Serial LTE Modem connects, then immediately disconnects

[tewii]

Hi all,

I've been playing around with the Nordic cellular boards (nRF9151, nRF9161, Thingy 91) and keep running into the same issue.

I'm using the Serial LTE Modem application on the nRF9151 and attempting to connect via LTE-M. The below shows the AT commands I send, subscribing to some notifications and then setting the modem into full functionality.

It seems that I connect (+CEREG: 5,...) but then the connection immediately drops and goes back to searching (+CEREG: 2,...). The second +CEREG: 2 is the last message that gets printed to the terminal:

Ready
> AT+CEREG=5

OK
> AT+CSCON=1

OK
> AT%XSYSTEMMODE=1,0,1,0

OK
> AT+CFUN=1

OK

+CEREG: 2,"1012","07A1D76E",7

+CSCON: 1

+CEREG: 5,"1012","07A1D76E",7,,,"11100000","11100000"

+CEREG: 2,"1012","07A1D76E",7,0,15

+CSCON: 0

+CEREG: 2

I've tried reflashing the modem and application firmware, and have used both iBasis and Onomondo SIM cards and the same issue occurs repeatedly. My cellular connection on my phone is very strong in the area (Three UK), and I am sure the other available networks will have good coverage in my area.

Please could you provide assistance as to why my connection is immediate disconnecting, and how to ensure the connection is maintined?

Thanks for any help!

[tewii]

For additional context, when I run a modem trace using Cellular Monitor, the LTE Connection stage fails, stating that "Evaluation failed, not registered".

I've attached the modem trace here:
trace-2024-11-25T12-31-20.339Z.mtrace

[Øyvind]

Hello, 

From the terminal output that you attached we can see that your device is being rejected by the network with reject cause 15. 

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.

I know from earlier this year that the O2 network has tightened what SIM can roam on their network. iBasis has been very affected by this. Wireless Logic SIMs should work better in the UK. Did you receive one with the nRF9151 DK?

My cellular connection on my phone is very strong in the area (Three UK), and I am sure the other available networks will have good coverage in my area.

Please note that the nRF91 series supports LTE-M and NB-IoT only which means that it does not use the same technology as your phone. The GSMA Mobile IoT Deployment map provides some information on what operators that provides support in the UK. Currently Vodafone for NB-IoT and Telefonica for LTE-m. You could reach out to them as well. 
You could also test with the Three UK SIM card in the nRF91 SIM slot to see what coverage you get.

Kind regards,
Øyvind

 

[tewii]

Hi Øyvind,

Thanks for the prompt reply!

Thanks for elaborating on the reject cause. Please could you provide the source for that cause description? It would be helpful for further debugging if new issues pop up.

I tried with the Wireless Logic Connexa SIM card and I get a similar issue: a rejected conenction with cause 15. Only this time, there is no CEREG: 5 to indicate that the modem connected in the first place:

Ready
> AT+CEREG=5

OK
> AT+CSCON=1

OK
> AT+CFUN=1

OK

+CEREG: 4

+CEREG: 2,"1012","07CA386E",7

+CEREG: 2,"1012","07A54B6E",7

+CSCON: 1

+CEREG: 2,"1012","07A54B6E",7,0,15

+CSCON: 0

+CEREG: 2

[Øyvind]

tewii said:

Please could you provide the source for that cause description? It would be helpful for further debugging if new issues pop up.

Of course. This is found in our AT command documentation for CEREG read command, Here it states:  EPS Mobility Management (EMM) cause value. See 3GPP TS 24.301 Annex A

Or you can use this webpage: https://telecompedia.net/lte-release-causes/

tewii said:

I tried with the Wireless Logic Connexa SIM card and I get a similar issue: a rejected conenction with cause 15. Only this time, there is no CEREG: 5 to indicate that the modem connected in the first place:

That is very strange. I fear that LTE-M is not supported very well, i.e. you will need to configure NB-IoT using XSYSTEMMODE e.g. AT%XSYSTEMMODE=0,1,1,0

Kind regards,
Øyvind

[tewii]

Thanks for the sources, they've already proved useful. I switched to NB-IoT as you advised and am now getting the following:

+CEREG: 2,"1BEE","0026E915",9

+CSCON: 1

+CEREG: 2,"1BEE","0026E915",9,0,13

+CSCON: 0

+CEREG: 2

From the list of reject codes, it looks like I'm now getting:

Cause #13 – Roaming not allowed in this tracking area

This EMM cause is sent to an UE which requests service, or if the network initiates a detach request, in a tracking area of a PLMN which by subscription offers roaming to that UE but not in that tracking area.