AMF to MME Idle Mode Handoff Call Flow

This section describes the AMF to MME Idle Mode Handoff call flow.

The following call flow shows the messaging that happens in the network.

AMF to MME Idle Mode Handoff Call Flow
AMF to MME Idle Mode Handoff Call Flow Description

Step

Description

1

A UE that was previously registered on an AMF in 5GC moves to EPC, and sends a TAU request to an MME. From the GUTI sent by the UE, the MME finds the identity of the AMF, and sends a ContextRequest over N26.

2

The ContextRequest reaches the AMF. MME sends the full TAU Request message as a part of the ContextRequest. The AMF does integrity checks on the request to ascertain the validity of the request.

If the request from the MME indicates that the MME has authorized the UE, AMF doesn’t do security checks on the received message.

If integrity checks fail, AMF rejects the request. Or else, the AMF retrieves the PDU sessions information from each of the SMFs that host PDUs for this UE and has allocated EBI for their sessions from the AMF.

3

The SMF responds to the SmContext Retrieve Request from the AMF.

4

AMF responds to MME by sending a ContextResponse message when AMF receives all the expected responses from SMFs.

It starts context-transfer-guard timer (configured with greater than zero (0)). On expiry of the context-transfer-guard timer, the source AMF performs the following:

Triggers the UDM Deregistration internally to clear the local ueContext.

5

The MME sends a ContextAcknowledgement message to the AMF. The AMF starts a guard timer to clear allocated resources in case the notification from the UDM to clear the registration doesn’t come through.

6

Since the MME is now the owner of the registration, the UDM notifies the AMF that the registration for 3GPP access is cancelled.

7

The AMF releases any local resources, and responds to the UDM.

If the guard timer is not running, the AMF releases any local resources, and responds to the UDM.

Otherwise amf responds to the UDM, and waits for the guard timer to expire before cleaning up the UEcontext.

8

The AMF clears the subscription to changes in subscription data at the UDM.

9

The UDM responds to the request from the AMF.