LANChapter02.docx

LANChapter02.docx

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LANChapter02

Chapter2.SegmentingLANs

Thischaptercoversthefollowingkeytopics:

∙WhySegmentLANs?

— 

DiscussesmotivationsforsegmentingLANsandthedisadvantagesofnotsegmenting.

∙SegmentingLANSwithRepeaters— 

Discussesthepurpose,benefits,andlimitationsofrepeatersinLANs.

∙SegmentingLANSwithBridges— 

Discusseshowbridgescreatecollisiondomainsandextendnetworks.AsthefoundationaltechnologyforLANswitches,thissectiondescribesthebenefitsandlimitationsofbridges.

∙SegmentingLANSwithRouters— 

Discusseshowrouterscreatebroadcastdomainsbylimitingthedistributionofbroadcastframes.

∙SegmentingLANSwithSwitches— 

Discussesthedifferencesbetweenbridgesandswitches,andhowswitchescreatebroadcastdomainsdifferentlyfromrouters.

Ascorporationsgrow,networkadministratorsfindthemselvesdeepinfrustration.Managementwantsmoreusersonthenetwork,whereasuserswantmorebandwidth.Tofurtherconfusetheissue,financesoftenconflictwiththetwoobjectives,effectivelylimitingoptions.Althoughthisbookcannothelpwiththelastissue,itcanhelpclarifywhattechnologyoptionsexisttoincreasethenumberofusersservedwhileenhancingtheavailablebandwidthinthesystem.NetworkengineersbuildingLANinfrastructurescanchoosefrommanyinternetworkingdevicestoextendnetworks:

repeaters,bridges,routers,andswitches.Eachcomponentservesspecificrolesandhasutilitywhenproperlydeployed.Engineersoftenexhibitsomeconfusionaboutwhichcomponenttouseforvariousnetworkconfigurations.Agoodunderstandingofhowthesedevicesmanipulatecollisionandbroadcastdomainshelpsthenetworkengineertomakeintelligentchoices.Further,byunderstandingtheseelements,discussionsinlaterchaptersaboutcollisionandbroadcastdomainshaveaclearercontext.

Thischapter,therefore,definesbroadcastandcollisiondomainsanddiscussestheroleofrepeaters,bridges,routers,andswitchesinmanipulatingthedomains.ItalsodescribeswhynetworkadministratorssegmentLANs,andhowthesedevicesfacilitatesegmentation.

WhySegmentLANs?

Networkdesignersoftenfaceaneedtoextendthedistanceofanetwork,thenumberofusersonthesystem,orthebandwidthavailabletousers.Fromacorporatepointofview,thisisagoodthing,becauseitmightindicategrowth.Fromanetworkadministrator'spointofview,thisisoftenabadthing,implyingsleeplessnightsandnoweekends.Evenso,howdoesanadministratorkeepeveryonehappywhilemaintainingpersonalsanity?

Astraightforwardtechnologyanswermightincludethedeploymentofahigherspeednetwork.Ifuserscurrentlyattachtoalegacy10Mbpsnetwork,youcoulddeployaFastEthernetnetworkandprovideanimmediatetenfoldimprovementinbandwidth.Changingthenetworkinfrastructureinthiswaymeansreplacingworkstationadaptercardswithonescapableof100Mbps.Italsomeansreplacingthehubstowhichthestationsconnect.Thenewhubsmustalsosupportthenewnetworkbandwidth.Althougheffective,awholesaleupgrademightbecostprohibitive.

SegmentingLANsisanotherapproachtoprovideusersadditionalbandwidthwithoutreplacingalluserequipment.BysegmentingLANs,theadministratorbreaksanetworkintosmallerportionsandconnectsthemwithsometypeofinternetworkingequipment.Figure2-1illustratesabefore-and-aftersituationforsegmentingnetworks.

Figure2-1ANetworkBeforeandAfterSegmentation

Beforesegmentation,all500userssharethenetwork's10Mbpsbandwidthbecausethesegmentsinterconnectwithrepeaters.（Thenextsectioninthischapterdescribeshowrepeatersworkandwhythisistrue.）Theafternetworkreplacestherepeaterswithbridgesandroutersisolatingsegmentsandprovidingmorebandwidthforusers.BridgesandroutersgeneratebandwidthbycreatingnewcollisionandbroadcastdomainsassummarizedinTable2-1.（ThesectionsonLANsegmentationwithbridgesandrouterslaterinthischapterdefinecollisionandbroadcastdomainsanddescribewhythisisso.）

Table 2-1.AComparisonofCollisionandBroadcastDomain

Device

CollisionDomains

BroadcastDomains

Repeater

One

One

Bridge

Many

One

Router

Many

Many

Switch

Many

Configurable

Eachsegmentcanfurtherdividewithadditionalbridges,routers,andswitchesprovidingevenmoreuserbandwidth.Byreducingthenumberofusersoneachsegment,morebandwidthavailsitselftousers.Theextremecasededicatesoneusertoeachsegmentprovidingfullmediabandwidthtoeachuser.Thisisexactlywhatswitchesallowtheadministratortobuild.

Thequestionremains,though,"Whatshouldyouusetosegmentthenetwork?

Shouldyouusearepeater,bridge,router,orLANswitch?

"Repeatersdonotreallysegmentanetworkanddonotcreatemorebandwidth.Theysimplyallowyoutoextendthenetworkdistancetosomedegree.Bridges,routers,andswitchesaremoresuitableforLANsegmentation.Thesectionsthatfollowdescribethevariousoptions.Therepeaterisincludedinthediscussionbecauseyoumightattacharepeater-basednetworktoyoursegmentednetwork.Therefore,youneedtoknowhowrepeatersinteractwithsegmentationdevices.

SegmentingLANswithRepeaters

LegacyEthernetsystemssuchas10Base5,10Base2,and10BaseThavedistancelimitationsforsegmentsasdescribedinChapter1,"DesktopTechnologies."Wheneveryoudesiretoextendthedistance,youcanuseaninternetworkingdevicelikearepeater.RepeatersoperateatLayer1oftheOSImodelandappearasanextensiontothecablesegment.Workstationshavenoknowledgeofthepresenceofarepeaterwhichiscompletelytransparenttotheattacheddevices.ArepeaterattacheswiresegmentstogetherasshowninFigure2-2.

Figure2-2InterconnectingLANSegmentswithaRepeater

Repeatersregeneratethesignalfromonewireontotheother.WhenStation1transmitstoStation2,theframealsoappearsonWireB,eventhoughthesourceanddestinationdevicecoexistonWireA.Repeatersareunintelligentdevicesandhavenoinsighttothedatacontent.Theyblindlyperformtheirresponsibilityofforwardingsignalsfromonewiretoallotherwires.Iftheframecontainserrors,therepeaterforwardsit.IftheframeviolatestheminimumormaximumframesizesspecifiedbyEthernet,therepeaterforwardsit.IfacollisionoccursonWireA,WireBalsoseesit.Repeaterstrulyactlikeanextensionofthecable.

AlthoughFigure2-2showstheinterconnectionoftwosegments,repeaterscanhavemanyportstoattachmultiplesegmentsasshowninFigure2-3.

Figure2-3AMultiportRepeater

A10BaseTnetworkiscomprisedofhubsandtwisted-paircablestointerconnectworkstations.Hubsaremultiportrepeatersandforwardsignalsfromoneinterfacetoallotherinterfaces.AsinFigure2-2,allstationsattachedtothehubinFigure2-3seealltraffic,boththegoodandthebad.

Repeatersperformseveraldutiesassociatedwithsignalpropagation.Forexample,repeatersregenerateandretimethesignalandcreateanewpreamble.PreamblebitsprecedetheframedestinationMACaddressandhelpreceiverstosynchronize.The8-bytepreamblehasanalternatingbinary1010patternexceptforthelastbyte.Thelastbyteofthepreamble,whichendsinabinarypatternof10101011,iscalledthestartofframedelimiter（SFD）.Thelasttwobitsindicatetothereceiverthatdatafollows.Repeatersstripalleightpreamblebytesfromtheincomingframe,thengenerateandprependanewpreambleontheframebeforetransmissionthroughtheoutboundinterface.

Repeatersalsoensurethatcollisionsaresignaledonallports.IfStations1and2inFigure2-2participateinacollision,thecollisionisenforcedthroughtherepeatersothatthestationsonWireBalsoknowofthecollision.StationsonWireBmustwaitforthecollisiontoclearbeforetransmitting.IfStations3and4donotknowofthecollision,theymightattemptatransmissionduringStation1and2'scollisionevent.Theybecomeadditionalparticipantsinthecollision.

Limitationsexistinarepeater-basednetwork.Theyarisefromdifferentcausesandmustbeconsideredwhenextendinganetworkwithrepeaters.Thelimitationsincludethefollowing:

∙Sharedbandwidthbetweendevices

∙Specificationconstraintsonthenumberofstationspersegment

∙End-to-enddistancecapability

SharedBandwidth

Arepeaterextendsnotjustthedistanceofthecable,butitalsoextendsthecollisiondomain.Collisionsononesegmentaffectstationsonanotherrepeater-connectedsegment.Collisionsextendthrougharepeaterandconsumebandwidthonallinterconnectedsegments.Anothersideeffectofacollisiondomainisthepropagationofframesthroughthenetwork.Ifthenetworkusessharednetworktechnology,allstationsintherepeater-basednetworksharethebandwidth.Thisistruewhetherthesourceframeisunicast,multicast,orbroadcast.Allstationsseeallframes.Addingmorestationstotherepeaternetworkpotentiallydividesthebandwidthevenfurther.LegacyEthernetsystemshaveashared10Mbpsbandwidth.Thestationstaketurnsusingthebandwidth.Asthenumberoftransmittingworkstationsincreases,theamountofavailablebandwidthdecreases.

Note

Bandwidthisactuallydividedbythenumberoftransmittingstations.Simplyattachingastationdoesnotconsumebandwidthuntilthedevicetransmits.Asatheoreticalextreme,anetworkcanbeconstructedof1,000deviceswithonlyonedevicetransmittingandtheother999onlylistening.Inthiscase,thebandwidthisdedicatedtothesingletransmittingstationbyvirtueofthefactthatnootherdeviceistransmitting.Therefore,thetransmitterneverexperiencescollisionsandcantransmitwheneveritdesiresatfullmed