Re: Google Summer of Code 2009 -- Linux wireless roaming project

[Helmut Schaa <helmut.schaa@googlemail.com>]

Am Dienstag, 24. M=C3=A4rz 2009 schrieb Holger Schurig:
> > Hmm, quick example: AP1 - STA - AP2
> >
> > We cannot consider the signal strength as constant as it
> > varies over time even when neither the STA nor the AP are
> > moving. Assume a threshold value of t=3D40. Furthermore, the
> > signal strength of AP1 and AP2 might alter between 35-50 which
> > means we have an average signal strength of 42,5 > t.
> > Nevertheless that would result in ping-pongs between AP1 and
> > AP2 because the signal might drop below t on both APs, while
> > it would be better to stick to one AP as the signal is already
> > quite bad (but still good enough to do some communication).
>=20
> Yeah, but if the client is moving, you have to live with that,=20
> more or less.
>=20
> And if the client is not moving (and roaming is a loadable kernel=20
> module), then simply don't load it :-)

Ah, ok. I was more referring to an ordinary laptop user who sits at his
desk and once in a while starts moving (for example to a conference roo=
m).
While he sits at his desk the optimal solution shouldn't trigger a scan=
 as
the chance that a better AP pops up is relatively low. Once he starts
moving scanning is desired.

> My ad-hoc approach that I already implemented (for non-mac80211)=20
> shows a quite number of scannings. But that is ok for my=20
> use-case (e.g. telnet connection via WLAN). "Connection lost" is=20
> way worse than one scanning/reassociation too much, especially=20
> if the scanning/association is done intelligently.
>=20
> So for now I wouldn't optimize here, but make non-sucking roaming=20
> possible in the first place. We can build upon this anyway.

=46ine with me. But too aggressive scanning might lead to unstable or
intermittent connections, especially when WPA-EAP without PMKSA-caching
is used where roaming from one AP to another can take up to several
seconds ;)

And additionally if the connection is idle, repeated scanning will
increase the power consumption which is not desired on battery driven
devices.

Helmut
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