Additional measurements and observations

We found that packet loss is highly dependant on internal queue size. The following graphs are a

comparison of runs with the same QoS parameters of two possible internal queue sizes (in each mobile node).

802.11e_old is with queue length of 50 packets

802.11e_new is with queue length of 2000 packets

Conclusions:

1) The packet loss rate depends on internal queue size.

2) We see a dramatic decrease of packet loss when increasing queue length, but it is not for free: while the average throughput doesn’t change at all, the average latency increases to unacceptable sizes of 7.5 seconds (!) when 20 voice connections are run.

In other words there is a tradeoff between two QoS parameters: latency and packet loss – and it is expected since the packet, which comes from the application layer can either be enqueued or dropped. In first case it will arrive after some time (latency is much higher) while in second case the packet will be lost. In our opinion when dealing with real-time applications (voice) it is worthwhile to lose packets but receiving others immediately than receiving more packets after a large delay.

This shows that our choice of queue-length of 50 packets was a reasonable one in terms of acceptable latency.