AP230 transmit beamforming TxBF

  • 1
  • Question
  • Updated 4 years ago
  • Answered
  • (Edited)
Greeting and salutations

I was testing beamforming on the AP230 yesterday and just trying to work out the mechanics.
equipment used
MBA 10.9.2 and 10.9.3
AP230
HM 6.1r5


According Matthew Gast's
802.11ac: A Survival Guide

802.11ac uses explicit TxBF and that both the AP and Client Supplicant must support it.

HM config allows you to choose auto or explicit.



According Matthew Gast's
802.11ac: A Survival Guide

TxBF will have no benefit if you are too close to the AP or at the edge of the cell. You have to be in the middle to see the rate over range benefit.

I ran some tests and captured some frames with explicit only
There seems to be a increase in speed due to shift up in modulation and coding.
TxBF turned off



TxBF turned on explicit only



I need to run the test a few more times, I am sure that result will vary.

I looked through the capture to find evidence of the TxBF

I found the following:

In beacon frames from AP230 VHT capabilities indicating that AP supports beamforming


and frames from supplicant to receiver
and frames from receiver to supplicant

these contained VHT information inside data frames

and



What I could not find was NDP announcements.

According Matthew Gast's
802.11ac: A Survival Guide

Explicit TxBF require the use of sounding frames, feedback matrices. Which uses NDP to kick things off

According to the CWAP

Null Data packet -can not be seen in protocol analyzer. Because they are PPDUs that contain no MPDU.

However, they go on to say that you might be able to prove the existence of TxBF by spotting NDP announcement frames and TRQ.

Anyway at this point I have not been unable to capture any frames containing NDP announcements or TRQ.

So if anyone is able to capture any such frames I would love to see them.

Cheers

A






Photo of Andrew MacTaggart

Andrew MacTaggart, Champ

  • 483 Posts
  • 86 Reply Likes
  • Zippy

Posted 4 years ago

  • 1
Photo of Gregor Vucajnk

Gregor Vucajnk, Official Rep

  • 74 Posts
  • 27 Reply Likes
Good post Andrew. I'll try to dig up some captures for you.

Gregor

Photo of Andrew MacTaggart

Andrew MacTaggart, Champ

  • 483 Posts
  • 86 Reply Likes
Thanks Gregor

I am limited with the tools I have

but it does look like Wireshark will need an update for 802.11ac

I have found an unknown control frame with subtype 0x15



so type 1 subtype 5

and I found this info from

http://fossies.org/dox/wireshark-1.11.3/packet-ieee80211_8c_source.html



where 0x15 is to be defined for VHT NDP Announcement

or at least that is how I am reading it.

Would be interested in seeing frames from omnipeek if anyone has it.

Cheers
A

Photo of Andrew MacTaggart

Andrew MacTaggart, Champ

  • 483 Posts
  • 86 Reply Likes
Just wanted to add some things I have learned along the way with wireshark

under preferences you can add columns to display other 802.11 useful info












Photo of J. Goodnough

J. Goodnough, Champ

  • 266 Posts
  • 32 Reply Likes
Very cool information, Andrew.