NB-IoT is configured, but LTE-M is selected

[Bard]

Hi

I have two custom build boards based on nRF9160-SICA. Both boards also have a nrf52840 running application code. The nr91's are running identical firmware (slightly modified serial_lte_modem sample).

Both boards have the same config (see below), but one connect using NB-IoT and one with LTE-M. This is verified with at+cereg? giving "9 – E-UTRAN NB-S1" and "7 – E-UTRAN" respectively. Even when CONFIG_LTE_NETWORK_USE_FALLBACK is set to n, this happens. Both modems are running modem firmware ver.1.2.0. Why the difference? Could it be that some configuration in the eSIM's override the configuration? eSIM's are not from same batch, but from same operator.

CONFIG_LTE_LINK_CONTROL=y
# CONFIG_LTE_AUTO_INIT_AND_CONNECT is not set
# CONFIG_LTE_LOCK_BANDS is not set
# CONFIG_LTE_LOCK_PLMN is not set
# CONFIG_LTE_UNLOCK_PLMN is not set
CONFIG_LTE_PSM_REQ_RPTAU="00000011"
CONFIG_LTE_PSM_REQ_RAT="00100001"
# CONFIG_LTE_EDRX_REQ is not set
CONFIG_LTE_EDRX_REQ_VALUE="1001"
# CONFIG_LTE_LEGACY_PCO_MODE is not set
# CONFIG_LTE_PDP_CMD is not set
# CONFIG_LTE_PDN_AUTH_CMD is not set
# CONFIG_LTE_NETWORK_MODE_LTE_M is not set
# CONFIG_LTE_NETWORK_MODE_LTE_M_GPS is not set
CONFIG_LTE_NETWORK_MODE_NBIOT=y
# CONFIG_LTE_NETWORK_MODE_NBIOT_GPS is not set
CONFIG_LTE_NETWORK_USE_FALLBACK=y
CONFIG_LTE_NETWORK_TIMEOUT=600
# CONFIG_LTE_LINK_CONTROL_LOG_LEVEL_OFF is not set
# CONFIG_LTE_LINK_CONTROL_LOG_LEVEL_ERR is not set
# CONFIG_LTE_LINK_CONTROL_LOG_LEVEL_WRN is not set
CONFIG_LTE_LINK_CONTROL_LOG_LEVEL_INF=y
# CONFIG_LTE_LINK_CONTROL_LOG_LEVEL_DBG is not set
CONFIG_LTE_LINK_CONTROL_LOG_LEVEL=3

[Heidi]

Hi!

The AcT value from +CEREG (9 – E-UTRAN NB-S1 and 7 – E-UTRAN) just indicates the access technology of the serving cell the UE is registered to. So I suppose the two devices are registering to different cells, where only one of the cells supports NB-IoT. 

To check which protocol the UE is using, you should read out AT%XSYSTEMMODE. 

Best regards,

Heidi

[Bard]

Both devices are at the same location, and the testing is done simultaneously with identical firmware. But one device ends up with %XSYSTEMMODE:0,1,0,0 and the other one with  %XSYSTEMMODE:1,0,0,0. The only significant difference is the eSIM from different batches.

Both devices and up at channel (20), with same <plmn>  -  Mobile Country Code (MCC) and Mobile Network Code (MNC).

But with different <tac> Tracking Area Code (TAC).

I tried to force the LTE-M device to use NB-IoT

AT+CFUN=4

AT%XSYSTEMMODE=0,1,0,0

AT+CFUN=1

But then CEREG returned <stat> = 4 – Unknown (e.g. out of E-UTRAN coverage), although the other device is well within coverage.

So, I'm still confused, why are not both devices on NB-IoT?

[Heidi]

Devices in the same location can actually register with different cells, if there are two towers with a similar distance from the UE.

Is the cell ID (<ci>) from the two devices different?

I suspect they are registering two different cells where one of the cells does not support NB-IoT and CONFIG_LTE_NETWORK_USE_FALLBACK is causing that device to switch SYSTEMODE to LTE-M.

To confirm, could you disable that config and see if the SYSTEMMODE still changes to LTE-M? 

[Bard]

Hi

Yes, different cell ID's are shown. But always the same on the same device. As I said in my first posting: Even when CONFIG_LTE_NETWORK_USE_FALLBACK is set to n, the device ends up in LTE-M mode. Anyway, shouldn't the device search for a cell that supports NB-IoT? I know there is one, since the other device connects to it.

[Heidi]

Oh, sorry, I missed that. Yes, you're right. It should. 

I'm not sure what's going on, but could you take a modem trace and I'll have a look. Maybe that will give us some more information.

What network provider are your SIM cards from?

[Bard]

is it possible to take modem trace on custom board? I have a serial port to a 52840 with NUS, sending all data over BLE. But no direct serial port to nrf91. By the way, the link over seems to not point to modem trace info.

eSIM fromTelenor

[Heidi]

Hi, I updated the link. Yes, it is possible to take a modem trace with a custom board but I'm not sure if you will need external hardware for that. I'll get back to you when I know more. 

[Heidi]

Hi again, R&D just commented on the case. 

I see you mentioned it in your post but just to confirm: both HWs are SICA (triple mode capable HWs)?

[Bard]

yes, both are SICA

[Heidi]

Hi, sorry about the delay. They're taking a look at your case right now, and I will let you know as soon as I have an answer for you. 

[Heidi]

Hi again! They are requesting the modem response when you send "AT+CEREG=3". Please also set the CONFIG_LTE_NETWORK_USE_FALLBACK to n before doing this and for any future testing they request. 

And for the modem trace, it should be the same procedure as from the DK. Do you have access to UART1 on the device? And you might need to unselect the "auto device/port filter" in the trace collector application. 

The modem does not change the system mode by itself nor due to any SIM configuration. So the question still open is that one of the HW cannot find the NB cell? Unless modem logs are provided (AT command log might help too), we cannot be sure. There may be some reject from the network.

Best regards,

Heidi