[PATCH] hostapd: add check for overlapping legacy BSS

Rajkumar Manoharan rmanohar
Tue May 13 11:09:09 PDT 2014

On Tue, May 13, 2014 at 05:32:56PM +0300, Jouni Malinen wrote:
> On Mon, May 12, 2014 at 11:29:12PM +0530, Rajkumar Manoharan wrote:
> > On Mon, May 12, 2014 at 06:28:26PM +0300, Jouni Malinen wrote:
> > > On Sat, May 03, 2014 at 07:56:07PM +0530, Rajkumar Manoharan wrote:
> > > > Before bringing up 20/40 MHz BSS, a scan is triggered to identify
> > > > overlapping BSS. If there is any legacy BSS present within the affected
> > > > range, then 40 MHz operation should be rejected and BSS should start
> > > > with 20 MHz.
> > > 
> > > Why would we do that? What's so different about legacy (as in "non-HT"
> > > in this case) BSS and HT 20 MHz BSS that would justify such difference?
> > > Can you please point me to the rule in the IEEE Std 802.11-2012 20/40
> > > coex requirements that this patch implements?
> > 
> > IEEE Std 802.11-2012 - 10.15.12: Switching between 40 MHz and 20 MHz
> > 
> > An FC HT AP 2G4 shall reevaluate the value of the local variable 20/40
> > Operation Permitted (see when either of the following events occurs:
> > 	? A BSS channel width trigger event TE-A is detected.
> > :
> > :
> > 
> > TE-A: On any of the channels of the channel set defined in Clause 19, reception
> > of a Beacon frame that does not contain an HT Capabilities element.
> That is not a rule defining when 40 MHz channel is allowed; that's a
> rule describing when that determination needs to re-evaluated. While I
> agree that TE-A is talking about the case of a non-HT BSS, I cannot find
> such language from which includes the rules on how to
> determine "20/40 Operation Permitted" value. I'd assume this non-HT 20
> MHz BSS would be in the OT_i set, if it were reported by an association
> station, but the source code change proposed here is not processing that
> information; it is processing results from a local scan performed by the
> AP. The applicable rule for that seems use language "at least one 20 MHz
> BSS that was detected within the AP's BSA" which does not say anything
> about HT vs. non-HT. Consequently, I do not understand what this patch
> is trying to do and how it matches the coex requirements.
Completely agree that it is not talking about HT or non-HT BSS.

S.5.2 Establishing a 20/40 MHz BSS

Before starting a 20/40 MHz BSS, an 40-MHz-capable HT AP is required by
the rules defined in 10.15.5 to examine the channels of the current regulatory
domain to determine whether the operation of a 20/40 MHz BSS might unfairly
interfere with the operation of existing 20 MHz BSSs. The AP (or some of its
associated HT STAs) is required to scan all of the channels of the current
regulatory domain in order to ascertain the operating channels of any existing
20 MHz BSSs and 20/40 MHz BSSs.

When the AP performs the scanning and the secondary channel for the 20/40 MHz BSS
has been selected, then the scan shall be performed over the set of channels that
are allowed operating channels within the current operational regulatory domain and
whose center frequency falls within the affected frequency range given by Equation (10-1).

So the scan results should be validated against all 20 MHz BSSs (both HT
& non-HT BSS) falling within the affected frequency range.

Please correct me If im wrong.


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