Question regarding wlan plugin's definition of signal quality...

[Brian Potkin]

James Westby <jw+xfce <at> jameswestby.net> writes:

> Hi,
>
> The plugin uses the same values as iwconfig, however it just combines
> them in to one number.

As you say, wavelan uses the Link Quality value from /proc/net/wireless and 
iwconfig simply displays this. Link Quality appears to be a number which is
entirely dependent on what factors the writer of the driver for the card
decides contribute to it. So, different drivers give different Link Quality
values.

The plugin does a calculation based on the Link Quality:

log(Link Quality)/log(92.0)*100.0

Why 92.0 is used and why the numbers are logged isn't apparent and isn't really 
explained in the code, but I expect there is a sensible reason for the technique.

> When iwconfig displays 78/100 it is showing value/maximum. The assumption is
> that this is a linear scale. However the plugin uses logarithms rather than a
> straight percentage calculation, hence the "strange" values. I am not sure
> why this is the case as it predates me. I guess as most will just assume that
> it is a simple percentage we could just change it to that.

I'd be wary of assuming Link Quality performs to a linear scale for any driver. 
Perhaps that is the reason for the logarithms. The original author of wavelan
(Benedikt Meurer?) may be able to shed some light on the reason for the
calculation above.

Also, assuming Link Quality is a simple percentage would not be warranted. The 
madwifi project, for example, says 'The "Quality" parameter reported by some of
the Wireless Tools such as iwconfig is used by MadWiFi to report the SNR
(RSSI). It should not be regarded as a percentage (ignore the /94 part).' That
is from http://madwifi.org/wiki/UserDocs/RSSI.

> If no-one objects to having their wireless signal strength reduced then I
> will release a new version when the FreeBSD fixes have been finalised,
> provided that doesn't take too long.

In the room I am in now my RT2500 driver reports 60/100 via iwconfig for Link
Quality and wavemon displays 90%. With an Atheros based card the data are 38/94
and 80%. Which should make me happier? Both are correct and the plugin is doing
a good job. Changing its behaviour to fit in with other software, whose own
processing of /proc/net/wireless may be open to criticism, may not be for the
best.

Brian.