Poll stations from AP transparently

Victor Aleo victor.aleo at estudiant.upc.es
Tue Oct 22 10:20:21 EDT 2002

Jacques Caron wrote:

> One solution might be to listen for all traffic received by the AP, 
> even from stations not associated. But I believe that means having the 
> card in monitor mode, and on the same channel as the neighboring 
> AP(s). In short, that means having several cards in each AP, only one 
> being used in AP mode, the others being in monitor mode. Then each AP 
> can know which stations it can "hear" (which doesn't necessarily mean 
> the station can "hear" the AP, though, but with proper thresholds on 
> signal levels it should be possible to get something consistent). 

Yes, it is a possible solution. However, in my scenario (3 APs with 
overlapping coverage cells using non-interference channels 1, 6 and 11) 
the APs only have one WLAN card.

>> 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.
> When you say "reject", do you mean at initial association, or later on 
> (forced disassociation)? 
Good question! The term "reject" is not adequate. The exact term is 
"disassociation". Thus, the APs distribute the load disassociating the 
stations, but not "rejecting" initial associations. The main reason: my 
algorithm that decides the load distribution takes into account the load 
of the last association.

> If the stations are more or less stationnary, you only care about 
> initial association, and you can be sure all stations are using active 
> polling rather than passive, then all APs should get the probe 
> request, after which you can decide which one should accept the 
> association (or even better, which one should send back a probe 
> response, though I don't believe this would be possible with hostap 
> mode). 

Yes, but in this case you are not considering that the traffic pattern 
in WLAN networks is highly dynamic. As I have said before, considering 
only initial associations does not take into account that each station 
can generate a very different type and quantity of traffic.

> If they are using passive scanning, you might try the following 
> approach, though I'm not sure it will work with all clients: when a 
> station tries to associate to a given AP, refuse. The station should 
> (?) try another one, which should refuse also. Once it has tried all 
> APs (and you thus have information about all APs it can reach, along 
> with signal level etc.), I guess it should start again (maybe after 
> some timeout?) and then you can pick which AP will accept the 
> association. Of course if the station is "too intelligent" and caches 
> those APs which rejected the association and never tries those again 
> you have a problem... 

Uhmm... it seems interesting, but what about these issues:

    1) Initial necessary time to get the available APs since it is 
required that the station must be rejected by     all the APs that can 
reach and therefore, it can represent a initial delay for the user to 
connect to the         network.

    2) It does not contemplate station movement (if the station moves to 
another position where only can     reach one AP this information will 
not be anymore valid).

Therefore, I still think it is necessary to find out this information 
whenever it is required and not only at some initial phase (as it it 
based your mechanism).

> Other than that, there might be other solutions for the AP to poll the 
> stations, though I have to admit I don't know very well all the 
> low-level mechanisms like RTS, CTS, ACK, CF-Poll and so on, but others 
> on the list probably do :-) 

Well, I guess the solution I am looking for is much related with those 
mechanisms. Anyway, thanks a lot for your comments. Overall, this is my 
final degree project in Electrical Engineering therefore I must 
consider, analyse and compare all the possible solutions to solve the 

> One thing that could help is to know what the scenario is exactly: 
> public or private use, pre-determined set of stations or not (i.e. 
> standardized enterprise-provided laptops or random student-owned 
> ones), ability to impose some settings or not, stationnary or moving, 
> etc. 

My scenario is composed of 3 APs with overlapped coverage areas placed 
in an indoor laboratory. There are not special requirements for the 
stations (this one of the main features of my solution) due to the load 
distribution system must be transparent for them. Therefore, all the 
modifications have to be implemented in the network side (at the APs). 
Indeed, I should consider the movement of the stations so the APs must 
know in advance if a station can reassociate with another AP in the same 


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