Context Replacement Call Flow

The SMF supports the context replacement procedure, when it receives a Create Session Request (CSReq) with the existing IMSI and EBI combination for which there’s an existing subscriber.

The SMF can receive a CSReq for an already existing session. When the new CSReq is received, the existing session gets replaced with the new session, based on the new request. The context replacement procedure also referred as the create-over-create procedure.

As a first step in this process, the disconnect procedure for an existing session gets initiated. Later, it establishes a session based on the new request.

The following image describes the context replacement procedure call flow.

Call Flow for the Context Replacement Procedure
Call Flow Description for the Context Replacement Procedure

Step

Description

1

The MME sends a Create Session Request to the S-GW.

2

The S-GW sends a Create Session Request to the SMF.

Note

A stale session was already present for this IMSI and EBI combination. The disconnect procedure gets started for the existing session.

3

The SMF sends a PFCP Session Deletion Request to the UPF.

4

The UPF sends a PFCP Session Deletion Response to the UPF.

5

The SMF sends an Async RADIUS Accounting-Request Stop to the AAA server.

6

The SMF sends an Async Gx CCR-T to the PCRF.

7

The SMF sends an Async Gy CCR-T to the OCS.

8

The AAA server sends a RADIUS Accounting-Response to the SMF.

9

The PCRF sends a Gx CCA-T to the SMF.

10

The OCS sends a Gy CCA-T to the SMF.

Note

After this action, the connect procedure continues for the new request.

Note

The order in which the response messages outlined in Steps 8–10 is subject to change depending on the availability.