计算机网络-Chapter_9_V7.0-课件.pptx
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1、Computer Networking:A Top Down Approach,A note on the use of these Powerpoint slides:Were making these slides freely available to all(faculty,students,readers).Theyre in PowerPoint form so you see the animations;and can add,modify,and delete slides(including this one)and slide content to suit your n
2、eeds.They obviously represent a lot of work on our part.In return for use,we only ask the following:,If you use these slides(e.g.,in a class)that you mention their source(after all,wed like people to use our book!)If you post any slides on a www site,that you note that they are adapted from(or perha
3、ps identical to)our slides,and note our copyright of this material.Thanks and enjoy!JFK/KWR All material copyright 1996-2016 J.F Kurose and K.W.Ross,All Rights Reserved,7th edition Jim Kurose,Keith RossPearson/Addison WesleyApril 2016,Chapter 9Multimedia Networking,9-1,Multimedia Networking,Multimed
4、ia networking:outline,9.1 multimedia networking applications9.2 streaming stored video9.3 voice-over-IP9.4 protocols for real-time conversational applications9.5 network support for multimedia,9-2,Multimedia Networking,Multimedia:audio,analog audio signal sampled at constant ratetelephone:8,000 samp
5、les/secCD music:44,100 samples/seceach sample quantized,i.e.,roundede.g.,28=256 possible quantized valueseach quantized value represented by bits,e.g.,8 bits for 256 values,time,audio signal amplitude,analogsignal,quantized value ofanalog value,quantization error,sampling rate(N sample/sec),9-3,Mult
6、imedia Networking,Multimedia:audio,example:8,000 samples/sec,256 quantized values:64,000 bpsreceiver converts bits back to analog signal:some quality reductionexample ratesCD:1.411 MbpsMP3:96,128,160 kbpsInternet telephony:5.3 kbps and up,9-4,Multimedia Networking,video:sequence of images displayed
7、at constant ratee.g.,24 images/secdigital image:array of pixelseach pixel represented by bitscoding:use redundancy within and between images to decrease#bits used to encode imagespatial(within image)temporal(from one image to next),Multimedia:video,frame i,frame i+1,temporal coding example:instead o
8、f sending complete frame at i+1,send only differences from frame i,9-5,Multimedia Networking,Multimedia:video,frame i,frame i+1,temporal coding example:instead of sending complete frame at i+1,send only differences from frame i,CBR:(constant bit rate):video encoding rate fixedVBR:(variable bit rate)
9、:video encoding rate changes as amount of spatial,temporal coding changes examples:MPEG 1(CD-ROM)1.5 MbpsMPEG2(DVD)3-6 MbpsMPEG4(often used in Internet,1 Mbps),9-6,Multimedia Networking,Multimedia networking:3 application types,streaming,stored audio,videostreaming:can begin playout before downloadi
10、ng entire filestored(at server):can transmit faster than audio/video will be rendered(implies storing/buffering at client)e.g.,YouTube,Netflix,Huluconversational voice/video over IP interactive nature of human-to-human conversation limits delay tolerancee.g.,Skypestreaming live audio,videoe.g.,live
11、sporting event(futbol),9-7,Multimedia Networking,Multimedia networking:outline,9.1 multimedia networking applications9.2 streaming stored video9.3 voice-over-IP9.4 protocols for real-time conversational applications9.5 network support for multimedia,9-8,Multimedia Networking,Streaming stored video:,
12、Cumulative data,time,9-9,Multimedia Networking,Streaming stored video:challenges,continuous playout constraint:once client playout begins,playback must match original timing but network delays are variable(jitter),so will need client-side buffer to match playout requirementsother challenges:client i
13、nteractivity:pause,fast-forward,rewind,jump through videovideo packets may be lost,retransmitted,9-10,Multimedia Networking,constant bit rate videotransmission,Cumulative data,time,client-side buffering and playout delay:compensate for network-added delay,delay jitter,Streaming stored video:revisite
14、d,9-11,Multimedia Networking,Client-side buffering,playout,variable fill rate,x(t),client application buffer,size B,playout rate,e.g.,CBR r,buffer fill level,Q(t),video server,client,9-12,Multimedia Networking,Client-side buffering,playout,variable fill rate,x(t),client application buffer,size B,pla
15、yout rate,e.g.,CBR r,buffer fill level,Q(t),video server,client,1.Initial fill of buffer until playout begins at tp,2.playout begins at tp,3.buffer fill level varies over time as fill rate x(t)varies and playout rate r is constant,9-13,Multimedia Networking,playout buffering:average fill rate(x),pla
16、yout rate(r):x r:buffer will not empty,provided initial playout delay is large enough to absorb variability in x(t)initial playout delay tradeoff:buffer starvation less likely with larger delay,but larger delay until user begins watching,variable fill rate,x(t),client application buffer,size B,playo
17、ut rate,e.g.,CBR r,buffer fill level,Q(t),video server,Client-side buffering,playout,9-14,Multimedia Networking,Streaming multimedia:UDP,server sends at rate appropriate for client often:send rate=encoding rate=constant ratetransmission rate can be oblivious to congestion levelsshort playout delay(2
18、-5 seconds)to remove network jittererror recovery:application-level,time permittingRTP RFC 2326:multimedia payload typesUDP may not go through firewalls,9-15,Multimedia Networking,Streaming multimedia:HTTP,multimedia file retrieved via HTTP GETsend at maximum possible rate under TCPfill rate fluctua
19、tes due to TCP congestion control,retransmissions(in-order delivery)larger playout delay:smooth TCP delivery rateHTTP/TCP passes more easily through firewalls,variable rate,x(t),TCP send buffer,videofile,TCP receive buffer,application playout buffer,server,client,9-16,Multimedia Networking,Multimedi
20、a networking:outline,9.1 multimedia networking applications9.2 streaming stored video9.3 voice-over-IP9.4 protocols for real-time conversational applications9.5 network support for multimedia,9-17,Multimedia Networking,Voice-over-IP(VoIP),VoIP end-end-delay requirement:needed to maintain“conversatio
21、nal”aspecthigher delays noticeable,impair interactivity 400 msec badincludes application-level(packetization,playout),network delayssession initialization:how does callee advertise IP address,port number,encoding algorithms?value-added services:call forwarding,screening,recordingemergency services:9
22、11,9-18,Multimedia Networking,VoIP characteristics,speakers audio:alternating talk spurts,silent periods.64 kbps during talk spurtpkts generated only during talk spurts20 msec chunks at 8 Kbytes/sec:160 bytes of dataapplication-layer header added to each chunkchunk+header encapsulated into UDP or TC
23、P segmentapplication sends segment into socket every 20 msec during talkspurt,9-19,Multimedia Networking,VoIP:packet loss,delay,network loss:IP datagram lost due to network congestion(router buffer overflow)delay loss:IP datagram arrives too late for playout at receiverdelays:processing,queueing in
24、network;end-system(sender,receiver)delaystypical maximum tolerable delay:400 msloss tolerance:depending on voice encoding,loss concealment,packet loss rates between 1%and 10%can be tolerated,9-20,Multimedia Networking,constant bit ratetransmission,Cumulative data,time,Delay jitter,end-to-end delays
25、of two consecutive packets:difference can be more or less than 20 msec(transmission time difference),9-21,Multimedia Networking,VoIP:fixed playout delay,receiver attempts to playout each chunk exactly q msecs after chunk was generated.chunk has time stamp t:play out chunk at t+q chunk arrives after
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