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I am using an ASUS RT-AX88U router and a Samsung Galaxy S10 to transfer 802.11ax packets. Using iperf3, I can get speeds of up to 900 (!) Mbps.

I use a laptop with an Intel AX200 chipset put in monitor mode to capture these packets.

Unfortunately, I can only capture a very small percentage of all the packets that are being transferred.

Note that I keep the laptop next to the sender (router) which was the best location to capture 802.11ac packets (which I have been doing as well and very successfully).

I have made sure that I am on the right channel with the correct channel width (I set the router to 80 MHz and then the interface in monitor mode to 80 MHz as well). Also, all packets I capture are 802.11ax (HE) packets.

For some reason I can capture all the packets when using the Netgear Nighthawk RAX80 router (But I only get a maximum throughput of 465 Mbps, roughly half of what I got with ASUS, though that could just be a Netgear firmware bug). With the same setup though I capture like 5% of the packets with the ASUS router.

Also, I see packets from MCS 8 to MCS 11 (256-QAM and 1024-QAM packets).

Anyone else who has come across this issue?

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  • Did you double check that you had your sniffer on the right channel, with the right channel width? Did you make sure the sniffer is not too close to either device? You can overload the front end of the radio with a signal that's too hot. Aim for a little below -40 dBm. Are there any clues in the RadioTap (or other radio meta-data) headers, like maybe you're only seeing VHT (AC style) packets, not HE (AX-style), or maybe you're only seeing 256-QAM or less, not 1024-QAM, or maybe only 1SS, not 2SS?
    – Spiff
    Mar 11, 2020 at 2:44
  • Answered some of your questions in my now edited question. What is the best way to check that the sniffer is below -40 dBm? I'll check the other things and get back to you.
    – Shivang
    Mar 11, 2020 at 2:54
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    The radiotap headers (or whatever radio metadata header format you're using) for each captured packet should tell the RSSI for that packet, so you can look there to make sure it's below -40 dBm.
    – Spiff
    Mar 11, 2020 at 6:40
  • I should have mentioned, I don't necessarily have a solution in mind, I'm just a wireless sniffer nerd and I'm intrigued by your issue and hoping I can somehow help you get to the bottom of it.
    – Spiff
    Mar 11, 2020 at 7:25
  • The RSSI was indeed much higher (-20 to -25 dBm) and I moved the sniffer away so that it was just below -40 dBm. But still it was capturing the same amount or even less packets. Then, interestingly, I switched out the ASUS router with a Netgear RAX80 router and I could see all 11ax packets in monitor mode. The problem now is I get only 450-500 Mbps throughput with this router.
    – Shivang
    Mar 11, 2020 at 18:12

1 Answer 1

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You will need to edit /etc/modprobe.d/iwlwifi.conf and set this option:

options iwlwifi amsdu_size=3

The module option amsdu_size=3 increases the incoming buffer size enough to receive 12 KB aggregates. It increases the driver's memory footprint, but without it, expect massive packet drops from the sniffer.

After applying that setting, please reboot and retest.

To see all module options, run this as root:

modinfo iwlwifi

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