PDU Session Establishment Call Flow

This section describes the PDU Session Establishment call flow.

The UE receives data services through a PDU session, which is a logical connection between the UE and core network.

During the PDU session establishment, UE establishes a PDU session for accessing data services. Unlike EPS, where a default PDU session is always created while the UE registers to the network, in 5G, the UE can establish a PDU session when the service is needed.

PDU Session Establishment Call Flow
PDU Session Establishment Call Flow Description

Step

Description

1

In order to establish a new PDU Session, the UE generates a new PDU Session ID and initiates 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 a PDU session ID, Requested PDU Session Type, Requested SSC mode, 5GSM Capability PCO, SM PDU DN Request Container, and Number of Packet Filters.

2

The AMF selects SMF based on slice-info and plmn-info provided by UE. SMF is selected by NRF query or by static configuration. AMF invokes the Nsmf_PDUSession_CreateSMContext Request towards SMF with SUPI, DNN, single or multiple 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, DNN Selection Mode. Subscriber data-store is modified to store PDU information. The AMF-service stickiness is maintained for the subscriber for the PDU establishment transaction.

3

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)).

4

The SMF sends Namf_Communication_N1N2MessageTransfer to the AMF. The N2 SM information carries information that the AMF shall forward 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.

5

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.

6

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

7

The gNB sends the N2 PDU Session Request Ack to the AMF.

The N2 PDU Session Response included PDU Session ID, Cause, N2 SM information (PDU Session ID, AN Tunnel Info, List of accepted/rejected QFI(s), User Plane Enforcement Policy Notification).

8

The AMF sends the Nsmf_PDUSession_UpdateSMContext Request (N2 SM information, Request Type) to the SMF. The AMF forwards the N2 SM information received from the RAN to the SMF.

9

The SMF sends the Nsmf_PDUSession_UpdateSMContext Response to the AMF.