PDU Session Establishment for Existing PDU Call Flow

This section describes the PDU Session Establishment for Existing PDU call flow.

If the UE starts the PDU Establishment Request for an existing PDU, the AMF performs local PDU release and sends the PDU release to SMF. It also initiates PDU Resource Setup Request. If the PDU release fails at SMF, the AMF sends the PDU reject.

PDU Session Establishment for Existing PDU Call Flow

The UE receives data services through a PDU session, which is a logical connection between the UE and the core network. The PDU Session Establishment procedure describes the procedures by which UE establishes a PDU session for accessing data services. In 5G, the UE can establish a PDU session when service is needed.

PDU Session Establishment for Existing PDU Call Flow Description

Step

Description

1

In order to establish a new PDU session, the UE generates a new PDU Session ID and starts the PDU Session Establishment procedure by the transmission of a NAS message containing a PDU Session Establishment Request within the N1 SM container. The PDU Session Establishment Request includes the following:

  • PDU session ID

  • Requested PDU Session Type

  • Requested SSC mode

  • 5GSM Capability PCO

  • SM PDU DN Request Container

  • Number of Packet Filters

2,3,4

If PDU exists, then clean up at AMF and SMF (SmContextReleaseRequest) is done and PDU Session Establishment procedure is performed.

5

The AMF selects SMF based on slice-info and plmn-info provided by UE. The SMF is selected by NRF query or static configuration. The AMF invokes the Nsmf_PDUSession_CreateSMContext Request towards the SMF with SUPI, DNN, S-NSSAIs, PDU Session ID, AMF ID, Request Type, PCF ID, Priority Access, N1 SM container (PDU Session Establishment Request), User location information, Access Type, PEI, GPSI, UE presence in LADN service area, Subscription For PDU Session Status Notification, and DNN Selection Mode. The subscriber data store is modified to store the PDU information. The AMF service stickiness is maintained for the subscriber for the PDU establishment transaction.

6

The SMF creates an SM context and responds to the AMF by sending Nsmf_PDUSession_CreateSMContext Response (Cause, SM Context ID or N1 SM container (PDU Session Reject (Cause)).

7

The SMF sends Namf_Communication_N1N2MessageTransfer to AMF. The N2 SM information carries information that the AMF forwards to the RAN. The N1 SM container contains the PDU Session Establishment Accept that the AMF provides to the UE. The Namf_Communication_N1N2MessageTransfer contains the PDU Session ID allowing the AMF to know which access towards the UE to use.

8

The AMF sends the NAS message containing PDU Session ID and PDU Session Establishment Accept targeted to the UE and the N2 SM information received from the SMF within the N2 PDU Session Request to the RAN.

If the Initial Context Setup is incomplete, the AMF sends the NAS message information as a part of the Initial Context Setup Request.

9

The AMF sends the Initial Context Setup Request to the gNB.

The RAN forwards the NAS message (PDU Session ID, N1 SM container (PDU Session Establishment Accept)) to the UE.

10

The gNB sends the Initial Context Setup Response to the AMF with the N2 PDU Session Response (PDU Session ID, Cause, N2 SM information (PDU Session ID, AN Tunnel Info, List of accepted or rejected QFIs, User Plane Enforcement Policy Notification)).

11

The AMF forwards the N2 SM information received from RAN to the SMF.

The Nsmf_PDUSession_UpdateSMContext Request includes the N2 SM information and Request Type.

12

The SMF responds with the Nsmf_PDUSession_UpdateSMContext_Response to the AMF.