Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Slide 45Slide 46Slide 47Slide 48Slide 49Slide 50Slide 51Slide 52Multimedia15-441 Computer Networks10/02/02Xavier AppéOutlinesDifference with classic applicationsDifference with classic applicationsClasses of multimedia applicationsRequirements/ConstraintsProblems with today’s Internet and solutionsCommon multimedia protocolsRTP, RTCPAccessing multimedia data through a web serverConclusionDifference with classic applicationsHighly delay-sensitivePackets are useless if they arrive too late Loss-tolerant (for the most part) Packet loss can be concealedOutlinesDifference with classic applicationsClasses of multimedia applicationsClasses of multimedia applicationsRequirements/ConstraintsRequirements/ConstraintsProblems with today’s Internet and solutionsCommon multimedia protocolsRTP, RTCPAccessing multimedia data through a web serverConclusionClasses of multimedia ApplicationsStreaming Stored Audio and VideoStreaming Live Audio and VideoReal-Time Interactive Audio and VideoOthersClass: Streaming Stored Audio and VideoThe multimedia content has been prerecorded and stored on a serverUser may pause, rewind, forward, etc…The time between the initial request and display start can be 1 to 10 seconds Constraint: after display start, the playout must be continuousClass: Streaming Live Audio and VideoSimilar to traditional broadcast TV/radio, but delivery on the InternetNon-interactive just view/listenCan not pause or rewindOften combined with multicastThe time between the initial request and display start can be up to 10 secondsConstraint: like stored streaming, after display start, the playout must be continuousClass: Real-Time Interactive Audio and VideoPhone conversation/Video conferencingConstraint: delay between initial request and display start must be smallVideo: <150 ms acceptableAudio: <150 ms not perceived, <400 ms acceptableConstraint: after display start, the playout must be continuousClass: OthersMultimedia sharing applicationsDownload-and-then-play applicationsE.g. Napster, Gnutella, FreenetDistance learning applicationsCoordinate video, audio and dataTypically distributed on CDsOutlinesDifference with classic applicationsClasses of multimedia applicationsRequirements/ConstraintsProblems with today’s Internet and Problems with today’s Internet and solutionssolutionsCommon multimedia protocolsRTP, RTCPAccessing multimedia data through a web serverConclusionChallengeTCP/UDP/IP suite provides best-effort, no guarantees on expectation or variance of packet delayPerformance deteriorate if links are congested (transoceanic)Most router implementations use only First-Come-First-Serve (FCFS) packet processing and transmission schedulingProblems and solutionsLimited bandwidthSolution: CompressionPacket Jitter Solution: Fixed/adaptive playout delay for Audio (example: phone over IP)Packet lossSolution: FEC, InterleavingProblem: Limited bandwidth Intro: DigitalizationAudiox samples every second (x=frequency)The value of each sample is rounded to a finite number of values (for example 256). This is called quantizationVideoEach pixel has a colorEach color has a valueProblem: Limited bandwidthNeed for compressionAudioCD quality: 44100 samples per seconds with 16 bits per sample, stereo sound44100*16*2 = 1.411 MbpsFor a 3-minute song: 1.441 * 180 = 254 Mb = 31.75 MBVideoFor 320*240 images with 24-bit colors320*240*24 = 230KB/image15 frames/sec: 15*230KB = 3.456MB3 minutes of video: 3.456*180 = 622MBAudio compressionSeveral techniquesGSM (13 kbps), G.729(8 kbps), G723.3(6.4 and 5.3kbps)MPEG 1 layer 3 (also known as MP3)•Typical compress rates 96kbps, 128kbps, 160kbps•Very little sound degradation•If file is broken up, each piece is still playable•Complex (psychoacoustic masking, redundancy reduction, and bit reservoir buffering) •3-minute song (128kbps) : 2.8MBImage compression: JPEGDivide digitized image in 8x8 pixel blocksPixel blocks are transformed into frequency blocks using DCT (Discrete Cosine Transform). This is similar to FFT (Fast Fourier Transform)The quantization phase limits the precision of the frequency coefficient.The encoding phase packs this information in a dense fashionJPEG CompressionVideo compressionPopular techniquesMPEG 1 for CD-ROM quality video (1.5Mbps)MPEG 2 for high quality DVD video (3-6 Mbps)MPEG 4 for object-oriented video compressionVideo Compression: MPEGMPEG uses inter-frame encodingExploits the similarity between consecutive framesThree frame typesI frame: independent encoding of the frame (JPEG)P frame: encodes difference relative to I-frame (predicted)B frame: encodes difference relative to interpolated frameNote that frames will have different sizesComplex encoding, e.g. motion of pixel blocks, scene changes, …Decoding is easier then encodingMPEG often uses fixed-rate encodingI PB B BB B BP PI B B B BMPEG Compression (cont.)MPEG System StreamsCombine MPEG video and audio streams in a single synchronized stream Consists of a hierarchy with meta data at every level describing the dataSystem level contains synchronization informationVideo level is organized as a stream of group of picturesGroup of pictures consists of picturesPictures are organized in slices…MPEG System Streams (cont.)MPEG System Streams (cont.)Problem: Packet JitterJitter: Variation in delayExample135 4 3 2SenderNo jitter125 466ReceiverJitterpkt 6pkt 5Dealing with packet jitterHow does Phone over IP applications limit the effect of jitter?A sequence number is added to each packet A timestamp is added to each packetPlayout is delayedDealing with packet jitterFixed playout delayFixed playout delayDealing with packet jitterAdaptive playout delayObjective is to use a value for p-r that tracks the network delay performance as it varies during a transfer. The following formulas are used: di =
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