Order of use of AMF interface protocols (authentication and call) in 5G

AMF (Access and Mobility Management Function) is a control plane (CU) functional unit in the 5G core network (CN). Radio network elements (gNodeBs) need to connect to AMF before they can access any 5G service. The connection between AMF and other units in the 5G system is shown in the figure below.

*Figure 1. Schematic diagram of AMF and 5G network element connection (solid lines in the figure represent physical connections, and dashed lines represent logical connections)

I. AMF Interface Functions

• N1[2]: The AMF obtains all connection and session-related information from the UE through the N1 interface.
• N2[3]: Communication between the AMF and the gNodeB related to the UE, as well as communication unrelated to the UE, is conducted through this interface.
• 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, and the AMF obtains this information through the N8 interface.
• N11[4]: The N11 interface represents the triggers for the AMF to add, modify, or delete PDU sessions on the user plane.
• N12: The AMF simulates an AUSF within the 5G core network and provides services to the AMF through the AUSF-based N12 interface. The 5G network represents a service-based interface, focusing on the AUSF and the AMF.
• 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.
• SBI[8]: The service-based interface is API-based communication between network functions.

II. AMF Application Protocols

• NAS[5]: In 5G, NAS (Non-Access Layer Protocol) is the control plane protocol on the radio interface (N1 interface) between the UE and AMF; it is responsible for managing mobility and session-related context within the 5GS (5G system).
• NGAP[6]: NGAP (Next Generation Application Protocol) is a control plane (CP) protocol used for signaling communication between the gNB and AMF. It is responsible for handling services related to the UE and services unrelated to the UE.
• SCTP[7]: Flow Control Transmission Protocol (SCTP) ensures the transmission of signaling messages between the AMF and the 5G-AN node (N2 interface).
• ITTI Messages[9]: Inter-task interface used to send messages between tasks.

III. Call Flow - UE Registration and Deregistration (Steps)

• The AMF first needs to register with the NRF to identify and communicate with the Network Function Location. When the UE powers on, it goes through a registration process. The AMF processes the registration and then receives the initial NAS UE message and registration request. This message is used to create an AMF identity for the UE.
• Then, the AMF checks the AMF the UE last registered with. If the old AMF address is successfully found, the new AMF will retrieve all UE contexts and initiate a deregistration procedure for the old AMF. The old AMF requests to release the SM context from the SMF and the UE context from the gNB.

IV. Terminal Authentication and Authorization

• If the new AMF does not detect any trace of the old AMF, it initiates the authorization and authentication process with the UE. It handles the identity verification process and requests an authentication vector from the AMF. It then sends an authentication request to the UE to set a security key and select a security algorithm for the channel, thereby ensuring secure data transmission.
• The AMF controls all NAS downlink/uplink transmission channels used for communication.