From: Helmut Schaa <hschaa@suse.de>
To: Dan Williams <dcbw@redhat.com>
Subject: Re: More thoughts about roaming ...
Date: Mon, 15 Sep 2008 16:18:18 +0200
Cc: linux-wireless@vger.kernel.org
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Am Montag, 15. September 2008 15:59:20 schrieb Dan Williams:
> On Mon, 2008-09-15 at 14:16 +0200, Helmut Schaa wrote:
> > Hi,
> >
> > As a result of the thread "Pondering: how to improve mac80211 roaming
> > ..." I thought a bit more about that topic. Here is a quick suggestion
> > for a first version:
> >
> > * Enhance mac80211 software scan to interrupt the current scan and switch
> >   back to the operating channel every once in a while (only if at least
> > one STA interface is associated) => background scan.
> >
> > * Enhance mac80211 to check the signal quality frequently (or any other
> >   indicator like beacon misses) and decide when the signal quality is too
> > low. Indicate that to the user space.
> >
> > * wpa_supplicant (or any other user space application) gets notified
> > about the low signal quality and triggers a scan in order to find a
> > better AP. wpa_supplicant already chooses the AP with the best signal
> > strength.
>
> I'm pretty sure it doesn't.  The code in wpa_supplicant_select_bss()
> doesn't have any logic to detect signal strength, nor does any driver
> report signal strength to the supplicant.  Drivers set the level (well,
> only wext/nl80211, ndis, iphone/osx, and bsd) but I'm pretty sure
> nothing in the supplicant uses it.

Once the supplicant receives the actual scan results it orders them by
security, rate, signal level and finally quality (see 
src/drivers/scan_helpers.c, wpa_scan_sort_results). Hence if multiple APs
have the same SSID the one with the best signal level (or quality) will
be the first in the scan results and wpa_supplicant just selects the first
matching AP. Works fine here ;)

> Maybe now is the time to switch that on.  That needs to be coupled with
> some additions to cfg80211 to turn firmware/driver roaming on and off,
> along with a capability bit somewhere to indicate that the
> driver/firmware support turning roaming on/off.

True. Which cards that can do roaming in driver/firmware?

Helmut