Tuning "roaming"

Dan Williams dcbw
Tue Sep 11 09:03:22 PDT 2012

On Mon, 2012-09-10 at 18:35 +0100, Ed W wrote:
> Hi, am I missing, or are there any plans to add some variables to tune 
> the roaming sensitivity? (ie the wpa_supplicant roaming, card is a 
> RT2870 usb)
> We have some customers in mobile configuration and whilst all is ok in 
> the lab, out in the field we seem to find that it's fairly easy to see 
> reasonable sized swings in signal strength which in turn causes regular 
> roaming jumps and interruption in service.  In fact several customers 
> say they are literally bouncing between access points every 30/120 
> seconds when using a config such as:
>      bgscan="learn:30:-64:120"

We've had exactly the same problem with some of our larger customers.
If you're using background scanning, and have a lot of access points,
you'll certainly hit this problem of ping-ponging between APs on each
scan which increases the possibility of an 802.1x auth failure.

If Jouni thinks that the current signal thresholds shouldn't be changed,
then a patch to let them be adjusted via the configuration is probably
the best way to go.  FWIW I'm using:

dbm < -75 :: min_diff = 4
dbm < -70 :: min_diff = 6
dbm < -65 :: min_diff = 8
else min_diff = 15

for a modified wpa_supplicant_need_to_roam() in events.c.  This may not
work that well if you're in a car but it certainly seems to be a lot
more stable at walking speeds.


> The mobile customers are actually on yachts and I think because of the 
> very subtle movement of the boat tilting, plus sometimes shadows due to 
> small physical obstructions that will be drifting in/out of the way, the 
> effect is fairly large implied changes in signal level
> What might be helpful would be to have a frequent scan interval, but 
> only roam if we have recorded an average signal strength over multiple 
> periods which is greater than some trigger threshold. Probably someone 
> smarter than me knows why this won't work, so just kicking around an idea?
> Thanks
> Ed W
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