Saturday, January 30, 2016

Is my 802.11ac wifi AP beamforming ?

I wanted to see whether my 802.11ac AP (Aerohive AP230) is beam forming. I enabled explicit beamforming. 802.11ac only supports explicit beam forming because 11ac beam forming is based on channel measurements which both the AP and the client need to support.

First thing, make sure the single user beamforming is advertised by the AP in beacons, I did this packet capture using wireless diagnostics tool of MacBook

Make sure that client supports single user beamforming in association request,

Now how do I know whether the AP is using beamforming when transmitting packets to a specific client. I was thinking I can check the SNR at the same client with and without beamfirming enabled. But every time you check RSSI or SNR at a client it keep changing, so it wouldn't really be a convincing test result. The next way to do this is to look at a packet capture to see whether the AP is using explicit beamforming procedure and transmit packets with wlan_radio.11ac.beamformed bit set to 1. The process for explicit beamforming is as follows,

You can see the packets for steps 1, 3, 4, 5 of the diagram above, but not step 2. Step 2 is NDP (Null Data Packet), this frame does not have a 802.11 mac header, it is only a physical layer packet. I think this packet is not passed to upper layers by wifi driver and that is why Wireshark does not display it. The packet capture below shows that the AP is using the beamforming process and sending frames using beamforming.

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