Poll stations from AP transparently

Victor Aleo victor.aleo
Tue Oct 22 05:58:37 PDT 2002

Jacques Caron wrote:

> If you want to know if a station can reach other APs, I think the only 
> solution (other than adding software on the clients) would be to have 
> your AP communicate with neighbouring APs, asking them to poll them 
> somehow.

Yes, you are right. Currently, in my system the APs communicate between 
each other (in order to share load metrics information). Therefore, 
communication among APs is not a limitation.

> But I'm not sure there are ways for an AP to poll a station associated 
> with another AP that will always work (it depends on CF-Pollable 
> settings and similar things).

Uhmm...If I am not wrong, the CF-Pollable settings are designed only for 
the PCF mode and the current WLAN cards only implement DCF mode. So I do 
not think this could be a solution.

But this is exactly the problem I want to solve: how an AP can poll a 
station that is associated to another AP? If somehow, the station answer 
to this "query" (note here that the query itself is not important at 
all, but the fact of finding out that the station was "able" to answer 
the query) the problem is solved!

> Alternatively, if the clients are using active scanning, the 
> neighboring APs could have a cache of recently seen probe requests. 
> But again, that won't work with clients using passive scanning only. 

Yes, and there is still another problem with this solution: the active 
scanning is not performed periodically. Therefore, it can happen that 
the potentially APs to associate changes (in function of the radio 
quality, multipath, etc.) and therefore the information stored at the AP 
will not be anymore representative.

> A slightly different question: why do you want your AP to reject 
> clients? A scalability issue to achieve some form of load-balancing? 

Rejecting stations from the AP is a mechanism to distribute the load (to 
achieve load balanced scenario in a WLAN network). For instance, if an 
AP "knows" that there are other APs in the same ESS less loaded, it will 
reject  a station  in order to achieve a better performance for each 
user and for the network.

Thanks a lot for your comments,


> At 12:38 PM 10/22/2002, Victor Aleo wrote:
>> Hi everybody!
>> I already know, from multiple emails with this list and especially 
>> with Jouni, that is not possible to know from the AP side if a 
>> station (client) can reach other APs in the same ESS. The reason to 
>> get this knowledge is very simple: if an AP knows that a STA can 
>> reach other APs it will be potentially possible to reject this 
>> station "safely". Thus, with the security that the station will be 
>> able to reassociate with at least another AP.
>> However, I still think that this can be implemented without the aid 
>> of any special software in the station side (transparent to the 
>> clients). Thus, only the AP will be "modified". The key question is 
>> how? It is clear to me that there are not specific methods to perform 
>> this task (mainly because the standard specified mobile based handoff 
>> but not network based). But I think there are "indirect" methods to 
>> find out the answer:
>> 1) If the AP can poll, by means of beacon frames for instance, the 
>> stations. In this case, the APs in the  same ESS send beacon frames 
>> to a specified STA. If the STA answer to this beacon frames, it means 
>> that can be associated to more than one AP. Therefore, if it is 
>> rejected by its current AP it will not have problems to reassociate 
>> with  another.
>> My questions are: is this feasible with Prism2? Have you thought 
>> about other mechanisms to find out this information?
>> Sorry to write such a long email, but I think this is a key issue to 
>> implement load balancing algorithms in WLAN networks and I am working 
>> in it :)
>> Thanks a lot for your comments or suggestions,
>> Victor
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> -- Jacques Caron, IP Sector Technologies
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