NAS (Non-Access Stratum) is the signalling protocol between the mobile device and the core network in LTE and 5G. It handles registration, authentication, security and session management, operating transparently over the radio access network via RRC messages.
What is NAS?
Non-Access Stratum: the protocol connecting your device to the mobile core network for registration and sessions.
Detailed explanation
The NAS protocol operates at Layer 3 between the UE and the MME (in LTE) or AMF (in 5G NR). Unlike RRC which manages the radio interface, NAS handles core network functions: device registration (Attach/Registration), authentication (AKA procedure), security mode activation and PDN/PDU session establishment for data connectivity.
In LTE, NAS is divided into two sub-layers: EMM (EPS Mobility Management) for registration and mobility, and ESM (EPS Session Management) for bearer and PDN connection management. The Attach procedure, which registers the device on the network, involves multiple NAS messages exchanged between UE and MME.
In 5G, NAS is updated to 5GMM (5G Mobility Management) and 5GSM (5G Session Management). New capabilities include network slicing selection (NSSAI), enhanced registration procedures and PDU session types supporting both IP and non-IP data. The NAS security is also strengthened with SUPI/SUCI privacy mechanisms.
NAS messages are carried within RRC messages over the radio interface but are not interpreted by the base station. They are passed transparently to the core network. This separation allows NAS security to be end-to-end between the device and the core, independent of radio-layer security.
Key NAS procedures
| Procedure | LTE (EPS) | 5G (5GS) |
|---|---|---|
| Registration | Attach Request/Accept | Registration Request/Accept |
| Authentication | Auth Request/Response | Auth Request/Response |
| Session | PDN Connectivity Request | PDU Session Establishment |
| Security | Security Mode Command | Security Mode Command |
How HiCellTek decodes NAS
The L3 Decoder module in HiCellTek captures and decodes all NAS messages (EMM, ESM, 5GMM, 5GSM) in real time. Each message is displayed with all IEs decoded, enabling engineers to diagnose registration failures, authentication issues and session establishment problems directly on the field.
Frequently asked questions
What does the NAS protocol do?
What is the difference between EPS NAS and 5GS NAS?
Why do NAS messages matter for field testing?
HiCellTek decodes all NAS and RRC messages directly on Android smartphones with full protocol analysis.