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Chapter 17. Using a Cable Modem

Chapter 17. Using a Cable Modem

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cablecompanieshavetheequipmentnecessaryto

providethisservice,ortheyarenotequippedto

extendpasttheirlimitationssincemostcable

providersoffer"shared"networkservices.Inthat

caseyoushouldconsiderusingxDSLforahighspeedconnectiontotheInternet.Oryoumaylivein

ametropolitanareathatoffersbothservices.Ifthat

isthecase,Isuggestyoureadthischapterand

Chapter16,"DigitalSubscriberLines(DSL)

Technology,"andmakeyourdecisionbasedonthe

topicscovered.Ifyouliveinanareawhereneither

serviceisavailable,youareoutofluck,because

"twotincansandastring"usesamplitude

modulationinsteadofdigitalsignaling.Justseeing

whetheryou'repayingattention!



Anotherfactoryoumightthinkaboutistheactualcostforcable

modemservices.Becausemanycablecompaniesarenow

tryingtomaximizetheirrevenues,sincethecableTVmarketis

prettymuchsaturated,youmightfindthatyoucangeta

specialdealwhensubscribingtobothcableTVandInternet

access.Ifanything,youmaybeabletogetafewmonthsof

cable-modemaccessatareducedprice.



HowCableModemsWork

Bytheirverynature,cabletelevisionnetworksarenot

constrainedbytheproblemscausedwhendataneedstopass

throughthepublicswitchedtelephonenetwork(PSTN).Cable

televisionnetworksdon'tusethePSTN.Instead,theyusea

shieldedcoaxialcable(andsomehigh-speedfiberlinkstoget

closertoyourhouse)todeliverqualityTVsignals,usuallyusing

6MHzbandsforeachdigitalchannel.Toprovideforcable

modemservice,allthatisnecessaryistosetasideoneormore

ofthesefrequencybandsandusethemasdeliverychannelsfor

Internetaccess.Again,theofferingisusuallyasymmetrical,

justlikeADSL.Byusingahigh-bandwidthchannelfor

downstreamcommunications,cablecanofferalarger

bandwidth(forallsubscribersonasharedsegment)tosend

datafromtheInternettoyourhome.Byusingtimeslots(some

dedicatedtoeachsubscriber,andsomecalledcontentionslots

thatanyonecantrytouse),cablecompaniesofferasmaller

upstreamdatacommunicationspath.BecausemostInternet

accessinvolvesalargeramountofdatatransferinthe

downloaddirection,thismakessense.Ifyouwanttooperatea

Websiteandneedahigheruploadbandwidththenresidential

cablemodemserviceisnotforyou.Instead,somecompanies

offerxDSLwithvaryingup-anddownloadbandwidths.These

servicesaregenerallyforbusinessusersandaremore

expensivethantheserviceprovidedbythetelcosandother

DSLproviders.

Cablemodemsaren'treallymodemsatall.Thetermmodemis

anacronymforMODulation/DEModulation,andatruemodemis

adevicethatconvertsdigitalsignalstoanalogsignalsfor

transmissionoverananalogtelephoneline.Thereceiving

modemdoestheopposite,convertingtheanalogsignalbackto

adigitalsignalyourcomputercanunderstand.Thetermcable

modemisjustaconvenientwayofindicatingthatyou'regetting



afastconnectionfromthecablecompany.No"analog"signalis

involvedintheconnectionwhatsoever.



Tip

IfyousubscribetobothcableTVserviceandcable

Internetaccess,youmightbeabletomakethemost

ofyourcableaccess.Useasplitter.Thisisasimple

devicethatplugstheincomingcableintooneside,

andontheothersidetherearetwooutlets.Youcan

pluginonecoaxcableandrunittoyourcable

modem,andrunalinefromtheotheroutlettothe

graphicscardonyourcomputer.Ifthesoftwarethat

camewithyourgraphicscardsupportsaTVtuner,

youcanthenwatchtelevisiononyourcomputer's

monitorwhilesurfingtheWebatthesametime.In

somecasesyoumightalsohavetouseadigitalfilter

withthecablesegmentthatattachestoyour

computer'sgraphicscard.



ProvidingIPAddressestoCableModems

Mostcablecompanieswon'tallocateastaticIPaddressbecause

(1)it'seasiertouseDHCPtodynamicallyassignaddresses,just

likemostnetworkadministratorsdoinatypicalLANsituation,

and(2)therejustaren'tthatmanyaddressesavailabletogrant

auniqueaddresstoeachnodeonthelargenetworkthatthe

cablecompanyoperates.UsingDHCP,whichisdiscussedin

Chapter28,"BOOTPandDynamicHostConfigurationProtocol

(DHCP),"thecablecompanydoesn'thavetoperformthe

manualtaskofassigningIPaddressesforeachcustomer.The

DHCPserver(onthecablecompany'send)performsthistask



automaticallybyinteractingwithDHCPclientsoftwareonyour

computer.Figure17.1diagramstheaddresstranslations

involvedinconnectingasinglecomputertoacablemodem.



Figure17.1.IPaddressesusingacablemodem.



[Viewfullsizeimage]



Note

DHCPcanbeusedtosupplyIPaddressesthatare

notvalidontheInternet,asdescribedpreviously.

However,youcanalsouseDHCPtoconfigureclients

usingarangeofaddressesthatarevalidonthe

Internet.Inabusinesssituation,acablecompany

cansupplyastaticIPaddresstoyourconnectionso

thatyoucanrunaWebserveroranotherInternet

applicationthatrequirestheaddresstostaythe

same.Thiscanbedonebyeditingthenetworking



informationonyourcomputertomanuallyconfigure

astaticIPaddressgiventoyoubytheinstaller.Itis

alsoeasytoreserveIPaddressesusingDHCPso

thattheyareallocatedtojustselectedclientsthat

needthem.



Inthefigureyoucanseethateachcabledropgoestoasingle

cablemodem,andthenisusedbyjustonecomputeronefor

youandoneforyourneighbor.Thisisexactlyhowthecable

company(aswellasxDSLcompanies)wantsthesystemto

operate.Thelesstrafficontheline,thelessbandwidththat

needstobeaggregatedbythecableproviderwhereitconnects

totheInternet.

However,DHCPcanbecascadedsothatmorethanonedevice

isprovidingaddressestocomputers(andothernetwork

devices)foryourhomenetwork.Thisisaccomplishedby

attachingthecableconnectiontoaswitch/routerinsteadof

directlytoyourcomputer.Theswitch/routerdoesthesame

thingthattherouteratthecablecompanydoes:provide

addressesthatarenotvalidontheInternettocomputers

attachedtotheswitch/router.InFigure17.2youcanseethe

waythiscascadingofaddressesworks.



Figure17.2.IPcascadingIPaddressesusinga

routerandacablemodem.



[Viewfullsizeimage]



Herethecablecompanyprovidesyouwithanaddressthatis

notvalidontheInternet.Theswitch/routerusesthisaddressto

communicatewithyourcableprovider.Theswitch/routerthen

usesDHCPtoassignaddressestoyourcomputer(s),eachof

whichisnotvalidontheInternet.ByusingDHCP,

communicationseventuallyflowthroughtheIPaddressesthat

thecablecompanyownsthatarevalidontheInternet.Of

course,withalargenumberofsubscribers,thecablecompany

willhavemorethanonevalidIPaddress,andusemultiple

routerstoconnectvarioussegmentsofitsnetworktothe

Internet.Again,Chapter28willgiveyouanunderstandingof

howDHCPoperatesandkeepstrackofwhichcomputeris

sending/receivingdatafromanothercomputerontheInternet.

Theimportantthingtokeepinmindhereisthatyoucan



connectmorethanonecomputertotheInternetusingacable

modem(orxDSL).IfallcomputersareaccessingtheInternet

atthesametime(asinanofficesetting),theymustsharethe

bandwidthprovidedbythecablemodem.Ifonlyonecomputer

isusingthenetworkatanypointintime,itshouldbeableto

achievethefullbandwidththatthecableprovidercanofferat

thattime.Notethatthisdoesnot,however,helpyouovercome

anybottlenecksthatareinheritintheInternetitself.For

example,ifyouaretryingtoaccessabusyWebsite,youwill

findthatthebandwidthofyourISPdoesn'tmakeany

difference.Theaccesswait-timewillbedependentontheWeb

site'scapabilities.



First-GenerationCableModemSystems

Earlyimplementationsusedaseparate,ordinarytelephone

modemconnectionfortheupstreamdatachannel.Ifyoustill

havethiskindofservice,youmightaswellconsiderusingthat

singletelephonetwisted-pairwiringforADSL(seeChapter16).

ThisisbecauseDSLwillprovidevoiceanddatacircuits,

whereasusingthephonelineforanuploadtotheInternet

eliminatesthevoicecircuit.Ifyoucangetonlythecable

modemservice,thenasecondphonelinefortheuploadtraffic

isagoodideaifyouarerunningahomebusiness,orifyou

workattimeswhenthehometelephoneisfrequentlyused.

Again,notethatthisisanoldtechnology.Thatinvestmentmust

berecoupedbysmallcablecompaniesbeforetheycanoutfit

theircablenetworkswithmoremoderntechnologies,suchas

thenewestversionofDOCSIS(DataOverCableService

InterfaceSpecification).Thisisdiscussedbrieflylaterinthis

chapter.







HowCableModemsDifferfromxDSLBroadband

Access

DSLisadedicatedconnectionfromyourlocationtothetelco's

centralofficeusingyourexistingphoneline.Acablemodem

givesyouaccesstoasharednetworkmedia,similartothe

olderEthernettechnology.Thissharednetworkmediais

nothingmorethanapartofthesignalthattravelsthroughthe

coaxialcablesusedbycablecompaniestodeliverserviceto

yourhomeorbusiness.Justasthebandwidthofordinary

twisted-paircoppercablesisactuallyusedforvoice

transmissions,analmost-similarsituationexistsforcoaxial

cablingfromyourcablecompany.Digitalcablesignalsuse

channels,whicharebasicallytimeslotsinwhicheachchannelis

sentinachannelofusually6MHz.Cablecompaniesneedonly

setasidesomeoftheseslotsforcableserviceinsteadoffor

channelsthatnooneeverwatchesanyway.

Cablemodemsalsogiveyouahigherbandwidthfor

downloadingfromtheInternet,whencomparedtoDSL

solutions.Ascablesegmentsbecomeoverpopulated,your

performancemayvary,dependingonotherusers'habitsand

thecapacityprovidedtoyourlocalneighborhood.Performance

mayvaryfromhourtohour,dependingonwhentheserviceis

beingusedbyyourneighbors.Thisisbecauseandthisis

importanttoremembercablesegmentsaresharedmedia.Ifyou

aretheonlypersoninyourneighborhoodusingacoaxialcable

thatterminatesatthecablecompany,you'llhavethebestcable

servicepossible!Ifyouhaveseveraldozenothercablemodems

onyoursegment,allmustsharetheavailablebandwidthonthe

sharedcoaxialcable.Don'tworry,howeveryourTVchannelswill

stillworkjustaswellasever!



Note



Don'tforgetthatifyouusearouter/switchto

connectmorethanonecomputertoacablemodem,

youmayalsoseeadecreaseinspeedifallofyour

computersareaccessingtheInternetatonetime.



Formanyusers,cablemodemsareawelcomesolutionfor

thosewhohavephonelinesthatcannotbeprovisionedforDSL

service.Almosteveryonewithinevenasmallcitycanobtain

cableTVservices,whetherornotthelocaltelcocanprovide

DSLservice.ByofferingInternetaccessoverexisting

equipment,acablecompanycanaddarouterandconnect

cablesegmentstoalargeInternetpipelineandgenerateextra

revenuefromexistingcustomers.OfferingcableInternet

servicealsoisagoodsellingpointforattractingnewcustomers.

BecausethecableTVmarketisnowprettymuchsaturated,

cablecompaniesneedanewtechnologytoofferinorderto

keepgrowing.

Forsomesmall-businessaccess,andespeciallyformanyhome

users,cablemodemaccessisagreatsolution.WhereasDSL

requiresthatthetelephonecompanyprovisiontheline(remove

bridgetaps,voicecoils,andsoon)andtestittodeterminethe

bandwidththatcanbesupported,cablemodemscanbe

purchasedinanelectronicsstoreandself-installed.Oryoucan

havesomeonefromyourcablecompanyinstallthecable

modem.AfteryouhavecableTVinstalled,addingacable

modemissimple.

Nomatterwhichtechnologyyouwanttouse,itreallycomes

downtothesimplematterofwhatisavailableinyourlocation.

InsomeplacesyoumaybeabletogetDSLaccess,andin

otherscablemodemaccess.Yetinmorecompetitive

environmentsyoumighthaveachoicebetweenthetwo.







TheDataOverCableServiceInterface

Specification(DOCSIS)

Newercablesystems,however,basedontheDOCSIS(Data

OverCableServiceInterfaceSpecification)standardsallowfor

bidirectional,asymmetricalcommunications.Version1.1and

2.0ofDOCSISincludethenecessaryauthenticationand

encryptionfeaturesthatmakecablemodemaccessassecure

asDSL.Ingeneral,ifyoucanseeothercomputersinyour

NetworkNeighborhood,yourcableproviderisusinganolder

systemandyoumightwanttotakestepstoprotectyourself,

suchasusingafirewallappliance.

DOCSISspecificationscanbefoundattheURL

www.cablemodem.com/.ThisistheWebsiteofanorganization

responsibleforestablishingspecificationsandtestinghardware

toensurethatitmeetsthestandardsforcabletelevisionCable

TelevisionLaboratories,Inc.(CableLabs).Thistestingensures

thatacablecompanycanbuyhardwarefromdifferent

manufacturersandbeassuredthatthesystemthatisinstalled

willwork.Forexample,theroutermaycomefromone

company,andthecablemodemsusedatcustomersitesmay

comefromanothermanufacturer.

DOCSISversion2.0isthemostrecentstandard.Thisversion

hasbeenadoptedbytheInternationalTelecommunications

UnionTelecommunicationSector(ITU-T)asaworldwide

standard.

DOCSIS2.0specifications(developedbybothCableLabsand

CableTelevisionLaboratories,Inc.)covermanyaspectsofusing

acablesystemforInternetaccess:

RadioFrequencyInterfaceSpecification:This

documentdescribesthephysicalmediaspecification,



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