[PATCH] hostapd: Don't force HT Mixed Mode for non-GF STAs

Helmut Schaa helmut.schaa at googlemail.com
Wed Mar 2 04:04:16 EST 2011


Am Mittwoch, 2. März 2011 schrieb Johannes Berg:
> On Wed, 2011-03-02 at 09:53 +0100, Helmut Schaa wrote:
> > Am Mittwoch, 2. März 2011 schrieb Johannes Berg:
> > > On Wed, 2011-03-02 at 09:37 +0100, Helmut Schaa wrote:
> > > > Currently hostapd will force HT Mixed Mode if at least one non-GF STA is
> > > > associated. This will force _all_ HT transmissions to be protected.
> > > > 
> > > > 802.11n-2009 doesn't require HT Mixed Mode to be used in case of non-GF
> > > > STAs but instead the HT information element contains a flag if non-GF
> > > > STAs are present. All STAs are required to protect GF transmissions in
> > > > that case. Hence, setting HT Mixed mode if non-GF STAs are present is
> > > > superfluous.
> > > 
> > > It seems like this would still be relevant for the AP's TX?
> > 
> > Why? hostapd will configure the new ht_opmode (including the non-GF-STA flag)
> > to the driver. And the driver should do the right thing and enable protection only
> > for GF transmissions in that case, no?
> 
> Not sure really, this HT stuff isn't as well defined as we'd like :-)

Yes, but this particular part looks well enough defined to me ;)

802.11n-2009, 9.13.3.1, page 143:

"When the HT Protection field is set to no protection mode or 20 MHz protection
mode and the Nongreenfield HT STAs Present field is set to 1, HT transmissions
that use the HT-greenfield format shall be protected."

So, in this case the driver used by the AP should also protect GF
transmissions as long as hostapd passes the new htopmode to it.

Helmut


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