Scan wifi card on linux when connected-

Dan Williams dcbw
Wed May 30 19:19:38 PDT 2007 

On Wed, 2007-05-30 at 13:55 -0700, hong zhang wrote:
> Your suggestion should be called progressive scan not
> normal scan. Some driver supports it. But I do not
> think madwifi supports progressive scan yet. For QoS
> feature, voice data interval is not more than 60ms.
> Otherwise, voice will be paused and quality is bad. To
> support WMM, client needs progressive scan to find APs
> near it so that it can roam if need. But normal scan
> would not work for QoS when connected. Timing is also
> an issue here too. 
> 
> Should AP know when client is scanning for normal
> scan?
> 
> Madwifif provides background scan. I believe it scans
> when client connected.
> 
> There are 3 kinds of scan. Normal scan, background
> scan and progressive scan. Madwifi currently supports
> first two.

That's in the driver.  iwlist and userspace tools don't care what type
of scan the driver does (except for active/passive scan) and probably
shouldn't have to care.

But user tools certainly expect to be able to get full scans whenever
they request a scan, and the driver needs to provide that.  Doesn't
matter whether the driver does a "normal" scan, background scan, or
progressive scan, whatever.  The driver should be smart enough that it
can provide scan results on request, and update those scan results in a
manner which is conducive to whatever userspace is doing at the time as
well.

For example, if you're doing a VOIP call at the time, and some other
userspace process requests a scan, then the driver might be able to scan
one or two channels then jump back to the current channel for another
dwell time, then one or two others, etc.  The ITU recommends that
conversational voice applications have a maximum delay of about 250ms,
not 60ms, so a dwell time of 100 - 200ms is quite within that range.

Dan

> 
> --- Dan Williams <dcbw at redhat.com> wrote:
> 
> > On Wed, 2007-05-30 at 12:44 -0700, hong zhang wrote:
> > > when driver scans, it will scan all channels as
> > > default. Driver supports about 32 channels on
> > a/b/g.
> > > Default scan time on each channel is 100ms. It
> > takes
> > > at least 3 seconds to complete scan job. If client
> > is
> > > associated with AP, for example at channel 6. If
> > scan
> > > is allowed, driver will leave channel 6 when it
> > scan
> > > on other channels. Unless AP konws driver is
> > canning
> > > and AP buffers packets for more than 3 seconds.
> > > Otherwise, packets will be lost.
> > > 
> > > So I think normal scan is not allowed when
> > connected.
> > 
> > So that's you answer then, as to why it's not
> > returning scan results
> > when it's connected.
> > 
> > If the driver is smart, it will send the AP a power
> > save enter frame, do
> > the scanning, then tell the AP it's woken up again,
> > then get all the
> > buffered packets from the AP.
> > 
> > Or, the driver could scan 2 or 3 channels, then hop
> > back to the
> > associated channel for a small amount of time to
> > talk to the AP, then do
> > the next set of channels.
> > 
> > The driver should be keeping a list of scan results
> > from previous scans
> > to return when iwlist gets called, and aging those
> > scan results to
> > delete each BSS when a certain amount of time has
> > elapsed since the last
> > beacon has been seen from that BSS.
> > 
> > In any case, if the driver doesn't allow scans while
> > connected, that
> > driver is dumb and broken and should be made
> > smarter.
> > 
> > Dan
> > 
> > > --- Dan Williams <dcbw at redhat.com> wrote:
> > > 
> > > > On Wed, 2007-05-30 at 10:31 -0700, hong zhang
> > wrote:
> > > > > Magda,
> > > > > 
> > > > > When client is already associated with AP,
> > iwlist
> > > > will
> > > > > not do scan (normal scan). You can
> > disaasociate
> > > > with
> > > > > the AP and run iwlist again. See any
> > difference.
> > > > To 
> > > > > verify what I said. You can put printk() on
> > > > > ieee80211_notify_scan_done(struct ieee80211vap
> > > > *vap)
> > > > > to prove scan is done when iwlist is executed.
> > > > 
> > > > Sounds like a driver bug; if you look in the
> > source
> > > > for the SIOCSIWSCAN
> > > > handler, does it dis-allow a scan when
> > connected?
> > > > 
> > > > Dan
> > > > 
> > > > > 
> > > > > ----henry
> > > > > --- magda chelly <sandra26899 at yahoo.fr> wrote:
> > > > > 
> > > > > > Hello,
> > > > > > 
> > > > > > I'm trying to scan a wifi card on debian
> > > > > > distribution,
> > > > > > (armel:nokia n800), but with iwlist scan, it
> > > > doesn't
> > > > > > show me all the networks, the connected one,
> > > > does
> > > > > > not
> > > > > > appear and others are missing.
> > > > > > Can someone help me please.
> > > > > > 
> > > > > > Thanks,
> > > > > > Magda,
> > > > > > 
> > > > > > 
> > > > > >      
> > > > > >
> > > > >
> > > >
> > >
> >
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