5G NR SA Registration/Attach Call Flow

UE interactions

• It performs the random access procedure to initiate communication with the gNB. 
• Setup the RRC connection with the gNB. 
• A signaling bearer is set up at this point. The UE also sends a Registration Request to the 5G Core. 
• It performs the NAS (Non-Access Stratum) level Authentication and initiates ciphering for the NAS messages with the Core. 
• Complete the AS (Access Stratum) security procedure with the gNB. 
• It handles the RRC Reconfiguration from the gNB.
• This message sets up the default PDU session. The message may add secondary cells.
• The UE concludes the registration procedure. 
• Data is flowing in the uplink and downlink directions. 

gNB interactions 

UDM- Unified Data management 

It is a centralized way to control network usage data. It is cloud-native and designed for 5G technology. The functions of UDM are similar to HSS (Home Subscriber Service). It generates 5G AKA Authentication vectors, user identification handling, this function stores the long-term security credentials used in Authentication for AKA. It stores subscription information.

• The UDM provides authentication vectors during the registration procedure. 
• The new AMF serving the user also registers with the UDM. 
• The UDM also provides Subscriber Data Management information to the new AMF. 

PCF - Policy Control Function

It supports the unified policy framework that governs the network behavior. It provides policy rules to enforce them. To facilitate this, the subscription information is gathered from the Unified data management function. It provides policy rules for control plane functions, which include network slicing, roaming, and mobility management. 

• The New AMF creates a policy associated with the PCF.
• The old AMF deletes the policy associated with the PCF.
• The PCF registers with the AMF so that it can be notified of events like location change and communication failure. 

SMF - Session Management Function

It is the fundamental element of the 5G core network architecture which is responsible for interacting with decoupled data plane, creating, updating, and removing PDU (Protocol data units). It manages the session context with the User plane function. The functions performed by SMF include UE IP address allocation and management, termination of interfaces, downlink data notifications, charging data collection, lawful intercept, roaming functionality, etc.

• The old AMF deregisters with the SMF.
• The new AMF updates the session management context. The SMF assigns an IP address and the tunnel ID to be used for sending uplink data. 
• The SMF selects the UPF to be used for the session. 
• The SMF updates the UPF using PFCP messages via the N4 control-data plane interface. 
• The new AMF also notifies the SMF when the session is ready for uplink and downlink data transfer. 

UPF - User Plane Function 

It is the fundamental component of a 3GPP 5G Core Architecture. It represents the data plane evolution of a control and user plane separation (CUPS). The functions of UPF include QoS handling for the user plane, packet routing, and forwarding, packet inspection, lawful intercept for the user plane, traffic accounting and reporting, etc.

• The UPF is a data plane component that handles user data.
• The UPF is completely controlled from the SMF using the N4 interface.
• The SMF uses the Packet FLow control protocol (PFCP) to update the data plane. 
• The first PFCP session Modification request prepares the UPF to receive uplink data. 
• The UPF also starts buffering the downlink data. 
• Once the default PDU session has been set up, the SMF updates the PFCP session to start sending downlink data.