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Unit PresentingMultimediaMostuniversittycampuswillbelimitedintthebandwidthavailableintheirnetworks,oftenabout100MbpsEthernets,whichmeansthe umavailabletoindividualusersislikelytobeabout10Mbps.Withtimefurtherbandwidthwilleavailableandthentheabilitytohandlemultimediainformationwillbemoreassured.隨著時間的推移，可用帶寬會逐漸增加，人們

UnitTwoscenarioswillbecommon:firstthepresentationofmultimediamaterialintheteachingspacewhichisbeingrelayedoverthelocalareanetworkfromanetworkserverandsecondlybytheuseofstandalonesystems. Unit

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Whenthenetworksareused,therearetwoprincipalmethodsinuse:eitherthematerialmaybemulticasttoanumberofsitessimultaneouslyovertheMBone,ormultipointISDNconfigurationscanbeused.Ineithercaseteachingcanbecarriedouttodistributionofsitescollaboratinginasharedteachingprogram.

TheMBoneoperatesontheInternet,andisdesignedtolinkanumberofsitesbutusespathwayswhichminimizethetrafficontheInternetbysharingroutesbetweensitesasmuchaspossible.TheMBonecanbeusedin permittingeachsitetocommunicatewitheveryotherinthesession. UnitThisisdifferenttomultipointISDNconferencesbetweenanumberofsitessimultaneously,whereallsitesareconnectedtoacentralMultipointControlUnit(MCU).UnitThisisdifferenttomultipointISDNconferencesbetweenanumberofsitessimultaneously,whereallsitesareconnectedtoacentralMultipointControlUnit(MCU).TheswitchingoftheMCUiscommonlyachievedbyvoiceactivationsothatthesitewheretheaudioisactiveistransmittingtoallreceivingsites.1 8

Thissystemuntilrecentlyrequiredthatallsiteswereoperatingatthesamebandwidth(e.g.,128kbpsforISDN-2),whichresultedinthewholenetworkhavingtooperateatthespeedoftheslowestpartner.Recentadvanceswillpermitmixedworkingthuspermittingsomesitestousetheirequipmentcapableofoperatingat384kbpsandhigherresolution,andatthesametimeallowingsmallersitestobepartoftheconferencewiththeirequipmentonlyoperatingat128kbps.2 9Unit

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在教學中提供的支持信息越復雜，Themultimediamaterialusedinteachingapplicationscanbedeliveredtotheuseroveravarietyoftypesofnetworks,whichcanbeinternaldepartmentalorcampusLANsortheycanbewideareanetworks(WANs)linkedtovariousregionsandinstitutions,supportedbytheacademicnetworks(JANETandMANs)orbypublicserviceprovidersuchasISDN.Eachof willbediscussedinthispaper.

Itisassumedthatthehigherthesophisticationofthesupportinginformationinteachingandlearningthebetteritisforthestudent.Thisgreatersophisticationrequiresmoreeffortintheproduction,andmoresophisticatedequipmentforthepresentation,sotheteachershouldalwaysaskwhatisthesimplestandclearestwayofpresentinginformationconsistentwiththeuseofthematerial.Unit Ahigh-resolutionpicturehasnoadvantagetothestudentiftheinformationcouldbepresentedperfectlyadequa yasalinedrawing.3Asmultimediaisexpensivetoproduceandtopresent,itsusemustbejustified.High-resolutionimagesaregoodtolookatbuttheaddeddetailmustcontributesomethingpositivetothestudent’sknowledge.

UnitTheadvantageofmultimediaistheabilitytobringtogetheranumberofdifferentmediainawaywhichpermitssynchronizationoffinformation,e.g.,visionandsound,andwhichoverallenhancestheinformation.Itshouldberememberedthatthe“people”themselvescanprovideinformationinseveralmedianaturally,e.g.,thepatientdemonstratingaparticularclinicalsituationprovidesvisual,tactileandevenolfactoryinformationofsignsandsymptoms,aswellasbeingabletodescribeandbequestionedabouttheeffectsofthedisease.4 UnitAnotherexampleistheabilitytolinkintotheteachingspaceoverremote linkswhichcanprovidevisualandaudioinformationfromadistance,e.g.,theapplicationofopenanddistancelearningtechniques.5

UnitInpracticethegooduseofmultimediagivestheteacheropportunitiestoprovideanumberofstimulitothestudents,whichcanmaketheteachingmuchmorecomprehensiveand提供多方位的刺激，這使得教學內(nèi)容 Theabilitytoprovidemorevariedstimulitotheteachingandlearningprocesscanalsoincludethestudentworkingonhis/herown;theabilitytopresentonthecomputerscreeninavarietyofcoloredimagesandsoundeffectscanenhancetheinformationbeingpresentedtothestudent.Theeyeisundoubtedlyattractedbycolorsandshapesandthesearemostfrequentlyusedtoattracttheinterestofthestudent.

Unit Network Iflegacynetwork,suchasopticalfiberscarrying audio-visualsignalsstillexistinaninstitutiontheycanbeusedtogoodpurpose.ForexampleintheTLTPINSURRECTprojectaSONYLVR discserverwasusedtostorestillandmovingimagesandtheLIVENETopticalfiberswereusedtotakethese ogimagesintotheteachingspace.6 Unit

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AlsotheycarriedtheimagestoCODECsfortransmissiontoremoteteachingsitesovertheSuperJANETATMnetwork.TheSONY serverstoredupto54,000imageframesaseitherstillimagesorshort sequences.ThisSONYsystemcouldbecontrolledusingRS232Acommands,transmittedeitherovertheLANorovertheINTERNETfromremotesites.

Thusitwaspossibletocontrolslideshowseasilyfromalaptopinthelecturetheatre,eitherwithinthesameinstitutionor sequencescouldalsobecontrolledinan ctivemanner:stop ,startingand andforwards.Inthefuturethedigitalnetworkwillincreasinglypervadethewholecampus,butitmustberememberedthatthe oguenetworkislikelytoprovideahigherbandwidthandthereforehigherresolutionimages.

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UnitIntheINSURRECTproject,LIVENET oguecircuitswereusedttotransmitvideoandaudiosignalsaroundtheUCLAcampus,andalsowerefedintoCODECstobetransmittedovertheSuperJANETATM networktotheremotesites.Onesite(Bristol)wasnotonSuperJANET,andTCP/IPwastransmittedovera100MbpsSMDSlinkverysuccessfully.Intheearlypartoftheexperimenttherewassomedifficultysynchronizingthevoicechannelwiththe butthiswasimprovedduringthelifetimeoftheproject.

EthernetLANsarecommoninuniversitycampuscapableofprovidinggoodresolutionsignalsusingIPintotheteachingspacebutthereisnobandwidth forthe andthusthequalityofthesignalwillvaryaccordingtothelevelofusageoftheEthernet.7 在將音頻通道與 UnitItisnotablethatcurrentLAN conferencingsystemsareusingIPforthecampusnetworks,butuseanISDNgatewaytoconnectwithremotesitesbecausetheguaranteedbandwidthavailableonISDNensuresthatthepicturequalitycanbe

UnitATM網(wǎng)絡常用作傳輸媒介，使用多種 ATMnetworkscanbeusedwithtypicalbandwidthsof25Mbpsto155Mbps,bothinLANsandWANs.ATMnetworksareoftenusedasatransportmediumcarryinganumberofprotocolssuchasIP,SMDSandLANEmulation.InsomeapplicationsnativeATMtechnologyisutilizedtotransmittheaudioand UnitInmanycampusnetworksthereissamixtureofdifferentinuse;atypicalscenariowouldbeanATMbackbonewithEthernetsconnected.AsfaraspossibletheATMbackboneshouldberoutedclosetomajorteaching

UnitWhenteachingthereisfrequentlyaneedtolinkmorethantwosites,andthenMboneandmultipointconfigurationarenecessary.Amultipointconferenceusesastarnetworkconfiguration,andiscapableofbeingin ctiveaseachportontheMCUwillsupporttwo-way andaudio.Theaudiosignalisusedtocontroltheswitchinginvoiceswitching UnitUnitWhatisISDNIntegratedServicesDigitalNetworkisa servicethatenablesyoutohavehigh-speeddataconnectionsthroughyourphoneline.ISDNisbasicallythe networkturnedcomple ydigital,usingexistingwiring.ISDNiscapableoftransmittinglargeamountsofdatarapidlybetweensites,anditispossibletosupportcompressedtransmissionbecausethebandwidthisguaranteedandthereforetheaudioand synchronyofthesignalis原則上ISDN是利用已有的聯(lián)線，將UnitMulti-castingisparticularlyrelevanttoIPnetworks,andtheMbonehasbeendevelopedtoeconomize/minimizethetrafficonthenetworkbychoosingroutesbetweenallsitessothatasmuchaspossibleofthetrafficcansharethesameroutesandbranchingoutinthefinalstagesofthelinks.8Ideallythemulti-castnetworkwilllinkallsitestoeachotherandfurtherthisnetworkcansupportin ctivetraffic.Unit

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ISDNismuchcheaperthanmanyothermethodsofmovingdataathighspeeds,butitisstillexpensiverelativetoanormalphoneline.Normalphonelinesthekindthatworkreliablywithyour14.4kbpsmodem,fax,andansweringmachine,areknowninsomecirclesasPOTS inold ephoneservice.Thereareseriousattemptstoprovide onthesecircuitsasexemplifiedby ephones,buttheseingeneraldonotprovidetheresolutionnecessaryforteaching.

AmajordrawbacktoISDNisthatbecauseitmovesdigitaldatainsteadof oguedata,itdoesn’tworkwithyourregularmodem,oransweringmachine.Youneedspecial,expensiveequipmenttoperformthosefunctionsatISDNspeeds.However,ISDNisfasterthanastandardmodem,andisavailableinvariouspartsoftheworld,includingAustralia,WesternEurope,Japan,Singapore,France,andportionsoftheU.S.WhyisbandwidthimportantinthedistancelearningusingHowcan ogtechniquesbeutilizedinthenetwork Whatarethetypesoflinksthatareusedinnetwork-basededucationsystemsasdiscussedinthetext?

Operationoftheradarisbasedonthemeasurementofthetimeittakesforapulsetransmittedfromanantennatogetreflectedbytheobjecttobedetectedandtoreturnattheantennaandthereceiver. Thedegradationofthetransmittedsignalisaresultofsignaldistortionduetoimperfectresponseofftthechannelandduetoundesirableelectricalsignals(noise)andinterference.Noiseandsignaldistortionaretwobasicproblemsofelectricalcommunication.

Theprecedingchaptershavedescribedhowtheautomaticcomputerperforms,buthavenotdescribedhowtheautomaticcomputercanbeputtousetododatahandlingworktoserveanorganization,norwhysuchworkisdoneinthewaythatitis.ForalongtimetheinternalspeakerwastheonlysoundsourceforthePC.Unfortuna y,theinternalspeakercanonlyproduceafewsimplebeeps.Notuntil1987,whenaudioadaptercardsbeganappearingonthemarket,didPCusersgetanopportunitytohearandcreatetruesound.

InanMP3encoder, zationisdoneviaapower-lawzerwithlargersamplevaluesautomaticallycodedwithlessaccuracy.The zedvaluesarecodedbyHuffmancoding,whichiscallednoiselesscodingbecausenonoiseisaddedtotheaudiosignal.TheHuffmancodetablesassignshortercodewordstosmallerandmorefrequentzed Ifthenumberofresultedbitsexceedsthenumberofbitsavailableforagivendatablock,theglobalgainisadjustedtoresultinalarger zationstepsizeandsmallerzedvalues.ThisoperationisrepeateduntiltheresultingbitdemandforHuffmancodingissmallenough.Thesystemstartswithadefaultfactorof1.0foreachband.Ifthe zationnoiseinagivenbandexceedstheallowednoiseas dbytheperceptualmodel,theIfthenumberofresultedbitsexceedsthenumberofbitsavailableforagivendatablock,theglobalgainisadjustedtoresultinalarger zationstepsizeandsmallerzedvalues.ThisoperationisrepeateduntiltheresultingbitdemandforHuffmancodingissmallenough.Thesystemstartswithadefaultfactorof1.0foreachband.Ifthe zationnoiseinagivenbandexceedstheallowednoiseas dbytheperceptualmodel,thescalefactorisadjustedtoreducethe zationnoise.Wesha