On Mon, 2008-11-24 at 10:43 +0100, Felix Fietkau wrote: > Benoit PAPILLAULT wrote: > > I did a similar test here and results is very strange. AP was my good > > old linksys WRT54G running an iperf server. Client was a laptop running > > either ath5k or madwifi/trunk and an iperf client. Channel is 5. Both > > drivers show the same behaviour. > > > > At the beginning, throughput was very low : 500 - 600 kbit/s. Suddenly > > (after few minutes), it jumps to 15 - 17 Mbit/s and then few minutes > > later (let's say 10 - 20 minutes maybe), it jumps back to 500 - 600 > > kbit/s. Using a fixed rate has no effect. > > > > I used my latest wireless monitoring tools and I did not saw lost of > > duplicates or lost packets. The only difference was the number of > > packets sent by seconds.... > > > > Looking a my syslog, I just saw few messages, unrelated in time with the > > throughput going up or down. They were: > > - ath5k : unsupported jumbo > > - switching to short barker preamble > > - switching to long barker preamble > > > > I can repeat the same test with iwl3945 as well, if needed. > While reading the code for calculating the frame duration, i noticed > something odd: It doesn't seem to be taking into account the short > vs long preamble distinction for ERP rates. IMHO this might be causing > issues like this. I've seen similar behaviour a long time ago when testing > iwl3945 against a Broadcom AP with exactly the same throughput drop (500- > 600 kbits/s). > When I analyzed the problem with an extra monitor mode card, I found out > that the throughput drop is caused by a huge number of retransmissions, > and if I remember correctly (I didn't look for this specifically back then), > the retransmissions went down the rate scaling table until they hit the > first non-ERP rate and that one worked on the first try. > > Johannes, does that sound like a probable cause? If so, it should be easy > to fix. I have no idea. Shouldn't that be easy to tell from the monitor? And if the short/long switching was _unrelated_ in time it seems like it wouldn't be a problem? johannes
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