Re: MCS field: RFA

[Bill Stafford <bill.stafford-BUHhN+a2lJ4@public.gmane.org>]

On Feb 2, 2010, at 11:54 AM, David Young wrote:
> On Mon, Feb 01, 2010 at 10:09:37PM +0000, Bill Stafford wrote:
>> 
>> So the bandwidth options for a packet when the device is tuned to a 40 MHz
>> channel are:
>> 40 MHz (full 40 MHz bandwidth packet)
>> 20L (20 MHz packet in the lower 20 MHz of the 40 MHz channel)
>> 20U (20 MHz packet in the upper 20 MHz of the 40 MHz channel)
>> 
>> When tuned to a 20 MHz channel, the packets can only be 20MHz
> 
> My recollection of 802.11n may be incorrect, but aren't 20L and 20U HT
> transmission channels aliases for the same center frequency and width as
> a 20 MHz HT/legacy channel?

As I understand it, those transmission modes, and rx attributes,
do represent packets on the the air that look just like packets
sent by a device tuned to the 20 MHz channels. So if a device is
on the 40 MHz channel that is the pair of {36,40}, then when it
transmits a 20L packet, it should be received by a device tuned
to the 20 MHz channel 36 just as if the transmitter had been
tuned to 36. So in that way you could think of it as an
alias---at least from what you would see on the air. But packet
reception would be from a radio that is really tuned to a 40 MHz
wide channel. I've got to assume that the transmit side would
also be operating in a 40 MHz mode as it generated the 20U or 20L
packet. So these transmit/receive modes are different than being
tuned to a 20 MHz channel.

I think it is an important piece of information for capture on
both transmit and receive. For instance, if a capture shows that
a receiver is getting a higher rate of packet errors when it
receives 20L packets from the 40 MHz channel pair {36,40}, than
it is when receiving 20 MHz packets while tuned to 36, then you
might be suspicious of how well the 20L decode is working. If the
capture just reported 20 MHz packets on 36 in both cases, you
would not have anything to go on. Same sort of example for tx.

Also, on use of RadioTap as an app level 802.11 packet format,
you would want to be able to make the distinction between
remaining on a 40 MHz channel and sending a 20L packet, and
actually switching to lower 20 MHz channel and sending a packet.

-Bill