A Brief Introduction to the Functions and Interfaces of the 5G System AMF

The Access and Mobility Management Function (AMF) is a Control Plane (CU) unit in the 5G core network (CN). In a wireless network, a gNodeB must connect to the AMF before it can access 5G services. The AMF is also the only Network Functional Unit (NF) (excluding interactions with the User Plane Function (UPF) during PDU session establishment) that allows the gNodeB to communicate with the 5G core network.

I. Extended MME AMF: The AMF in 5G performs most of the functions of the MME (Mobility Management Entity) in 4G. The establishment of the terminal (UE) PDU session is performed by the Session Management Function (SMF) unit, while authentication and security-related functions are performed by the Authentication Server Function (AUSF) in 5G; thus achieving the separation of the control plane and user plane in the 5G architecture.

II. AMF Functions: Its functions are defined in relevant 3GPP protocols as including:

1. Registration Management – ​​The AMF manages the registration and deregistration of the terminal (UE) in the 5G system; the terminal (UE) must complete the registration process to access 5G services.

2. Connection Management - Establishes and releases control plane (CP) signaling connections between the UE and AMF via the N1 interface.

3. Mobility Management - The AMF updates the UE's location in the network. This is achieved through the UE's periodic registration.

4. NGAP Signaling Flow - Includes paging procedures, NAS message transmission, PDU session management, UE context management, and other message transmissions.

III. 5G (NR) System Internal Interfaces (Functions)

• N1/N2: The AMF obtains all connection and session-related information from the UE through the N1 and N2 interfaces.
• N8: All user and specific UE policy rules, session-related subscription data, user data, and any other information (such as data exposed to third-party applications) are stored in the UDM. The AMF retrieves the UDM through the N8 interface.
• N11: This interface represents a trigger for adding, modifying, or deleting PDU sessions through the AMF on the user plane.
• N12: This interface simulates an AUSF within the 5G core network and provides services to the AMF through the AUSF-based N12 interface. 5G networks represent service-based interfaces, focusing on AUSF and AMF.
• N14: This reference point is located between two AMFs (Access and Mobility Management Functions). UE context is transmitted through this interface during handover and other processes.
• N15: The transmission and removal of access and mobility policies are performed through the N15 interface between the AMF and PCF.
• N17: An emulated Device Identity Register (EIR) is created within the 5G core network and provided to the AMF via an interface based on N5g-EIR services. This interface supports device identity verification services.
• N22: The AMF selects the best Network Function (NF) in the network using the NSSF. The NSSF provides Network Function Location Information to the AMF through the N22 interface.
• N26: This interface is used to transmit UE authentication and session management context when the UE handovers between 5G and 4G (EPS).