Revision history [back]

Please, can someone tell me is it actually possible AT ALL to capture the high rate frames of 802.11n and 802.11ac, and how I can troubleshoot this issue?

Yes, it is possible. However, the higher the datarate, the harder it will be. Here is an example of some relatively high datarate frames collected:

They are all the same frame, just captured with three different WiFi adapters (there is a fourth adapter that didn't pick it up and I have no idea why). They also don't report the same information - two say it is LDPC encoded, one BEC. Which is correct? Not always obvious to tell (I suspect LDPC is correct). Missing traffic; inconsistent information; behavior that changes as the Linux kernel changes or MacOS updates - such is 802.11 packet capture.

If you search here you will see this comes up often, and many of those discussions provide the root cause: the capture capability envelope has to be at least as big as the test traffic to have any chance. I don't know why you need these frames - solve a problem for work? For fun? Any which way, I recommend a different approach to these types of problems: understand what you need the tool to do, then buy a tool that meets those requirements.

So will you ever be able to see the traffic you need to? No one here can answer that because we have no idea what you need to capture. Once the problem is understood, we can set about looking for a solution. (And yes, there are edge cases that just sometimes do not to work, even if they should).

The master table for 802.11 n/ac is big. I have never seen a device support the whole thing, only a subset. So troubleshooting steps:

What do you need to capture? What are the modulations that need to be collected? We need datarates, spatial streams, channel widths, MCS Index, aggregation, and guard interval (some of these are inter-related). Until you can answer this, we don't know what system you need to collect this traffic. Capturing more than 2 spatial streams gets significantly more complicated.

Be close enough to the test traffic. High datarate frames do not travel very far.

Suggest to use a high end wifi system that can tune traffic - disable/enable certain features so we can calibrate the response of the capture system to gain confidence in it's capabilities. For instance, test with LDPC on and off. Test with 4, then 3, then 2, etc., spatial streams.

The beacons and probe & association request/response frames provide a lot of information related to capabilities of the AP and the client. Typically, systems will try to use the maximums in common between the two to attain the highest performance. These are low datarate frames, so capturing these is easier and mapping out the capabilities from a packet capture will show you most, if not all, of the required information needed to set your test requirements. Look at HT, VHT, and HE information elements.