I've got an interesting puzzle for you:

  • I've got two computers connected to the same LAN wirelessly.
  • Computer A is using 802.11ac wireless protocol. Computer B is using 802.11n.
  • I begin uploading files from computer A to computer B.
  • Initially the upload speed is 70 Mbps but a few minutes later it drops to 3Mbps.
  • When the upload speed decreases:
    • It only affects computer A. Computer B works just fine.
    • Internet speeds are limited in the same way (as reported by http://www.speedtest.net/).
    • Download speeds don't seem to be impacted at all.
  • Disconnecting and reconnecting computer A to the wireless network fixes the problem for a few minutes. Then it returns.
  • As far as I can tell, my router does not have any QoS enabled. I am using OpenWrt 18.06 on a Linksys 1900ACS v2.
  • The NIC on computer A is called "D-Link DWA-192 AC1900 Wi-Fi USB 3.0 Adapter".
  • Both Computer A and B are running Windows 10.0.17134.228.

Any ideas what could be causing this and how to fix it?

  • How are you testing speed between PCs what protocol you use and what instrument to measure? Are you sure you are not measuring some other bottleneck? For example your disk read/write speed? – Art Gertner Aug 17 '18 at 11:23
  • When you say you "internet speeds" are limited. Are you sure you are not on ADSL? Are these speeds in line with what your ISP advertises? – Art Gertner Aug 17 '18 at 11:26
  • @ArtGertner I am monitoring the transfer speed using Task Manager while running rclone.org (basically copying files across a network share). I don't think disk speed is a factor because both ends use an SSD and the problem goes away immediately if I reconnect the Wifi. For the internet speed, I am saying that I normally I get 25Mbps down, 10 Mbps up but when the connection goes into a bad state I get 25Mbps down, 3 Mbps up until I reconnect. Again though, the problem is reproducible on the LAN without even going to the internet. – Gili Aug 17 '18 at 11:45
  • How do you measured speed? If you coping bunch of small files it can slowdown transfer. Try to measure just network speed with iperf3 – Alex Aug 17 '18 at 14:39
  • @Alex I am using the built-in Task Manager and I trust it because when the slowdown begins on the LAN I can see the exact same slowdown on speedtest.net. Task Manager shows an upload speed of 3Mbps and speedtest.net reports the same. – Gili Aug 18 '18 at 1:14

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