fted-T random variables with a mean the delay of arriving packets is often assumed to be delay of 50 ms and a standard deviation of 25 ms. a shifted-T distribution. Expressions for E() and p(1) Figure 3.5-2 plots PSNR versus transmitted bit rate for can be derived in terms of the Bernoulli loss probabilities, a heuristic, prioritized ARQ system and for R-D opti- the cumulative distribution functions for the F-distributed mized system. In the ARQ system, the client requests delays, the transmission policies and transmission histo- retransmissions for packets that do not arrive by a time ries, and the data units' arrival deadlines. These deriva- interval after they are expected, and the server transmits tions are straightforward, but because the resulting these requested packets with priority as long as the expressions are cumbersome, they are omitted here. requested packet may still reach the client in time for The scheduler reoptimizes the entire policy TT at each playout. When the capacity of the channel falls below the transmission opportunity to take into account new in- source rate for the enhanced stream, the ARQ system formation since the previous transmission opportunity sends only the base layer packets. Both the ARQ and the and then executes the optimal TT for the current time. An R-D optimized system use an initial preroll delay of exhaustive search to find the optimal TT is generally not 400 ms. By continuously optimizing its packet trans- tractable; the search space grows exponentially with the mission choices, the optimized system makes use of the number of considered data units, M, and the length of SNR and temporal scalability of the source encoding to the policy vector, N [14]. Even though rates and distor- finely tune the source rate to the available channel ca- tion reductions are assumed to be additive, the graph of pacity, yielding substantial gains. packet dependencies leads to interactions, and an ex- Several techniques have been proposed to further haustive search would have to consider all