5G Terminal Call - SMF Protocol Interface Usage

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