RE: [RFC] VHT fields

> On Tue, 2012-06-12 at 15:05 +0000, Alwin Beukers wrote:
> > Hi everyone,
> >
> > This is a proposal for adding support for 802.11ac. Four fields were
> added to the suggested-fields section:
> >
> > http://www.radiotap.org/suggested-fields/SU_VHT
> > http://www.radiotap.org/suggested-fields/MU_VHT
> > http://www.radiotap.org/suggested-fields/GROUP_ID
> > http://www.radiotap.org/suggested-fields/PARTIAL_AID
> >
> > A Wireshark patch for parsing SU_VHT, GROUP_ID and PARTIAL_AID is
> attached.
> 
> What's the reason for splitting it up?

Johannes,

The reason for splitting it up is to avoid extra overhead related to MU-MIMO 
when describing VHT SU PPDUs. The separation is based on the different PLCP 
header formats (VHT-SIG-A1/A2/B) for SU and MU PPDUs:

SU PPDUs:
- Only one NSTS, MCS and Coding field.
- Partial AID identifies recipient
- Group ID is always 0 (addressed to AP/mesh STA) or 63 (addressed to STA)

MU PPDUs:
- NSTS, MCS and Coding fields for up to four users
- Partial AID is not applicable
- Group ID is 1..62

All fields common to both SU and MU are in the SU_VHT field. The MU_VHT field 
contains additional information for MU PPDUs. The GROUP_ID and PARTIAL_AID
fields have varying value or applicability.

So for SU PPDUs you would likely have SU_VHT and PARTIAL_AID fields, and maybe
a GROUP_ID field (for SU the GROUP_ID field only provides direction info that
could also be determined from PARTIAL_AID and/or MAC header).
For MU PPDUs you would have SU_VHT (for the common fields as well as 
NSTS/MCS/Coding for the first user), MU_VHT (for NSTS/MCS/Coding for
additional users), and GROUP_ID.

By combining fields we can reduce complexity at the expense of some increased
overhead, what are your thoughts on his? Also, I was wondering if we need all
of the 'known' flags in SU_VHT. The MCS field (after which SU_VHT was modeled)
also has these, but I don’t know the rationale behind them. Can one assume a
driver has full access to the PLCP header and thus can fill in most of fields
in the SU_VHT field?

Thanks,
Alwin
 
> johannes
>