[ath9k-devel] Unrealistic RSSI values being reported

[Daniel Smith <viscous.liquid@gmail.com>]

On 11/23/2011 1:06 AM, Mohammed Shafi wrote:
>
> Hi Daniel,
>
> sorry was busy with some other urgent work. a value upto 127 seems to
> be valid for Atheros chipsets, bad -128
> http://en.wikipedia.org/wiki/Received_signal_strength_indication  :)
> further the negative value should be caught by the check in
> ath9k_process_rssi unless my_beacon is 'false' hope i had not missed something.
>
>          if (rx_stats->rs_rssi<  0)
>                  rx_stats->rs_rssi = 0;
>
> i will test this behavior and do further investigation.
>
>

Hey Shafi,

No worries, I completely understand.

So while 127 is a valid value for the chip, the reality is that it would 
not be (legally) possible. if the noise floor is -90dBm and the RSSI was 
127 then the signal power at the antenna would have been 37dBm. Even if 
the TX antenna was right next to the RX antenna that would still be more 
powerful than what is legally allowed by the FCC. In this case the 
devices creating the offending traffic are Android smartphones (a HTC 
and a Motorola; based off of MAC), which typically have a max tx power 
around 15dBm, which again with a -90dBm noise floor would have a RSSI of 
105 in a zero-loss free space. All those numbers are theoretical so for 
a baseline I set a Nokia N900 right next to the antenna (antenna is ~5' 
away and has a 6' coax whip) the largest signal strength (rssi + noise) 
I saw was -29dBm.

For testing, take a look at the analysis that Adrian and I have been 
pursuing. It seems that the invalid RSSI occur for aggregate frames when 
there are still additional frames left in the aggregate, i.e. rs_isaggr 
and rs_moreaggr are both set.

Thanks for taking some time to help look at this!

Daniel