计算机网络：自顶向下 配套PPTChapter_9_V7.0.pptx

Computer Networking: A Top Down Approach ;Multimedia networking: outline;Multimedia: audio;Multimedia: audio;video: sequence of images displayed at constant rate e.g., 24 images/sec digital image: array of pixels each pixel represented by bits coding: use redundancy within and between images to decrease # bits used to encode image spatial (within image) temporal (from one image to next);Multimedia: video;Multimedia networking: 3 application types;Multimedia networking: outline;;Streaming stored video: challenges;;Client-side buffering, playout;Client-side buffering, playout;playout buffering: average fill rate (x), playout rate (r): x r: buffer eventually empties (causing freezing of video playout until buffer again fills) 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 ;Streaming multimedia: UDP;;Multimedia networking: outline;Voice-over-IP (VoIP);VoIP characteristics;VoIP: packet loss, delay;;VoIP: fixed playout delay; sender generates packets every 20 msec during talk spurt. first packet received at time r first playout schedule: begins at p second playout schedule: begins at p’;Adaptive playout delay (1);also useful to estimate average deviation of delay, vi :;Q: How does receiver determine whether packet is first in a talkspurt? if no loss, receiver looks at successive timestamps difference of successive stamps 20 msec --talk spurt begins. with loss possible, receiver must look at both time stamps and sequence numbers difference of successive stamps 20 msec and sequence numbers without gaps -- talk spurt begins.;VoiP: recovery from packet loss (1);another FEC scheme: “piggyback lower quality stream” send lower resolutionaudio stream as redundant information e.g., nominal stream PCM at 64 kbpsand redundant streamGSM at 13 kbps ;interleaving to conceal loss: audio chunk