[ath9k-devel] strange MPDU loss pattern

[Ali Abedi <a2abedi@uwaterloo.ca>]

Thank you for sharing the story.
Even if I consider interference as a possibility, still I can't justify 
the higher
chance of frame loss in the second half of the aggregate frame.

We use

PCI-express 3 antenna dual band cards
product: AR93xx Wireless Network Adapter
and/or
Atheors AR5B97 which is a 2.4 GHz dual antenna internal card in a laptop

we also tried TP-LINK TL-WDN4200 N900 
<http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0CAUQjhw&url=http%3A%2F%2Fwww.newegg.com%2FProduct%2FProduct.aspx%3FItem%3DN82E16833704195&ei=AglMVOw-w8TxAfW4gZAH&bvm=bv.77880786,d.b2U&psig=AFQjCNFYng4rfQG-HxRTr2hrjqAkrhLpTQ&ust=1414355559112616> 
as the receiver.

However we see the same results.
we mostly use MCS 20-23, sgi = 0, 20 MHz channels.

The loss pattern is something like this
(each line is an imaginary aggregated frame and each bit is the fate of 
the MPDU)

11111111111100011000000000000
11111110001101011010000000000
11111000000000000000000000000
11111111111010100000000000000
11111100101010000000000000000

The interesting part is that with the start of the next frame error rate 
goes down initially
then it goes up again in the second portion of the packet.

Best,
Ali




On 25/10/2014 2:30 PM, Adrian Chadd wrote:
> On 25 October 2014 08:28, Ali Abedi <a2abedi@uwaterloo.ca> wrote:
>> Hi Adrian,
>>
>> We have a high end spectrum analyzer. So we are sure there is no background
>> interference
>> We run our experiments in the 5 GHZ spectrum. The channel conditions can
>> still vary due to
>> the movement of the people in the vicinity of the experiment setup. We
>> select a rate that
>> experiences at least 20% error on average. Since if the error is 100% or 0%
>> it's not interesting
>> for us.
>>
>> My point is if the channel conditions change the distribution of failed
>> packets should be uniform.
>> The first and second  half of the packets have the same chance to be
>> received successfully.
> Here's a little story.
>
> My first wifi contract had me spend months trying to figure out why an
> AP was losing its mind. It'd get stuck in a "stuck beacon" loop and
> only a hard powercycle of /all/ of the access points in an area would
> clear it.
>
> It turned out that the PCB design had some non-grounded /
> non-populated tracks that just "happened" to form a 2GHz resonator.
> Once we grounded those tracks, the APs started behaving themselves.
>
> The company in question spent months with high end spectrum analysis
> kit in the lab (where it never happened) and underground (where it did
> happen.) It's only after they stuck the spectrum analyser probe
> _inside the access point_ right up close to the NIC did they see it.
>
> Here's the spectrum analyser traces. You can see the peak.
>
> http://www.creative.net.au/ath/
>
> So, weirder crap has happened.
>
> Which NICs and which MCS rates are you using?
>
>
>
> -adrian

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