CharacteristicsofLANs
ThemainCharacteristicsofaLANincludeitsthroughput,delays,wiringtypeanddistances,security,andreliability.Wediscussthesecharacteristicsbelow.WethencomparethecharacteristicsofmajorLANs.ThroughputThethroughputistheaveragetotaltrans
missionratewhentheLANisheavilyl
oadedbymanynodes.InasharedLAN,thethroughputisafractionoftherateofthetransmitters.ThisfractioniscalledtheefficiencyoftheMACprotocl.Forinsyance,anetworkthatusesaMACprotocolwithanefficiencyoh65percentand10-Mbpstransmittershasathroughputof6.5Mbps.Thesevaluesaretypicalforashared10-MbpsEthernet.ThethroughputofatokenringnetworkandthatofFDDIareclosetotheirthransmissionrate.LatencyThelatencyisthetimeapackettakesfromsitsarrivalatanetworkinterfaceuntilitreachesthedestinationnode.WeexplaininSection4.8thatthislatencyisalmostaiwayslessthanafractionofasecondandisoftenmuchsmaller.WiringTypeandDistanceSomeLANsusetwistedwirepairs,othersuseopticalfibers,OlderLANsandinstallationsinspecialenvironmentsusecoaxialcables.ThemaximumlengthoftheconnectionsdependsontheLANandonthe
techanology.Typicalvaluesare100mfortwistedpairsandafewkilometersformultimodefibers.SecurityAnopticalfiberismoredifficulttotapintothanatwistedpair,whichimprovesthe“physical”security.However,eavesdroppers
rarelyneedtotapdirectlyintoalinktogain
accesstoinformation.Amorecommonattack
istouseoneofthecomputersattachedontheLANandconfigureittoreadallthepacketsthattravelonthenetwork(thisiscalledsnooping).Ifsensitivepiecesofinformationsuchasuserpass
wordsaretransmittedintheclear(nonencrypted),thentheeavesdropperobtainsentrypointsintocomputers.Asarule,aswitchedLANismoresecurethanasharedLANbecauseacomputeronsuchanetworkseesonlythepacketsintendedforit.ReliabilityWedependincreasinglyonnetworks.Accordingly,reliabilitybecomesmoreessential.Aswementionedintheopeningremarksofthischapter,FDDAisdesignedtosurvivealinkornodefailure.AnEthernetkeepsonworkingwhensomeofitsnodesorlinksfail.Atokenringnetworkcanbewiredwithbypasselectronicstohaveasimilarreliability.NetworkDevicesTextTherearemanytypesofmediaforthenetwork,andeachonehasadvantagesanddisadvantages.Oneofthedisadvantagesofthetypeofcablethatweprimarilyuse(CAT5UTP)iscablelength.ThemaximumlengthforUTPcableinanetworkis100meters(approximately333feet).Ifweneedtoextendournetworkbeyondthatlimit,wemustaddadevicetoournetwork.Thisdeviceiscalledarepeater.Thetermrepeatercomesfromtheearlydaysofvisualcommunication,whenamansituatedonahillwouldrepeatthesignaltothepersononthehilltohisright.Italsocomesfromtelegraph,telephone,microwave,andopticalcommunications,allofwhichuserepeaterstostrengthentheirsignalsoverlongdistances,orelsethesignalswilleventuallyfadeordieout.Thepurposeofarepeaterisregenerateandretimenetworksignalsatthebitleveltoallowthemtotravelalongerdistanceonthemedia.Watch