Re: Scan while TX/RX'ing a lot of data

[Johannes Berg <johannes@sipsolutions.net>]

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On Thu, 2009-05-14 at 14:54 -0400, Dan Williams wrote:

> Libertas splits scans up into 3 parts with a short return to the
> operating channel between each part.  There's nothing that requires
> cfg80211 for that to work...

Yeah that was my idea too, just return to the operating channel after
having scanned a channel or two and wait for the next beacon, and
possibly receive traffic if indicated in that beacon. It takes a bit of
synchronisation and isn't easy to implement, but it's definitely
possible.

> The problem here is that at any time an application (say, wifi location
> app) could ask for the list of access points.  If you don't scan
> periodically, all APs other than your associated AP (and others on the
> same channel) will gradually drop off because their beacons are
> received.  Hard to wifi position or get area statistics if there's only
> one AP in the list.

The other thing we should do is bump the AP list timeout, I think -- 10
seconds is very small. But then again we really need such apps to query
NM anyway.

> Secondarily, scanning is a tradeoff between better roaming latency and
> continuous high throughput.  If you don't scan, you have no idea what's
> around, and when you move and the current AP becomes marginal, you
> *have* to take the hit no matter what, so you can scan and find a new AP
> to associate with.

Yeah, that too.

> I would have though that the periodic scanning would be more of an
> annoyance when doing VOIP or SSH other latency sensitive tasks, but when
> just downloading a file, a few second drop in transfer rate gets lost in
> the bucket in the grand scheme of things.

ssh isn't too bad, at least not after I fixed the timings... before I
got very annoyed with ar9170. OTOH with VoIP it would suck to have a
small hiccup every 2 minutes -- maybe then we should postpone
indefinitely and only do it if the signal strength fluctuates?

johannes

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