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Sebastian Robitzsch a écrit :
> Hi Benoit,
> Thanks for your reply. Comments in line...
>> - what you think is turbo mode might not be turbo mode. AFAIK,
>> turbo mode is not related to data rate. Turbo mode is linked to
>> using 40 MHz of frequency bandwidth rather than 20 MHz (it's
>> OFDM, so I think the data rate is linked to the radio modulation
>> over each sub carrier X coding rate X number of data sub
>> carrier. Right?).
> Sorry... but you are wrong. I'm talking about "Dynamic Turbo",
> i.e., two 20MHz bandwidth radio channels. And finally you are
> getting 108 Mbps nominal datarate.
> (Page 12)
You might be right as well and only the spectrum analyzer can see the
difference (send more screenshot!). 2 channels of 20 MHz can be used
in several ways....
>> Could you provide a capture file done with tshark for instance?
> No need to give you the tshark logfile. I used the following
> command # tshark -i mon0 -n -T fields -e radiotap.datarate
> the result (without sending user data) was just: 12 12 12 12 12 12
> 12 ...
> with user traffic just: 108 108 108 108 108 108 ...
Ah!!! I am correct on this point then. I just type the same command on
an intel 3945 abg radio cards and got the very same value. The IEEE
802.11 datarate is numerically the double of the actual rate in order
to be able to encode 5.5 Mbit/s as the value 11. So 108 is 54 Mbit/s
and 12 is 6 Mbit/s which is the basic rate set for 802.11a. HOWEVER,
according to my reading in the Atheros source code, Turbo mode is not
reported in the datarate so if you are using Turbo mode or not, the
maximum datarate reported will still be 108 (ie 54 Mbit/s).
> Please have a look to the attached gnuplot picture. The guy on
> channel 36 was sending IPERF generated traffic. That is more than
> just noise - this is definitely turbo mode.
36 is not a meaningful channel (it's the first one). I'm not even sure
than Turbo is available on this channel. Try channel 44 and narrow the
SPAN of your spectrum analyzer to say 50 MHz only. According to your
screenshot, you are using channel 36 with 20 MHz (if we assume a
transmit filter at -30dB). Other observed peaks are either noise or
interference generated by channel 36 but below the rejection filter.
Do you use the MAX HOLD settings on the spectrum analyzer? My
suggestion is then : redo your test with channel 44 + SA centered on
channel 44 (5.22 GHz) + SPAN 50 MHz + MAXHOLD and send a new screenshot.
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