Adaptive Media Streaming - Scalable Continuous Media Streaming Systems - page 150

received during rebuffering periods. Playback of these frames is delayed so their amount
represents the quality degradation resulting from buffer underflow. Average video bit-rate,
on the other hand, represents video quality. Higher average video bit-rate generally produces
better visual quality during playback.

8.6.1 Sensitivity to Prefetch Duration
The presented rate adaptation algorithm makes use of knowledge of the client's initial prefetch
duration in estimating the client buffer occupancy. However, if this is not known, then it simply
assumes no prefetch is performed. To investigate the performance impact of such knowledge,
we run two sets of simulations for all 94 traffic traces, one set with the prefetch duration
known to the server and the other set simply assuming no prefetch. In both cases the client has
a prefetch duration of 5 seconds.
Figure 8.5 shows the rebuffering ratio and average video bit-rate for all 94 traces for the
two cases. The result is a bit surprising as the rebuffering ratios of all traces drop by 50% on
average, when the prefetch duration is not known, while the average video bit-rates for the case
when the prefetch duration is not known drops only 2% on average. Not knowing the prefetch
duration, the algorithm will simply assume the client does not prefetch, which means that the
estimated client buffer occupancy will be lower than the actual one. As a result, the algorithm
will become more conservative and the client buffer occupancy will be maintained at a higher
level than expected, thus reducing the likelihood of buffer underflow.

0.12

0.1

0.08

0.06

0.04

0.02

0

0 10 20 30 40 50 60 70 80 90 100

Trace No.

AVS (prefetch_known) AVS (prefetch_unknown)

Figure 8.5a Effect of knowledge of prefetch on the rebuffering ratio