SMF-Initiated PDU Session Modification Call Flow
This section describes the SMF-Initiated PDU Session Modification call flow.
The PDU Session Modification is required when one or several of the QoS parameters exchanged between the UE and the network need to be modified.

Step |
Description |
---|---|
1 |
The SMF starts the PDU Session Modification to the AMF through Nsmf_PDUSession_UpdateSMContext (N2 SM information (PDU Session ID, QFI(s), QoS Profile(s), Session-AMBR), N1 SM container (PDU Session Modification Command (PDU Session ID, QoS parameters, Session-AMBR)). The N2 SM carries information that the AMF provides to the RAN. It includes the QoS profiles and the corresponding QFIs to notify the RAN that one or more QoS flows were added, or modified. It can also include only QFI(s) to notify the RAN that one or more QoS flows were removed. The N2 SM information provided to the RAN includes information for establishment of User Plane resources. The N1 SM container carries the PDU Session Modification Command that the AMF provides to the UE. |
2 |
The SMF invokes Namf_Communication_N1N2MessageTransfer (N2 SM information (PDU Session ID, QFI(s), QoS Profile(s), Session-AMBR), N1 SM container (PDU Session Modification Command (PDU Session ID, QoS rule(s), QoS parameters, Session-AMBR))). |
3 |
The AMF sends the N2 PDU Session Request (N2 SM information received from SMF, NAS message (PDU Session ID, N1 SM container (PDU Session Modification Command))) Message to the RAN. |
4 |
The RAN issues AN-specific signaling exchange with the UE that is related with the information received from SMF. |
5 |
The RAN acknowledges N2 PDU Session Request by sending a N2 PDU Session Ack (N2 SM information (List of accepted/rejected QFI(s), AN Tunnel Info, PDU Session ID), User location Information) Message to the AMF. |
6 |
The AMF forwards the N2 SM information and the User location Information received from the AN to the SMF through Nsmf_PDUSession_UpdateSMContext service operation. The SMF replies with a Nsmf_PDUSession_UpdateSMContext Response. |
7 |
If the RAN rejects QFI(s) the SMF is responsible of updating the QoS rules and QoS Flow level QoS parameters if needed for one or more QoS Flows associated with one or more QoS rules in the UE accordingly. |
8 |
The UE acknowledges the PDU Session Modification Command by sending a NAS message (PDU Session ID, N1 SM container (PDU Session Modification Command Ack)) message. |
9 |
The RAN forwards the NAS message to the AMF. |
10 |
The AMF forwards the N1 SM container (PDU Session Modification Command Ack) and User Location Information received from the AN to the SMF through Nsmf_PDUSession_UpdateSMContext service operation. The SMF replies with a Nsmf_PDUSession_UpdateSMContext Response. |