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From: Johannes Berg <>
To: Guy Harris <guy-FrUbXkNCsVf2fBVCVOL8/>,
Cc: Simon Barber <>,
	Richard Sharpe
Subject: Re: Correct radiotap header for 802.11ad
Date: Thu, 17 Sep 2015 18:37:59 +0200
Message-ID: <> (raw)
In-Reply-To: <1135A126-6A5A-4C84-A52D-13C0387609CC-FrUbXkNCsVf2fBVCVOL8/>

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.

> "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", 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

> 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.


  parent reply index

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-08-22 19:41 Richard Sharpe
     [not found] ` <>
2015-08-22 20:59   ` Guy Harris
     [not found]     ` <38F46E1D-1C4A-48DC-A906-9522006E8474-FrUbXkNCsVf2fBVCVOL8/>
2015-08-22 22:06       ` Richard Sharpe
     [not found]         ` <>
2015-08-22 22:24           ` Guy Harris
2015-08-22 23:34       ` Guy Harris
     [not found]         ` <1606812C-649C-4C06-ABE0-AE2F4474BCD0-FrUbXkNCsVf2fBVCVOL8/>
2015-08-24  7:40           ` Johannes Berg
     [not found]             ` <>
2015-08-24 16:21               ` Richard Sharpe
     [not found]                 ` <>
2015-08-24 16:28                   ` Johannes Berg
2015-08-26 22:59                   ` Simon Barber
     [not found]                     ` <>
2015-08-27  1:17                       ` Guy Harris
     [not found]                         ` <126B842D-05EA-4510-BC9B-DB1A4AABEC12-FrUbXkNCsVf2fBVCVOL8/>
2015-09-10 18:25                           ` Guy Harris
     [not found]                             ` <1135A126-6A5A-4C84-A52D-13C0387609CC-FrUbXkNCsVf2fBVCVOL8/>
2015-09-17 16:37                               ` Johannes Berg [this message]
2019-12-10 23:51                                 ` Guy Harris
2019-12-11  8:32                                   ` Johannes Berg
2019-12-11  9:39                                     ` Krishna Chaitanya
2019-12-11 12:57                                       ` Johannes Berg
2019-12-11 13:20                                         ` Krishna Chaitanya
2019-12-20 21:56                                     ` Guy Harris
2015-09-17 16:32                       ` Johannes Berg
2015-08-26 22:56       ` Simon Barber

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