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Chapter 14. Leasing a (Network) Roadway Between Two Points

Chapter 14. Leasing a (Network) Roadway Between Two Points

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theytypicallyconnectdevicesthatarerelativelyclosetogether.

Asitturnsout,WANsandLANshaveanotherdifferencea

differencethatmattersalottobasicunderstandingofhoweach

isbuilt.WithLANs,youareincontrol.Youcanbuycable,runit,

connectittotheswitches,pluginthingswhereyouwantthem

intheswitch,anditworks.WithWANs,youcan'tjustruna

cable.EveniftheWANlinkisamilelong,youtypicallycan'trun

acableoverthestreetstoyourotherbuilding.It'stypically

illegal,evenifyoucouldrunthecableunderthestreetwhere

hopefullynoonewouldmesswithit.

Inshort,tobuildWANs,youmustleaseaphysicalnetwork

fromsomeonewhocanrunacablebetweenthebuildingsthat

youwanttoconnect.ThischapterfocusesonWANsthatwork

wellbetweenapairofsites.







LeasingtheCableWhenYouCan'tRunthe

Cable

Iftworoutersintwocitiesneedtoforwardpacketstoeach

other,theyneedsomesortofphysicalmediumoverwhichto

sendthepackets.BackinChapter4,"HowtoBuildaLocal

(Network)Roadway,"youlearnedthatyoucouldjustrunan

Ethernetcross-overcablebetweentwodevices,andthetwo

devicescouldcommunicatewithEthernet.Thatworkswell,but

wearen'tallowedtorunEthernetcablesthroughotherpeople's

property.

ThissectionintroducesthegeneralideaofaWANlink,which

conceptuallyisequivalenttoanEthernetcross-overcable.

Figure14-1depictsthegeneralidea.



Figure14-1.WANLinkItQuacksLikeanEthernet

Cross-OverCable



[Viewfullsizeimage]



IfyouremembermuchaboutthephysicaldetailsofEthernet,

Figure14-1shouldlookfamiliar.WhenR1sendsoutan

electricalsignaloverthecable,R2needstoreceivethatsame

signalonthewiresthatitexpectstoreceivedata.Likewise,R1



needstoreceivewhatR2transmits.Becausethephysical

interfacesontheroutersusethesamepinstotransmit,the

cableconnectsthetwistedpairusedfortransmittingbyR1over

toR2'sreceivepins,andviceversa.Ineffect,thecableinthe

figurelookslikeanEthernetcross-overcable.Allthatthetwo

routersneedisacablebetweenthem,withtransmitand

receivepairsofwires,sothattheycansendandreceive

anytimetheywant.

ThetemptationmightbetosimplyuseEthernet.However,two

problemsstopyoufromusingEthernetinthiscase:

Youaren'tlegallyallowedtorunacablebetweenAtlanta

andCincinnati(thesitesinthefigure).

Thosewhocanrunthecablenamely,thetelephone

companies(telcos)don'tleaseorsell450-mileEthernet

cross-overcables.

Asyoumightimagine,thereisasolutionthatactslikeusingan

Ethernetcross-overcablebetweentworouters;thedetailsare

justalittledifferent.Thenextsectiondescribesthedetails.



note

ManytelephonecompaniesarebeginningtoofferWANservicesbased

onEthernet.However,thevastmajorityofexistingWANconnections

worklikewhatisdescribedinthischapterandChapters15,"Leasinga

(Network)RoadwayBetweenLotsofPlaces,"and16,"Drivingfrom

HomeontotheGloballyInterconnected(Internet)Roadway."



YouCan'tLeasetheCross-OverCable,SoLease

SomethingAlmostJustLikeIt

We'veestablishedthefactthatyoucan'tbuyorleaseanactual

Ethernetcross-overcablefromthetelco.Butwhatifthetelco

wouldleasetoyousomethinglikethefollowing?

Useaservicebetweentworoutersthatactslikeacable

withfourwires(twopairs)init.

Whenaroutersendsononepair,thatpairiscrossedtothe

otherpairbeforeitgetstotheotherendcable.Thatway,

whenoneroutersendsononepair,theotherrouter

receivesontheotherpair.

Becausetheservicerequiresthatyouphysicallyencode

dataalittledifferentlythanonEthernet,yourroutershave

touseadifferenttypeofinterface,calledaserial

interface.



Theendresultwillbethattworouterscansendandreceive

datatoandfromeachother.

Well,theendresultisthattherouterscanforwardpacketsto

eachother,andthat'sexactlywhatthetelcooffers.Thetelco

essentiallycanleaseyoua4-wirecable,or4-wirecircuit,

betweentwopoints.Althoughit'snotexactlyEthernet,itsure

actsalotlikeanEthernetcross-overcable.

Thetelcoshavebeenwellpreparedtooffer4-wirecircuitsto

theircustomersfordecades.Theyhavealreadyruncables

betweenalmosteverytownandcity.Theyhaveoffices,called

centraloffices(COs),almosteverywhereaswell.Inaddition,

telcoshavetheright-of-way,whichisthelegalrighttodigup

roadsandputcablinginthegroundandoverhead,allforthe

commongoodofthepopulace.Ineffect,thetelcocanrunthe

cableforyoutocreatethis4-wirecircuit.



It'sNotReallya450-MileCable,butItWorksLikeOne

YouliketheideaofthisnewWANlinkfromAtlantato

Cincinnati,soyoumeetwiththetelcosalesman.Yousignthe

papers,andthenhismoretechnicalcolleague,typicallycalleda

salesengineer(SE),happensby.Youaskhimwhatthe

salesmanreallymeantwhenhesaid,"It'sjustlikewe're

runninga450-milecableforyou."TheSEsayssimply,"We

didn'trunnostinkin'cable,butwhatwediddoactsjustlikewe

didrunacableforyouatleastthat'sthetechnicaldescription."

Ifthetelcoactuallyhadtorunacablefor450miles,you

wouldn'tbeabletoaffordtopaytheinstallationcharges.So,

thetelcodoessomeotherthingsinsideitsnetwork,allofwhich

ishiddenfromthecustomers.Intheend,theservicereally

doesactlikea4-wirecable.

Tobuildthisservice,thetelconeedstoinstalla4-wirecable



betweenyourofficebuildingandtheirs,asshowninFigure142.



Figure14-2.EstablishingaPhysicalPathfromthe

CustomertotheTelco



OneendofthecableisinsideaCO,whichiswherethetelco

keepstheequipmentusedtocreatethetelephonenetwork.

Genericallyspeaking,theswitchesintheCOarecalledtelco

switches,phoneswitches,orWANswitches.Therearelots

ofothernamesforspecifictypesofequipmentthatthetelco

usesintheCO.Thecableispluggedintoaphysicalinterfacein

atelcoswitchintheCO,withtheotherendofthecable

connectingtotherouteratthecustomersite.

ThecablerunsfromtheCO,typicallyunderground,andthen

comesupintothecustomerbuilding.Installingthecable

requirestheright-of-waytogounderthestreet,blocktraffic,

anddigupastreetoryardtorunthecable.Thinkingahead,

thetelcooftenrunsextrawiresbetweentheCOandlarge

buildings,officecomplexes,andotherbusinesscenters,justso

itdoesn'thavetointerrupttrafficanddiguptheearthallthe

time.



TocompletethepathbetweenAtlantaandCincinnati,you'll

needtwomorethings.First,youneedasimilarcablerunfrom

theofficebuildinginCincinnatiwheretheotherroutersitsand

thetelcoCOinCincinnati.Theotherrequirementisthatthe

telconeedstosomehowgettheelectricalsignalsbetweenthe

AtlantaCOandtheCincinnatiCO.Todoso,thetelcodoes

somethinglikewhatisshowninFigure14-3.



Figure14-3.ALeasedLine,ALeasedCircuit,A

Point-to-PointLink,andAWANLink



[Viewfullsizeimage]



BeforeyouneededthisnewWANlink,thetelcohadthought

aboutsupportinglotsofcustomersinAtlantaandCincinnati.In

anticipationoffuturebusiness,theyinstalledgearand

additionalcabling.Thatequipmentandcablingtogethercan

takethebitssentbyR1overthecabletotheAtlantaCO,

deliverthemacrossthetelconetwork,andsendthebitstoR2

inCincinnati.Ofcourse,thetelcodoesthesamewithbits

comingintotheCincinnatiCO,deliveringthosetotheAtlanta

COandthenouttoR1.

RegardlessofwhathappensbetweentheCOs,asfarasthe

routersareconcerned,whentheysenddataonthatcable,the



otherroutergetsit.Thedetailsofhowthetelcodoesthings

insideitsnetworkareimportantifyouworkforthetelco,but

thegoalofthischapterissimplytobrieflyhighlighthowthe

tworouterscouldcommunicate.



TheManyPersonalitiesofaWANLink

I'vebeenusingthetermsWANlinkand4-wirecircuitto

describetheservicethatactslikea4-wirecablebetween

routers.However,severalothertermsmeanthesamething.

Hey,whyuseonenamewhenyoucanletpeoplepicktheir

favorite?

Table14-1summarizesthenames,alongwithsomeofthe

reasonswhythetermsareused.

Table14-1.DifferentNamesforWANLink

Term



Rationale



Leasedcircuit



Intelcolingo,whenyoupickupthephoneandmakea

voicecall,thetelcosetsupatemporarycircuit.Whena

customerpaysmoneytohaveapermanentlyupcircuit

betweentwosites,thetelcoconsidersthecircuittobe

leased.



Leasedline



Theterm"leased"isusedforthesamereasonsasfor

leasedcircuit.Theterm"line"referstothefactthatthe

serviceissupposedtoalwaysbeworking,soit'slikea

cable,oraline,betweenthetwosites.



Point-to-pointlink Theterm"point-to-point"referstothefactthatexactly

twoendpointsexistonthecircuit.Theterm"link"refers

tothewaythetwoendpointsarelinkedtogethersothat

theycancommunicate.

WANlink



Theterm"link"isusedforthesamereasonsasinpointto-pointlink.Theterm"WAN"justreferstothelinkbeing

usedforwide-areanetworkingatarelativelylong

distance.



Seriallink



Theterm"serial"referstothefactthattheroutersuse

serialinterfacesoneachendofthelink.



4-wirecircuit



The"4-wire"partreferstothenumberofwiresinthe

cablebetweeneachCOandeachsite;atelcooftenuses

theterm"circuit"torefertoacommunicationpath

betweentwocustomerdevices.



Seriallink



AWANlinkconnectstoserialinterfacesonrouters,hence

thename"seriallink."



DifferencesBetweenaCross-OverCableanda

LeasedCircuit

IfyouinstallanEthernetcross-overcablebetweentworouters

andplugthecableintosometypeofEthernetinterfaceonthe

routers,thetworouterscanforwardEthernetframestoeach

other.IfyouinstallaWANlinkbetweentworouters,thetelco

doescreatesomethingsimilartoacross-overcable,butafew

detailsaredifferent.Incaseyou'rewondering,therearelotsof

legitimatereasonsfordoingthingsdifferentlyforleased

circuits,mostofwhichhavetodowithsupportingtheservice

overalargeandvariedtelconetwork.



IFeeltheNeed,theNeedforSpeed

Ethernetcansupporttransmissionspeedsof10Mbps,100

Mbps,1000Mbps(1gigabit/second),andeven10Gbpswith

themostrecentversionofEthernet.AsyoureadinChapter6,

"ReducingCongestionandDrivingFasterontheLocal(Network)

Roadway,"thespeedscanoftenbeautomaticallynegotiated,so

aPCthatisconnectedtoaswitchcannegotiatetouse100

Mbps,aslongasboththePCandtheswitchsupportthat



speed.

WithWANlinks,manydifferenttransmissionspeedsare

supported.TherearevariousWANtransmissionspeeds,buta

lotofWANlinkstodayusespeedsthataremultiplesof64Kbps.

(Whenthetelcosbuilttheirfirstdigitalnetworks50yearsago,

theyneeded64Kbpstosupportasinglevoicecall;that'swhy

WANlinksaremultiplesof64Kbpstoday.)Thetelcocanoffer

multiplesof64Kbps,upthroughtheequivalentof24times64

Kbps,whichisabout1.5Mbps.Intelcojargon,theservicein

whichyougettosend1.5MbpsofdataiscalledaT1circuitor

T1line.

Forexample,imaginethatyouthinkyouneed128Kbpsforthe

seriallinkbetweenAtlantaandCincinnati.Whenyouorderthe

leasedlinefromthetelcoandsignthecontracts,thecontracts

statethespeedofthelink.Ifyoutrytogofasterorslower,the

linkwon'twork,soyouhavetoconfiguretherouter(or

CSU/DSUmoreonthatinthenextsection)torunattheright

speed.

Youshouldbeawareoftwoothertermsthatrefertothespeed

atwhichbitsaresentoveraWANlink:clockrateand

bandwidth.Clockratereferstotheactualtimingofhowoftena

bitissent;forinstance,abitissentevery1/64,000ofasecond

ona64-Kbpslink.Bandwidthisanothertermthatrefersto

thetransmissionspeedofthelink.

Thetelcooffersspeedsfasterthan1.5Mbpsaswell.Youcan

orderlinksinmultiplesof1.544Mbps,whichistheexactspeed

ofaT1.Youcanalsoorderevenhigherspeedsthatgenerally

comeinmultiplesof51.84Mbps.Sufficeittosay,youhave

tonsofoptions,foralargevarietyofrequirements,andallat

differentpricesaswell.Followingarethekeypointsyoushould

rememberaboutthespeedofWANlinks:

Manydifferentspeedsarepossible,someofwhicharen't



mentionedhere.

Youspecifythespeedwhenyouordertheleasedline.

Youneedtoconfiguretherouters(orCSU/DSU)tousethe

rightspeed.



TheNeedtoControltheSpeed

SometypesofEthernetNICsandrouterinterfacescanrunat

variousEthernet-supportedspeeds.ForthoseNICsand

interfaces,youcaneitherconfigurethePCorroutertouseone

speed,orthedevicecanautonegotiatethespeed.

Seriallinkscanrunatdifferentspeeds,butthespeedmustbe

preconfigured.Acomponentofaserialinterfacecalledthe

CSU/DSU(channelserviceunit/dataserviceunit)controlsthe

speed.TheCSU/DSUfunctions,whichincludeseveralother

thingsbesidessettingthespeed,canbedonewithanexternal

deviceorasafunctionoftheserialinterfacecardontherouter.

IfyouuseaCSU/DSUthatsitsoutsidetherouter,youmust

configurethespeedonthatexternaldevice;iftheCSU/DSUis

builtintotherouter,youmustconfigurethespeedoftheWAN

linkontherouter.

Whenyouorderarouter,youcanorderaserialinterfaceeither

withorwithouttheCSU/DSUontheserialinterface.Ifyou

orderaserialinterfacewithaCSU/DSU,it'scalledaninternal

CSU/DSU,andifyouorderarouter'sserialinterfacewithouta

CSU/DSU,youhavetouseanexternalCSU/DSU.Figure14-4

showsanexampleofbothaninternalandanexternal

CSU/DSU.



Figure14-4.CablingwithInternalandExternal

CSU/DSUs



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