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[Ham-80211] diversity techniques and long link timing...

David Young dyoung at pobox.com
Fri Dec 16 12:12:41 UTC 2005

On Fri, Dec 16, 2005 at 02:59:11AM -0700, Chris D wrote:
> My first post... Hope I'm doing it right...
> ----- Original Message ----- 
> From: "Frank" <kg6jve at pasadena.net>
> To: "'TAPR Mailing List for Ham Radio Use of 802.11'"
> <ham-80211 at lists.tapr.org>
> Sent: Thursday, December 15, 2005 12:09
> Subject: RE: [Ham-80211] diversity space or not
> >
> > The diversity on most of the less expensive gear does not do
> "real"
> > diversity. On the Senao and probably the Linksys, one port is
> TX/RX and the
> > other RX only.
> I've always wondered about how the wifi gear does diversity... As
> far as I know, each antenna must have its own receiver -at least
> down to the "IF" section (whatever that is with wifi!). And then
> some fancy comparator... Is that how it is done in wifi? Or do the 2
> nodes test their single receiver on one antenna at a time, and then
> just settle on the one with the best SNR? And hang on that
> combination for...... the next... 5 minutes?

In a typical WiFi card, the modem has control of an antenna switch.
During the first several microseconds of packet reception, the modem flips
between antennas to sample the signal on each.  When the antenna-selection
period is over, the modem fixes on whichever antenna had the great sum
of squared samples (or some other measure of the energy).

> That leads to another question: I've read that most wifi has its
> timing set for VERY short links and fast turnarounds... How do you
> know that your units are indeed adjusted to expect a
> "15-miles-delayed" response, and deal with it efficiently? Can most
> cards be adjusted for unusual long link delays with some specialized
> soft? Or is there a table somewhere that lists the timing for ea
> model?

Today, it must be an unusual card where it is not physically possible to
adjust the ACK timeout, however, documentation is not always available.
If it helps, the Atheros "Hardware Abstraction Layer" has an API call for
the purpose, ath_hal_setacktimeout(); you can adjust the timeout using
"sysctl" in *BSD and Linux.


David Young             OJC Technologies
dyoung at ojctech.com      Urbana, IL * (217) 278-3933

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