in some conditions, device does not try to change RAT type to maximize chance of connection

[barbiz]

Hello,

we have developed an application using nrf9160.

We have set the nrf9160 as LTE-M + nbiot with preference for nbiot.

We have noticed that, in some countries, nbiot is present BUT operator is not allowing the radio to register into the network. This happens, for instance, because no roaming agreement is set.

In such condition, device attaches to multiple cells and repeatedly gets rejected. RAT type changes between 9 (nbiot) and 0 multiple times in a minute.

In this case, device is never attempting to connect to LTE-M even if the netowrk is present.

Can you please explain this behaviour?

Thanks much,

Marco

[Achim Kraus]

Do you use a multi-IMSI-SIM-card? With such SIM-cards I had also that experience. The SIM card observes the modem trying to register at mobile-networks. If that isn't successful for some time (in my experience it starts with SIM cards using 2 minutes, I have also some with 5 and 10 minutes), then the SIM-card changes the IMSI and the modem restarts the network search with that new IMSI.

nRF9160, mfw 1.3.5, PERIODICSEARCHCONF and automatically switching multi IMSI SIM cards

The strategy to overcome that, is to swap/switch the "LTE-M/NB-IoT" priority after sometime of trying to register without success.