nRF9160 connectivity & configuration help

[JonathanF]

Hello,

We are working to improve our connectivity globally, and have some questions about how we might improve our firmware.

We are using the nRF9160 to connect via LTE-M (CAT-M1) using a SIM that allows us to roam across multiple networks. This SIM has a feature that allows it to rotate through several profiles in order to find a network that we can connect to.

The initialization of the nRF9160 modem (FW 1.2.0) is done via the following AT commands:

• AT%CESQ=1
• AT+CPSMS=1,,,"01100000","00000000"
• AT+CSCON=3
• AT+CNEC=24
• AT%XSYSTEMMODE=1,0,0,0
• AT+CGDCONT=0,"IP","[APN]"
• AT+CEREG=5
• AT+CFUN=1

Our questions are:

1. What should we do about +CEREG status = 4 (“Unknown”)?
   Whilst the modem is searching and attempting to connect to a network (+CEREG: 2), we occasionally see +CEREG status messages indicating “Unknown” status.
   Example message: +CEREG: 4,"FFFE","FFFFFFFF",7,0,15,"11100000","11100000"
   Our question is what to do in this case? Is this an error, meaning we should try again (maybe turn the modem off, then on again), or should we just let the modem continue to search? Our interpretation of this above message is that the network search has completed, but no available networks were found (reject_cause=15, indicates “No suitable cells in tracking area”). Will the modem continue to search? We’ve also seen reject_cause=11 (“PLMN not allowed”) and have similar questions.
2. Is it possible to do a factory reset for the modem?
   One thing that makes debugging difficult is that we aren’t sure what settings are stored/cached in the modem’s NVM. This makes it difficult to compare device behavior across multiple devices.
3. When should we transition into AT+CFUN=0 and when should we transition to AT+CFUN=4?
   Several of the AT commands state that settings are written to NVM when transitioning to CFUN=0. Is there guidance on when it’s best to transition to off vs. airplane mode? Our approach has been to transition to airplane mode when we wish to save power, but when an unpredictable reset happens (via a user event), we want to be able to reasonably quickly connect to the network again.
4. Why does it sometimes take about 60 seconds from CFUN=1 to the first status update message (example: %CESQ: 14,0,0,0 followed within 1 millisecond by +CEREG: 2,"0411","00EF090F",7,0,0,"11100000","11100000"). Is there something we can/should be doing to force the modem to begin searching immediately?

Any help would be appreciated!

Thank you,
Jonathan

[Momchil_MRP01]

I think we just reached resolution to this issue stated by JonathanF by having our SIM provided changing the IMSI rules affecting profile rotation and connection registartion using specific IMSI to a tower.

The suggestion by Heii-Irene was in fact the issue 

You're using a SIM card that has several profiles and is able to switch between those. What could be happening is that some of the location status events are not being forwarded from the modem to the SIM card and thus the SIM card is not able to perform a SIM refresh to change its profile and allow LTE registration. 

The SIM provide had rules setup for specific IMSI that caused Service Rejection (from Cell tower provider) rather than Home Rejection (from the SIM card provider). Service Rejection was then not translated to SIM card rotation and at that physcal location the device was only in search and connect to tower state but never registered to the provider or changed IMSIs.

Example message: +CEREG: 4,"FFFE","FFFFFFFF",7,0,15,"11100000","11100000"

Cause #15 – No suitable cells in tracking area

      This EMM cause is sent to the UE if it requests service, or if the network initiates a detach request, in a tracking area where the UE, by subscription, is not allowed to operate, but when it should find another allowed tracking area or location area in the same PLMN or an equivalent PLMN.