Re: Correct radiotap header for 802.11ad

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

On Thu, 2015-09-10 at 11:25 -0700, Guy Harris wrote:
> 
> The ones that aren't in the MCS field or the A-MPDU field are:
> 
> 	HT Length - is that just the length of the data following 
> the radiotap header?

I don't think so - at least not with A-MPDU.

> 	Smoothing
> 
> 	Not Sounding
> 
> 	CRC - is this of any interest?
> 
> 	Tail Bits - always 0, so these are presumably not of 
> interest
> 
> Can:
> 
> 	whether "20" means "20", "20L", or "20U";
> 
> 	HT mixed vs. greenfield;
> 
> both of which are available from the MCS field, be determined from 
> the PLCP header?
> 
> Or is what "20" means not fully determinable for received packets, so 
> that "20L", "20U", and "40" are indistinguishable, similar to what 
> the Radiotap spec for the VHT field says?

I have a hard time seeing 20L/20U being distinguishable if you're
hearing those while tuned to a 20 MHz channel, but if you're tuned to a
40 MHz channel then perhaps that's possible - likely not from the PLCP
though since that would have to look like a regular 20 MHz transmission
for purposes of somebody picking it up on a 20 MHz channel? I'm not
really all that familiar with the PHY details though.

> 20.3.9.5.4 "HT-greenfield format HT-SIG" says "The content and format 
> of the HT-SIG of an HT-greenfield format frame is identical to the HT
> -SIG in an HT-mixed format frame, as described in 20.3.9.4.3.", so if 
> that information can be determined for received frames, it sounds as 
> if greenfield vs. mixed *can't* be determined from the HT-SIG bits.

You can know whether or not it was preceded by a compat preamble
though.

> For *transmitted* frames, I guess you have more control over how it's 
> transmitted, which is presumably why there's a VHT field rather than 
> just a PLCP header field.
> 
> I.e., as radiotap is used for transmission, it needs to include at 
> least some of the parameters to the PHY service's transmission 
> operation, which may not be available in the PLCP header.

That's also a good point - having to build a PLCP signal field for
transmission would be somewhat painful, especially if you just want to
control some part of the transmission.

> And is there anything more that would be needed for *injected* 11ad 
> frames?  If so, that's an argument for a "DMG" field containing the 
> interesting information from the PLCP header combined with 
> information necessary for injected frames?

Not being familiar with DMG, I can't really comment on this.

It does sound like we need *some* new field though, be it either a DMG
field or a PLCP SIGNAL field, or perhaps even both.

Going back to the original thread though, I think using the MCS field
is quite wrong.

johannes