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NAS : PDU Session Establishment

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The Non-Access Stratum (NAS) Protocol Data Unit (PDU) Session Establishment is a procedure in 5G that involves the establishment of a PDU Session, which is a data communication session between the User Equipment (UE) and the 5G Core Network (5GC). The NAS layer is responsible for the signaling between the UE and the 5GC for various functions, including session establishment.

Let's break down the NAS PDU Session Establishment procedure into technical details:

1. Background:

  • NAS: The Non-Access Stratum is responsible for the signaling between the UE and the core network. It includes functions related to mobility, session management, security, and more.
  • PDU Session: A Packet Data Unit (PDU) Session is a communication session that carries user data between the UE and the 5GC. It is established to enable data transfer for various services, such as internet access, multimedia streaming, and other applications.

2. Initiation of the Procedure:

  • Trigger: The PDU Session Establishment procedure is triggered when the UE needs to initiate a data communication session, for example, when the user wants to access the internet.
  • User Request: The UE or an application on the UE requests the establishment of a PDU Session to initiate data transfer.

3. Procedure Steps:

a. Attach Procedure:

  • If the UE is not already attached to the 5G network, an Attach procedure may precede the PDU Session Establishment.

b. Registration:

  • The UE registers its presence and capabilities with the 5GC.

c. Authentication and Security Setup:

  • The UE undergoes authentication and security setup to ensure a secure connection.

d. PDU Session Establishment Request:

  • The UE sends a NAS message to the 5GC indicating the request to establish a PDU Session. This message includes information such as the type of service requested, QoS requirements, and other session parameters.

e. Network Capability Exchange:

  • The 5GC responds with a message, and there may be an exchange of capabilities and parameters between the UE and the network to negotiate session characteristics.

f. PDU Session Establishment Acceptance:

  • Upon successful negotiation, the 5GC sends a NAS message to the UE, indicating the acceptance of the PDU Session Establishment.

g. Bearer Setup:

  • The 5GC sets up the necessary bearers to facilitate the data transfer. This involves the establishment of paths and configurations for user data transmission.

h. PDU Session Establishment Complete:

  • The UE acknowledges the successful establishment of the PDU Session to the 5GC.

4. Session Management:

  • Session Context: The 5GC maintains a context for the established PDU Session, storing relevant information such as IP addresses, QoS parameters, and security keys.
  • UE Mobility: The NAS PDU Session Establishment procedure includes mechanisms to handle UE mobility, allowing the session to be maintained or transferred seamlessly as the UE moves between cells.

5. QoS and Policies:

  • Quality of Service (QoS): The QoS parameters negotiated during the PDU Session Establishment ensure that the data transfer meets the required performance levels, considering factors like latency, reliability, and bandwidth.
  • Policies: The 5GC may enforce policies during the establishment to control the access and resources allocated to the UE based on subscription profiles and network policies.

6. Security Considerations:

  • Authentication: The authentication process ensures that the UE is legitimate and authorized to access the 5G network.
  • Encryption: Security mechanisms, including encryption, protect the confidentiality and integrity of user data during the PDU Session.

7. Use Cases:

  • Internet Access: PDU Sessions are commonly established for providing the UE with internet access.
  • Application Services: PDU Sessions support various application services, including video streaming, messaging, and other data-intensive applications.

8. Release and Termination:

  • Release Procedure: When the UE or the network decides to terminate the PDU Session, a release procedure is initiated to gracefully close the session.
  • Session Termination: The UE sends a request to release the PDU Session, and the 5GC acknowledges the request, releasing the resources associated with the session.

9. Logging and Monitoring:

  • Logging: The network and UE may log events related to the PDU Session Establishment, aiding in troubleshooting and monitoring.
  • Monitoring: Operators can monitor the ongoing PDU Sessions for performance and quality assurance purposes.

In summary, the NAS PDU Session Establishment procedure in 5G involves a series of signaling steps between the UE and the 5GC to establish a data communication session. This procedure ensures secure, efficient, and QoS-compliant data transfer for various services and applications supported by the 5G network.