nRF9151 stuck in disconnected, unable to reconnect to LTE-M

[teba99]

Hi,

We are testing connection resilience on our nRF9151 product. Generally, LTE-M connectivity (Germany, on Telefonica and/or Telekom) works fine.

However, under certain circumstances the nRF9151 appears unable to reconnect to the network. The only way to get it reconnect is with a full reboot.

One situation is when the SIM is briefly inactive for whatever reason (for example, an outage on the carrier side or because the network assignments change and the SIM is reset). The nRF9151 will loose connectivity immediately (as expected) and will try to connect later, but fails forever with "<err> modem_info_params: Link data not obtained: 5 -5". Only a reboot resolves it, after which the nRF9151 will immediately reconnect (no change on the SIM/network side).

In the same situation, we've also seen the modem actually re-attach to the network (as seen from the signallng logs) but the nRF9151 network stack apparently fails to establish a PDN session repeatedly with "nrf_cloud_transport: Could not connect to nRF Cloud MQTT Broker mqtt.nrfcloud.com, port: 45858. err: -111". Here also after a reboot it immediately reconnects. 

I understand the FPLMN List may come into play here, but I've also tested this by adding a new LTE-M network to the SIM that the device has not seen since the reboot, so it should not be in the FPLMN. But the nRF9151 is still unable to connect even to that new network (until a reboot).

My interpretation is that once the nRF9151 has disconnected it gets into a state where some part of the stack (modem, network stack, link manager) is stuck and fails to reconnect even though it could (SIM is active and suitable networks are available). A reboot resolves that state.

Unfortunately, a reboot is not a good option for us because we would then loose the data queued in RAM.  

Note: We are using the nRF9151 with the latest modem FW and nRF SDK, the application is based on the multi service sample.

Is this a known issue? Can I provide more information to help you analyse it? Do you have recommendations to deal with this situation?

Thanks much!

[Menon]

Hello,

Thank you for sharing the details. I will need to investigate this issue internally. Please collect the modem trace and share it with us, and also share any logs (if available) that might help narrow down the issue.

Kind regards,

Abhijith

[teba99]

Hi Menon ,

Ok, will get modem traces but it may be 1-2 weeks.

[teba99]

Hi Menon ,

In the meantime, I am also seeing inability to reconnect even in the case of a simple connection loss due to poor signal.

The device starts up and connects right away without problems. Later, I add 30 dB attenuation to the antenna path to simulate a poor connection. The device disconnects after a few seconds, as expected. Then I remove the attenuation and I would expect the device to reconnect because the signal is now good again. However, the device is unable to reconnect. The logs I get:

[00:00:47.145,690] <inf> app_cloud_connection: Disconnected from nRF Cloud

[00:00:47.145,690] <inf> app_cloud_connection: Retrying in 30 seconds...

[00:01:17.145,812] <inf> app_cloud_connection: Waiting for network ready...

[00:01:17.253,692] <err> modem_info_params: Link data not obtained: 5 -5

[00:01:17.253,723] <wrn> app_message_sender: Could not obtain information from modem, error: -5

[00:01:17.254,486] <inf> app_cloud_connection: Could not send a device message - no connection

[00:01:25.806,854] <wrn> lte_lc: Registration rejected, EMM cause: 9, Cell ID: 23438850, Tracking area: 59672, LTE mode: 7

This continues for a long time (many minutes) and the device only connects again after a reboot. Note that I made no changes on the network side and the SIM is always active and the network should be accessible. 

Again, this looks like the connectivity stack get stuck in some state after it looses the connection the first time and then is unable to reconnect by itself.

I will work on getting modem traces.