Re: [RFC] VHT MU-MIMO correction

[Johannes Berg <johannes-cdvu00un1VgdHxzADdlk8Q@public.gmane.org>]

I guess this has passed a three-week minimum review time - bump, I'll
make those changes unless I hear objections within ~a week?

johannes

On Wed, 2015-10-21 at 11:15 +0200, Johannes Berg wrote:
> Hi,
> 
> As we're looking into MU-MIMO again, we found some issue with the way
> the VHT radiotap field is defined. The issue is the following: the VHT
> SIG-A field defines the NSTS (NSS) for each user, and it would be good
> to have that shown. However, radiotap ties the validity of the MCS to
> the existence of the NSS, which is incorrect.
> 
> Therefore, I suggest to make the following edit:
> 
> Change this language:
> 
>     If the NSS field for a user is zero, the user is not present and
>     the MCS and coding (in the coding field) associated with that user
>     are not valid. For SU PPDUs, only the first user will have a
>     nonzero NSS field.
> 
> To read:
> 
>     If the NSS field for a user is zero, the user is not present and
>     the MCS and coding (in the coding field) associated with that user
>     are not valid. If the NSS field for a user is non-zero, but the MCS
>     is not known (for example due to receiving only data for a single
>     user), the MCS shall be set to 15.
>     For SU PPDUs, only the first user will have a nonzero NSS field.
> 
> 
> The coding should be known for all, since it's also part of VHT-SIG-A,
> so I'm not suggesting to add validity flags for that. Does anyone think
> that is necessary?
> 
> A wireshark patch won't really be necessary since it will already
> display the MCS as "15 (invalid)" in this case, which seems reasonable
> though it could be changed to just "not known" instead.
> 
> johannes
>