Non-Access Stratum Protocols

Description

The Non-Access Stratum (NAS) protocols form a crucial layer in the 3GPP protocol stack, operating between the User Equipment (UE) and core network entities such as the Mobility Management Entity (MME), Serving GPRS Support Node (SGSN), Mobile Switching Centre (MSC), and Access and Mobility Management Function (AMF). Unlike Access Stratum (AS) protocols, which terminate in the radio access network (e.g., eNodeB, gNodeB), NAS protocols are transparent to the RAN and are carried as payload over the radio interface. They are defined in specifications such as TS 24.301 for EPS and TS 24.501 for 5GS, with overarching definitions in TS 21.905.

The NAS protocol stack is divided into two main sublayers: the EPS Mobility Management (EMM) or 5GS Mobility Management (5GMM) sublayer, and the EPS Session Management (ESM) or 5GS Session Management (5GSM) sublayer. The mobility management sublayer handles procedures such as attach, tracking area update, authentication, security mode control, and identity management. The session management sublayer is responsible for establishing, modifying, and releasing packet data sessions (PDN connections or PDU sessions), including bearer or QoS flow management. These sublayers interact with the AS to trigger radio resource control (RRC) connections and to transport NAS messages.

NAS messages are encapsulated within RRC messages for transmission over the air interface. For example, in LTE, a NAS Attach Request is sent as part of an RRC Connection Setup Complete message. The RAN (eNodeB or gNodeB) forwards these NAS messages to the appropriate core network node without interpreting them, ensuring separation of concerns between radio access and core network functionalities. This separation allows the core network to evolve independently of the radio access technology, facilitating interworking between different access technologies (e.g., E-UTRAN, UTRAN, GERAN, NR) and enabling seamless mobility.

Security is a fundamental aspect of NAS protocols. NAS security includes integrity protection and ciphering of NAS signalling messages, as well as authentication of the UE by the network. The security context is established during procedures like Attach or Registration and is managed by the core network. NAS also supports security mode control procedures to activate and modify security algorithms. In 5G, NAS security is enhanced with additional features such as unified authentication framework and support for network slicing.

NAS protocols have evolved significantly from their introduction in Release 8 for EPS, extending to 5G in Release 15 and beyond. They continue to be enhanced to support new services, network slicing, and improved mobility management. The protocols are specified in a set of 3GPP technical specifications, primarily TS 24.301 (EPS NAS) and TS 24.501 (5GS NAS), with definitions and general aspects in TS 21.905. Understanding NAS is essential for telecom engineers as it governs the essential control plane interactions between the UE and core network, enabling mobile services.

Purpose & Motivation

The NAS protocols were created to provide a clear separation between the radio access network and the core network, allowing independent evolution and multi-access interworking. Before NAS, mobility and session management functions were tightly coupled with the radio access technology, making it difficult to introduce new radio technologies or core network architectures. NAS solves this by defining a set of protocols that are transparent to the RAN, enabling the core network to manage UE mobility and sessions regardless of the underlying access technology.

Historically, in pre-Release 8 networks (e.g., GPRS/UMTS), similar functions existed but were not as clearly separated. The introduction of EPS in Release 8 brought a clean architectural split with NAS protocols between UE and MME, which was a significant advancement. This allowed the core network to support multiple access technologies (E-UTRAN, UTRAN, GERAN) through a common set of NAS procedures, simplifying interworking and mobility. It also facilitated the adoption of all-IP flat architectures and improved scalability.

In 5G, NAS protocols continue to serve the same fundamental purpose but are extended to support new capabilities such as network slicing, enhanced security, and service-based architecture. The separation of NAS from AS remains critical for enabling diverse deployment scenarios, including non-standalone and standalone 5G, and for ensuring backward compatibility with legacy networks. Thus, NAS protocols are foundational to the success of 3GPP mobile systems, providing a robust and flexible framework for control plane communication.