Re: How many SSIDs can be listed?

[Robert Moskowitz <rgm htt-consult com>]

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).
>   
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.