Modems disconnecting when signal level is low

Mon Aug 14 09:37:09 UTC 2023

Hi all,

I am currently trying to use two SIMCOM SIM8300G-M2 5G modems to test cellular performance (coverage, latency, throughput) from two different operators in a rural area. I have been experiencing some stability issues with the modem connections. It seems that it is related to the low signal level observed in the area where I am measuring. Whenever the signal level is low, or there is no coverage, the network interface is deleted, the modem disappears from mmcli list, and it reattaches automatically after a bit, when the signal level improves. This causes long (or at least longer than I would wish) interruptions in my tests. I include some logs below as an example of what I see whenever this occurs.

I have tried in different hosting devices (Lenovo laptop, NUC Pro 12) and a newer version of the modems (SIMCOM SIM8380G-M2), and the problem persists. The behavior is quite erratic, I do not always see the error described above happening, but it is also appearing often enough so that it affects my measurements.

Output from ip monitor:

Deleted 4: wwan1    inet 100.77.182.96/26 brd 100.77.182.127 scope global noprefixroute wwan1

       valid_lft forever preferred_lft forever

Deleted broadcast 100.77.182.127 dev wwan1 table local proto kernel scope link src 100.77.182.96

Deleted local 100.77.182.96 dev wwan1 table local proto kernel scope host src 100.77.182.96

Deleted 0.0.0.0 dev wwan1 lladdr 08 NOARP

4: wwan1: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc fq_codel state DOWN group default

    link/none

Deleted 0.0.0.0 dev wwan1 lladdr 08 NOARP

Deleted inet wwan1

Deleted inet6 wwan1

Deleted 4: wwan1: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN group default

    link/none

inet wwan1 forwarding on rp_filter loose mc_forwarding off proxy_neigh off ignore_routes_with_linkdown off

inet6 wwan1 forwarding off mc_forwarding off proxy_neigh off ignore_routes_with_linkdown off

8: wwan1: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default

    link/ether b2:8e:24:1d:25:20 brd ff:ff:ff:ff:ff:ff

Deleted inet wwan1

Deleted inet6 wwan1

Deleted 8: wwan1: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default

    link/ether b2:8e:24:1d:25:20 brd ff:ff:ff:ff:ff:ff

inet wwan1 forwarding on rp_filter loose mc_forwarding off proxy_neigh off ignore_routes_with_linkdown off

inet6 wwan1 forwarding off mc_forwarding off proxy_neigh off ignore_routes_with_linkdown off

Output from dmesg, where it seems that the modem is physically disconnected?:
[Mon Aug 14 10:21:34 2023] qmi_wwan 1-3.2:1.5 wwan1: register 'qmi_wwan' at usb-0000:00:14.0-3.2, WWAN/QMI device, b2:8e:24:1d:25:20
[Mon Aug 14 10:21:36 2023] usb 1-3.2: USB disconnect, device number 7
[Mon Aug 14 10:21:36 2023] option1 ttyUSB6: GSM modem (1-port) converter now disconnected from ttyUSB6
[Mon Aug 14 10:21:36 2023] option 1-3.2:1.0: device disconnected
[Mon Aug 14 10:21:36 2023] option1 ttyUSB7: GSM modem (1-port) converter now disconnected from ttyUSB7
[Mon Aug 14 10:21:36 2023] option 1-3.2:1.1: device disconnected
[Mon Aug 14 10:21:36 2023] option1 ttyUSB8: GSM modem (1-port) converter now disconnected from ttyUSB8
[Mon Aug 14 10:21:36 2023] option 1-3.2:1.2: device disconnected
[Mon Aug 14 10:21:36 2023] option1 ttyUSB9: GSM modem (1-port) converter now disconnected from ttyUSB9
[Mon Aug 14 10:21:36 2023] option 1-3.2:1.3: device disconnected
[Mon Aug 14 10:21:36 2023] option1 ttyUSB10: GSM modem (1-port) converter now disconnected from ttyUSB10
[Mon Aug 14 10:21:36 2023] option 1-3.2:1.4: device disconnected
[Mon Aug 14 10:21:36 2023] qmi_wwan 1-3.2:1.5 wwan1: unregister 'qmi_wwan' usb-0000:00:14.0-3.2, WWAN/QMI device
[Mon Aug 14 10:21:45 2023] qmi_wwan 1-3.1:1.5: nonzero urb status received: -EPIPE
[Mon Aug 14 10:21:58 2023] usb 1-3.2: new high-speed USB device number 8 using xhci_hcd
[Mon Aug 14 10:21:58 2023] usb 1-3.2: New USB device found, idVendor=1e0e, idProduct=9001, bcdDevice= 5.04
[Mon Aug 14 10:21:58 2023] usb 1-3.2: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[Mon Aug 14 10:21:58 2023] usb 1-3.2: Product: SDXLEMUR-SD-MTP _SN:AE2F01F4
[Mon Aug 14 10:21:58 2023] usb 1-3.2: Manufacturer: SIMCOM
[Mon Aug 14 10:21:58 2023] option 1-3.2:1.0: GSM modem (1-port) converter detected
[Mon Aug 14 10:21:58 2023] usb 1-3.2: GSM modem (1-port) converter now attached to ttyUSB6
[Mon Aug 14 10:21:58 2023] option 1-3.2:1.1: GSM modem (1-port) converter detected
[Mon Aug 14 10:21:58 2023] usb 1-3.2: GSM modem (1-port) converter now attached to ttyUSB7
[Mon Aug 14 10:21:58 2023] option 1-3.2:1.2: GSM modem (1-port) converter detected
[Mon Aug 14 10:21:58 2023] usb 1-3.2: GSM modem (1-port) converter now attached to ttyUSB8
[Mon Aug 14 10:21:58 2023] option 1-3.2:1.3: GSM modem (1-port) converter detected
[Mon Aug 14 10:21:58 2023] usb 1-3.2: GSM modem (1-port) converter now attached to ttyUSB9
[Mon Aug 14 10:21:58 2023] option 1-3.2:1.4: GSM modem (1-port) converter detected
[Mon Aug 14 10:21:58 2023] usb 1-3.2: GSM modem (1-port) converter now attached to ttyUSB10
[Mon Aug 14 10:21:58 2023] qmi_wwan 1-3.2:1.5: cdc-wdm1: USB WDM device
[Mon Aug 14 10:21:58 2023] qmi_wwan 1-3.2:1.5 wwan1: register 'qmi_wwan' at usb-0000:00:14.0-3.2, WWAN/QMI device,

Modems monitoring shows the modem being removed and added again with a different argument/number to access it from the cli:
    /org/freedesktop/ModemManager1/Modem/1 [QUALCOMM INCORPORATED] 0
    /org/freedesktop/ModemManager1/Modem/0 [QUALCOMM INCORPORATED] 0

(-) /org/freedesktop/ModemManager1/Modem/0 [QUALCOMM INCORPORATED] 0
(+) /org/freedesktop/ModemManager1/Modem/3 [QUALCOMM INCORPORATED] 0

Has anyone experienced this behavior before and have an idea of what could it be due to? And if so, is there a solution?

Thanks a lot in advance.

Best Regards,
Melisa
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