I am working on Fine Time Measurement and I can use IW command to measure RTT from two AX200/201/210 cards. However, I notice that I cannot change the preamble even though I have set preamble=VHT. Using a sniffer I can see that it got stuck with 80211n. The preamble=legacy/ht/vht does not take effect. (My system: ubuntu 22.04, I tried ax200, ax201, ax210 and hostapd). I will appreciate your help a lot. thanks.

Welcome to LQ, vutran86. I look forward to your insightful contributions, because you're clearly no newbie.

It would be good to post the exact command you're using, and what output you're receiving. You're there, clearly frustrated but the onus is on you to give us the detail we need, if we are to help you. What are the "two AX200/201/210 cards"? Nics or wifi? I'm guessing because you're talking about IW, and not IP, it's wifi. But you make no mention of channel loading, etc. Why not just use ping? No preamble.

Actual conditions for wifi are affected by loading, reception, weather, other activity sharing your channel, etc. The preamble is a fixed time interval which should not affect relative measurement greatly, if at all.

Hi business_kid,

I admit that my question was unclear (also I am newbie in kernel programming). So I will rephrase it. I am using iwlwifi backport driver to test Fine Time Measurement using a pair of Intel WiFi chips AX201 and AX210 (AX201 used as an AP with hostapd). I used IW for FTM measurements as follows:

$iw measurement ftm_request request.conf

request.conf: xx:xx:xx:xx:xx:xx bw=20 cf=5745 asap preamble=vht

With xx:xx:xx:xx:xx:xx is my AP MAC address.

I tried to change the field preamble in the request.conf but it does not change the preamble in the FTM packets (always use HT for 20MHz. Confirmed using an AX200 as a sniffer). I tried compiling IW and can see that it did take the preamble setting and write it into the driver. Now I can vaguely find out that the driver seems to set the preamble based on the bandwidth but I need to do some experiments at 20MHz bandwidth using different preambles (in the end, actually I need to test it with HE -- WiFi6 preamble). I am trying to get some ideas from the driver. So if you could give me any suggestions to solve it, I would really appreciate it.

FYI: Channel loading is extremely low in my case as I only use the AP to test FTM packets (only 1 user and a packet in a few seconds). Also, FTM measures the travelling time of RF waves in sub-nanoseconds, so Ping won't work. FTM is introduced in 802.11mc. The preamble I mention is the preamble in WiFi Phy layer packets (e.g. HT for 802.11n, VHT for 802.11ac, HE for 802.11ax)