Registration with AMF Change Call Flow
This section describes the Registration with AMF Change call flow.

Step |
Description |
---|---|
1 |
UE builds a registration message with type set to Mobility Updating or Initial Registration and sends it to the gNB. At gNB, the message becomes the payload of an INITIAL UE message (if the UE is in ECM_IDLE) or an UPLINK NAS TRANSPORT message when the UE is in ECM_CONNECTED (Only for Mobility Updating). When the UE is in ECM_CONNECTED, there's a N2 handover procedure that precedes this part, and the context transfer steps of the call flow are omitted. |
2 |
The new AMF analyses the GUTI that is send by the UE and determines whether it’s allocated by a different AMF. The new AMF determines the old AMF using parameters from the GUTI and constructs a transfer request. The whole message body that is received by the new AMF is part of the request to the old AMF. The new AMF sets the type of transfer request based on whether the UE is registering for initial registration or mobility updating. |
3 |
The old AMF checks the integrity of the message that is received from the new AMF. If the integrity check passes, the old AMF packages the attributes of the UE that is available in the response to the transfer request. If the request was for mobility updating, PDU session information present in the old AMF is sent to the new AMF. |
4 |
The new AMF sends a transfer update message to the old AMF. |
5 |
If the new AMF decides not to use the current PCF, the old AMF clears the PCF associations created by it. In the case of initial updating, the AMF clears all the PDU sessions. The new AMF interacts with UDM to register as the node responsible for the UE. The AMF also interacts with the UDM to register for changes to subscription data for the UE, and these steps are the same as the one executed by the AMF during initial registration. |
6 |
Once the new AMF registers with the UDM, the UDM notifies the old AMF that its registration has been cancelled. |
7 |
The old AMF acknowledges the notification from the UDM. |
8 |
Since the old AMF is no longer interested in changes to the subscription information, it sends a cancel for subscription for changes to SDM subscription. |
9 |
The UDM clears the subscription and responds to the old AMF. The old AMF clears any state it has on the UE. The new AMF sets up policies in the PCF, and these steps are the same as those done during initial registration. |
10 |
If AMF policies are to be set up with the PCF, the AMF sends a request to create the policies in the PCF. |
11 |
PCF responds to the AMF request. |
12 |
For each PDU session that the new AMF has taken over, it sends a message to the SMF to change the AMF for the session. |
13 |
SMF responds to the AMF. |
14 |
AMF sends a Registration Accept to the UE with new GUTI and a Tracking Area List. These steps are same as done during the AMF initial registration. |
15 |
If the registration type is Mobility Updating, AMF ignores FollowOn IE and does not initiate UE CONTEXT RELEASE COMMAND. |