Establishment of a PDU Session in 5G/NR involves a structured multi-step process to facilitate communication between the User Equipment (UE) and the network's Data Network (DN) through the 5G Core (5GC). This ensures that the UE can access services such as internet and Voice over New Radio (VoNR). Below are the key steps involved, with additional details for each phase:

1. PDU Session Establishment Request: The process starts when the UE initiates a request for a PDU session. This request can be triggered by user actions (such as launching a data service) or by the network itself (for services like IMS emergency calls). The request includes parameters such as the desired PDU session type (IPv4, IPv6, or Ethernet), requested QoS, and the Data Network Name (DNN) the UE wants to connect to.
2. Initial Registration: Before establishing a PDU session, the UE must register with the 5G network. During registration, the UE and the network perform mutual authentication using credentials stored in the USIM and the Authentication Server Function (AUSF). The Access and Mobility Management Function (AMF) manages this process, establishing a secure signaling connection and updating the UE's location in the network.
3. Session Management Function (SMF) Selection: Once registration is complete, the AMF selects an appropriate SMF based on the UE's subscription profile, requested services, and network policies. The SMF is responsible for managing the lifecycle of the PDU session, including resource allocation and policy enforcement.
4. Session Establishment: The SMF coordinates with the User Plane Function (UPF) to set up the user plane path for the session. This involves configuring routing rules, establishing tunnels, and ensuring that the data path between the UE and the DN is ready. The SMF also interacts with the Policy Control Function (PCF) to apply relevant policies and QoS rules.
5. Allocation of Network Resources: The SMF allocates necessary network resources, such as IP address assignment for the UE, and configures the UPF for data forwarding. This step may involve interaction with the DHCP server or internal IP address management functions. The network ensures that the allocated resources meet the requested QoS and security requirements.
6. PDU Session Establishment Acceptance: After successful resource allocation and path setup, the SMF sends a PDU Session Establishment Accept message to the UE via the AMF. This message contains session parameters, assigned IP address, and QoS information. The UE acknowledges receipt, and the session becomes active.
7. Data Transfer: With the PDU session established, the UE can now send and receive data packets to and from the DN. The UPF handles packet routing, forwarding, and QoS enforcement. The data path is optimized for low latency and high throughput, supporting various services such as web browsing, video streaming, and VoNR.
8. Session Management and Modification: Throughout the session, the SMF monitors and manages the PDU session. It can modify session parameters in response to changes in network conditions, user mobility (handover between cells), or updated QoS requirements. The SMF can also initiate session release when the service is no longer needed.

This process is defined by the 3rd Generation Partnership Project (3GPP) standards, ensuring interoperability, security, and efficiency across different network vendors and configurations. The PDU session establishment procedure is fundamental to enabling flexible, high-performance data services in 5G networks.

Key Network Functions Involved:

• UE (User Equipment): The device requesting the PDU session.
• gNB (Next Generation NodeB): The 5G base station providing radio access.
• AMF (Access and Mobility Management Function): Handles registration, authentication, and mobility management.
• SMF (Session Management Function): Manages session establishment, modification, and release.
• UPF (User Plane Function): Forwards user data packets between the UE and DN.
• PCF (Policy Control Function): Provides policy and QoS rules.
• AUSF (Authentication Server Function): Performs authentication procedures.
• DN (Data Network): The external network (e.g., internet, IMS) the UE connects to.

References: For detailed signaling flows and message formats, refer to 3GPP TS 23.502 and TS 24.501.