On Tue, May 01, 2018 at 10:22:22AM +0100, Stuart Henderson wrote: > On 2018/05/01 10:48, Stefan Sperling wrote: > > > On 2018/04/30 11:08, Stefan Sperling wrote: > > > > Derp. A dBm value of -10 would of course be better than -60. > > > > > > > > Whatever the numbers shown by tcpdump really mean, the probe response's > > > > one is better!!! > > > > > > Better as in "more accurate". But as the reported value is ridiculously > > > high rather than too low, why wasn't 5GHz selected anyway? > > > > What I wrote in my first mail was not very clear (I wrote it in > > a hurry before leaving to catch a train). > > > > The kernel compares the RSSI values which are shown in debug output. > > For reference, the scan debug printfs below again show a 2GHz beacon > > vs. a 5GHz "low power" beacon, where measured RSSI on 2Ghz is represented > > as "58" and on 5Ghz is represented as "6": > > > > + b8:ee:0e:cb:b3:08 1 +58 54M ess privacy rsn "ESSID" > > + b8:ee:0e:cb:b3:09 112 +6 54M ess privacy rsn "ESSID" > > > > The "non-reduced" Tx power frames, i.e. probe responses or beacons while > > a client is associated, closely matched Tx power seen on 2GHz: > > > > + b8:ee:0e:cb:b3:08 1 +58 54M ess privacy rsn "ESSID" > > + b8:ee:0e:cb:b3:09 112 +61 54M ess privacy rsn "ESSID" > > What do the values represent here? > > I've been reading them as sign-flipped dBm signal strength because > (before the +6 outlier) the numbers all matched that (i.e. -61dBm for > 5GHz and -58dBm for 2GHz looks right for an access point at typical > power levels about 30m away).
I've done some quick digging: The values come from iwm(4). Which means they are derived as dBm from values reported by hardware and then converted into a percentage. See iwm_calc_rssi() and its caller iwm_rx_rx_mpdu(). I've been wondering if we should make iwm(4) report values in dBm instead of as a percentage. But a percentage may be more intuitive to users. I'll leave this as it is -- since pirofti@ told me he might be working on fixing the RSSI situation I'd rather leave such decisions to him. > I didn't interpret them as % in this case because it would be unusual > for the 5GHz signal to be stronger than the 2GHz though that is > possible and if that's the case, your diff makes sense. It seems % would be the correct interpretation. I was sitting right next to the AP and I'm not surprised that both bands show similar values at such close range.
