TABLEOFCONTENTS:
1.VideoConferencing
2.ComponentsofaV.C.system
3.Classification
4.Architecture
5.Protocols
6.NetworkTopologies
7.Standards
8.IssuesinImplementation
9.Applications&Conclusion
VideoConferencing
NetworkTopologiesofVideoconferencingSystems:
Itispossiblethateachparticipant(incaseofmorethantwo)
islocatedindifferentlocations.Insuchascenario,the
synchronizationbetweenalltheparticipatingunitshastobe
done.Therearetwowaysthismaybedone.First,eachunit
makesanindividualdirectlinktoeveryotherlinkand
maintainsconnectionwiththemthroughoutthesession.This
methodisparticularlyoverburdeningforanynetwork
equipmentandincurssignificantnetworkbandwidthand
costs.Theapparentadvantageofthismethodistheselectivity
thatcanbeprovidedtoeachuserforprovisioningad-hoc
connections.Also,thereisnosinglepointoffailure;ifalink
betweentwoparticipants,sayAandBbreaks,thenthe
topologybeinglikethatofamesh,wouldnotdisturbanyotherconnections.Thevideorelayedbetweenpointsis
ofbetterqualitybecauseoftheabsenceofanycentralmanagerthrottlingbandwidth.Thisdecentralizedmultipoint
architectureusesH.323standards.
TheotherarchitecturethattakestheloadofftheterminalequipmentusesaMultipointControlUnit(MCU).An
MCUperformsthefunctionofabridge,interconnectingthecallsfromdifferentsources.Theterminalequipment
maycalltheMCUortheMCUmayinitiatetheconnectiontoalltheparties.Thus,thetopologynowchangesto
thatofastar.ThisMCUcanpurelybesoftware,oracombinationofhardwareandsoftware.Itcanbelogically
dividedintotwomainmodules:AMultipointControllerandaMultipointProcessor.Thecontrollerworksonthe
signallingplaneandcontrolstheconferencecreation,closingetc.,negotiateswitheveryunitintheconference
andcontrolsresources.ThemixingandhandlingofmediafromeachterminalisdonebytheMediaProcessor
whichresidesintheMediaPlane.Itcreatesthedatastreamforeachterminalandredirectsittothedestination
endpoint.Thepresenceofacentralmanagercanhelpshapingthebandwidthuseduponeachlink.
WhenbandwidthisatapremiumatechniquecalledVoiceactivatedSwitch(VAS)canbeused.Soina
conference,whenonepartyatalocationisspeaking,onlythatpartyismadevisibletootherparticipants.But,
problemsmayariseifmorethanonepersonstartstalkingsimultaneously.Inthatcase,itbecomesacontingency
problemfortheonewiththeloudestvoicewillbegivenpreference.TheothermodeisContinuousPresenceMode,
wheretheMCUcombinesthevideostreamsfromalltheendpointsintoacommonstreamandtransmitsittoall
theendpoints.Thisway,everyparticipantcanseeeveryoneelsesimultaneously.Theseunifiedimagesareoften
called‘layouts’.Howeverinboththecasesthevoiceistransmittedtoeveryendpointinafullduplexmode.
Codec(Coder-Decoder):
Evenwiththebestavailablenetworksandbandwidths,itwouldbeimpracticaltosendvideoinitsuncompressed
form.So,somekindofvideocompressionhastobeinplacetocompressthevideotoreducethesizeofthebit
streamtobetransmitted.Thisisachievedwiththeuseofcodec(Coder-Decoder).Now,videocompressioncan
bedonethroughtwoapproaches.First,tofindtheinformationthatisrepetitive,andthen,replacetherepetitive
informationbyashortcutbeforetransmission.Thisshortcutcanthenbereplacedbyoriginalinformationatthe
receivingendrestoringthevideotoitsoriginalform.(Justlikeamacroinprogramming)Otherapproachinvolves
eliminationofunimportantdatafromtheframes,sothatonlytheinformationperceptibletothehumaneyesis
visible.Thiscandrasticallyreducethesizeofdigitaldatatobetransmitted,butcanresultinverypoorquality
video.Twomajormethodshavebeenemployedtoachievevideocompressiontominimizelossesandsizeatthe
sametime:
-BlockBasedCompression:Eachframeofthevideo,whichisasingleimage,isdividedintosmall
blocksofinformationcalledpixelsandthealgorithmthenkeepsatrackofhowthevaluesateach
pixelvarieswitheachframeandtime.
-ObjectBasedCompression:MoreadvancedCodecalgorithmclassifytheobjectsintheframesand
keeptrackofmovableandstationaryobjects.Thuslessdatamaybeusedtostoretheinformationof
stationaryobjects,andmoredetailofthemovingobjectsbeprovided.Suchtechniquesaremore
efficientthatthesimplerblockbasedcompressionmethods.
Inordertostandardizethecompressionmethods,theMovingPictureExpertsGrouphascomeupwithseveral
standardsliketheMPEG-4standard.
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