Digital Signal Processing Reference
In-Depth Information
Figure 15-4.
Lost and late speech packets as a function of the bit error rate; adaptive vs. not-
adaptive technique, maximum number of retransmissions set to zero and two.
frame every 20 ms adapting the payload size (244 or 95 bits of speech data,
which correspond to 704 or 560 bits at the physical layer) to the channel state.
A bit error rate of for the channel in the “good” condition, and a BER
of for the “bad” condition, that represents a channel fade, are used.
The transition probability of the Gilbert model has been kept constant at
the value 0.6 so that the average sojourn time in the bad state (with a time slot of
20 ms) is
time slots. The
value has been changed to reflect
different BER's as in Eq. 2.
Figure 15-4 compares adaptive and fixed-rate transmission at the same av-
erage bit-rate for the cases of maximum number of retransmissions zero and
two, respectively. With a bit error rate at the physical level of and
two retransmissions, the adaptive solution nearly halves the number of lost and
late packets from 2.9% to 1.7%. The plot also demonstrates that if the network
allows a higher number of retransmissions the gap between constant bit rate
transmission and adaptive transmission increases.
