4G to 5G Handover Call Flow

This section describes the 4G to 5G handover call flow.

4G to 5G Handover Call Flow Description

Step

Description

1

As the UE transitions to 5GC, the SRC-ENodeB (source eNB) sends a Handover Required message to the SRC MME (source MME).

2

The MME does target-ID analysis, and chooses the TGT AMF (target AMF). The source MME sends a Forward Relocation Request to the TGT AMF (target AMF).

3

The target AMF creates associations with the SMF for the PDU sessions that are acceptable on the target AMF.

4

The SMF responds to the Create Request from the target AMF.

5

The target AMF selects the TGT GNB (taget gNB) that serves the UE, and sends a Handover Request to the target GNB.

6

After the GNB has allocated resources to the UE, it responds with a Handover Request ACK to the target AMF.

7, 8

The target AMF updates the SMF with information from the target GNB.

8

The SMF responds to the target AMF.

9

The target AMF responds to the request from the source MME using a Forward Relocation Response message.

10

The source MME sends a Handover Command to the SRC ENodeB (source eNB) to complete the handover.

11

Target gNB sends a Handover Notify to the target AMF when the UE handover gets completed.

12

The target AMF notifies the source MME using a Forward Relocation Complete notification.

13

The source MME acknowledges the message with a Forward Relocation Complete acknowledge.

14, 15

The target AMF notifies the SMF on the completion of the handover.

The SMF acknowledges the notification.

Note

When AMF receives the Registration Request message, it performs the following:

  • When the message is received in Uplink NAS Transport, it assumes that the HO is successful and doesn't send the Context Request to the MME.

  • When the message is received in Initial UE message, it sends the Context Request to the MME.