Re: How many SSIDs can be listed?

Dan Williams wrote:
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.
This is a problem when you are moving around a lot. Well maybe not so much a problem if you are always wanting to connect to SSID ietf-a, regardless of which AP. But a problem if you are moving around in an area with a lot of open networks and you are looking for something to ride on...
The problem is that wireless is hard,
Tell me about it. I work on the standards. Will be in Orlando next week for the 802 plenary meeting.
 and sometimes cards/drivers miss beacons.
Of course. Until we change 'everything' with 802.11s, scanning requires the radio to listen to each channel, one at a time, and hope to catch the BEACON for that channel. And not just a BEACON, but all the APs using a given channel. The standard does not allow for a radio to listen on all channels. 802.11n does change this a bit. 11s basically requires it (well for the mesh nodes at least).
Often they will not report all the APs that are known to be
around at a given time.
Because they frequently have table limit sizes and can only record so many.
  So NetworkManager takes a composite of the last few scans as the scan list.
Ouch. Not good for an actively moving device. A person walking can easily encounter a few APs for a given SSID on the same channel. Which one is really current? So when you do an ASSOCIATE on a given channel, which AP do you put in as the destination BSSID?
0.6.x also combines APs with the same SSID in the UI.
As it should. People don't understand lots of APs in an SSID unless they install them!
0.7 splits them out at the NetworkManager layer,
AH, so NetworkManager controls the ASSOCIATE, not the device driver?
 while the applet combines APs that are
similar based on more than just SSID (SSID, security settings, band,
Channel/band? well other than b/g vs a vs n. And within an SSID you cannot have different security settings, per the spec.
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?
If they are 'hidden' (which is a myth, read my paper on this), they are not of interest. Hidding an SSID is a waste of effort. And it seriously breaks AP roaming.

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