Re: How many SSIDs can be listed?

[Derek Atkins <warlord MIT EDU>]

There's another problem.  The wireless extensions have a size limit
for the scan results data.  The buffer size is a u16, which means you're
limited to 65535 bytes.  The network manager buffer increase algorithm
keeps doubling the buffer size, so you get 2k, 4k, 8k, 16k, 32k, 64k..
but 64k mod 2^16 == 0!  Meaning you never actually get to try a full 64k
buffer.

A workaround to this issue is to change the NM code to max out at
65535 instead of 65536 or "100000" (which is the current limit)..

This is being a MAJOR problem to a bunch of us at the IETF because
we can easily hear well over 100 APs most of the time.

-derek

Quoting Dan Williams <dcbw redhat com>:

> On Mon, 2008-03-10 at 23:40 -0400, Robert Moskowitz wrote:
> > Ver 0.6.4  in Centos 5.1
> >
> > I am seeing 17 SSIDs in the current list.  But I am not seeing one that
> > I expected to see.  And some of the listed SSIDs are 'stale'; that is
> > they were visible in the part of the hotel I was in a couple minutes
> > ago, but not in this part.  So I guess a second question is how do you
> > force a scan to produce a current SSID list?
>
> You don't force a scan.  NetworkManager will periodically scan with a
> backoff algorithm; it will start at 20 seconds and back off to 2
> minutes.  APs are kept in the scan list for a maximum of 6 minutes
> before being culled.
>
> The problem is that wireless is hard, and sometimes cards/drivers miss
> beacons.  Often they will not report all the APs that are known to be
> around at a given time.  So NetworkManager takes a composite of the last
> few scans as the scan list.
>
> 0.6.x also combines APs with the same SSID in the UI.  0.7 splits them
> out at the NetworkManager layer, while the applet combines APs that are
> similar based on more than just SSID (SSID, security settings, band,
> channel).
>
> > Perhaps the question may be how many APs can be handled and then those
> > are turned into the SSID list (when more than one AP per SSID is found
> > as in the case of some of these SSIDs).
>
> Are any of the APs hidden?
>
> Dan
>
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--
      Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory
      Member, MIT Student Information Processing Board  (SIPB)
      URL: http://web.mit.edu/warlord/    PP-ASEL-IA     N1NWH
      warlord MIT EDU                        PGP key available