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Chapter 49. Network Testing and Analysis Tools

Chapter 49. Network Testing and Analysis Tools

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Basics:TestingCables

Anetworkconsistsofend-userworkstationsconnectedto

serversbywhatmightappearatfirsttobeatangledwebof

wiresandcables.Ifthebuildingorcampusiswiredcorrectly,

however,thisisnotajumbleofcablesjoinedtogetherina

spaghettifashion,butisanorderlycollectionofcomponents

muchlikeaspider'sweb,fanningouttoconnecteveryoneina

hierarchicalmanner.Inaddition,wirelessnetworking

componentshaveaddedanentirenewterritory,andtoolsare

currentlybeingdevelopedtoaddresstroubleshootingthisnew

areaofnetworktechnology.

Forthemostpart,whenyoubegintobuildanetwork,thefirst

thingyouhavetodoisinstallthecablesthatwillconnectthe

serversandworkstations.Thiscanbedonewhenabuildingis

beingconstructed,asisthecaseinmostofficebuildingstoday.

Oritcaninvolveplacingcableductsinceilingsandknockingout

areasofthewallstoinstallfaceplateswherethecables

terminate.Eitherway,beforeyoubegintoconnectendusersto

thenetwork,youfirsthavetotesttheinstalledcablestobe

suretheyareperformingasexpected.

Devicesthatcanbeusedtotestcables(bothcopperwireand

fiber-opticcables)rangefromveryinexpensivehandheld

devicesthatacableinstallercanusetocheckhiswork,tovery

expensivedevicesthatrequireaskilledtechniciantoperform

thetestsandunderstandtheresults.Thingsthatareusually

testedincludethefollowing:

CablelengthThephysicalnetworktopologyrestrictsthe

lengthofcertainsegmentsinthenetwork.Ifyoumakeyour

owncables,acommonerrormayresultfromtryingto

stretchthelimitsofthetopologyandcreateacablethat's

justafewmeterstoolong.Ifadesktopisjustafewmeters



fartherfromaswitchthanthestandardallows,youmayget

complaintsfromthatuser!

ResistanceElectricityencountersresistanceasittravels

alongacopperwire.

NoiseInterferencecancomefromothercablesthatare

bundledtogetherorfromoutsidesources,suchas

fluorescentlighting,nearbywelding,strongsourcesof

electromagneticfrequencies,andotherhigh-voltage

electricalsourceslocatednearthenetworkcabling.

AttenuationAsthecableencountersresistancetraveling

downthewire,andaspartofthesignalradiatesoutofthe

wire,thesignalweakens.Thisisanormalsideeffectof

usingcopperwiringinsteadoffiberoptics.Youcanexpect

copperwiringtoworkbestatthestandardizedlengths,and

takeyourchancesatextendingthatlength.

Near-endcross-talk(NEXT)Fromthetransmissionend

ofacable,itisnecessarytoremovethesurrounding

materialthatenclosesthecopperwiresandattacheach

wiretoapininthecableconnector.Becausethestrengthof

signalisstrongestattheendofthecablewherethe

electricalsignalisgenerated,thereisagreaterpotentialfor

interferencebetweenthewiresatthisendofthecable.

Togetabetterunderstandingofthekindsofproblemsyoumight

experience,seeChapter6,"WiringtheNetworkCables,Connectors,

Concentrators,andOtherNetworkComponents."



Twobasicinstrumentsareusedfortestingcables.Thefirstis

thesimplecablechecker,whichisusedtodeterminethatthe



cableactuallyprovidesanelectricalpathfromheretothere.

Thesecondisthecabletester,whichdetermineswhetherthe

cablehasbeeninstalledcorrectlytosupportthetopologyof

yournetwork,takingintoconsiderationthingssuchascable

lengthandcross-talk.



HandheldCableCheckers

Acable-checkerdeviceisusuallyasmallbattery-operatedunit

thatisusedtocheckSTPorUTPcables.Thissimpletestis

usuallydonewhencablesarefirstinstalledasaquickcheckto

besurethattheprocessofpullingthecablesthroughthe

ceilingorwallshasnotdamagedthem.

Ifthecableisalreadyattachedtoanetworkdevice,youhave

todisconnectitandattachittotheunit.Acablechecker

operatesbyplacingavoltageonawireanddetermining

whetheritcanbedetectedattheoppositeend.Thiscanbe

usedtodeterminewhetherthecablehasabreakanywhere

alongitspathandwhetheryouarelookingatthesamecable

onbothendswhenseveralcablesaretraversingasinglepath.

Mostcablecheckersconsistoftwocomponents,whichyou

attachtooppositeendsofthecable.



CableTesters

Acabletesterisasmallstepupfromthebasicchecker.This

devicecanbeusedtomeasureNEXT,attenuation,impedance,

andnoiseonaline.Somecabletestersevenperformlength

measurements,ofboththetotalcableandthedistancetoa

faultonthecable,suchasakinkinthewirethatiscausing

reflectionsofthesignaltoradiatebacktothetransmittingside

ofthecable.Anotherfunctionyoumightseeiswire-mapping,

whichcheckstobesurethatthecorrectwire-pairsinacable



havebeenmappedtothecorrectpinsontheconnector

attachedtotheendofthecable.Incablesusedfor10BASE-T

networks,forexample,thestandardspecifiesspecificpairsof

wiresinthecablethatmustbeusedfortransmittingand

receivingdata.Theactualdecisionsaboutwhichpinsare

chosenforaparticularconnectorarenotmadearbitrarily.Ifthe

wiresarenotcorrectlymappedtothepin-outontheconnector

specifiedbythestandard,thecablemightgenerateerrorsdue

tonoiseorcross-talk.

SmallhandheldinstrumentsliketheseusuallyhaveLEDlights

thatindicateapassorfailconditionforthetestyouare

performing.Theydonotrequireakeyboardormonitorto

displaydata.Somehaveasmallscreenthatdisplayslimited

text,sometimesshowingthesuspectedtypeoferrorthathas

causedafailcondition.Mostarebatterypoweredandcanuse

anACadapter,whichmakesthemusefulportableinstruments

forinstallingortroubleshootingcabling.

Whenyoubegintogoupthepriceladderforthesetypesof

instruments,youwillfindsomethatcanperformmore

advancedmonitoringfunctions,suchasshowingnetworkuse

andEthernetcollisions.Anotherusefulfeaturetolookforifyou

canafforditisthecapabilitytologdatatoamemorybufferfor

laterreview.Somecabletestersareevencapableofconnecting

toaPCorprintertoproduceawrittenreport.Thisallowsyouto

leavethedeviceconnectedforawhiletomonitoraline.

Dependingonthecapabilitiesoftheparticulardevice,youcan

expecttopayfromseveralhundreddollarsuptoathousandor

moreforagoodcabletester.Whenevaluatingproducts,besure

tocomparefeatures.Pricedoesn'talwaysreflectthequalityof

adevice.Andyoushouldcarefullychecktheliteratureand

documentationthatisavailableforeachdevicewhenmakinga

purchasingchoice.Althoughsomefeatures,suchasthe

capabilitytoproduceawrittenreport,mightsoundgreat,do

youreallyneedthatcapability?Inalargenetwork,probablyso;

inasmallone,probablynot.



BitErrorRateTesters(BERT)

Datatravelsthroughthewire(orthefiber)asaseriesof

signalsthatindicateasinglebit,representingeitherzeroor

one.Thestatisticcalledbiterrorrate(BER)iscalculatedasa

percentageofbitsthathaveerrorswhencomparedtothetotal

numberofbitssampled:

numberofbiterrorsduringsamplinginterval

BER=--------------------------------------------totalnumberofbitstransmitted



WhereasLANanalyzersoperateondatacapturedfromthewire

inunitsofframes(dependingontheLANprotocol,suchas

EthernetorToken-Ring),abiterrorratetester(BERT)performs

amorebasicfunctiontodeterminewhetherthelineiscapable

ofcarryingthenetworksignalingatthebitlevelwitha

minimumoferrors.

Thiskindofinstrumentisnormallyusedwheninstallinga

connectiontoanetworkserviceprovider,anditmightbeused

todemonstratethequalityofservicethattheprovider

establishesforyourlink.

Theinstrumentusedtoperformthiskindoferrordetection

usuallydoessobygeneratingaspecificbitpatternontheline

andthencheckingitatanotherlocationtocomparethe

generatedsignalwiththatwhichisreceived.Apseudorandom

binarysequence(PRBS)ofbitsisproducedbytheinstrument.

Itispseudorandombecauseitsimulatesrandomdata.However,

becausethepatternisalsoknownbythereceivingconnection

sothatitcanmakethecomparison,it'snottrulyrandom,but

insteadisapredefinedpattern.Othertestsincludesequences

ofspecificbits,eitherzerosorones,forextendedperiods,or

specificuser-definedbitpatterns.



Whenyouhavealinethatexhibitsahighbit-errorrate,usinga

slowertransmissionspeedusuallyimprovesperformance.This

isbecausewhenyoulowerthenumberoferrorsthatoccur,

higher-levelprotocolsdonothavetoresendpacketsasoftento

compensate.Althoughonebiterrorinaframeusuallyiseasily

recoveredbyanetworkprotocolusinganerrorcorrectioncode

(ECC)technique,multiplebiterrorsmightbeallthatittakesto

causeanentireframeofseveralhundredthousandbitstobe

re-sent.



TimeDomainReflectometers

Asignalusuallypropagatesdownawireataconstantspeed,

providedthattheimpedanceofthecableisthesame

throughoutitsjourney.Whenthesignalrunsintoafaultinthe

wire(suchasakinkorasplice)orreachestheendofthewire,

partorallofthesignalisreflectedbacktoitsorigin.Similarto

radar,instrumentsthatusetimedomainreflectometry(TDR)to

makecablemeasurementsarebasedonpreciselytimingthe

signalpulseasittravelsthroughthecableandback.

Ofalltheinstrumentsyoucanusetotestcables,TDRisoneof

themostaccurateandfastest.Itcanhelplocatefaultsdueto

variouscauses,suchasthese:

Wiresthathavebeensplicedtogether

Moisturetrappedinthecable

Cablesthathavebeencrushedorhavekinksinthem

Shortcircuits

Problemsinthesheathsurroundingacable



Looseconnectors

YoualsocanuseTDRtomeasurethelengthofacablethathas

nofaults.Thiscanbeusefulforinventoryfunctionsbecauseyou

canevenuseittomeasurethelengthofacablewhileitisstill

onareeltodeterminewhetheryouhaveenoughorneedto

orderadditionalstockbeforebeginningamajorwiringproject.

TDRcanbeusedtotakemeasurementsontwisted-paircables,

coaxialcables,andevenfiber-opticcables.Becausefiber-optic

cablingisthemostexpensive(butlargestbandwidth)media

today,youshouldconsiderinvestinginaTDRthatsupports

fiber-opticmediaifyouareplanningonawidedeploymentof

fiber-opticcabling.Attheveryleast,youshouldexpectathirdpartyinstallerofalargecableplanttoprovidesuchan

instrument.Youshouldalsomakeitpartofyourprojectplanto

recordthestatisticsprovidedbytheinstaller.Thus,ifafuture

needdictates,youmighthaveaparticularcablesegmentreevaluated.Besuretostipulatesuchthingsastheperformance

ofanycablesegmentsinyouragreementwithaninstaller.If

youareperformingtheinstallationyourself,bepreparedtouse

aTDRtocheckacablesegmentshouldperformancedegrade.

Forexample,forelectricalcabling,otherdevices(evencables)

mightbeinstalledlaterthatinterferewithyouroriginal

installation.Inthatcase,movingcablestonewlocationsshould

solvetheproblem.

Themoreexpensivemodelsofthisinstrumentcanbeequipped

withaCRTorLEDdisplaythatshowsthewaveformofthe

signalandanyreflectedsignals.Themorecommoninstrument

displaysthenumberoffeettotheendofthecableorafault,

andmighthaveanindicatorthattellsyouthetypeoffault.By

showingthenumberoffeettoaperceiveddefect,youcantrace

yourinstalledcabling(orunrollyouron-the-rollcabling)sothat

youcangettothepointwherethedefectoccurs.

UsingaTDRinthismethodcanhelpyouwithinstallednetwork

cabling(whereanewproblemhasbeenintroduced)andhelp



youcheckoutcablingspoolsbeforeyouacceptthem.



Impedance

Whenconductorsmadeofmetalareplacedincloseproximityto

eachother,asinatwisted-pairorcoaxialcable,theeffectthey

haveoneachotherisknownasimpedance.Whenthewiresare

perfectlyseparatedbyaconstantdistance,theimpedance

remainsthesamethroughoutthecable.Whensomething

happensalongtheway,suchasdamagecausedbyacrushed

cable,theimpedancechangesatthatpoint.Changesin

impedancecausepartsofthesignaltobereflectedbackto

whereitstarted.

Cablesthatareusedinlocalareanetworks(LANs)needtobe

manufacturedtostrictspecifications,ensuringthatthe

dielectricmaterialthatseparatesthewireswithinthecable

remainsconstant.Iftherearerandomvariationsduetopoor

manufacturingprocedures,thecablewillsufferfromproblems

causedbysignalreflections,whichmightrenderitunsuitable

foryournetwork.Thus,TDRcanbeusednotonlyasafaultfinderwhentroubleshootingawiringproblem,butalsoto

ensurethatyou'vereceivedwhatyoupaidforwhenyou

upgradeorexpandyournetwork.



SettingaPulseWidth

MostofthegoodTDRinstrumentsallowyoutoselectthepulse

width,whichisusuallyspecifiedinnanoseconds.Thelargerthe

pulsewidth,themoreenergythatistransmittedfromthe

deviceandthusthefartherdownthewirethesignalwilltravel.

Agoodtipforsettingthisvalueistostartwiththesmallestthat

theinstrumentallowsandmakesubsequentmeasurements,



graduallyincreasingthepulsewidth.Ifthefaultinthecableis

onlyashortdistanceawayfromthemeasuringinstrument,a

smallpulsewidthwillbeadequatetolocateit.However,ifthe

faultisminor,asmallburstofenergymightnotbeenoughto

traveltothefaultandsendbackareflectionstrongenoughto

beaccuratelymeasured.Byvaryingthepulsewidthandmaking

severalmeasurements,youcanmoreaccuratelydeterminethe

locationofafaultinthecable.



Velocity

Lighttravelsataconstantspeedof186,283milespersecondin

avacuum.Whenmeasuringthevelocityatwhichanelectrical

signaltravelsthroughawire,itisexpressedasapercentageof

thespeedoflight,whichisconsideredtobeslightlylessthan

100%,oravalueof1.Forexample,atwisted-paircablethat

hasaVOP(velocityofpropagation)of.65wouldconductan

electricalsignalat65%ofthespeedoflight,oraboutthespeed

Idriveontheinterstate(milesperhour,ofcourse).

Manufacturersusuallysupplythisvaluetocustomers,anditwill

mostlikelybefoundonthespecificationsheetforthecableyou

arepurchasing.BecauseTDRmeasuresthetimeittakesfora

signaltotraveldownawireandmakethereturntrip,youhave

toknowtheVOPofthecablebeingtestedbeforeyoucanmake

accuratemeasurements.

Ifyouhavecablesthatyouareunsureabout,youcantest

themfirsttodeterminetheVOP.Dothisbymeasuringaspecific

lengthofcabletogetitslengthandthenusingtheTDR

instrumenttotestforthelengthofthecable,varyingtheVOP

untilthetesterreadsthecorrectlength.Ofcourse,this

assumesthatthesegmentofcableyouuseforthistestisin

goodcondition!



NetworkandProtocolAnalyzers

Thefirstlevelofnetworktestingconsistsofmakingsurethat

theunderlyingphysicalcablingstructureisperformingas

expected.Thenextlevelistomonitorandtestthenetwork

trafficandmessagesgeneratedbythenetworkprotocolstobe

surethatyouhaveahealthynetwork.Networkanalyzer

productsoperatebymonitoringthenetworkattheDataLink

andTransportlayersintheOSIreferencemodel.



Note

TheOSIreferencemodelseparatesthecomponents

ofanetworkprotocolstackintomodularlayers,

eachofwhichperformsaspecifictaskforthelayer

aboveorbelowitinthemodel.AppendixA,

"OverviewoftheOSISeven-LayerNetworking

ReferenceModel,"coversthismodel,discussing

eachlayerindetail.You'llheartermssuchas

Networklayer,DataLinklayer,andTransportlayer

frequentlywhendiscussingnetworking,soit'sa

goodideatohaveabasicunderstandingofwhat

thesetermsmeanandthenetworkingfunctionsthey

represent.



Again,youwillfindthatthetoolsyoucanselectforprotocol

analyzersrangefromtheveryinexpensive(free)tothevery

expensive(severalthousanddollars).Onedifferencebetween

thesekindsoftoolsandthoseusedtocheckcables,however,is

thatyouneedtohaveagoodunderstandingofthenetwork

structureandprotocolsusedbeforeyoucanmakemeaningful

judgmentsaboutthedatayoucollect.TheLANanalyzerallows



youtointerceptnetworktrafficasitpassesthroughthewirein

realtimeandsavethedataforanalysis.Agoodanalyzershould

beabletoproducemeaningfulstatisticsaboutthetrafficonthe

network,decodetheprotocolsthatareused,andprovidea

goodfilteringcapabilitysothatyoudon'tgetboggeddownin

anoverwhelmingamountofdata.

Youshouldconsidermanyfactorswhendecidingonanetwork

analyzerproduct.Themostbasicfactoriswhetheryouwanta

portabledevicethatcanbetransportedtodifferentsitesorone

ormoredevicesthatcanbeplacedatstrategiclocationsinthe

networktoperformcontinuousmonitoring.Otherfeaturesto

considerincludethefollowing:

PriceOfcourse,thisisalwaysafactorwhenpurchasing

equipmentforanetwork.

SoftwareorhardwareDoyouneedadedicatedhardware

instrumentthatcanperformintenseanalysisandconnectto

multiplesegments,orcanyoulivewithasoftware

implementationthatrunsonanexistingnetwork

workstation?

NetworkinterfaceDoyouneedtoconnecttojusta

100BASE-T(orevenhigherbandwidthdevices)

environment,ordoyouneedadevicethatconnectsto

othertopologiessuchasFDDIorToken-Ring?

ProtocolstacksupportIsyournetworkhomogeneous,or

doesitsupportmultiplenetworkprotocols?

StatisticsWhatkindofstatisticaldatadoestheinstrument

support?Themostbasicisframes-per-second.Others

includeutilizationandusage.Utilizationisameasurement

oftheactualamountofbandwidththatyournetworkmedia

issupportingatanypointintime.Usagestatisticscantell



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