Package: firmware-iwlwifi Version: 20190114-1 Severity: normal Dear Maintainer,
This seems a bizarre failure mode, but after lots of debugging it seems to be what is happening. I have a Thinkpad X220 with a Centrino Wireless-N 1000: 03:00.0 Network controller: Intel Corporation Centrino Wireless-N 1000 [Condor Peak] It works fine most of the time, but occasionally I get sudden spikes of transmit errors (`iwconfig wlan0` shows `Tx excessive retries` going very quickly), accompanied by long packet roundtrip times (can be upwards of 20, 60, maybe even 200 seconds long), visible to ICMP `ping`. I have determined this correlates very closely with periods of very high signal strength. E.g. when I am sat at my desk right next to my access point (a good strong Ubiquiti Ubifi), the signal strength gets as strong as I've ever seen it, and suddenly transmit errors spike hugely and network performance plumets. Usually at that point I would restart the network card which would fix it for a while. One day I happened to notice the correlation that it never seemed to occur when I was anywhere else in the house, more than a couple of meters away from the AP and sometimes with a wall inbetween. This problem rarely happens when travelling to other places (e.g. friends houses, most coffee shops, etc...) but I have seen it also from time to time elsewhere. I have applied packet dumping on both sides of the network during one of these periods of breakage, and it seems entirely the laptop->AP direction (at least at the IP / UDP layer) where the delays happen. Any frames the AP wanted to send to the laptop arrive promptly, but often the frames transmitted by the laptop towards the AP will arrive very late. So I tried an experiment. I moved the AP further away. This fixes it. I don't yet have exact scientific numbers of recording the reported signal strength during OK and broken periods, but I wonder if a possible explanation could be that the firmware/hardware is performing some kind of dynamic gain control, turning down its own transmit power if it sees the incoming signal strength is very high. Perhaps it turns that down so far that it doesn't have a lower bound and accidentally drops off way too quietly, for the AP to hear it, whereupon a reset or moving the AP away thus dropping the incoming strength, will fix it. -- System Information: Debian Release: buster/sid APT prefers testing APT policy: (990, 'testing'), (500, 'unstable') Architecture: amd64 (x86_64) Foreign Architectures: i386 Kernel: Linux 4.19.0-2-amd64 (SMP w/4 CPU cores) Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8), LANGUAGE=en_GB:en (charmap=UTF-8) Shell: /bin/sh linked to /bin/dash Init: systemd (via /run/systemd/system) LSM: AppArmor: enabled firmware-iwlwifi depends on no packages. firmware-iwlwifi recommends no packages. Versions of packages firmware-iwlwifi suggests: ii initramfs-tools 0.133 -- no debconf information
