Here's the relevant entry from my handy dandy 802.11 spec:
11.1.4 Adjusting Station Timers and Coalescing
In the infrastructure network, stations shall always adopt the timer in a
Beacon or Probe Response coming from the AP in their BSS.
In an ad hoc network, a station shall always adopt the information in the
contents of a Beacon or Probe Response frame when those frames contain a
matching ESSID and the value of the time stamp is greater than the station's
TSF timer. A station may return to it's previous BSS, if any, and transmit a
Beacon with the newly adopted information.
End quote from spec.
note - the ESSID mentioned above was later changed to just a "SSID", but I
wanted to quote the passage verbatim. (my spec is a circa 1996 copy)
(I have been assured by one of our firmware folks who participates in 802.11
that this behavior has not changed)
On Wed, Aug 29, 2001 at 10:52:58AM -0700, Jean Tourrilhes wrote:
> I never checked with 2 aironet cards, but I think they
> won't. I wrote the coalescence in SWAP, so I know what it takes ;-)
What is SWAP?
> Moreover, if they were listening to beacon each other, how do
> they negociate which BSSID to keep and which BSSID to throw away ?
> There is no provision in 802.11 for that.
> Anyway, having this restricted to a single frequency is pretty
> limitative. The whole point of IBSS mode as opposed to demo-ad-hoc is
> that you don't care about the frequency and let the card choose
> whatever frequency it believes is best (for example based on noise
> level or load). Having to care about the frequency would be a step
> backward, IMHO.
I specifically said if they were on the same frequency and using the
same SSID and any WEP keys were the same - THEN they would coalesce.
If they originally started out on different channels and they can't hear
each other - then they will not.
> This is exactly why I was asking Ben a few weeks ago about
> cell scanning in ad-hoc mode. You basically do regular scans (on all
> frequencies) and if you find another cell you like, you join
> it. That's a bit more complex than that, but I've got details figured
> out ;-)
Our radios have always scanned for an existing cell on all frequencies
before giving up and starting their own.
They ONLY use the configured frequency when forced to start their own cell,
otherwise they always adopt the existing BSS or IBBS's frequency.
Aironet firmware prior to 45c425_10 had a bug that could have prevented
this from occurring properly.
| | Jim Veneskey
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