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

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

  • 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.

  • Hello

    Thank you for contacting DevZone at NordicSemi.

    Are you using: LTE_LC_SYSTEM_MODE_PREFER_NBIOT?
    Which actually prefers the NBIOT network whenever it is available, and as such it appears (in your case) the network is available but not accepting.

    How you are handling connections to the network in your application?
    Should that kind of situation not be handled in your application? Once your application knows (lets say N times) a connection is rejected then it should switch the network. We have LTE Link Control library that provides such functionality. You may check the network-registration status, and based on that handle to switch the network in you application.


    Are you using LTE_MODE_PREFERENCE_NBIOT_PLMN_PRIO config? This should do the job as per your case. It will make NBIOT as a preference, but will switch to LTEM if NBIOT is not available. You can try this as well, but once again, the issue could be not the unavailability of the network.

    You can also use the connection fallback mode, which will connect to the other network after defined timeout.

    Maybe changing operator or talking to them regarding roaming might also help solve the issue. 

    With regards, 

    Naeem

  • Hello Naeem,

    thank you for the explanation.

    However, it is not clear to me why LTE_MODE_PREFERENCE_NBIOT_PLMN_PRIO should help to solve the issue. Can you please better elaborate on this?

    Thanks much,

    Marco

  • The point is, you need to now your SIM card and the configuration stored in it.

    Some SIM cards came with a list of "Home Networks" and or "Priorized Networks".

    e.g. one of my SIM card has

    CRSM eq. home plmn: 23410,310041,50501,45400,20408,26201,26003,21403,20801
    CRSM home plmn sel: 23410,310041,50501,45400,20408,26201,26003,21403,20801
    CRSM operator plmn sel: 23455,42505,42503,310011,23203,20610,28401,21910,28010,23003,23801,24801,24491,20205,21630

    but other SIM cards don't use that lists.

    If you select now  LTE_MODE_PREFERENCE_????_PLMN_PRIO then the modem first checks to find an network in that list. In my case, the 26201. And then it tries to use LTE-M or NB-IoT with that network.

    +CEREG: 2,"67B9","01CC2B00",7
    I 95.141 : iccid: 8944477100000445083F (new)
    I 95.141 : imsi: 204047795921998
    +CEREG: 2,"D325","01CC2B0C",9
    +CEREG: 2,"D325","01CC2B0C",9,0,15
    +CEREG: 2
    +CEREG: 2,"D325","01CC2B0C",9
    I 16.211 : iccid: 8944477100000445083F
    I 16.212 : multi-imsi: 208090063613998 (204047795921998, 121 seconds)
    +CEREG: 2,"D325","01CC2B0C",9,0,15
    +CEREG: 2
    I 45.322 : Modem connects for 30 s of 300 s(multi imsi)
    I 75.324 : Modem connects for 60 s of 300 s(multi imsi)
    I 05.327 : Modem connects for 90 s of 300 s(multi imsi)
    I 35.329 : Modem connects for 120 s of 300 s(multi imsi)
    +CEREG: 2,"D325","01CC2B0E",9
    I 37.370 : iccid: 8944477100000445083F
    I 37.370 : multi-imsi: 204047795921998 (208090063613998, 121 seconds)
    +CEREG: 2,"D325","01CC2B0E",9,0,15
    +CEREG: 2
    I 65.260 : Modem connects for 150 s of 300 s(multi imsi)
    I 95.262 : Modem connects for 180 s of 300 s(multi imsi)
    I 25.265 : Modem connects for 210 s of 300 s(multi imsi)
    I 55.268 : Modem connects for 240 s of 300 s(multi imsi)
    +CEREG: 2,"D325","01CC2B0C",9
    I 57.878 : iccid: 8944477100000445083F
    I 57.879 : multi-imsi: 208090063613998 (204047795921998, 120 seconds)
    +CEREG: 2,"D325","01CC2B0C",9,0,15
    +CEREG: 2
    I 85.270 : Modem connects for 270 s of 300 s(multi imsi)
    +CEREG: 2,"D325","01CC2B0C",9
    I 77.292 : iccid: 8944477100000445083F
    I 77.292 : multi-imsi: 204047795921998 (208090063613998, 120 seconds)
    +CEREG: 2,"D325","01CC2B0C",9,0,15
    +CEREG: 2
    I 06.470 : Modem connects for 30 s of 300 s(multi imsi)
    I 36.401 : Modem connects for 60 s of 300 s(multi imsi)
    I 66.403 : Modem connects for 90 s of 300 s(multi imsi)
    I 96.406 : Modem connects for 120 s of 300 s(multi imsi)
    +CEREG: 2,"D325","01CC2B0C",9
    I 98.545 : iccid: 8944477100000445083F
    I 98.546 : multi-imsi: 208090063613998 (204047795921998, 120 seconds)
    +CEREG: 2,"D325","01CC2B0C",9,0,15
    +CEREG: 2
    I 26.408 : Modem connects for 150 s of 300 s(multi imsi)
    I 56.339 : Modem connects for 180 s of 300 s(multi imsi)
    I 86.342 : Modem connects for 210 s of 300 s(multi imsi)
    I 16.344 : Modem connects for 240 s of 300 s(multi imsi)
    +CEREG: 2,"D325","01CC2B0C",9
    I 18.959 : iccid: 8944477100000445083F
    I 18.959 : multi-imsi: 204047795921998 (208090063613998, 120 seconds)
    +CEREG: 2,"D325","01CC2B0C",9,0,15
    +CEREG: 2
    I 46.347 : Modem connects for 270 s of 300 s(multi imsi)
    +CEREG: 2,"67B9","01CC2B06",7
    I 79.723 : iccid: 8944477100000445083F
    I 79.723 : multi-imsi: 204047795921998 (208090063613998, 120 seconds)

    But as you can see, it still tries to search for one RAT quite long, even if only cells of a provider from that list are tested and not all. In my case, the SIM card then changes the IMSI after a time without network registration. And that causes again to search for the wrong RAT. And finally, it tries with the other RAT and has success. So in my experience, even using ???_PLMN_PRIO stick more to RAT than a user would assume.

  • Thanks Achim for this information, this is very interesting.

    I will check the settings of my SIM cards here.

    By the way, a little OT I need to understand:

    • how are you extracting home PLMN list?
    • how can you know the provider from the CEREG notification, which is just telling TAC and CID?

    Thanks much!!

    Marco

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