APs and multiple BSSID support (OT?)

Jouni Malinen jkmaline at cc.hut.fi
Sat Feb 10 22:38:44 EST 2007


On Tue, Feb 06, 2007 at 11:05:50PM -0700, Bob Carlson wrote:

> The problem with mult. SSID/BSSIDs is that there is no way to make the
> hardware aware of 2 different BSSIDs for things like timing derived from
> the beacons. I believe I can connect to two different BSSIDs if they are
> supported by the same physical AP. Beacons from the same physical AP will
> share many attributes such as timing, channel, rates, etc.

There is no guarantee that Beacons from the same physical radio will
share these attributes.. As far as channel is concerned, it will most
likely be the same and also TSF is likely to be shared. However,
advertised rate sets could be different..

Which timing problems are you worried about? If I understood correctly,
you did not require power save support which makes this much easier..
Even WMM could be mostly doable assuming the APs are using more or less
compatible settings (or the client radio supports large enough set of TX
queues).

I would expect that this type of virtual client could associate with
multiple BSSes from multiple APs as long as these BSSes are using the
same channel. Whether everything works exactly in the same way as it
would with multiple radios (one per client STA) is another question and
how important this is depends on the use.

> That's a long winded explanation for a simple need. Because I can only
> successfully connect to 2 different BSSIDs if they are supported by the
> same physical AP, I would like a way to determine that. The Atheros and
> Cisco address methods allow me to do that. However, if other vendors choose
> their multiple BSSIDs differently (I have seen a proposal that the BSSID be
> generated randomly in the local space) I don't see any way to determine
> "same physical AP."

I don't think there is generic mechanism for determining whether there
is single physical radio or not.. As an example, I'm aware of multiple
different ways of doing this with Atheros hardware and I know that more
than one mechanism has been deployed.

> If there is another way to determine whether 2 beacons or probe responses
> with different BSSIDs come from the same physical AP, I would love to hear
> of it.

This would get complex if all cases would need to be caught without
co-operation from the APs. I'm not sure whether this is worth the
problem taken into account the virtual clients are likely to work well
enough with multiple APs for most uses.

-- 
Jouni Malinen                                            PGP id EFC895FA



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