I was wondering how to do it.

In airodump it seems to be possible to count the data packets leaving a device. It should be possible to analyze the encrypted size of each packet, discard the encryption overhead to have a rough guess of the currently used bandwidth of the network. Maintain some logging and it should be possible to guess the maximum bandwidth.

Sadly, tcp sequences are encrypted, else it would be possible to guess response times..

Anyone know of a program that does this ?