Force Network Re-registration

[tmihajloski]

Hi,

I am using the nRF9160 with a multi IMSI SIM card in the US. The SIM supports the 3 major cell operators in the US. I have a case where the modem registers to a network successfully, however, the performance is sub-optimal due to poor reception, which is manifested as lost TCP packets, after retries. If I force manual operator selection I can switch to a different operator that has better reception in my location and the TCP issues are resolved.

How does the automatic operator selection decide which operator to connect to?

Is there a way to force a re-registration?

[Charlie]

Hi tmihajloski,

The IMSI change should and have to be controlled by the multi IMSIs SIM card. The SIM card will switch its SIM profile (IMSI) and send a UICC reset from the SIM to the nRF9160 modem. 

The multi-IMSI SIM provider should provide a platform for IMSI management. The vender EMnify has a description of the process How changing IMSIs works. Please try to check with your SIM card provider.

Best regards,

Charlie

[tmihajloski]

Thanks!

I have access to the IMSI management platform. The case I am working on is on a single IMSI, which is allowed to roam on multiple networks. In this case does the modem have a prioritization scheme or is it still up to the SIM to decide which network it connects to?

[Charlie]

Hi Todor,

Yes, it is still mostly decided by the SIM card IMSI profile, and it provides an OPLMN list with priority order which is decided by the home network MNO. If the home network is not available, it will try to connect to the next preferred network. For the device side, signal strength only comes into play when the modem is choosing which cell to connect to. It will always try the strongest cell of the PLMN it tries to connect to. There are limited SIM cards on the market that offer UPLMN, see How and Why You Can Set Super SIM’s Network Attach Priority List | Twilio. You need to check with your SIM card provider if UPLMN is supported.

Update the UPLMN list is done with AT+CRSM command which can also be found from the nRF9160 AT command guide:

Syntax: AT+CRSM=<command>[,<fileid>[,<P1>,<P2>,<P3>[,<data>[,<pathid>]]]]

Length of every record is 5 bytes. Example of updating the first record in the UPLMN list:

Writing command:
AT+CRSM=214,28512,0,0,5,"65F7208000"

Where:
<command>: UPDATE BINARY = 214,
<fileid>: EF_PLMNwAcT = 28512,
<P1/P2>: Start offset = 0,
<P3>: Bytes to write = 5,
<data>: PLMN ID: 567/02, Access Technology: EUTRAN (M1 & NB) = (mcc2mcc1 mnc3mcc3 mnc2mnc1 acc1 acc2) = "65F7208000" . 'F' is used for mnc3 in case MNC has only 2 digits.

Response:
+CRSM: 144,0,""
OK

which means "OK, no response data"

You can confirm the writing as follows:

AT+CRSM=176,28512,0,0,5
OK

Response:

+CRSM: 144,0,"65F7208000"
OK

A network may have several PLMN IDs so you may want to add more than one PLMN ID to the UPLMN list. This can be done by modifying the start offset and bytes to be written. Please see more details in the AT command guide.

And when the modem enters some unwanted network, you can send AT+COPS=0 after which the modem will perform a new PLMN selection.

Best regards,

Charlie

[tmihajloski]

Thanks! This is what I was looking for.

[Eric Jager]

I am running in exactly the same issue with our roaming SIM, I added the list of UPLMN's in the SIM, but the NRF9160 keeps reconnecting to the previous registered PLMN. How can this RPLMN be reset?