Similar to previous generations of mobile communication, the services supported by the terminal (UE) are stored in the core network. The UE can only be executed by the radio network after completing authentication and encryption actions upon power-on. In 5G (NR) systems supporting NSSF (Network Slice Selection Function), after "RRC connection establishment, UE context, UE ID allocation, and security authentication," the terminal (UE) will obtain specific subscription data based on the activation status and perform user plane settings. The specific process is as follows:
I. Subscription Data Acquisition: The AMF searches for the NSSF (Network Slice Selection Function) through the N22 interface to select the best available network slice for the user's requested service. Then, it searches the UDM to retrieve all subscription data related to AM (Access Management), SM (Session Management), and UE (Terminal). The AMF connects to the UDM via the N10 interface to obtain subscription data. The process (message) is as follows:
[21] Fill in the slice information in the PDU session establishment accept message
[8] Obtain the AMF context based on the UE identifier
[8] Obtain the SMF context from the mapping
[20] Set the SMF context in the AMF context
[8] The AMF creates a new UE context
---The AMF configures the PCF (Policy Control Function) to retrieve the AM policy through the N15 interface accessible to the UE, and the SMF allocates services accordingly.
---The AMF has collected all UE contexts, and now it creates another identifier for the UE, the AMF UE NGAP ID, to add it to the network.
II. User Plane Setup The AMF selects the SMF (which performs all session management operations in the 4G system MME (as well as SGW-C and PGW-C)) to manage all session management operations itself. Message exchange between the AMF and SMF is conducted through the N11 interface. The SMF then finds the best UPF (User Plane Function) for the UE and creates a session during the UL and DL data streams. Interaction between SMF and UPF is performed via PFCP (Packet Forwarding Control Protocol) on the N4 interface; the specific process (message) is as follows:
[3] Check the session ID of the existing PDU session
[3] Send a PDU session establishment acceptance message to the UE and gNB
[3] Send a PDU session resource establishment request message to the gNB
[4] Process the PDU session resource establishment response
[4] Process the PDU session resource release response
[20] AMF processes PDU session establishment rejection
[20] Send a PDU session rejection message to the UE
[3] Set the session AMBR
[20] Update the IP address information in the SMF context and send a downlink transmission message with a 5GMM reason to the gNB
[3] [5] Retrieve the user QoS profile and UPF GTP TEID IP address from the SMF context
[1] Send an activation PDU session context request message
[5] Add a security header to the AMF PDU session transmission request
[3] [6] Generate a new AMF NGAP UE ID
[8] Notify NGAP of the new AMF NGAP ID
