Cause #27 - N1 mode not allowed - registration reject

Hello Experts.

_5gmm=27 (N1 mode not allowed). Can anybody please let me know why I am facing this registration reject?

This 5GMM cause is sent to the UE if it requests service, or if the network initiates a de-registration request, in a PLMN where the UE by subscription, is not allowed to operate in N1 mode.

Thanks for the response!

Can it be because of a SIM?

May be not.
But, your UDM (HSS) may not have been provisioned for this SIM to access this service.

Ok thanks!

No, I don’t think so, as my testing station is 5G capable and other devices are camping to 5G.
Difference in other devices and mine is that I inserted yesterday newly allocated SIM cards.
So suspecting SIM issue.
Anyway, thanks for the response.

I will try to find out its resolution and i will share the same here if i find.

This is SA failure in Registration.

N1 mode is when you use gNB to access 5G core.

As @vikram said, failure is related to SIM info issues.

We had this problem here, where the operator allowed one PLMN for EPC but not 5GC

24501 is your guide

If you check 24.501, you can see this error described.

N1 mode: A mode of a UE allowing access to the 5G core network via the 5G access network.

So, this means that network denying access to the subscriber, 5GC cannot be accessed via the gNB. There is an issue with the credentials of this subscribers (AUSF/UDM) or some rules on the PCF are prevent the UE to access network. Most likely this is related to PLMN (Check techplayon link below).

5G NAS - Registration Reject Cause - Techplayon - NAS Signalling.

Ask carrier to open core traces to see messages on AUSF/UDM/PCF error logs.

Maybe due to: terminal support SA/NSA, it try to attach to SA network first as higher priority.
But SIM is not registered in CN for SA service. So attach reject.