[svlug] real basic wireless access question!!

Brian J. Tarricone bjt23 at cornell.edu
Tue Jun 5 10:21:39 PDT 2007

On Tue, 5 Jun 2007 09:45:55 -0700 Akkana Peck wrote:

>Related question: with my prism54 card, when I iwlist eth0 scan,
>I only see ten or fifteen networks. Amazingly enough, in downtown
>Mountain View, that's nowhere near the whole list, and if I run
>iwlist eth0 scan five times in a row, I'll get five different
>lists, with some overlap. Is there a way to get the whole list of
>available networks? Is it a function of the wireless card driver
>(e.g. something atheros can do while prism54 can't?)

Assuming the card just reports receiving beacons to the driver, maybe
it's a driver limitation where there's a max # of BSSIDs it can keep
track of in response to a query from userspace?  Or maybe it's
kernel-dependent -- perhaps the 'TTL' for throwing away an AP if it
hasn't seen a beacon in X seconds is shorter?

How is the signal strength on most of the networks?  If there are a lot
of them that are very weak, it's not too surprising that many disappear
in successive scans.  If the strong APs are disappearing between scans,
though, that doesn't quite fit with this theory.

More data points: my Broadcom card in my PowerBook (on Linux) hasn't had
problems with 25-30 APs in the past.  Ditto for Atheros.  I work for a
wireless networking company, so we have tons of APs active for testing
purposes; right now my Atheros card sees a max of 42 APs (it's the
Answer!).  If I do successive scans, that number fluctuates and can
drop as low as 35 or so; that seems pretty normal to me.

So: expect some variation in the results, especially if there are a lot
of APs with very weak signals you can see.  Otherwise, maybe it's a
driver limitation, but that sounds kinda silly to me.  I can't think of
a reason off the top of my head that the hardware itself would be
causing the trouble, especially since it can actually see all (or most)
of the APs.


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