N2 Handover without AMF Change Call Flow

This section describes the N2 Hanover without AMF Change call flow.

N2 Handover without AMF Change Call Flow
N2 Handover without AMF Change Call Flow Description

Step

Description

1

With signaling from the UE, the source gNB starts the handover procedure by sending a HANDOVER REQUIRED message to the AMF.

2

The AMF finds a gNB that can support the signaled TargetId from the gNB. AMF rejects the message when it can’t find gNB. The AMF creates a ModificationRequest and sends it to the SMF.

3

The SMF analyzes the TargetID and takes appropriate actions. The SMF then responds.

4

The AMF finds the gNB corresponding to the Target ID, and the NGAP EP that serves that gNB. The AMF then sends a handover required message to the target gNB.

5

Target gNB sets up the resources required for the handover and responds with an ACK message. This ACK message contains the PDU resources that failed to setup as well.

6

The AMF constructs a Sm Context Modify message to update the target gNB tunnel endpoint IDs to the SMF. The AMF starts a guard timer and forwards the message to the SMF.

7

The SMF updates the information in associated UPFs and responds to the AMF.

8

The AMF builds a HandoverCommand message and sends it to the source gNB.

9

The UE now completes the handover at the target gNB. The target gNB sends a HANDOVER NOTIFY message to the AMF.

10

The AMF constructs a Sm Context Modify request to inform the SMF that the handover is complete.

11

SMF responds to the update.

12

The Handover procedure ends. The source gNB receives a UE context release command.

13

If there are PDU sessions that fail to setup at the target gNB are now released at the SMF.