Hey Felix, just wanted to bump on this issue again, as it has not been applied yet - the patch seems still valid, and Mathias results appear to show that as well. What do you think? Thanks, Simon On Fri, Dec 07, 2012 at 01:35:49PM +0100, Mathias Kretschmer wrote: > Hi all, > > On 10/30/2012 01:43 PM, Felix Fietkau wrote: > >On 2012-10-30 1:07 PM, Simon Wunderlich wrote: > >>From: Mathias Kretschmer <mathias.kretschmer@xxxxxxxxxxxxxxxxxxx> > >> > >>According to 802.11-2007 17.3.8.6 (slot time), the slot time should > >>be increased by 3 us * coverage class. The code only increased the > >>ack timeout, which is fixed by this patch. > >> > >>We have noticed in our long shot scenario that we see less collisions > >>with this patch. > >At some point I had the slot time increase in the driver, but noticed a > >massive throughput degradation on 10-20 km links. Leaving the slot time > >alone and changing only the ACK timeout fixed this. What distances did > >you test? > > > >- Felix > > > > We've run some tests on a 5km .11a link to verify the proper > functioning of this patch (slot time depending on coverage class) > and the recent patch to ensure the shorter (9us) slot time is used > in .11a adhoc mode. > > According to the standard, the slot time should be calculated as follows: > > slottime = 9us + 3 * ah->coverage_class; > > For our link, this would be > > slottime = 9 us + 3 * 10 = 39 us. (coverage class 10: up to 4950 m) > > If you look at the below TSFT histogram and try to determine the peaks > (first column: diffTime, second column: frameCount), the slot time > turns out to be roughly 39us, which fits pretty nicely with the > expected result. > > The TCP throughput (wget -O /dev/null) was very constant between 1.7 > and 1.8 MByte/s. Which, I'd say, is pretty decent for such a link > (without TxOp, etc). > > Similar measurements for a 10km link, reveal a slot time of about > 71us, which also matches the theoretical figure pretty well: > > slottime = 9us + 3 * 21 = 72 > > Therefore, both patches seem to ensure a proper MAC timing while > yielding proper throughput. > > Cheers, > > Mathias > > ------------ > > === TSFT Histogram (times in us) of 00:00:00:00:00:00, age 66s === > 411,945 > 412,5921 > 413,1534 > 450,1271 > 451,4286 > 452,1804 > 490,5592 > 491,731 > 528,773 > 529,3482 > 530,1177 > 567,693 > 568,3071 > 569,1022 > 599,552 > 607,3659 > 608,486 > 637,266 > 638,737 > 639,437 > 645,782 > 646,2103 > 647,929 > 677,1525 > 678,560 > 684,463 > 685,2337 > 686,589 > 715,276 > 716,1630 > 717,704 > 754,424 > 755,1518 > 756,889 > 794,2347 > 795,796 > 832,420 > 833,1800 > 834,891 > 975,315 > 1014,329 > 1015,257 > > > >
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