Hello,
Hopefully this will be a quick(ish) one since I think I found the answer, but just need confirmation.
To preface (My setup):
- nRF9151DK
- Modem firmware: mfw_nrf91x1_2.0.2
- nRF Connect SDK / Toolchain: 2.9.0
- Terminal app: PuTTY
- Application: Custom app based on the HTTP Client sample
- Carrier: Verizon
In our device logs, I observe the following lines when using Verizon. Take note of the line in bold.
+CEREG: 1,"Redacted","Redacted",7,0,18,"11100000","11100000"
[00:00:06.579,101] <wrn> lte_lc: Registration rejected, EMM cause: 18, Cell ID: Redacted, Tracking area: Redacted, LTE mode: 7
01-01-1970 00:00:06.579 MODEM: (sysworkq) LTE Registration: Registered Home
Full log snippet below from our custom application:
*** Booting nRF Connect SDK v2.9.0-7787b2649840 *** *** Using Zephyr OS v3.7.99-1f8f3dc29142 *** 01-01-1970 00:00:00.399 HBEAT: (main) Reset reason(s): (RESETPIN) 01-01-1970 00:00:00.400 MODEM: (main) Bringing network interface up 01-01-1970 00:00:00.401 HBEAT: (logging) Starting new log file 01-01-1970 00:00:00.666 MODEM: (main) Provisioning TLS certificate 01-01-1970 00:00:00.715 MODEM: (main) TLS certificate match 01-01-1970 00:00:00.715 MODEM: (main) Connecting to the network +CEREG: 2,"Redacted","Redacted",7 01-01-1970 00:00:05.182 MODEM: (sysworkq) LTE Registration: Searching +CSCON: 1 01-01-1970 00:00:05.228 MODEM: (sysworkq) RRC mode: Connected +CEREG: 1,"Redacted","Redacted",7,0,18,"11100000","11100000" [00:00:06.579,101] <wrn> lte_lc: Registration rejected, EMM cause: 18, Cell ID: Redacted, Tracking area: Redacted, LTE mode: 7 01-01-1970 00:00:06.579 MODEM: (sysworkq) LTE Registration: Registered Home 01-01-1970 00:00:06.579 MODEM: (sysworkq) Network registration status: Connected - Home network
From my research, this is normal and expected behavior for Verizon. Per Verizon documentation (See highlighted part):
EMM cause 18 definition per 3GPP TS 24.301 V19.2.0 (2025-03):
Cause #18 – CS domain not available
This EMM cause is sent to the UE if the MME cannot service an UE generated request because CS domain is not available and SMS in MME is not supported.
Question 1: Is my understanding of EMM cause 18 correct, and can you confirm that it should have no impact to the network performance of the nRF9151 modem when using Verizon?
Question 2: Why does the link controller library log "Registration rejected, EMM cause: 18" when the registration was successful but the network doesn't support the legacy CS (Circuit-switched) domain? For someone not intimately familiar with the attach process, this can be confusing.
Question 3: Can I manually set the EPS attach type to "EPS Attach" instead of the default of "Combined EPS/IMSI attach?" Is there some disadvantage to doing this if so?
Thanks,
Derek