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1. In a 5G system, one function of SMF (Session Management Function) is to be responsible for the transmission of user control plane (CP) information; it works with UPF to manage the relevant context of terminal sessions; it is responsible for creating, updating and deleting sessions, and assigning IP addresses to each PDU session, providing all parameters and supporting various functions of UPF; the interface between SMF and other network elements is shown in Figure (1).   *Figure 1. Schematic diagram of SMF connection with other network elements (solid lines in the figure represent physical connections, and dashed lines represent logical connections).   II. Application protocols in SMF include: PFCP[2]: All communication between SMF and UPF is managed by PFCP (Packet Forwarding Control Protocol); it is one of the main protocols separating the user plane and the control plane. UDP[3]: User Datagram Protocol, a transport layer protocol that provides source and destination port addressing for multiplexing/demultiplexing of higher-level applications. This protocol is responsible for data transmission between gNB and UPF. SBI[4] (Service-Based Interface): This is an API-based communication method between network functions.   III. Terminal Session Call Flow During 5G terminal session establishment: First, the SMF registers with the NRF to locate other network functions. If a user wants to access 5G data services, a PDU session must be established with the network. The UE sends a PDU session establishment request to the core network (i.e., the AMF). The AMF selects the best SMF in the network to maintain its session-related information. After selecting the best SMF, it requests the SMF to create an SM context. The SMF obtains SM subscription data from the UDM and generates an M context. Then, the SMF and UPF initiate the PFCP session establishment process and set default values ​​for session-related parameters. Finally, the AMF sends session information to the gNB and UE to establish the default PDU session value.   Session establishment interface uses (sequential) message content: [22] Send NF registration [22] Retry sending NF registration [6] Set NF configuration file [22] Send NF discovery service AMF [5] Process PDU session establishment request [4] Build GSM PDU session establishment rejection [30] Send PDU session establishment rejection [28] HTTP POST SM context - Receive Create SM context [31] Process PDU session SM context creation [22] Send NF discovery UDM [27] Get SM context [10] Build/Set created data [2] Initialize SMF context [2] Get DNN information [4] Build GSM PDU session establishment acceptance [22] Send NF discovery PCF [10] PCF selection [24] Send SM policy association creation [29] SM policy in application decision [16] Create UPF list for selection [16] Sort UPF list by name [16] Select UPF and assign UE IP [15] Select UPF by DNN [16] Get UPF name by IP [16] Get UPF node ID by name [16] Get UPF node by IP [16] Get UPF ID by IP [18] Construct PFCP association establishment request [17] Process PFCP association establishment request [19] Send PFCP association establishment request [18] Construct PFCP session establishment request [19] Send PFCP session establishment request [20] Send PFCP request [18] PFCP creates PDR, FAR, QER, BAR [10] Add PDR to PFCP session [13] [16] Generate default data path [16] Generate data path [15] Add data path [15] Generate Terminal Equipment Identifier (TEID) [2] [10] Assign Local System Equipment Identifier (SEID) [10] Select session rule [15] Select UPF parameters [15] Add PDR, FDR, BAR, QER [29] Process session rule [3] Activate tunnel and PDR [3] Activate uplink/downlink tunnel [16] Select uplink path source [30] Activate UPF session [30] Establish PFCP session [18] Build PFCP session establishment response [19] Send PFCP session establishment response [20] Send PFCP response [18] Build PFCP association establishment response [19] Send PFCP association establishment response [2] Get user plane information [16] Get default user plane path through DNN and UPF [3] Get UPF ID, node IP, UL PDR, UL FAR [3] Copy the first data path node [25] Get UE PDU session information through HTTP [15] Get interface to get UPF interface information [15] Get UPF node through node ID [15] Get UPF IP, ID, PDR ID, FAR ID, BAR ID, QER ID [2] Get UE default path pool [30] Notify UE - send all data paths to UPF and send the results to UE [10] Send PDU address to NAS [12] Create UE data path node [2] Initialize SMF UE routing [7] Build PDU session resource establishment request transmission [8] Handle PDU session resource establishment failure transmission [8] Handling PDU session resource establishment response transmission  

In a 5G system, when the NG interface or certain parts of the NG interface need to be reset, the NG-RAN node will be notified; when the AMF processes overload, an overload message will also be sent to the NG-RAN node to notify the gNB to start the load management process; the specific definitions of these messages are as follows;   1. NG reset messages are sent by NG-RAN nodes and AMF to request the reset of the NG interface or certain parts thereof.   Message direction: NG-RAN node → AMF and AMF → NG-RAN node   2. The NG RESET acknowledgment message is jointly sent by the NG-RAN node and the AMF as a response to the NG RESET message.   Message direction: NG-RAN node → AMF and AMF → NG-RAN node   3. NG RESET Confirmation Message: This message is jointly sent by the NG-RAN node and the AMF as a response to the NG RESET message.   Message direction: NG-RAN node → AMF and AMF → NG-RAN node   4. Error indication messages are sent by NG-RAN nodes and AMF to indicate that an error has been detected in the node.   Message direction: NG-RAN node → AMF and AMF → NG-RAN node 5. The overload start message is sent by the AMF to indicate to the NG-RAN node that the AMF is overloaded.   Message direction: AMF → NG-RAN node   6. The overload stop message is sent by the AMF to indicate that the AMF is no longer overloaded.   Message direction: AMF → NG-RAN node      

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.