NAS — The protocol between the device and the core, which the RAN only carries
The device — identity, state, mobility
Where it sits
What it is
What it is. The Non-Access Stratum — the protocol between the device and the AMF, which the radio network carries and is required not to understand. TS 24.501. The name is the definition: everything not in the access stratum, the access stratum being RRC and everything below it.
The split, stated plainly:
| Access stratum | Non-access stratum | |
|---|---|---|
| Between | device and base station | device and AMF |
| Protocol | RRC (TS 38.331) | NAS (TS 24.501) |
| About | the radio — measurements, bearers, BWPs | the subscription — registration, sessions, mobility |
| Changes when you change cell | yes, constantly | no |
| Ciphered by | PDCP | the AMF itself |
NAS is carried inside RRC over the air, and inside N2 beyond it — and neither leg reads it. That is why N1 is drawn as a reference point between the device and the AMF even though no such link physically exists. The base station relays an opaque blob.
And it is separately ciphered. TS 23.501 §6.2.1 gives the AMF "NAS ciphering and integrity protection" — so NAS is protected between the device and the core, independently of the PDCP ciphering protecting it over the radio. A compromised base station sees neither.
Why the layering pays. A device changing cell renegotiates the entire access stratum and changes nothing about NAS: it stays registered, its sessions stay up, its security context stands. Mobility becomes a radio-layer event that the subscription layer never sees.
Two protocols inside it, and they behave differently — TS 24.501 defines both:
5GMM, mobility management, which the AMF does read; and 5GSM, session management, which the AMF forwards without reading to the SMF. One protocol family, two state machines, two audiences.
Read on
This concept is read as part of one argument in ref-ue-state, alongside the rest of its group.
Before this concept, the hierarchy says to learn the following — the full chain, in order: