Re: Atheros abg / madwifi trouble

[Dan Williams <dcbw redhat com>]

On Sun, 2005-01-30 at 17:11 -0500, Sven wrote:
> NetworkManager does not work well with my mini Atheros abg card. it can
> associate ok, but then roughly every minute it looses the connection and
> the card is scanning/cycling through all channels for about 10 secs or
> so. during that time iwconfig gives "FF:...:FF" as the AP. it cycles
> through the channels 3 times, and the associates with the original AP
> again.

Sven,

Ok, I took a look with this card.  The behavior you're seeing is pretty
much expected, that's the card running its wireless scan so we can
update our list of access points.  The thing you may not be aware of is
that the card supports so many frequencies that it takes a _long_ time
to scan all the channels (somewhere around 10s it seems, from this
card).  Obviously, since the card isn't associated during that time, it
cannot send or receive traffic.

Some observations:

1) NetworkManager starts with a 20s scan interval, and if the aggregate
list of access points does not change with each successive scan, it
backs the scan interval back to 60s.  If the list changes (an AP drops
off after the 2m "last seen" check, or a new one appears), the scan
interval jumps back to 20s.

2) Since the card takes so long to scan, and since no traffic can flow
during the scan, it doesn't work very well from a user's perspective.

3) I don't mind tuning the scan algorithm.  I can figure out how many
channels the card supports, and if its over 14 that means its an a/b/g
card right now.  I then know that it will take a while to scan, since
the time that a card must remain on a channel to sense the AP's beacon
does have a lower bound (a couple hundred ms, something like 150ms -
200ms).  Perhaps if the card is an a/b/g card, I should step the scan
interval back 20s rather than 10.

4) There are smarter ways of doing this.  For example, no card will do
well with streaming video/audio when scanning.  So perhaps
NetworkManager should monitor the amount of traffic on the card, and if
its above a certain level, then put off the scan.  If more than 4 scans
(for example) have been deferred, then force a scan at the next
interval.  This would mean far fewer interruptions.  I assume the kernel
has some way of relaying traffic statistics to us.

There are really just two things to consider.  We can't _never_ scan,
and we can't always scan.  We need to find some algorithm for NM of
balancing the two, and make that algorithm tunable on-the-fly.  We want
to do a few scans on startup of course, to build the initial list.  And
perhaps we want to scan more often when the card isn't in use or when
its not associated, for example.

I think I've got a few ideas on how to do this, but at the moment there
are a few other things that need to get done first...  Patches would be
accepted though.  If you'd like to file a bug at bugzilla.gnome.org or
bugzilla.redhat.com just to keep me reminded, that would be helpful.
Say something like "RFE: Tuneable scanning algorithm".

Dan