nRF9160 does not select between NB-IoT and LTE-M

[philrittenhouse]

Hi,

We are testing in two cellular service regions, one with LTE-M support and one with NB-IoT support. Following the instructions from this ticket devzone.nordicsemi.com/.../nrf9160-confirming-support-of-both-nb-iot-and-lte-m-in-same-firmwares-without-reflashing we are able to connect to the local cellular network in each region.   The problem is that it only works if we enable only one of LTE-M or NB-IoT at a time.

That is  AT%XSYSTEMMODE=1,1,1,0 does not work (can't get LTE connection status green in the LTE Link Monitor) in either region but AT%XSYSTEMMODE=0,1,1,0 or AT%XSYSTEMMODE=1,0,1,0 does work (in the corresponding region)

Since we need to support both networks and we would prefer not to have two different products with different firmware, this is a problem.  Has anyone else run into this?   Is there something we are doing wrong?

We are running modem f/w version 1.3.3

So far we have only verified this issue using the AT commands.   I assume we would have the same problem with the CONFIG_LTE_NETWORK_MODE_LTE_M_NBIOT_GPS option.   Is it possible to switch between LTE-M only and NB-IoT only options at run time instead of compile time?

Thanks,
Phil

[Achim Kraus]

philrittenhouse said:

The problem is that it only works if we enable only one of LTE-M or NB-IoT at a time.

In my experience, that may be caused by multi-IMSI cards with short auto-switching timeout. Here in south Germany it takes a little more than about 2 minute to search for a network in NB-IoT or LTE-M. If both are enabled, the modem switches after that 2 minutes to the other technology and search for a network again. If now a auto Multi-IMSI is used, that may interfere with that. The SIM card may switch the IMSI after 2 minutes and cause the modem to restart the network search. It never switches to the other technology or didn't search long enough with that. The timeouts of these Multi-IMSI cards differs, in my experience from 2 to 10 minutes. And some SIM card provider also start to provide multi IMSI cards with and additional "imsi select" mode.

[philrittenhouse]

Thanks Achim!

Can you share your work-around for the problem?   Have you written f/w for the nRF9160 that is able to detect the failure and switch modes automatically?   Do you send your own AT commands or is there an API to do it?

Best regards

[Achim Kraus]

I guess, the first point will be to clarify, if your SIM card is really a multi IMSI card.

I developed a CoAP/DTLS 1.2 CID client in order to demonstrate the "power (saving)" of RFC 9146. That client is available at Zephyr - Coaps Demo Client with Eclipse/TinyDtls . The specific function is not isolated, so it will not help too much. I use a lot of API calls and a "hand full AT commands", especially to read more data from the SIM card.

You may start with implementing to read the IMSI "frequently" and see, if that is changing. If so, I usually stop a unsuccessful network search after 5 minutes and revert the preference of NB-IoT and LTE-M.

To be frank, I also replaced in the meantime this auto Multi-IMSI SIM cards by SIM cards with an IMSI select mode. That works in my experience just more smooth.