Exfo EXpert IPTV Test Tools (FTB-200v2) Uživatelský manuál stáhnout pdf (Strana 49)

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Asaframeoftenspansmultiplepackets,andatypicalvideostreamincludesinterpolated

frames(P‐framesandB‐frames),agivenpacketlossratecanresultinaframelossratesix

timeshigher[9].SeeFigure2‐1above.

InordertoaccommodateIPTVtransmissionnetworkdelayandlowlevelsofdelayvar

iation,

aplayoutbufferisusedtotemporarilystoreincomingframes.Forstreamingvideo,suchas

DVBorIPTV,itispermissibletoapplyarbitrarydelays,andhencetheplayoutbuffercanbe

quitelarge.Interactivevideoconferencingrequiresarelativelylowdelay,sinceaparticipant

needstorespondimm

ediatelytoquestionsoropinionsoftheotherparticipants.Packets

arrivingtoolatemaybediscardedandwillappearaslostpackets.

Traditionally,videoqualityismeasuredbycomparingthevideosequencethathasbeen

processedbyavideosystemtotheoriginalreferencevideosequence.Itisnotfeasibleto



implementamid‐streamreal‐timemonitoringsystemusingthisapproach.Thisisbecauseit

needstodirectlyaccessthevideosourceanddecodevideocontent,andhashigh

computationalcomplexity.

Toovercomethesedisadvantages,itisthereforedesirabletomonitortransmi ssion

impairmentsinordertoensuregoodvideoqualit

y.VQmon/HDisanon‐intrusivemonitoring

technologythatcanpassivelymeasurethecharacteristicsoflivepacket‐basedvideo streams

andreportqualityscoresinreal‐time.VQmon/HDmodelsthewaythattime‐varying

impairments,mostnotablyburstpacketlossandpossibleplayoutbufferdiscards,affect

perceivedvideoquality.

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A.1.1.1 ImpactofPacketLossandJitter

onVideoQuality

Commonvideocodecs,suchasMPEGandH.263implementations,useacombination

ofintra‐andinter‐framecodingmethods.Forintra‐frameencoding(Iframe)theimageframe

isdividedintoblocks,aDiscreteCosineTransformisusedtoconverteachblocktoasetof

coefficientsandthen

variablelengthcodingisapplied.Agroupofblocksarecombinedintoa

singleentity(slice),whichcanbecarriedwithinasinglepacket.Ifatransmissionerroroccurs

thenthewholegroupmaybelost,creatinga“stripe”withinthedecodedimage.Forexam

ple,

becausetheDCcoefficientswithineachblockarepredicativelyencodedfromthefirstblockin

theslice,anerrormakesthisinformationunusablefortheremainderoftheslice.Someerrors

maydamagetheframestructureandrenderthewholeframeunusable.

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Exfo EXpert IPTV Test Tools (FTB-200v2) Uživatelský manuál Strana 49