N1 Interface
The reference point between the UE and the AMF carrying NAS signaling for registration, authentication, and session management procedures.
N1 is a bit different from the other Nx reference points: it's a logical interface, not a physical link. It carries NAS signalling between the UE and the AMF — registration, authentication, security mode, service requests, and session-management messages — but there's no direct UE-to-AMF wire. Those NAS messages ride inside RRC over the radio (the N1 stack sits on top of RRC), then get relayed by the gNB to the AMF over N2/NGAP.
There are really two NAS layers multiplexed on N1: mobility-management NAS, which the AMF handles, and session-management NAS, which the AMF passes straight through to the SMF without reading. When you're staring at a registration trace, the N1/NAS messages are what tell you how far the UE got — attach accepted, authentication failed, slice not allowed — so it's where a lot of attach-side debugging actually happens.
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