Xn-based Inter NG-RAN Handover

This section provides details regarding the Xn-based inter NG-RAN handover without UPF reallocation.

The handover preparation and the execution stages are implemented as specified in 3GPP TS 38.300. When performing the handover in a shared network, the source NG-RAN determines a PLMN to be used in the target network as specified in 3GPP TS 23.501. If the serving PLMN changes during the Xn handover, the source NG-RAN node indicates the selected PLMN ID to the target NG-RAN node.

If the AMF generates the N2 downlink signalling and receives a rejection to an N2 interface procedure due to the ongoing Xn handover procedure, the AMF reattempts the same N2 interface procedure either when the handover is complete or the handover is deemed to have failed. The failure is known by expiry of the timer guarding the N2 interface procedure.

Upon reception of an SMF-initiated N1 and/or N2 request(s) with an indication that the request has been temporarily rejected due to the ongoing Xn handover procedure, the SMF starts a locally configured guard timer. The SMF holds signalling messages targeted towards the AMF during the handover preparation phase unless it detects that the handover is completed or the handover has failed or cancelled. The SMF reattempts, up to a pre-configured number of times, when either it detects that the handover is completed or has failed using message reception or at expiry of the guard timer.

The Xn-based inter NG-RAN handover is used to hand over a UE from a source NG-RAN to target NG-RAN using Xn when the AMF is unchanged and the SMF decides to keep the existing UPF.

The following figure depicts the call flow of the Xn-based inter NG-RAN handover without the UPF reallocation.

Xn-based Inter NG-RAN Handover without UPF Reallocation
Xn-based Inter NG-RAN Handover Call Flow Description (Without UPF Reallocation)

Step

Description

1a

During the handover execution, the source NG-RAN node provides RAN usage data Report to the AMF. The source NG-RAN node provides this report only when the target NG-RAN has confirmed handover over Xn interface.

This report includes N2 SM Information (Secondary RAT usage data), Handover Flag, and Source to Target transparent container. The Handover Flag indicates that the report needs to be buffered by the SMF.

1b

The target NG-RAN sends an N2 Path Switch Request message to the AMF to inform that the UE has moved to a new target cell. The NG-RAN provides a List Of PDU Sessions To Be Switched. The N2 SM Information includes the AN Tunnel Info for each PDU Session to be switched.

2

The AMF sends N2 SM information by invoking the Nsmf_PDUSession_UpdateSMContext request service operation for each PDU session in the lists of PDU Sessions received in the N2 Path Switch Request.

3

The SMF sends an N4 Session Modification Request message to the UPF. The SMF may notify the UPF that originated the Data Notification to discard downlink data for the PDU Sessions and/or to not provide further Data Notification messages.

4

The UPF returns an N4 Session Modification Response message to the SMF after the requested PDU sessions are switched.

5

The UPF sends one or more "end marker" packets for each N3 tunnel on the old path immediately after switching the path. The UPF starts sending downlink packets to the target NG-RAN.

6

The SMF sends an Nsmf_PDUSession_UpdateSMContext response (CN Tunnel Info) to the AMF for PDU sessions which have been switched successfully.

Important

Step 6 can occur any time after the receipt of N4 Session Modification Response at the SMF.

7

Once the Nsmf_PDUSession_UpdateSMContext response is received from all the SMFs, the AMF aggregates the received CN Tunnel Info and sends this aggregated information as a part of N2 SM Information along with the Failed PDU Sessions in N2 Path Switch Request Ack to the target NG-RAN. If none of the requested PDU sessions have been switched successfully, the AMF sends an N2 Path Switch Request Failure message to the target NG-RAN.

8

The target NG-RAN confirms success of the handover by sending Release Resources message to the source NG-RAN.

9

The UE initiates Mobility Registration Update procedure if one of the triggers of registration procedure applies.

The following figure shows the detailed call flow of the Xn handover without UPF reallocation.

Xn Handover Without UPF Relocation Call Flow
Detailed Call Flow Description for the Xn Handover Without UPF Relocation

Step

Description

1

The NF Service Consumer (AMF) requests the SMF to switch the user plane connection of the PDU session. The AMF sends a POST request with the following information:

  • The toBeSwitched indication.

  • N2 SM information received from the 5G-AN (PDU session path switch request transfer IE), including the new transport layer address and tunnel endpoint of the downlink termination point for the user data for this PDU session.

  • User location and user location timestamp.

  • Other information, if necessary.

2

The SMF switches the N3 tunnel of the PDU session after receiving the request. The SMF initiates PFCP session modification procedure toward the UPF with downlink FAR updated with the following option:

  • Forwarding Action is enabled with the remote node “forwarding parameters” details, such as the IP address and GTP-U F-TEID.

3

The SMF marks the PDU handover as successful after receiving the successful response from the UPF node.

4

The SMF initiates the 200 OK response. This response includes the N2 SM information, which has the transport layer address and tunnel endpoint of the uplink termination point for the user plane data for this PDU session, that is UPFs GTP-U F-TEID for the uplink traffic.