Average one-way delay
Comments:
- One-way delay increases linearly with the
packet size both with and without best-effort
traffic;
- Without best-effort traffic the PB size
does not impact the average latency as expected,
since in these case a premium packet is
serviced by the scheduler as soon as it arrives
in the queue, so the premium queue never
builds up. For this reason, the queue depth is
irrelevant since the queue is always almost
empty.
With best-effort traffic the average latency
is again constant for a given packet size, with
the only exception of when the PB size is
of 1500 bytes: in this case, for any packet
size the average latency is approximately
1.8 msec larger.
- The slope of graph representing the average rate is the same with
and without best-effort traffic.
- As expected, when best-effort traffic is added,
the average delay increases of 1.7 msec (
figure 3).
An explanation of this could be that when a premium
packet is dequeued a best-effort packet could be
under transmission in the tx queue. However,
the additional delay is less than the transmission
time of a best-effort packet (4 msec for 1000 bytes at a
line rate of 2 Mbps), in fact the one-way delay plotted in
figure 2 is the average.
- The exception case for 1500 bytes with
best-effort traffic
(figure 2) is under investigation.
Figure 1: average one-way delay versus packet size for different
premium buffer sizes, without best-effort traffic.
Figure 2: average one-way delay versus packet size for different
premium buffer sizes, with best-effort traffic. Delay
is the same independently of the buffer size, with the only
exception of the case where PB = 1500 bytes.
Figure 3: comparison of one-way delay with and without best-effort
traffic.
Last modified: Nov 22, 1999