Initial Non-Access Stratum Message

Description

The Initial NAS message is a fundamental concept in the Non-Access Stratum (NAS) protocol used between the User Equipment (UE) and the Core Network (CN). According to 3GPP specifications 24.301, 24.501, and 24.801, a NAS message is considered an initial NAS message if it can trigger the establishment of a NAS signalling connection. In EPS (4G), the ATTACH REQUEST is a prime example; in 5GS (5G), the REGISTRATION REQUEST serves this role. These messages are the first NAS messages sent by the UE to initiate communication with the network for procedures like initial attach, registration, or service request.

The NAS protocol operates between the UE and the Mobility Management Entity (MME) in EPS or the Access and Mobility Management Function (AMF) in 5GS. The initial NAS message is encapsulated in an RRC message (e.g., RRC Connection Setup Complete) and sent over the radio interface. The network responds with a NAS message that may accept or reject the request, and may also establish a signalling connection for further NAS message exchange. The initial NAS message often includes parameters such as UE identity, UE network capability, and requested services.

The concept of an initial NAS message is crucial for connection management. When the UE has no NAS signalling connection, it must send an initial NAS message to establish one. This triggers the setup of a signalling connection between the UE and the CN, which may also involve the establishment of a radio connection. The network can then authenticate the UE, set up security, and manage mobility and session contexts. The initial NAS message is also used in procedures like Tracking Area Update (TAU) and Service Request, where the UE needs to communicate with the network.

In 5G, the initial NAS message is specifically defined as one that can trigger the establishment of an N1 NAS signalling connection. The N1 interface is between the UE and the AMF. The REGISTRATION REQUEST is the primary initial NAS message in 5G, used for initial registration, mobility registration update, and periodic registration update. Other messages like SERVICE REQUEST and DEREGISTRATION REQUEST can also be initial NAS messages depending on the context. The 3GPP specifications define the exact conditions and message types that qualify as initial NAS messages.

The handling of initial NAS messages involves security considerations. The network must be able to process the initial NAS message before security is fully established, so it may use null security or a temporary identity. The initial NAS message may also be used to carry a UE policy container or other information. Over releases, the definition has evolved to reflect changes in network architecture, particularly the shift from EPS to 5GS, but the fundamental concept remains the same.

Purpose & Motivation

The Initial NAS message exists to provide a standardized way for the UE to initiate communication with the core network when no NAS signalling connection is active. Without it, the UE would have no means to request services, update its location, or attach to the network. It solves the problem of connection establishment by defining a specific set of messages that can trigger the creation of a signalling connection, ensuring that both the UE and the network understand the start of a NAS procedure.

Historically, in earlier generations like GSM and UMTS, similar concepts existed (e.g., Location Update Request, Attach Request). The term 'initial NAS message' was formally introduced in 3GPP Rel-8 with the advent of EPS, to generalize the concept across different NAS procedures. This allowed for a more flexible and extensible framework where any NAS message that requires a signalling connection can be treated as initial. The definition was later adapted for 5G in Rel-15, where the N1 interface and REGISTRATION REQUEST became the primary examples.

The initial NAS message is essential for mobility management, session management, and security procedures. It enables the network to identify the UE, authenticate it, and establish the necessary contexts. It also allows the UE to request specific services, such as PDN connectivity or PDU session establishment. By defining which messages are initial, 3GPP ensures interoperability and consistent behavior across different UE and network implementations. The concept continues to evolve with new releases, supporting features like network slicing and enhanced security.