Network-initiated Modification Call Flow for Inactive User Plane and UE in CM-Idle State

This section describes the network-initiated modification procedure when the UE is in CM-Idle state and the User Plane (UP) context is deactivated.

The SMF supports the following use cases during the network-initiated PDU session modification procedure:

  • When the UE turns active with the service request for PDN activation

  • When the UE turns active with the control service request

Use case 1: When the UE turns active with the service request for PDN activation

  1. The PCF sends a policy update notification to the SMF for a PDU session with rules and QoS change. The SMF handles the updated policy rules when received in a notification from the PCF.

  2. The SMF returns the “200 OK” status code to the PCF.

  3. The SMF sends only N1 message PDU Session Modification Command to the UE with the modified rules and QoS change, using the NAMF Communication N1 N2 Message Transfer service operation towards the AMF.

  4. The AMF sends the NAMF Communication N1 N2 Message Transfer response to the SMF. This response includes the “200 OK” status code and the “ATTEMPTING_TO_REACH_UE” cause.

  5. The SMF stops the retransmission of the N1 - PDU Session Modification response message to the UE. Further, it stops the N1 PDU Modification Command retransmission timer and waits for a response from the UE.

    Note

    The N1 PDU Modification Command retransmission timer is configurable. Use the n1 t3591-pdu-mod-cmd timeout timeout max-retry retry_count command in Access Profile Configuration mode to configure the timeout value and maximum attempts for the retransmission of N1 PDU Modification Command. The default timeout value is 2 seconds and the default retry count is 2.

  6. The UE receives the paging request from the AMF and initiates the requested service to activate the PDU session. The UE includes the PDU Session ID in PDU Session-to-Activate list only if the UP context needs to be activated.

    The SMF initiates the Idle-to-Active PDU Session transition procedure and suspends the current modification procedure.

  7. After the Idle-to-Active procedure is complete, the SMF restarts the modification procedure and sends both the N1 and N2 content in N1 N2 transfer message and waits for both N1 and N2 response from the UE and gNB respectively.

  8. The SMF receives the N2 response from gNB, and the N1 response from the UE respectively.

  9. The SMF sends N4 Session Modification request to the UPF updating the User Plane tunnel modified rules and the QoS details.

  10. The UPF sends the N4 Session Modification response for the PDU session.

Use case 2: When the UE turns active with the control service request

  1. The PCF sends a policy update notification to the SMF for a PDU session with rules and QoS change. The SMF handles the updated policy rules when received in a notification from the PCF.

  2. The SMF returns the “200 OK” status code to the PCF.

  3. The SMF sends only N1 message PDU Session Modification Command to the UE with the modified rules and QoS change, using the NAMF Communication N1 N2 Message Transfer service operation towards the AMF.

  4. The AMF sends the NAMF Communication N1 N2 Message Transfer response to the SMF. This response includes the “200 OK” status code and the “ATTEMPTING_TO_REACH_UE” cause.

  5. The SMF stops the retransmission of the N1 - PDU Session Modification response message to the UE. Further, it stops the N1 PDU Modification Command retransmission timer and waits for a response from the UE.

  6. The AMF initiates the paging procedure towards the UE and the UE turns active with the Service Request for control message.

  7. The SMF receives the N1 response from the UE.

  8. The SMF sends N4 Session Modification request to the UPF updating the User Plane tunnel modified rules and the QoS details. Then, the SMF sets the Forwarding Action Rule (FAR) action for the new rules as ‘drop’.

  9. The UPF sends the N4 Session Modification response for the PDU session.