NAS

NAS is the conversation between the device and the core that the radio never reads. It rides over the access network but is logically end-to-end between the UE and the AMF (in 5G) or the MME (in 4G), carrying the procedures that aren't about the radio itself: registration/attach, authentication, the security mode that turns on NAS ciphering and integrity, session management, and mobility like tracking-area updates. The "non-access" in the name is the key idea — it's the stratum above the access-specific signalling (which is the AS, the RRC layer).

In 5G the NAS splits into two: 5GMM for mobility/registration and 5GSM for session management, with SM messages transported inside MM messages. NAS gets its own security context from the key hierarchy, separate from the AS keys, so signalling between the UE and the core is protected independently of the radio link. On a trace, NAS is where you watch a device register, authenticate, and establish a PDU session — the high-level control dialogue, distinct from the RRC chatter managing the air interface.