5GS MM Cause #43: N1 Mode Not Allowed — 5G SA Access Denied Diagnosis

5GS MM cause #43 (“N1 mode not allowed”) is returned by the AMF in a Registration Reject when the UE is not authorized to use N1 mode — the 5GS NAS operating mode over 3GPP access (5G NR radio). It is the 5G SA equivalent of a fundamental access authorization failure and is distinct from slice-related rejections (cause #62) or PLMN-level restrictions (cause #11).

Cause #43 is most frequently encountered during 5G SA rollout validation, when USIMs are being migrated from 4G-only to 5G-capable provisioning profiles, or when a 4G SIM is tested in a 5G SA device.

Technical Reference

What N1 Mode Means in 5G SA

In 3GPP TS 24.501, N1 mode refers to the UE capability to use 5GS NAS signaling (TS 24.501) over 3GPP radio access (NR or LTE for 5GC). It is the mode that enables:

• 5GS registration (Registration Request/Accept)
• Network slicing (NSSAI)
• 5G QoS model (5QI)
• VoNR (Voice over New Radio)
• 5G SA-specific features (network slicing, URLLC, LADN)

A UE that supports N1 mode has a 5GS-capable USIM (provisioned with 5GS subscription data in the UDM) and a modem that implements TS 24.501. Both conditions must be met for successful 5G SA registration.

UE Behavior After Receiving Cause #43

Per TS 24.501 §5.5.1.2.5 (Registration Reject handling):

• Set 5GS update status to 5U3 ROAMING NOT ALLOWED
• Consider N1 mode as not allowed in this PLMN for 3GPP access — the UE shall not initiate a new Registration Request in N1 mode in this PLMN over 3GPP access
• Enter state 5GMM-DEREGISTERED
• If LTE coverage is available and the device supports EPS NAS: the UE may attempt registration on LTE (EPS NAS, TS 24.301) as a fallback
• Non-3GPP N1 mode access (e.g., 5GS NAS over WLAN) may still be attempted if not separately restricted

Important: Cause #43 only restricts N1 mode over 3GPP access in the current PLMN. A SIM that receives cause #43 from PLMN A may still successfully register in N1 mode in PLMN B if the subscription in the UDM includes 5GS services there.

Field Scenarios

USIM provisioned for 4G only. A USIM whose UDM subscription contains EPS Subscription Data (TS 29.272) but no 5GS Subscription Data (TS 29.503) will receive cause #43 when the device attempts 5G SA registration. The AMF queries the UDM, finds no 5GS subscription context, and rejects with cause #43. The UE should fall back to LTE automatically if available.

USIM migration delay during 5G SA rollout. When an operator launches 5G SA and begins migrating existing 4G subscribers to 5G-capable profiles, the migration is done in batches. Subscribers whose USIMs have not yet been migrated receive cause #43 until the UDM provisioning is updated. This can produce cause #43 systematically for a subset of the subscriber base in the same coverage area.

Incompatible USIM generation. Some older USIM formats (UICC with limited capacity) cannot store the additional EFs (Elementary Files) required for 5GS subscription data. These SIMs structurally cannot be provisioned for N1 mode and will always receive cause #43 in 5G SA networks.

Test device with 5G SA modem and 4G-only SIM. During 5G SA network acceptance testing, engineers sometimes insert production 4G SIMs into test 5G SA devices. Every Registration Request in N1 mode receives cause #43 — the SIM must be replaced with a 5G-provisioned USIM.

Step-by-Step Field Diagnosis

Step 1 — Confirm cause #43 in the Registration Reject. In the 5GS NAS log, locate the Registration Reject and verify the 5GMM cause IE value is 0x2B. This immediately identifies the failure as N1 mode authorization, not a slice, PLMN, or radio issue.

Step 2 — Verify the USIM supports 5GS. Check whether the USIM contains 5GS EFs (EF_5GS3GPPLOCI, EF_5GSN3GPPLOCI, EF_SUCI). Absence of these files on the USIM confirms it is a 4G-only SIM. A SIM card reader or DIAG-level USIM access (via ATR/APDU exchange) reveals the EF structure.

Step 3 — Test with a 5G-provisioned USIM. Insert a SIM confirmed to be 5G SA-provisioned in the same device on the same cell. A successful 5G SA registration confirms the issue is USIM-side, not network configuration.

Step 4 — Verify UDM provisioning. On the operator side, confirm the IMSI’s subscription in the UDM (TS 29.503) includes 5GS Subscription Data. The relevant fields are: Subscribed S-NSSAIs, 5GS DNN configurations, and the 5G-AN Node Access Policy.

Step 5 — Observe LTE fallback behavior. After cause #43, a compliant UE should initiate an LTE Attach Request (EPS NAS, TS 24.301) if an LTE cell is available. The LTE attach may succeed with a 4G-only USIM, confirming the diagnosis: 5G SA not provisioned, 4G functional.

Capturing Cause #43 in the Field

5GS NAS Registration Reject messages are captured via DIAG log code 0xB821 (NR5G NAS OTA). The decoded Registration Reject shows the 5GMM cause IE 0x2B labeled “N1 mode not allowed.” The subsequent EPS NAS Attach Request (if LTE fallback occurs) is visible in DIAG log code 0x713A, allowing the complete RAT fallback sequence to be correlated in one trace.

Related Cause Codes

• Cause #7 in 5GS NAS — 5GS services not allowed (broader rejection, not mode-specific)
• Cause #62 — No network slices available (slice-level rejection, assumes N1 mode is authorized)
• Cause #11 — PLMN not allowed (PLMN-level restriction in 5GS context)
• EMM cause #7 in LTE NAS — EPS services not allowed (the LTE analog: EPS services not provisioned for this USIM)