Digital Signal Processing Reference
In-Depth Information
Figure 13-3.
PSNR values as a function of mean packet size, for various channel conditions;
is equal to 2.
at least one retransmission at the MAC level is needed to obtain an acceptable
video quality for realistic error probabilities.
Figure 13-3 shows the effect of the mean packet size on video quality, for
various channel conditions, with equal to 2. When the channel error proba-
bility is low the mean packet size has a limited influence on
the video quality, thus larger video packets can be used, minimizing the MAC
header overhead without incurring in an excessive quality degradation. For high
error probabilities it is better to create smaller video pack-
ets during the encoding process so that the resulting packet error probability is
minimized.
We now evaluate the impact that the following aspects of the system have on
the quality of the video service: i) the interfering TCP traffic, ii) the error prob-
ability over the radio channel, iii) the setting at the MAC layer of the maximum
number of allowed transmissions per MPDU.
Figure 13-4 shows the average delay perceived by UDP packets versus the
slot error probability over the wireless channel when various numbers of inter-
fering TCP sources are considered. The maximum number of transmissions per
MPDU, is set to 2 in the MAC layer of the video traffic source. As the
error probability over the wireless channel increases, the number of transmis-
sions performed per MPDU increases so that longer times are needed to deliver
