UE-initiated PDU Session Release Call Flow

The UE-initiated PDU session release procedure allows the UE to request the release of the PDU session. In the case of Local Breakout (LBO), the procedure is as in the case of non-roaming with the difference that the AMF, the SMF, the UPF, and the PCF are located in the visited network.

The following figure depicts the UE-initiated PDU session release procedure to support EPS interworking on the SMF as specified in 3GPP TS 23.502, section 4.3.4.2.

UE-initiated PDU Session Release Call Flow
UE-initiated PDU Session Creation Call Flow Description
Step Description

1, 2

The UE sends PDU_SESSION_RELEASE_REQUEST in NAS message to the AMF through the RAN. The AMF sends the message to the SMF in SmContextUpdateRequest.

3, 4

The SMF sends N4SessionReleaseRequest to the UPF. The UPF sends response for the same.

5

The SMF sends SmContextUpdateResponse message with N1 and N2 content.

  • N1: PDU_SESSION_RELEASE_COMMAND

  • N2: N2_PDU_SESSION_RESOURCE_RELEASE_COMMAND. exclude if the SMF is in IDLE mode. Also, skip the steps 8, 9, and 10.

6, 7

The AMF exchanges the message with RAN. The RAN forwards it to the UE.

8, 9, 10

The RAN sends N2 release response to the AMF. The AMF sends N2 release response (N2_PDU_SESSION_RESOURCE_RELEASE_RESPONSE_TRANSFER) in N11 SmContextUpdateRequest message. The SMF responds back to the AMF as SmContextUpdateResponse.

11, 12, 13, 14

The UE sends N1 release response in NAS message to the AMF through the RAN. The AMF sends N1 release response (PDU_SESSION_RELEASE_COMPLETE) in N11 SmContextUpdateRequest message. The SMF responds back to the AMF as SmContextUpdateResponse.

15, 16

The SMF sends delete charging request to the CHF. The CHF responds back to the SMF with delete response.

17, 18

The SMF sends SmContextStausNotify to the AMF. The AMF responds back with SmContextStausNotifyResponse message.

19, 20

The SMF sends delete request to the PCF. The PCF responds back to the SMF with delete response

21, 22

The SMF sends UDM deregistration request. The UDM responds back to the SMF with deregistration response.