iwspy gives strange results (error in code?)

Craig Foster craig at wiw.org
Tue Feb 25 12:12:12 EST 2003


I can't speak to the algorithm used to calculate these numbers.  However, the 
signal/link levels do not apply to Master mode since they reflect the 
strength of the connection with the associated AP.  (After all, if 12 clients 
were associates with your machine in Master mode, then which levels would it 
display?)

-- 
Craig Foster
craig at wiw.org

Michael Koshelev <michael at neonweb.ru> said:

> 
> Problem:
> --------
> 
> When issuing iwspy command on host working in Master mode, output looks 
> like:
> 
> wlan1     Statistics collected:
>     00:02:2D:5E:23:66 : Quality:235/92  Signal level:-91 dBm  Noise 
> level:-97 dBm (updated)
>     typical/average   : Quality:0  Signal level:0  Noise level:0
> 
> i.e. it displays wrong value for link quality. Seems it happends if 
> ( signal - noise < 10 dBm ).
> Two computers, involving in this experiment, located at the same room.
> 
> 
> On the client side, iwconfig outputs:
> eth1      IEEE 802.11-DS  ESSID:"test-wlan1"  Nickname:"HERMES I"
>           Mode:Managed  Frequency:2.457GHz  Access Point: 00:40:05:AE:CD:1E
>           Bit Rate:11Mb/s   Tx-Power=15 dBm   Sensitivity:1/3
>           Retry limit:4   RTS thr:off   Fragment thr:off
>           Power Management:off
>           Link Quality:27/92  Signal level:-71 dBm  Noise level:-98 dBm
>           Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
>           Tx excessive retries:0  Invalid misc:0   Missed beacon:0
> 
> 
> Even if client computer is placed near AP computer, iwspy output on AP
> won't show signal and quality values more than:
> wlan1     Statistics collected:
>     00:02:2D:5E:23:66 : Quality:16/92  Signal level:-81 dBm  Noise 
> level:-100 dBm (updated)
> 
> 
> 
> AP configuration:
> Linux kernel v. 2.4.18 (redhat 7.3) + Wireless extentions 15
> HostAP - CVS snapshot
> wireless-tools v.25
> Hardware - D-Link DWL-520  pri 1.1.0/sec 1.4.9 in Master mode
> 
> client configuration:
> 1) D-Link DCF-650 + hostap CVS
> 2) Avaya Wireless PCMCIA + orinoco_cs 0.13b
> kernel, wireless-tools - the same
> 
> 
> 
> So, I have two questions:
> 
> 1) Is signal and noise values in iwspy output on AP looks reasonable? Is 
> it possible to get such low signal/noise ratio even if two cards is placed 
> near one another, or it seems like some hard/software problem?
> 
> 2) I checked some code in HostAP (see below) which produced wrong 
> iwspy quality values. Is algorithm used is some sort of euristics? Or it 
> is documented eslewhere?
> 
> 
> Some analysys:
> 
> Besides other issues, it appears to be a problem in HostAP code about 
> stations link quality calculation.
> More specifically, in file hostap_ap.c, function prism2_ap_get_sta_qual,
> quality level for stations is calucated like this:
> 
>    if (sta->last_rx_silence == 0)
>       qual[count].qual = sta->last_rx_signal < 27 ?
>          0 : (sta->last_rx_signal - 27) * 92 / 127;
>    else
>       qual[count].qual = sta->last_rx_signal -
>          sta->last_rx_silence - 35;
> 
> So, when signal/noise ratio is low , it is possible for quality value 
> to be negative which results in wrong iwspy display.
> 
> 
> ---
> Michael Koshelev
> 
> 
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