Chapitre 7 Multimedia Networking

Transcription

1 Chapitre 7 Multimedia Networking Laurent Schumacher (UNamur) Dernière mise-à-jour : 19 novembre 2013 Materials used with permission from Pearson Education J.F Kurose and K.W. Ross, All Rights Reserved

2 Outline Definition and requirements Taxonomy of multimedia applications Which transport protocols for multimedia applications? Multimedia applications in best-effort Internet Real-time interactive applications RTP, RTCP Streaming stored audio and video RTSP Session Initiation Protocol SIP 2

3 Multimedia Networking Applications Multimedia applications Network audio and video Continuous media Quality of Service (QoS) Network provides application with level of performance needed for application to function Quality of Experience (QoE) The end-user opinion Sources: jibtel, AT&T U-Verse 3

4 Multimedia Networking QoS vs. QoE Quality of Service (QoS) Quality of Experience (QoE) (Claiming to be) objective Quantitative Subjective Qualitative Throughput/goodput Latency Packet interarrival times/jitter Packet loss rate Availability (99.999% = 5 nines ) Mean Opinion Score (MOS) Bad / poor / fair / good / excellent Peak Signal-to-Noise Ratio (PSNR) Automatic perceptive metrics Perceptual Evaluation of Audio Quality (PEAQ) Perceptual Evaluation of Speech Quality (PESQ) 4

5 Outline Definition and requirements Taxonomy of multimedia applications Which transport protocols for multimedia applications? Multimedia applications in best-effort Internet Real-time interactive applications RTP, RTCP Streaming stored audio and video RTSP Session Initiation Protocol SIP 5

6 Taxonomy of Multimedia Applications Peterson and Davie, 1996 Time critical Elastic /opportunistic Tolerate packet delays Do not tolerate packet losses Error intolerant Remote device operation Interactive Telnet, X, NFS Interactive bulk FTP Asynchronous , fax Delay sensitivity Applications Real-time/ playback/ streaming Error tolerant Multimedia applications 6

7 Quantitative application requirements Fidelity Ideal requirements Application Bi tr at e Latency No latency neither delay nor jitter High fidelity neither error nor loss MAX bit rate (bandwidth) In reality, optimise trade-off between available resources and application requirements 7

8 Taxonomy of Multimedia Applications Requirements of elastic applications Delay Bit rate Interactive SSH, X, NFS Minimise Low Interactive bulk FTP Can be high High Loss TCP recovery Asynchronous Can be high Can be high , fax Delays should be low when transactions are short and/or a human operator waits for the answer. Although packet losses can be recovered, they increase the E2E delay. 8

9 Taxonomy of Multimedia Applications Requirements of elastic applications Delay Bit rate Loss Error tolerant Multimedia Minimise High Can be tolerated Error intolerant Remote device operation Minimise Can be high Minimise Multimedia applications elastic applications * Sensitive to mean delay and delay jitter * Loss-tolerant (to some extent) Three broad classes * Streaming stored audio/video (e.g. VoD, catch-up TV, podcast) * Streaming live audio/video (e.g. Live TV) * Real-time interactive audio/video (e.g. Gaming) 9

10 Multimedia Networking Applications Streaming Definition Playout at client begins before download completed Challenge Achieve continuous playout at client Critical latency contraints for still-to-be transmitted content Arrive «on time» for playout Restore original timing despite varying network delays Types Stored content: VoD, catch-up TV Mostly unicast traffic VCR-like interactivity (play, pause, forward) Live content: broadcast TV Suited for multicast Limited interactivity with server 10

11 Multimedia Networking Applications Interactive Examples VoIP Video-conferencing Instant messaging Massively Multiplayer Online Role-Playing Game (MMORPG) Virtual worlds Challenges Constraints on E2E delay to preserve interactivity Session management How does callee advertise its IP address? What format/encoding does s/he support? 11

12 Multimedia NetworkingApplications E2E Delay Requirements of Interactive Apps Application E2E Delay Requirements Scientific computing Automated trading VoIP 1-10 µs 100 µs 1 ms < 150 ms not perceived < 400 ms still acceptable Gaming < 200 ms 12

13 Multimedia Networking Applications Audio compression Analog signal sampled at constant rate Telephone: samples/s CD music: samples/s Each sample is rounded to one of a finite set of quantisation values Each quantised value is represented by k bits Example: samples/sec, 256 quantised values (8-bit quantisation) 64 kbps in Pulse Code Modulation (PCM) Receiver converts it back from digital to analog signal with some quality reduction Example rates Telephony: 64 kbps Internet telephony: 5.3, 6.4, 8 kbps (G.723.3, G.729) GSM: 13 kbps CD: Mbps MP3: 96, 128, 160 kbps 13

14 Multimedia Networking Applications Acoustic to Electric Conversion Sample Sample and Hold and Hold Amplitude Pulse Code Modulation (PCM) Quantification 125 µs Time 160 Bytes Compression 20 ms Packetisation 14

15 Multimedia Networking Applications Image compression A digital image is an array of pixels Each pixel represented by bits One 4,096 x 2,304 image, 8-bit RGB = 226 Mbits! A video is a sequence of images displayed at constant rate (e.g. 24 images/s) One second of video = 5.4 Gbits! Way too much information! Double redundancy Spatial JPEG Temporal MPEG Standards MPEG 1 (CD-ROM) 1.5 Mbps MPEG2 (DVD) 3-6 Mbps MPEG4 (object-oriented) 15

16 Multimedia Networking Applications Still-image compression (JPEG) Source : 16

17 Multimedia Networking Applications Video compression Moving objects are identified in the original video sequence A reference image (I-Frame) is sent Rest of GoP is predicted from the motion vectors applied to the I-Frame Only the difference between the actual images and the predicted ones are transmitted along the motion vectors Motion estimation Group of Pictures (GoP) 17