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

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. |