he decoding time of the first B-frame. shifted-I with shift K = 10 ms, mean u = 40 ms, and If the P-frame arrives later, however, it should not be standard deviation o = 23 ms. The sequence was discarded. It may still be useful for decoding subsequent encoded using a two-layer SNR-scalable H.263+ [9], at B-frames or at least for decoding and displaying the 10 fps with prediction structure I-P-P-P. and GOP Network adaptive media transport CHAPTER 3.5 length of 20 frames. The base and enhancement layer bit the dependence among delays, however. In the Internet, rates and PSNRs were similar to those of the sequence in successive packets usually travel along the same path, Section 3.5.3. might experience a similar backlog while waiting in the In Figure 3.5-7, PSNR-versus-rate curves compare the same queues, and rarely arrive out of order. This results multiple-deadline schemes and the single-deadline in strongly dependent delays of successive packets. scheme, as well as a heuristic scheme. The heuristic In streaming simulations that employ measured In- scheme uses prioritized, deadline-limited ARQ in which ternet delay traces, we have observed that the iid model base layer retransmissions had highest priority, followed can lead to suboptimal scheduling performance. For ex- by base layer transmissions, enhancement layer trans- ample, Figure 3.5-8 shows simulation results when mission, and enhancement layer retransmissions. packets were delayed according to a delay trace mea- Retransmissions were triggered when packets were not sured over a 14-hop Internet path with a cable modem acknowledged within the 0.90 point of the cumulative last hop, as described in [10]. At transmission rates above distribution function of the round-trip time. Figure 3.5-7 80 kbps, the multidead-line R-D optimizing formulation shows that the multiple-deadline formulation yields up described in Section 3.5.4 is outperformed by the simple to a 3-dB improvement over a single deadline. The single- heuristic ARQ scheme (also described in Section 3.5