Re: How many SSIDs can be listed?

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

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.


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,

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?


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      Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory
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