Multimedia Networking Reading: Sections 3.1.2, 3.3, 4.5, and 6.5Goals of Today’s LectureDigital AudioAudio ExamplesAudio CompressionDigital VideoSlide 7Video Compression: Within an ImageVideo Compression: Across ImagesTransferring Audio and Video DataStreaming Stored Audio and VideoPlayout BufferInfluence of Playout DelayRequirements for Data TransportStreaming From Web ServersInitiating Streams from Web ServersUsing a Streaming ServerTCP is Not a Good FitBetter Ways of Transporting DataRecovering From Packet LossForward Error Correction (FEC)Interactive Audio and VideoVoice Over IP (VoIP)Traditional Telecom InfrastructureVoIP GatewaysVoIP With an Analog PhoneSkypeSkype Network ArchitectureChallenges of Firewalls and NATsData TransferSilence SuppressionSkype Data TransferVoIP QualityPrinciples for QoS GuaranteesSlide 35Slide 36Slide 37Quality of ServiceConclusions1Multimedia NetworkingReading: Sections 3.1.2, 3.3, 4.5, and 6.5 COS 461: Computer NetworksSpring 2007 (MW 1:30-2:50 in Friend 004)Jennifer RexfordTeaching Assistant: Ioannis Avramopoulos http://www.cs.princeton.edu/courses/archive/spring07/cos461/2Goals of Today’s Lecture•Digital audio and video–Sampling, quantizing, and compressing•Multimedia applications–Streaming audio and video for playback–Live, interactive audio and video•Multimedia transfers over a best-effort network–Tolerating packet loss, delay, and jitter–Forward error correction and playout buffers•Improving the service the network offers–Marking, policing, scheduling, and admission control3Digital Audio•Sampling the analog signal–Sample at some fixed rate –Each sample is an arbitrary real number•Quantizing each sample–Round each sample to one of a finite number of values–Represent each sample in a fixed number of bits4 bit representation(values 0-15)4Audio Examples•Speech–Sampling rate: 8000 samples/second–Sample size: 8 bits per sample–Rate: 64 kbps•Compact Disc (CD)–Sampling rate: 44,100 samples/second–Sample size: 16 bits per sample–Rate: 705.6 kbps for mono, 1.411 Mbps for stereo5Audio Compression•Audio data requires too much bandwidth –Speech: 64 kbps is too high for a dial-up modem user–Stereo music: 1.411 Mbps exceeds most access rates•Compression to reduce the size–Remove redundancy–Remove details that human tend not to perceive•Example audio formats–Speech: GSM (13 kbps), G.729 (8 kbps), and G.723.3 (6.4 and 5.3 kbps)–Stereo music: MPEG 1 layer 3 (MP3) at 96 kbps, 128 kbps, and 160 kbps6Digital Video•Sampling the analog signal–Sample at some fixed rate (e.g., 24 or 30 times per sec)–Each sample is an image•Quantizing each sample–Representing an image as an array of picture elements–Each pixel is a mixture of colors (red, green, and blue)–E.g., 24 bits, with 8 bits per color7The 320 x 240handThe 2272 x 1704hand8Video Compression: Within an Image•Image compression–Exploit spatial redundancy (e.g., regions of same color)–Exploit aspects humans tend not to notice•Common image compression formats–Joint Pictures Expert Group (JPEG)–Graphical Interchange Format (GIF)Uncompressed: 167 KB Good quality: 46 KB Poor quality: 9 KB9Video Compression: Across Images•Compression across images–Exploit temporal redundancy across images•Common video compression formats–MPEG 1: CD-ROM quality video (1.5 Mbps)–MPEG 2: high-quality DVD video (3-6 Mbps)–Proprietary protocols like QuickTime and RealNetworks10Transferring Audio and Video Data•Simplest case: just like any other file–Audio and video data stored in a file–File downloaded using conventional protocol–Playback does not overlap with data transfer•A variety of more interesting scenarios–Live vs. pre-recorded content–Interactive vs. non-interactive–Single receiver vs. multiple receivers•Examples–Streaming audio and video data from a server–Interactive audio in a phone call11Streaming Stored Audio and Video•Client-server system–Server stores the audio and video files–Clients request files, play them as they download, and perform VCR-like functions (e.g., rewind and pause)•Playing data at the right time–Server divides the data into segments–… and labels each segment with timestamp or frame id–… so the client knows when to play the data•Avoiding starvation at the client–The data must arrive quickly enough–… otherwise the client cannot keep playing12Playout Buffer•Client buffer–Store the data as it arrives from the server–Play data for the user in a continuous fashion•Playout delay–Client typically waits a few seconds to start playing–… to allow some data to build up in the buffer–… to help tolerate some delays down the road13Influence of Playout Delay14Requirements for Data Transport•Delay–Some small delay at the beginning is acceptable–E.g., start-up delays of a few seconds are okay•Jitter–Variability of packet delay within the same packet stream–Client cannot tolerate high variation if the buffer starves•Loss–Small amount of missing data does not disrupt playback–Retransmitting a lost packet might take too long anyway15Streaming From Web Servers•Data stored in a file–Audio: an audio file–Video: interleaving of audio and images in a single file•HTTP request-response–TCP connection between client and server–Client HTTP request and server HTTP response•Client invokes the media player–Content-type indicates the encoding–Browser launches the media player–Media player then renders the file16Initiating Streams from Web Servers•Avoid passing all data through the Web browser–Web server returns a meta file describing the object–Browser launches media player and passes the meta file–The player sets up its own connection to the Web server17Using a Streaming Server•Avoiding the use of HTTP (and perhaps TCP, too)–Web server returns a meta file describing the object–Player requests the data using a different protocol18TCP is Not a Good Fit•Reliable delivery–Retransmission of lost packets–… even though retransmission may not be useful•Adapting the sending rate–Slowing down after a packet loss–… even though it may cause starvation at the client•Protocol overhead–TCP header of 20 bytes in every packet–… which is large for sending audio samples–Sending ACKs for every other packet–… which may be more feedback than needed19Better Ways of Transporting Data•User Datagram Protocol
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