Re: MCS field - STBC and Ness

[Simon Barber <simon-vp0mx6+5gkqFX2APIN6yfw@public.gmane.org>]

One possible solution would be to adopt your proposal to add a single 
bit STBC, but also add a second bit to the 'knows' byte - STBC2. If this 
second bit is set, then it indicates STBC=2. This would leave one 
remaining bit in the 'knows' byte, to indicate the presence of Ness 
information, and 2 bits in the 'flags' byte for 2 bits of Ness information.

Copying the MCS definition from the radiotap.org website:

The known field indicates which information is known:

known	definition
0x01	bandwidth
0x02	MCS index known (in mcs part of the field)
0x04	guard interval
0x08	HT format
0x10	FEC type
0x20	STBC - STBC is known
0x40	STBC2 - 2 STBC streams are present
0x80	Ness - Number of extra spatial streams is known

The flags field is any combination of the following:

flag	definition
0x03	bandwidth - 0: 20, 1: 40, 2: 20L, 3: 20U
0x04	guard interval - 0: long GI, 1: short GI
0x08	HT format - 0: mixed, 1: greenfield
0x10	FEC type - 0: BCC, 1: LDPC
0x20	STBC - 0: no STBC streams, 1: 1 or 2 STBC streams (see STBC2)
0xc0	Ness - Number of extra spatial streams.

Simon



On Fri 11 May 2012 12:04:51 AM PDT, Wojciech Dubowik wrote:
> Hello,
> Atheros families 9002 and 9003 support only one STBC stream. I think I
> have seen a Ralink chipset
> which supports 2 stream at least on receive. I am not sure about
> transmiter.
> On Atheros you get STBC flag and two bits Ness field for received
> packets but In the patch there is is only flag reading but you could
> add Ness yourself it's only two bits left from the flag.
>
> Myself I have tested only one stream but I did it only by looking
> whether other party is setting rx descriptor bit. I don't know how it
> looks "in the air". I trust manufacturer;o)
>
> It's true that only one bit was allocated in wireshark extension.
> Should have been two but it's a bit waste. My proposal wasn't accepted
> but in the end I think it's ok becasue I have managed to get all info
> in vendor namesapes where I can do what I want.
>
> Br,
> Wojtek
>
>>
>> I am writing a Wireshark extension to visualise 802.11 frames on a
>> timeline. In order to do this I need to be able to calculate the
>> duration from start of TX to end of TX for each frame.
>>
>> For 802.11n I need to know the number of HT-DLTF and HT-ELTF training
>> symbols that are sent in the header - I have everything else needed
>> to calculate the duration. Given the MCS and the STBC and the Ness
>> (Number of extension spatial streams) I can calculate the number of
>> training symbols.
>>
>> Both STBC and Ness are encoded in the HT-SIG transmitted at the start
>> of each HT frame.
>>
>> The current proposal (or has it not yet been accepted?) for STBC
>> allocates just 1 bit - but the STBC field can have values or 0, 1 or 2.
>>
>> Ness can be 0-3, so the remaining 2 bits could be allocated to Ness,
>> but this does not solve the problem of how to encode the modes with 2
>> spatial streams and 2 additional streams for STBC.
>>
>> Does anyone know what current hardware supports - STBC values or
>> 0,1,2? Ness values of 0-3?
>>
>> Any suggestions for how to convey these
>>
>> Simon
>