Re: How to find out the candidate APs of an associated station?

From: Victor Aleo (
Date: 2002-09-13 22:06:56 UTC

Jouni Malinen wrote:

> On Fri, Sep 13, 2002 at 10:44:35AM +0200, Victor Aleo wrote:
> > Do you know if it is possible to find out, from the AP side, the
> > candidate APs that a station can reach? Maybe in the capabilities field?
> Not without modifying each station to make this information available..

I guessed that... :(

> > The problem I have is that I cannot kick a station (from an AP) if I do
> > not previously know if this station will be able to reassociate with
> > another AP in the ESS. Of course, it is possible to do it but in my case
> > I "need" to be sure that the station will be able to reassociate with
> > another AP before rejecting it.
> I don't think an AP can do this without resorting to some additional
> functionality. Anyway, it is the station that selects which AP to use,
> so I would recommend a bit different approach for this. Each AP could
> advertise its load level to associated stations and stations would then
> have a component that would collect these advertisements and signal
> quality info together and make roaming decisions based on this data.

Yes, as you have mentioned this is a possibility from the client (station) side. However, I am implementing a solution from the AP side, so all the solutions based in the client side are not feasible for me.

> Another option would be to add a component on each station to tell scan
> results to the associated AP and wait for a request from an AP to change
> to use another AP (again, this would need an additional protocol since
> 802.11 does not include such functionality).

Yeah, you are right! After reading carefully the standard I think this is the only possible solution that can be implemented. I thought that, somehow, the station kept a list of all the candidate APs in range and that this list could be polled by the AP. Because, as you have said, it seems this is not feasible I will implement a MIB in the station (the "component" that you have mentioned) to store the APs in range. Then, via SNMP, the AP will request this information from each station that is associated with it. In this way, the AP will not reject a station that does not have any candidate AP to associate with.

My question now is crystal clear: Why when I try to get the list of APs in range from the client side with the command: 'iwlist wlan# ap' or 'iwlist wlan# accesspoints" I get:

"wlan0 Interface doesn't have a list of Access Points"

I am running Prism2-2002-05-19 with kernel 2.4.18-3 (RH-7.3) and wireless-tools 23-2. You have said before that: "Scanning ioctls were added in Wireless Extensions v14, so you will need a newer kernel version." I have to recognize I am a bit lost :) What exactly do I need to get the list of candidate APs from the station?

  1. New version of your driver (hostap-2002-09-12) or / and,
  2. New version of the kernel (which one?) or / and,
  3. Wireless Extensions v14 (in this case, how can I get them?, I thought they were included with the Wireless Tools, so if I've installed wireless-tools-23.2 I should have already installed some version of the wireless extensions, isn't it?)

Sorry for this long email, but I have not got the whole picture, yet. :)

And thanks a lot, Jouni, for all your comments!


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