandardsdevelopedbythe802LAN/MAN

committeefromthiswebsite,thoughprintingallof

themwillrequirealotofpaper!



Thecommitteeismadeupofvariousworkinggroupsand

technicaladvisorygroups.Forexample,IEEE802.3isthe

workinggroupforstandardEthernetCSMA/CDtechnology,

whereasIEEE802.3zisthestandardforGigabitEthernet,which

isanupdated,fasterversionoftheoriginal802.3.

DifferentformsofEthernetarealsoreferredtousinganaming

schemethatlinksthenetworkspeed,theword"BASE"(for

basebandsignaling),andanalphaornumericsuffixthat

specifiesthenetworkmediaused.Anexampleofthisis

10BASE-T,whichisbrokendownlikethis:

10=Ethernet,runningataspeedof10Mbps

BASE=basebandsignaling

T=overtwisted-pair(T)wiring



TodaythereisawideassortmentofEthernetsolutionsfrom

whichtochoose.Originally,Ethernetusedcoaxialcable

(10BASE-5)thatwas"tapped"intowhenanewworkstationwas

addedtothebusnetwork.Later,thinnet(10BASE-2)was

developedandallowedasmaller,moreflexiblecoaxialcableto

beusedtoconnectthenetwork.Withthinnet,BNCconnectors

wereintroduced,makingitunnecessaryto"tap"intothecoaxial

cable.ThemostrecentversionsofEthernetusetwisted-pair

wiringandfiber-opticcablesandcentralizedwiring

concentrators,suchashubsandswitches.

TheoriginalEthernetIInetworkoperatedatablazinglyfast

speedof10Mbps.Themostrecentstandardthatisintegrated

intodesktopsystemsisGigabitEthernet,nowthatithasbeen

standardized.Andasifthatweren'tenough,the10Gigabit

Ethernetstandardhasbeenfinished,andproductsarereadily

availableforhigh-speednetworks.

Thesearethemostcommonstandards-basedEthernet

solutionsfromthepasttothepresent:

10BASE-5Oftencalled"thickwire"or"thicknet,"this

standardusesthickcoaxialcable.The10inthisname

indicatesthespeedofthenetwork,whichis10

megabits/second(Mbps).Asmentionedearlier,theterm

"Base"referencesthetechnologyused,whichisBaseband.

Thenumber5inthenameindicatesthatthemaximum

lengthallowedforanysegmentusingthistopologyis500

meters.10BASE-5networksusedthickcoaxialcable.To

installanodeonthenetwork,itisnecessarytousewhatis

commonlyreferredtoasa"vampiretap."Thatis,you

attachaconnectortothebackbonethicknetcoaxialcable

bypunchingintothewire.Adropcableisthenruntothe

workstationthatisbeingaddedtothenetwork.Ifyouare

stillusingthistechnology,it'stimetoupgradebecauseyou

willfinditverydifficulttofindpartsandsupportforthis

outdatedtechnology.



10BASE-2Oftencalled"thinwire"or"thinnet,"this

Ethernetstandardrunsatthesamespeedasa10BASE-5

network(10Mbps)butusesasmaller,moreflexiblecable.

Thenumber2inthenameindicatesamaximumsegment

lengthof200meters.Thisisabitmisleading,becauseitis

actuallyroundedupfromthetruemaximumsegment

lengthof185meters,butitsurewaseasierthancallingit

10BASE-1.85.Itiscommontoseeoldernetworks

composedofmultiportrepeaters,witheachportusing

thinnetcablestoconnectoneormultiplecomputers.Each

repeaterisjoinedusinga10BASE-5thicknetcable.Usinga

BNCT-connector,itispossibletocreateasimpledaisychainbususing10BASE-2.Again,ifyouarestillusingthis

technology,youarelivinginthepast!Itiseasiertofind

partsandsupportforthisoldertechnology,butwiththe

inceptionofTwistedPaircabling,youwillfinditveryhardto

findcoaxial-basednetworks,whetherThickorThinnet

based.

10BASE-36ThisrarelyusedEthernetspecificationuses

broadbandinsteadofbasebandsignalingdespitethefact

thatthenameimpliestheuseofbaseband.Thecoaxial

cableforthistechnologyusesacoaxialcablethathasthree

setsofwires,eachforaseparatechannel,andeach

channeloperatesat10Mbpsandcanextendoveradistance

ofabout3,600meters.

10BASE-TThenetworkconnectionismadefrom

workstationstoacentralhuborswitch(alsoknownasa

concentrator),usingaphysicalstartopology.Theuseof

twisted-pairwiring(hencethe"T"inthename),whichis

cheaperandmuchmoreflexiblethanearliercoaxialcables,

makesroutingcablesthroughceilingsandwallsamuch

simplertask.Centralizedwiringalsomakesiteasiertotest

forfaultsandisolatebadportsormoveusersfromonearea

toanother.Ifyouarestillusingthistechnology,youcanget



by,butyou'dfindyourjobaloteasierifyouupgradedtoat

least100BASE-T.

ThetopologiesusedbyvariousformsofEthernetarediscussedin

Chapter2,"OverviewofNetworkTopologies."



10BASE-FLThisversionofEthernetalsooperatesat

10Mbps,butinsteadofusingcopperwires,fiber-optic

cables(FL)areusedspecifically,multimodefibercable

(MMF),witha62.5micronfiber-opticcoreanda125

micronoutercladding.Separatestrandsoffiberareused

fortransmitandreceivefunctions,allowingfull-duplexto

operateeasilyacrossthiskindoflink.Thistechnologyhas

alsobeenrelegatedtohistory,althoughit'sverycommonto

findthistechnologyonmostlegacynetworkstoday.

100BASE-TXUsesCategory5wiring(seeChapter6,

"WiringtheNetworkCables,Connectors,Concentrators,and

OtherNetworkComponents")toallowadistanceofupto

100metersbetweentheworkstationandthehub.Four

wires(twopairs)inthecableareusedforcommunications.

Thistechnologyisstillusedwidelytoday,andwillprobably

bearoundforawhileuntilapplicationsmandatethe

necessityofupgradingyournetworktouseGigabitEthernet

tothedesktop.

100BASE-T4UsesCategory3orCategory5wiringtoallow

foradistanceofupto100meters(328feet)betweenthe

workstationandthehub.Fourwires(twopairs)inthecable

areusedfordatacommunications.Thisisanother100Mbps

technologythatwasusedtoprovideanupgradepathfor

installationsthathadnotyetupgradedtoCategory5

cabling(orbetter).Ifyoustillusethistechnology,it'stime



toconsideranupgradeifyoufindthatnetworkcongestion

andexcessiveerrorsareoccurring.

100BASE-FXUsesmultimodefiber-opticcablestoallowfor

adistanceofupto412metersbetweentheworkstationand

thehub.Onestrandofthecableisusedfortransmitting

datawhiletheotherisusedforreceivingdata.

1000BASE-SXThe802.3zIEEEstandardsdocument,

approvedin1998,definesseveralGigabitEthernet

networkingtechnologies.1000BASE-SXisintendedto

operateoverfiberlinksusingmultimodefiber,operating

withlasersthatproducelightatapproximately850

nanometers(nm).The"S"inthenameimpliesashort

wavelengthoflight.Themaximumlengthforasegmentof

1000BASE-SXis550meters.

1000BASE-LXThisfiber-basedstandarddefinesEthernet

whenusedwithsingle-modeormultimodefiber.The"L"in

thenameimpliesalongerwavelengthoflight,from1,270

to1,355nanometers.Themaximumlengthforasingle

segmentof10BASE-LXis550metersusingmultimodefiber,

andupto5,000metersusingsingle-modefiber.

1000BASE-CXThisstandardallowsforGigabitEthernet

acrossshieldedcopperwires.Itisdesignedprimarilyfor

connectingdevicesthatareonlyashortdistanceaway25

metersorless.

1000BASE-TTheIEEEstandard802.3abaddedtothe

PhysicallayerofGigabitEthernetCategory5unshielded

twisted-pairwirecables.Themaximumdistanceforany

segmentusing1000BASE-Tis100meters.

It'simportanttorememberthatwhenworkingwithnetwork

cabling,youneedtomakesureyouareusingthecorrecttype.



Category5,5e,and6arethemostcommonlyusedcabletypes

today.NotethatyouneedtouseCategory5-ratedcableor

higherwithGigabitEthernettechnologies.Tolearnmore,please

seeChapter6,"WiringtheNetworkCables,Connectors,

Concentrators,andOtherNetworkComponents."



Collisions:WhatAreCSMA/CAandCSMA/CD?

IntheoriginalPARCEthernet,themethodusedtoexchange

dataonthenetworkmediawascalledCarrierSenseMultiple

Access(CSMA).TheEthernetIIspecificationaddedCollision

Detect(CSMA/CD)tothistechnique.Acollisionoccurswhen

twoworkstationsonthenetworkbothsensethatthenetworkis

idleandbothstarttosenddataatapproximatelythesame

time,resultinginagarbledtransmission.Thetermcollision

itselfseemstoimplythatsomethingiswrong.Insome

technicalliterature,thiskindofeventiscalledastochastic

arbitrationevent,orSAE,whichsoundsmuchlesslikeanerror

thandoescollision.However,collisionsareexpectedinolder

Ethernetnetworks.Onlywhentheybecomeexcessiveisittime

tosearchforthesourcesofthecollisionsandrearrangesome

workstationsornetworkdevicesasappropriate.



Note

Thecollisiondomainhasprettymuchbeenrelegated

tohistory.Hubsandhalf-duplexconnectionsstilluse

CSMA/CD,butifyournetworkusesFastEthernet

switches,infull-duplexmode,thenCSMA/CDno

longercomesintoplay.Instead,full-duplexswitches

useseparatewirepairsinthecablesothatthe

switchportcansenddatatotheattachedcomputer,

whilereceivingdatafromthatcomputeronanother

wirepair.Whencreatinganewnetworktoday,the

costofnetworkadaptersandswitchesmakesita

veryinexpensivepropositiontousefull-duplex

networkadaptercardsandswitches.TheCSMA/CD

technologyisdiscussedinthischaptertoletyou

understandhowEthernethasevolved,andto

provideinformationforthosewhostillhavelegacy

Ethernetequipmentinstalled.



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