Significant loss of WiFi bandwidth when Smartphone is plugged via USB

Superhansel · 2023-03-26 22:52:38

I observe a strange phenomena with my notebook where I'd appreciate any helpful explanation (or even solutions):

Whenever I plug in my smartphone via USB (3.0), I see a significant drop in my WiFi link speed. This happens despite the fact, that I do only charge the Smartphone via USB and do not enable data transfer (either by turning it off in Android or eject it inside the nautilus file manager).

This is the link speed without the phone being connected (as reported by iw dev wlp0s20f3 link):

        SSID: MyWifiSSID5G
        freq: 5300
        RX: 1050789041 bytes (688280 packets)
        TX: 13648962 bytes (148813 packets)
        signal: -79 dBm
        rx bitrate: 175.6 MBit/s VHT-MCS 2 80MHz VHT-NSS 2
        tx bitrate: 195.1 MBit/s VHT-MCS 2 80MHz short GI VHT-NSS 2

        bss flags:      short-slot-time
        dtim period:    1
        beacon int:     100

And then this is the speed reported when the phone is connected:

Connected to 38:10:d5:af:13:4b (on wlp0s20f3)
        SSID: MyWifiSSID5G
        freq: 5300
        RX: 83759037 bytes (55126 packets)
        TX: 2413008 bytes (26943 packets)
        signal: -72 dBm
        rx bitrate: 6.5 MBit/s VHT-MCS 0 VHT-NSS 1   <-- This obviously varies but usually does not exceed 30 MBit/s
        tx bitrate: 9.0 MBit/s                       <-- This obviously varies but usually does not exceed 30 MBit/s

        bss flags:      short-slot-time
        dtim period:    1
        beacon int:     100

I first thought it to be some kind of software-related issue and had a look at journalctl -f to see if there is any network-related actions that become effective when connecting the phone. But this is not the case (at least nothing that meets the eye).

I then found this Ask Ubuntu post that kind of describes the issue that I have and also has a plausible explanation: Drop of WiFi performance due to RF interference. However, this is explicitly related to 2.4 GHz WiFi connections, whereas I use a 5 GHz connection. I couldn't find any information about whether this is also affected.

I hope anyone can help out with an online resource that describes this phenomena or has experience with this kind of issue. I do wonder why there is seemingly so few information to be found online about it, if its really related to charging a device through USB (which I think is quite a common thing to do).

Last edited by Superhansel (2023-03-26 22:54:05)

seth · 2023-03-27 07:41:38

Did you try to disable the phones radio (wifi/BT)?

Superhansel · 2023-03-27 13:32:32

Oh yes, good point. I did turn off WiFi and BT of the Smartphone as well. But it doesn't seem to make any difference at all.

seth · 2023-03-27 13:59:09

I guess you're not getting this when merely putting the phone there (w/o plugging it)?
Do you get the symptoms when shutting the phone down (and only charge it)?

ewaller · 2023-03-27 14:38:11

As a sanity check, does the output of ip addr change when the phone is connected?

Superhansel · 2023-03-27 16:30:13

Apologies. I may have acted to quickly by posting here.

seth wrote:

I guess you're not getting this when merely putting the phone there (w/o plugging it)?

Correct, and yesterday it was a reproducible behavior. The connection speed *instantly* dropped (or raised) after plugging the phone in (or out).

seth wrote:

Do you get the symptoms when shutting the phone down (and only charge it)?

So this is were I am puzzled now. I just shut my phone down , plugged it in and saw only a minor drop in WiFi speed.

Before the phone was plugged in:

        rx bitrate: 263.3 MBit/s VHT-MCS 6 80MHz VHT-NSS 1
        tx bitrate: 234.0 MBit/s VHT-MCS 3 80MHz VHT-NSS 2

After the phone was plugged in (note, that this is still a higher bitrate than I got in my opening post without the phone being connected, which is perfectly fine)

        rx bitrate: 175.5 MBit/s VHT-MCS 4 80MHz VHT-NSS 1
        tx bitrate: 234.0 MBit/s VHT-MCS 3 80MHz VHT-NSS 2

Even after turning on the phone again, the speed stays at the same level.

So I guess it is not only about the phone being connected. I will further watch this issue and also check which channel my router is on when it happens.. As of now, for instance, the router is on Channel 52, whereas yesterday (when this problem manifested) it was on Channel 56. Yesterday and today it was the only router sending on this channel, but I think it has likely to do with some outside interference from the neighboring buildings or flats..

ewaller wrote:

As a sanity check, does the output of ip addr change when the phone is connected?

The IP address stays the same, although there was some journalctl output related to Avahi when connecting/disconnecting the phone.

Thanks to both of you! I'll keep an eye on the issue and try to identify the root cause. I'll keep you updated.

Arch Linux

#1 2023-03-26 22:52:38

Significant loss of WiFi bandwidth when Smartphone is plugged via USB

#2 2023-03-27 07:41:38

Re: Significant loss of WiFi bandwidth when Smartphone is plugged via USB

#3 2023-03-27 13:32:32

Re: Significant loss of WiFi bandwidth when Smartphone is plugged via USB

#4 2023-03-27 13:59:09

Re: Significant loss of WiFi bandwidth when Smartphone is plugged via USB

#5 2023-03-27 14:38:11

Re: Significant loss of WiFi bandwidth when Smartphone is plugged via USB

#6 2023-03-27 16:30:13

Re: Significant loss of WiFi bandwidth when Smartphone is plugged via USB

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