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CHAPTER4
UTILITIES&FEATURES
Revised: 25 Jan 17 4-6 ESTeem Horizon Series
VLANOPERATIO N
VirtualLAN(VLAN)systemsarerapidlybecomingthedesiredconfiguration onlargerEthernetnetworks.TheESTeemHorizon
softwarewillallowtheEthernetmodemstotransparentlypasstheVLANpacketsforthesenetworks.TheHorizoncurrentlycan
notbeusedasaVLANswitch,butwillpassVLAN taggedEthernetpacketsthroughthewi
relessbridge.
IGMPSNOOPING
InternetGroupManagementProtocol(IGMP) SnoopingallowstheESTeemHorizontooperatemoreefficientlyinnetworkswith
highMulticast (UDP,etc.)traffic.IGMPSnoopingwilldefine thedestination fortheMulticast traf fi c and sendthedatatothe
correctwirelessEthernetmodem,nottheentirenetwork.ThislimitingofMulticasttraffictospecif
icdestinationsgreatlyincreases
theoverallnetworkefficiency.
TheproblemwithusingmulticasttrafficoverawirelessconnectionisthatmulticastpacketsdonotrequireanAcknowledgeon
theprotocollayer. IfthewirelessnetworkmissesadatapacketontheRFnetwork,thatmulticastdatapacketislost.
TheoryofOperation
TherearetwotypesofEthernetpacketsonanetwork,unicastandmulticast.Unicastisintendedforexactlyonerecipient(and
ignored byall others).Multicast isintended to bereceived bymultiple recipients.Interested partiescan listen for particular
multicast packets,but mostnodes ignore it
.Inanycase,the networkmedium isstillutilizedno matterhowmany nodesare
listening.Broadcastpacketsar easpecialtypeofmulticasttr a f ficwhichallnodesalwa ysreceive.Theseareparticularlyusefulfor
globalannouncements(Hey,I'mAlice!)andqueries(Hey,I'mAliceandI'mlookingforBob!).Morere
levantly,itishowhostson
anetworkfindouteachother'saddresses,andareabsol utelycrucialtotheproperfunctioningofanetwork.
Asthenetworkgrowsphysically,itencounterssomegrowingpains.Thefirstismorephysical,relatingtothecablinglimitations.
Bridgessolvethisproblembyjoiningtw
ophysicalnetworkstogethersotheyappeartobeasinglelargenetwork.Throughuseof
bridges,anetworkcanscaletohundredsoreventhousandsofhosts.Thereisadownside,however.Wellbeforethephysical
limitationsofbridgi nghit,youstarttorunintoefficiencyproblems,asal
ltraffichastotraveleverywhereonthenetwork.Theraw
carryingcapacityofthissharedmedium,oftenreferredtoasbandwidth,isthesecondscalingproblem.
Smartbridges,orswitches,helpalleviatethebandwidthproblembyonlypassingtrafficacrossthebridgeifthedestinationhost
was on the ot
her side.This greatly increases the capacity and efficiency of the network by allowing two pairs of hosts
communicatingsimultaneouslyoneachsideoftheswitchedbridge.Onlywhenthepacketneedstogototheothersideisboth
mediumsutilizedsimultaneouslyforthesam e packet.
Unfortunately,broadcastandmult
icasttrafficbyitsverynaturemustalwaysberelayedacrossthebridge.Whilethenumberof
network nodes may grow linearly, the multicast traffic tends to grow exponentially.This isn't generally a problem for wired
networks,as theyhaveaconsiderableamountofbandwidthtospare,butwirelessnetworks have,atbest,anorderofmagnitude
les
sbandwidthtobeginwith.Ifawireless network isbridgedwithawirednetwork,whiletheabsolutenumber s arethesame,a
much higher proportion of the wireless network’s availa ble bandwidth is used up by multicast traffic.This effect is further
magnifiedifm
ultiplewirelessrepeatersareinuse.
Thepreferredwayofdealingwithexcessmulticasttrafficistoputthewirelessnetworkonadifferentsub‐network(subnet)and
usearoutertojoinittothewirednetwork.Thisensuresthatonlyunicasttrafficintendedforthe wireless networkcrossesov
er,
asmulticastandbroadcasttrafficstayswithi nitslocalsubnet.Normallythisisfine,buttherearetimeswhereyouneedmulticast
traffictospansubnets.ForIPtraffic,thiscanbeobviatedbyusingamulticast‐capablerouter,butforlegacymulticastprotocols
thatwerenotdesignedtoberouted,anothe
rsolutionmustbefound.