Configuration for - UDP over NB-IoT

Question

Dear community 
 I'm new to the nRF9160 and to cellular connectivity related functionality. 
 We're in the concept phase of a product that implements cellular cloud connectivity. The solution will finally be lauched in several countries all over the world, and therefore must be capable of communicating via LTE-M and NB-IoT. 
 I managed to install the "nRF9160: UDP" example and to successfully connect to a UDP custom server and to send some dummy data (via LTE-M). That's already a great start, but now, I would like to achieve the same result based on NB-IoT transport. So far I didn't find the right configuration to achieve this. 
 Here's my setup: 
 
 nRF9160-DK Board (PCA10090 1.0.1 2021.25) 
 nRF Connect SDK 2.1.0 
 nRF9160 SiP modem firmware 1.3.3 
 Example Software: According to https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/nrf/samples/nrf9160/udp/README.html 
 iBASIS eSIM card ("GLOBAL ACCESS FOR THINGS") 
 Running the test in central Switzerland (should support LTE-M and NB-IoT, to my knowledge)

In order to switch to NB-IoT I slightly changed the prj.conf as follows (the example doesn't mention any config for NB-IoT mode): 
 
 CONFIG_LTE_NETWORK_MODE_LTE_M =n 
 CONFIG_LTE_NETWORK_MODE_NBIOT =y 
 
 However, with this config change the system seems not able to establish a stable connection anymore (although it looks like initially a connection gets established). 
 The sample software produces the following output: 
 *** Booting Zephyr OS build 3afbbacd2ea2 *** UDP sample has started LTE cell changed: Cell ID: 16971977, Tracking area: 600 RRC mode: Connected RRC mode: Idle LTE cell changed: Cell ID: -1, Tracking area: -1 LTE cell changed: Cell ID: 2643304, Tracking area: 21110 RRC mode: Connected RRC mode: Idle LTE cell changed: Cell ID: -1, Tracking area: -1 
 (here the connection attempt seems to stuck...) 
 
 I would be happy for any hint's (or maybe examples) on how I need to tweak the configuration to successfully communicate via NB-IoT 
 Thanks a lot in advance :-)

Didrik Rokhaug · Accepted Answer

Hi, 
 It doesn't look like iBasis have roaming agreements for NB-IoT in Switzerland: 
 https://ibasis.com/solutions/iot-connectivity/network-coverage/ 
 iBasis eSIM International Coverage Update 
 What happens in the log is probably that it finds two different cells, tries to connect to them, but get rejected. 
 If you add CONFIG_LTE_LC_LOG_LEVEL_DBG=y to the prj.conf file, you should get more logs, to confirm that. 
 Best regards, 
 Didrik

Didrik Rokhaug · Answer

StefanL said: I followed your advice and get more Details now (but it's quite cryptic to me :-D ). It can be a bit difficult if you don't know the code or the AT commands. The interesting lines in this case is D: +CEREG notification: +CEREG: 2,"5276","00285568",9,0,15< >< >< > (and the one that is almost identical, but a different cell). The line tells me that it tried to connect to a cell, but got rejected with reject cause "15". The spec has this to say about 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. Again, not the easiest to understand, but it basically means that your subscription does not allow you to connect to the cell (but you might be allowed to connect somewhere else/other cells on the same network). Which is what we would expect when you are allowed to use LTE-M on the network, but not NB-IoT.

Configuration for - UDP over NB-IoT

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