Difference between config options CONFIG_LTE_MODE_PREFERENCE_LTE_M_PLMN_PRIO and CONFIG_LTE_MODE_PREFERENCE_LTE_M?

[leo_nam]

Hello,

I want my nrf9160DK to use LTE-M, but when that is not available to switch over to NB-IoT.

I have enabled both in my prj.conf using:

CONFIG_LTE_NETWORK_MODE_LTE_M_NBIOT=y

However I don't know which CONFIG_LTE_MODE_PREFERENCE_.... to use because I don't know what their description means, specifically what PLMN is and how it plays a part in the preference of LTE-M over NB-IoT or vice versa. 

CONFIG_LTE_MODE_PREFERENCE_LTE_M
      LTE-M is preferred over PLMN selection. The modem will prioritize to
      use LTE-M and switch over to a PLMN where LTE-M is available whenever
      possible.

Does this mean connecting to a LTE-M network is more important than signal strength of cellular networks in the area? 

CONFIG_LTE_MODE_PREFERENCE_LTE_M_PLMN_PRIO
      LTE-M is preferred, but PLMN selection is more important. The modem
      will prioritize to stay on home network and switch over to NB-IoT
      if LTE-M is not available.

Does this mean connecting to a stronger cellular network (whether it be LTE-M or NB-IoT) is more important than connecting to a network with LTE-M?

Why not just have a single config option for preferring LTE-M, or NB-IoT, over the other in terms of preference when both modes are supported in the application? Does it have to do with the fact that there is some network areas have overlap and provide both LTE-M and NB-IoT? 

Thank you

[helsing]

Hi Leo, sorry for the delay.

Here is an explanation from another case, Question about #XSYSTEMMODE behavior:

Charlie said:

I get more explanation about the system mode preference settings from our modem team.

In system mode 1 & 2 (i.e. one system preferred over the other regardless the networks available in the area) the preferred system will be always prioritized. For example, in system mode 1 LTE-M will always be used in case any suitable network is available. If there is no suitable network available, only then modem starts to search for network in NB-IoT. System modes 3 & 4 behave differently: PLMN selection is prioritized over the system. In this case, for example, when modem starts to search for PLMN A it will search it from both systems before searching PLMN B.

In summary: System modes 1 & 2 make sure that the device uses always the preferred system when possible, whereas system modes 3 & 4 make sure that device is always in the highest priority network.

PLMN is short for Public Land Mobile Network (PLMN).

The source code for the various Kconfigs, CONFIG_LTE_NETWOR_MODE_, that you mention, can be found in nrf\lib\lte_link_control\lte_lc.c.

From the source code you may see that the Kconfigs set the supported mode and priority using AT%XSYSTEMMODE, as described in the AT commands reference guide. The guide might give you a better understanding of the priority.

You may also fire this AT command manually, however, the command is only allowed before activating the modem.

For example, AT%XSYSTEMMODE=1,1,1,1 enables both LTE-M and NB-Iot while LTE-M is the preferred mode.

The description of the <LTE_preference> parameter of the XSYSTEMMODE command might help you understand the priority as the AT manual is using other words than the Kconfig reference manual.

Please let me know if things are still unclear.

[leo_nam]

Hello it cleared some things for me but I still have one more thing I'm not clear about.

Preference between LTE-M or NB-IoT is manually set by the user. 

With PLMN selection, where do the network priorities come from? Are these priorities already preset in the SIM card? 

Thank You

Edit: Yes the priorities are preset in the SIM.