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,

Could you share some more details, such as the nRF Connect SDK version and the modem firmware version? Are there any logs or error messages that could provide more information about the issue? Would you be able to collect the modem traces and share them here?

Kind regards,
Abhijith

[teba99]

Hi Menon 

Thanks for the reply. Build info below. Right now we only have UART debug logs with error messages as per above as well as SIM signaling logs from the carrier.

Modem traces are a good idea, we'll try to create those.

Thanks,
-- Terrence

 

nRF Connect SDK v2.9.0

Using Zephyr OS v3.7.99

Modem mfw_nrf91x1_2.0.2

CONFIG_NETWORKING=y

CONFIG_NET_SOCKETS=y

CONFIG_NET_SOCKETS_OFFLOAD=y

CONFIG_NET_MGMT_EVENT_STACK_SIZE=2048

CONFIG_NET_CONNECTION_MANAGER_MONITOR_STACK_SIZE=1024

CONFIG_NET_IPV4=y

CONFIG_NET_IPV6=y

CONFIG_NET_IPV6_NBR_CACHE=n

CONFIG_NET_IPV6_MLD=n

CONFIG_NET_CONNECTION_MANAGER=y

CONFIG_NRF_MODEM_LIB_NET_IF=y

CONFIG_NRF_MODEM_LIB_NET_IF_AUTO_DOWN=y

CONFIG_NRF_MODEM_LIB_NET_IF_DOWN_DEFAULT_LTE_DISCONNECT=y

CONFIG_LTE_PSM_REQ=n

CONFIG_NRF_MODEM_LIB=y

CONFIG_NRF_CLOUD_MQTT=y # This also enables FOTA by implicitly setting CONFIG_NRF_CLOUD_FOTA=y

CONFIG_NRF_CLOUD_MQTT_SHADOW_TRANSFORMS=y

CONFIG_MQTT_KEEPALIVE=120

CONFIG_LTE_LINK_CONTROL=y

CONFIG_SETTINGS=y

CONFIG_SETTINGS_FCB=y

CONFIG_FCB=y