[PATCH] roaming: start a scan if the SNR is below customized threshold

Holger Schurig hs4233 at mail.mn-solutions.de
Mon Sep 21 10:23:15 EDT 2009


> Right, that's useful.  I'm also always thinking about how this
> gets presented in the UI for users that have no idea what dBm
> or RSSI are, nor do they care.

The nl80211-equivalent "iw phy0 info" could give us min/max 
range, and anything outside min-max would then automatically 
be "don't know". That would enought for a user-space app to 
present a GUI.

I't won't hurt if NL80211_CMD_GET_WIPHY would also report if the 
signal is dBm or some other house-number. :-)


> If there's *any* out-of-band data that the supplicant
> controller has to know (ie, "I know this card has an antenna
> with X gain and the card reports in dBm so the bounds are Y
> and Z") that doesn't work for me because too much a-priori
> knowledge is required.

I don't think is will be necessary, when it's just because of 
roaming. Only when you want to be an AP and stay compliant you'd 
need to know the gain of the antenna and reduce the TX power 
accordingly.

Suppose well get the min/max/current signal level using different 
cfg80211 command. Then a GUI could draw something like this:

+-----------------------------------------+
|########################                 |
+--------^--------------------------------+
-96 dmB  -60dBm       -40dBm           0dBm

The "^" could be the marking for automatic roaming.

Now, if someone put's an antenna with a huge gain, the
picture would be

+-----------------------------------------+
|##################################       |
+--------^--------------------------------+
-96 dmB  -60dBm              -20dBm    0dBm

Now, the "^" is at the same position, there's no automatic code 
that moves the thresshold. However, the user should be allowed 
to change this into both directions --- or disable the automatic 
mode at all. Use case for this: at home I have my Fritzbox on 
the second floor. I have concrete floor with steel fleece 
between each storey (right name?  "etage"). In the basement I 
have a very weak signal. I don't want any roaming code here, 
because I know that I have only one AP anyway, so constantly 
scanning&probing for a better network won't be useful here.

However, in a typical warehouse I have 15 to 50 APs, so here 
automatic roaming is highly desirable (it's actually absolutely 
necessary, without it the WIFI connection is useless).



> Its fine if the supplicant somehow 
> provides the unit that the driver reports so that NM can pick
> the right set of defaults, or if the supplicant says that the
> SNR will always change in a linear fashion between two bounds
> X and Y as reported by the driver or soemthing.

Do you think that linearity is a needed property?

I actually think that the "Quality" isn't very linear at all. 
Once beacon losses are detected, I think it's degrading much 
steeper. But this is just a feeling, I haven't measured that. I 
don't really like the "quality" at all and don't use it in any 
of my roaming code. It doesn't adapt to a condition becoming 
better (e.g. user with handheld moved away from AP to fetch 
something, then he comes back ... the number of beacon-losses 
stays the same, degrading the quality permanently).


-- 
http://www.holgerschurig.de


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