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  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).

In 5G (NR), AMF units do not need to be interrupted or restarted when making configuration changes or updates; they only need to notify the relevant network units. For mobile terminals (UEs) within their coverage area, the changes will be notified via the gNB in ​​the radio network, and the AMF will determine whether the UE needs to re-register with the AMF. The update definition process is as follows:   I. Configuration Update Process: As shown in Figure (1), the AMF determines whether the UE needs to reconfigure or register with the AMF based on the changes. That is, when the AMF detects a change in the configuration previously sent to the UE, it will initiate the configuration update process. In response to the UE's confirmation request, the AMF will send configuration update completion information to the AMF.   Figure 1. AMF Configuration Update Notification Flowchart   II. AMF Configuration Update Interface (Message)   [12] Construct Downlink RAN ​​Configuration Transmission [13] Send Downlink RAN ​​Configuration Transmission [12] Construct Downlink RAN ​​Status Transmission [13] Send Downlink RAN ​​Status Transmission [12] RAN Configuration Update Failed [13] Send RAN Configuration Update Failed [12] RAN Configuration Update Confirmation [13] Send RAN Configuration Update Confirmation [7] Construct Configuration Update Command [8] Send Configuration Update Command [12] Construct Downlink UE-Associated NRPPA Transmission [13] Send Downlink UE-Associated NRPPA Transmission [12] Construct Downlink Non-UE-Associated NRPPA Transmission [13] Send Downlink Non-E-Associated NRPPA Transmission [9] Configuration Update Complete [12] Construct AMF Configuration Update [13] Send AMF Configuration Update

The AMF unit plays a crucial role in the 5G core network; it is responsible for processing NAS messages transmitted transparently through the RAN (gNB) from the terminal (UE). The registration and authentication and mobility management of the terminal (UE) during initial access are completed by the AMF independently or jointly with other relevant network elements, as follows:   I. The order of AMF interface and message usage for 5G terminal authentication is shown in Figure (1); Figure 1. Message usage order of UE authentication AMF interface in 5G.     [11] UE authentication request [11] UE response [17] NRF discovery AUSF [25] Initialize SCP NF instance [11] NAMF Nausf authentication request [11] 5gAKA [11] Av5gAka contains authentication vector 5gAKA method [11] Amf_ue->SUCI [11] 5g AKA confirmation URL [11] SEAF starts authentication process [11] SUPI and Kseaf [11] Authentication successful [11] (or) Authentication failed   II. Mobility Management 5G networks provide high-speed and reliable connectivity for mobile users and devices, including vehicles, smartphones, and IoT devices. During mobility, the AMF is responsible for the transmission and processing of terminal-related information. Its interface (protocol) is used as follows; Figure 2. Order of AMF interface messages used when the UE moves in 5G   [5] Process registration request [5] UE sends initial NAS message to AMF [5] Set 5GS registration type: KSI, TSC [5] AMF new GUTI [5] Copy stream number, NR-TAI, NR-CGI from ran_ue [5] Check TAI[5] The algorithm selected by AMF should be the same as the NAS security algorithm [5] 5GMM request accepted [5] 5GMM processes registration update [5] 5GMM processes service request [6] The initial NAS service request message should contain security header type, ngKSI, TMSI and security header type [6] 5GMM processes service update[17] NRF discovers AUSF [25] Initialize SCP NF instance [5][6] AMF NAUSF authentication response, then confirm [5] Identity response SUCI[6] 5GMM status registered [13] NGAP handles path switching request [13] NGAP handles switching request [13] NGAP handles switching notification [13] NGAP handles Ran configuration update [5][6] 5GMM handles UL NAS transmission [5] 5GMM handles deregistration request [5] Set 5GS deregistration type [5] AMF sbi release all sessions [5] Clear paging information [5] Clear SM context [5] Unassociate NG with NAS  

  The UPF (User Plane Function) is one of the most important units in 5GC. It is a key unit that the Radio Network (RAN) interacts with during PDU data transmission. The UPF is also an evolution of CUPS (Control Plane and User Plane Separation), responsible for inspecting, routing, and forwarding packets within QoS flows in subscription policies. It uses SDF templates sent by the SMF through the N4 interface to enforce uplink (UL) and downlink (DL) traffic rules. When the service ends, it will allocate or terminate the QoS flow in the PDU session; the order of use of UPF interface session update and deletion is as follows; please refer to the order of use of UPF interface (protocol) and terminal call in 5G.   I. Session Modification Terminal-specific QoS flow is allocated through the PDU session modification process; additional dedicated QoS flow supports traffic with higher QoS requirements (such as voice, video, game traffic, etc.); the application of session modification (update) in UPF is shown in Figure (1); Figure 1. UPF interface usage order of terminal session modification (update) in 5G   [6] N4 processes session modification request [6] Remove existing PDR [6] Update PDR [6] Update FAR [6] Update URR [6] Update QER [6] Update BAR [6] Set up GTP node [6] Set up N3 TEID and QFI [6] [7] PFCP sends session modification response [5] N4 constructs session modification response [5] PFCP request accepted [5] PDR buffer initialized [5] PDR has been created [6] Send buffered data packets to gnB (if necessary) II. Session deletion When the terminal service session ends, QoS flow will be allocated or terminated in the PDU session. The session deletion usage order in the UPF interface is as follows: Figure 2.5G Terminal deletion UPF related interface usage order   [6] N4 processes session deletion request [6][7] PFCP sends session deletion request [5][1] Session URR usage status full report [1] Last report timestamp [1] Time trigger [1] Quota validity period report [1] Capacity trigger [1] Capacity quota report [5][1] UPF session URR snapshot (total bytes, total data packets, including uplink and downlink) [6][1] UPF session deletion [1] UPF session URR account all deletion: validity period deletion, quota time deletion, threshold time deletion. [13]PDR all deleted [13]FAR all deleted [13]URR all deleted [14]QER all deleted [13]BAR all deleted [13]From SEID

User Plane Function (UPF) is one of the most important Network Functions (NFs) in the 5G core network. It is the second network function that the NR RAN interacts with during PDU flows. UPF is an evolution of CUPS (Control Plane Separation from User Plane), specifically responsible for inspecting, routing, and forwarding packets within QoS flows in subscription policies. It also uses SDF templates sent by the SMF through the N4 interface to enforce UL (Uplink) and DL (Downlink) traffic rules; when the corresponding service ends, it allocates or terminates QoS flows in the PDU session.   Figure 1.5G SMF and its interface (protocol)   I. UPF Interfaces and Protocols include the following: N4[5] After the user plane is established, the session management context and necessary parameters are transmitted from the single-mode fiber (SMF) to the user plane function (UPF). PFCP[7] Any communication between the SMF and UPF is managed by the packet forwarding PFCP (control protocol); it is one of the main protocols separating the user plane and the control plane. GTP[3] The GPRS tunneling protocol (GTP) is responsible for providing seamless interconnection and carrying traffic between roaming or home users and key network interfaces in 4G, NSA (5G non-standalone), SA (5G standalone), and mobile edge computing architectures. In 5G, GTP tunnels are also used for the N9 interface. II. Call Flow (Session Establishment and UPF Initialization) During PDU session establishment, the SMF connects to the UPF via PFCP (N4 interface). This PFCP session carries an SDF template containing information such as PDR, QFI, URR, and FAR. The UPF will allocate a default QoS (non-GBR) flow during the initial session establishment.   III. Terminal (UE) Call Interface Usage Sequence [6] N4 processes session establishment request [6] PFCP processes PDR creation [6] [12] Check existing PDI of PDR [6] [12] Check TEID [6] [12] Check source interface [6] [12] Check previous SDF filter ID [6] [12] Set all filter flags: BID, FL, SPI, TTC, FD [6] PFCP processes FAR creation [6] Create URR [6] Create BAR [6] Create QRR [6] Set N3 TEID and QFI [4] UPF Initialization [4] PFCP Context Initialization [1] Initialize UPF Context [1] Set User Plane Functional Characteristics: FTUP, EMPU, MNOP, VTIME, UPF Attribute Length [6] [7] Session Establishment Response [5] N4 Build Session Establishment Response [5] Node ID [5] PFCP Request Accepted [5] F-SEID [5] PDR Existence Checked [5] PFCP Build Message FTUP: The UP function supports the allocation/release of F-TEID. EMPU: The UP function supports sending end-of-file packets. MNOP: The UP function supports measuring the number of packets in the URR, which is performed via the "Measure Number of Packets in URR" flag. MNOP (Packet Count Measurement): When set to "1", it indicates that in flow-based measurements, in addition to measuring in bytes, the uplink/downlink/total number of packets transmitted is also requested. VTIME:UP functionality supports the quota validity period feature. If UP functionality supports the VTIME feature, it requests UP functionality to send a usage report after the validity period expires. After the quota validity period expires, if data packets are received on the UPF, the UPF should stop forwarding data packets or only allow forwarding of limited user plane traffic, depending on the operator's policy in the UP functionality. Abbreviations: FL: Flow Tag TTC: TOS (Traffic Category) SPI: Security Parameter Index FD: Flow Description BID: Bidirectional SDF Filter

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.

As mobile communication networks become increasingly complex, performance optimization and user experience improvement are crucial for operators. Previously, optimization engineers primarily relied on drive tests to perform (physical) measurements of the network to understand and control wireless coverage and performance. However, this testing method is costly, time-consuming, and not always comprehensive.   I. Minimum Drive Testing (MDT) is a wireless network measurement method designed by 3GPP for mobile communication networks. MDT allows the network to collect actual performance data directly from the User Equipment (UE) side, thereby reducing the need for manual drive testing. It is specifically divided into Logged MDT and Immediate MDT (iMDT).   II. Immediate MDT, as defined in 3GPP, refers to the real-time reporting of network performance data by the terminal equipment (UE) during a radio connection session. Unlike logged MDT, which stores data on the device for later upload, immediate MDT sends measurement results to the network, enabling operators to:   Identify network problems such as radio link failures (RLFs) in real time. Collect data at specific locations during the real-time session. Improve user performance in real time.   III. Key Points of Immediate MDT The Immediate MDT process during a connection session between the UE and the network mainly includes: MDT Configuration: The UE obtains the MDT configuration from the network. This configuration specifies which types of data need to be collected (e.g., RSRP, RSRQ, SINR, or call events). Measurement Timing: In a connected state, the UE periodically performs measurements based on specified conditions. Measurement parameters may include signal strength, quality metrics, and location data. Coverage Dead Zones and Radio Link Failures (RLF): If the UE finds itself in a coverage dead zone, an RLF may occur, prompting the MDT process to record signal strength and location for further analysis. Logger and RLF Indication: During an RLF event, the UE logs key information such as signal strength and location coordinates. After the RRC connection is re-established, an RLF log indication is created and sent. Re-establishment and Reporting: The UE needs to re-establish the RRC connection to reconnect. After the RRC reconnection, the UE sends the RLF log indication along with the recorded information. This helps the network identify the location and cause of the RLF, which is very useful for network optimization.

I. PDU Session Resource Notification (PDU SESSION RESOURCE NOTIFY) is a 5G system notification to the core network element AMF that a QoS flow or PDU session established for a specific terminal (UE) has been released, is no longer being executed, or is being re-executed by an NG-RAN node controlled by a request notification. This procedure is also used to notify the NG-RAN node of QoS parameters that were not successfully accepted during the path handover request procedure. The entire procedure uses UE-related signaling.   II. PDU Session Resource Success Notification: As shown in Figure 8.2.4.2-1, the PDU session resource success operation is initiated by the GN-RAN node.     III. Key information for PDU session resource notification includes:   The NG-RAN node initiates this process by sending a PDU session resource notification message. The PDU SESSION RESOURCE NOTIFY message should contain information about PDU session resources or QoS flows that have been released, are no longer being executed, or have been re-executed by the NG-RAN node. For each PDU session where some QoS flows have been released, are no longer being executed, or have been re-executed by the NG-RAN node, a PDU session resource notification transport IE should be included, containing: A list of QoS flows released by the NG-RAN node (if any) in the QoS flow release list IE. If no other QoS flows are associated with the existing bearer after release (e.g., splitting the PDU session), the NG-RAN node and 5GC should consider the associated NG-U transport bearer to have been removed, and the associated NG-U UP TNL information to be available again. A list of GBR QoS flows that the NG-RAN node no longer executes or has re-executed by the NG-RAN node (if any) in the QoS flow notification list IE, along with the notification reason IE. For QoS flows indicated as no longer satisfied, the NG-RAN node may also indicate the alternative QoS parameter sets that can currently be satisfied in the Current QoS Parameter Set Index IE. For QoS flows indicated as no longer satisfied, the NG-RAN node may also indicate RAN feedback in the TSC Traffic Characteristics Feedback IE. A list (if any) of QoS flows whose QoS parameters have been updated but cannot be successfully accepted by the NG-RAN node during a path handover request should be included in the QoS Flow Feedback List IE, which may be associated with values ​​that can be provided. For each PDU session resource released by the NG-RAN node, a PDU session resource notification transmission released should be included in the "PDU Session Resource Notification Released Transmission IE" and the release reason should be included in the "Reason IE". If the User Plane Error Indication IE is set to "Received GTP-U Error Indication", the SMF (if supported) should consider the PDU session released due to receiving a GTP-U error indication through the NG-U tunnel, as described in TS 23.527. The NG-RAN node (if supported) should report the UE location information in the User Location Information IE in the PDU SESSION RESOURCE NOTIFY message. Upon receiving a PDU SESSION RESOURCE NOTIFY message, the AMF should transparently transmit a PDU Session Resource Notify Transfer IE or a PDU Session Resource Notify Released Transfer IE to the SMF associated with the relevant PDU session for each PDU session indicated in the PDU Session ID IE. Upon receiving the PDU Session Resource Notify Transfer IE, the SMF typically initiates the corresponding release or modification procedure on the core network side for PDU sessions or QoS flows that are identified as no longer satisfying. For each PDU session, if its PDU Session Resource Notification Transfer IE or PDU Session Resource Notification Released Transfer IE contains a Secondary RAT Usage Information IE, the SMF should process this information in accordance with TS 23.502. If the PDU Session Resource Notification message contains a User Location Information IE, the AMF should process this information in accordance with TS 23.501.