Wireshark · Wireshark-dev: Re: [Wireshark-dev] Patch: Prepare for some cleanups of 802.11ad

[Richard Sharpe <realrichardsharpe@xxxxxxxxx>]

On Sat, Aug 22, 2015 at 1:32 PM, Guy Harris <guy@xxxxxxxxxxxx> wrote:
>
> On Aug 22, 2015, at 1:09 PM, Richard Sharpe <realrichardsharpe@xxxxxxxxx> wrote:
>
>> Also, I now see that sections 9.7.5a.4 and 9.7.5a.5 imply that MCS
>> headers are appropriate for an 802.11ad capture,
>
> I'm not so sure about that.
>
> 802.11ad-2012 has Clause 21, which says:
>
>         The DMG PHY supports three modulation methods:
>
>         -- A control modulation using MCS 0 (the control PHY; see 21.4)
>
>         -- A single carrier (SC) modulation using MCS 1 to MCS 12 (the SC PHY; see 21.6) and MCS 25 to MCS 31 (the low-power SC PHY; see 21.7)
>
>         -- An OFDM modulation using MCS 13 to MCS 24 (the OFDM PHY; see 21.5)
>
> so it has its own MCS values, independent of 11n and 11ac, so there should probably be a DMG field with, among other items, an MCS subfield, containing a value between 0 and 24.
>
> Don't be confused by the name of the "MCS" field; it really *should* have been called the "HT" field, as it has subfields for more than just the MCS, and as its MCS values are specific to the High Throughput PHY - i.e., the 11n PHY.  The page for it on the radiotap site says:
>
>         The mcs field indicates the MCS rate index as in IEEE_802.11n-2009.
>
> which, if we update it to say "as in Clause 20 of IEEE 802.11-2012", says it has values from 0 to 76, with modulations different from the ones in Clause 21, i.e. the MCS field is *not* appropriate for 11ad.
>
>> so the radiotap
>> dissector will need to change to use the frequency set the PHY type.
>
> No, I'd add a DMG field to radiotap, containing, among other values, an mcs subfield, with a Clause 21 MCS value in it.

Yes, in an ideal world. However, there will likely be captures out
there for a while yet that do not have such a DMG field because it is
not yet defined.

However, I agree with the below.

> I'm a software engineer, not an electrical engineer, so I'm not even remotely close to an authority on what radio-layer information would be useful, but a quick look at Clause 21 suggests that it might want to include a flag to indicate whether "Static Tone Pairing" or "Dynamic Tone Pairing" was used.

Can you make that suggestion on the radiotap mailing list?

I can then communicate it to the appropriate parties and perhaps get
them to join that mailing list as well.

-- 
Regards,
Richard Sharpe
(何以解憂？唯有杜康。--曹操)