N2-based Inter NG-RAN Handover
The source NG-RAN decides to initiate an N2-based handover (HO) to the target NG-RAN. Initiation of this procedure could be due to any of the following reasons:
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New radio conditions
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Load balancing
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If there is no Xn connectivity to the target NG-RAN
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An error indication from the target NG-RAN after an unsuccessful Xn-based handover (that is, no IP connectivity between Target RAN (T-RAN) and Source UPF (S-UPF))
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Based on dynamic information learnt by the Source RAN (S-RAN)
The source NG-RAN determines the availability of a direct forwarding path and indicates the same to the SMFs. If the IP connectivity is available between the source and target NG-RAN and security association is in place between them, a direct forwarding path is available. If a direct forwarding path is not available, use the indirect forwarding. The SMFs use the indication from the source NG-RAN to choose the data forwarding path.
When performing the handover in a shared network, the source NG-RAN determines a PLMN for use in the target network as specified by 3GPP TS 23.501. The source NG-RAN indicates the selected PLMN ID to the AMF as part of the Tracking Area sent in the HO Required message.
If the AMF generates the N2 downlink signalling and receives a rejection to a N2 interface procedure due to the ongoing N2 handover, the AMF reattempts the same N2 interface procedure either when the handover is complete or the handover is deemed to have failed. If the Inter NG-RAN node handover changes the serving AMF, the source AMF terminates any other ongoing N2 interface procedures except the handover procedure.
If the AMF is still the serving AMF, the AMF pauses non-handover related N2 interface procedures and resumes them after the N2 handover is complete.
If the AMF detects that it needs to be changed, the AMF rejects any SMF-initiated N2 request and includes an indication that the request has been temporarily rejected due to the ongoing N2 handover procedure.
The following figure depicts the call flow for the preparation phase of the N2-based inter NG-RAN handover procedure.

Step |
Description |
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1 |
The Source NG-RAN (S-RAN) sends the Handover Required message to the Source AMF (S-AMF). This message includes the following:
The Source to Target transparent container includes NG-RAN information for use in Target RAN (T-RAN), and is transparent to 5GC. It also contains the corresponding User Plane Security Enforcement information, QoS flows/DRBs information subject to data forwarding. If direct data forwarding is available, the SM N2 info includes Direct Forwarding Path Availability. Direct Forwarding Path Availability indicates whether direct forwarding is available from the S-RAN to the T-RAN. This indication from S-RAN is based on the presence of IP connectivity and security association between the S-RAN and the T-RAN. |
2 |
When the S-AMF cannot serve the UE anymore, the S-AMF selects the T-AMF as described in clause 6.3.5 on "AMF Selection Function" in TS 23.501. |
3 |
The S-AMF initiates Handover resource allocation procedure by invoking the Namf_Communication_CreateUEContext service operation towards the T-AMF. The Namf_Communication_CreateUEContext Request includes the following:
When the S-AMF can still serve the UE, this step and step 12 are not needed. |
4 |
For each PDU session indicated by S-RAN, the AMF invokes the Nsmf_PDUSession_UpdateSMContext Request to the associated SMF. However, if the S-NSSAI associated with PDU session is not available in the T-AMF, the T-AMF does not invoke Nsmf_PDUSession_UpdateSMContext for this PDU session. If the T-AMF detects that the UE moves into a restricted area based on Service area restrictions, the T‑AMF notifies that the UE is only reachable for regulatory prioritized services to each NF consumer which has subscribed for UE reachability event. |
5 |
Based on the Target ID, the SMF checks the acceptance of N2 handover for the indicated PDU session. The SMF also checks the UPF Selection Criteria. If the UE has moved out of the service area of the UPF connecting to NG-RAN, the SMF selects a new intermediate UPF. |
6a |
If the SMF selects a new UPF to act as intermediate UPF for the PDU session, and the different CN Tunnel Info need to be used, the SMF sends N4 Session Modification Request message to UPF (PDU Session Anchor (PSA)). If the SMF allocates the CN Tunnel Info, it provides the CN Tunnel Info on N9, and the UPF (PSA) associates CN Tunnel Info with UL Packet detection rules. |
6b |
The UPF (PSA) sends an N4 Session Establishment Response message to the SMF. If the UPF (PSA) allocates CN Tunnel Info (on N9) of UPF (PSA), it provides CN Tunnel Info (on N9) to the SMF. The UPF (PSA) associates the CN Tunnel Info (on N9) with UL Packet detection rules provided by the SMF. |
6c |
If the SMF selects a new intermediate UPF (T-UPF) and if the T-UPF allocates the CN Tunnel Info, the SMF sends an N4 Session Establishment Request message to the T-UPF. This request enables the Packet detection, enforcement, and reporting rules to be installed on the T-UPF. The T-UPF receives the CN Tunnel Info (on N9) of UPF (PSA) for this PDU session, which is used to set up N9 tunnel. |
6d |
The T-UPF sends an N4 Session Establishment Response message to the SMF with DL CN Tunnel Info and UL CN Tunnel Info (that is, N3 tunnel info). The SMF starts a timer to release the resource of S-UPF, which is to be used in step 13a of the Execution Phase. |
7 |
If N2 handover for the PDU session is accepted, the SMF includes the N2 SM Information in the Nsmf_PDUSession_UpdateSMContext response. The N2 SM Information contains the N3 UP address and the UL CN Tunnel ID of the UPF and the QoS parameters indicating that the N2 SM Information is for the Target NG-RAN. If the N2 SM information received at step 4 does not include the Direct Forwarding Path Availability and the SMF knows that there is no indirect data forwarding connectivity between source and target, the N2 SM Information includes a Data forwarding not possible indication. If the N2 handover for the PDU session is not accepted as described in step 5, the SMF does not include the N2 SM Information to avoid establishment of radio resources at the target NG-RAN. The SMF provides a reason for non-acceptance. If the SMF receives notification from T-AMF that UE is only reachable for regulatory prioritized service, the SMF deactivates the PDU session. |
8 |
The AMF supervises the Nsmf_PDUSession_UpdateSMContext Response messages from the involved SMFs. At the expiry of maximum wait time or when all Nsmf_PDUSession_UpdateSMContext Response messages are received, the AMF continues with the N2 Handover procedure (Handover Request message in step 9). |
9 |
If the subscription information includes Tracing Requirements, the target AMF provides the target RAN with Tracing Requirements in the Handover Request. The Handover request includes Source to Target transparent container, N2 MM Information, N2 SM Information list, and Tracing Requirements. The T-AMF determines T-RAN based on Target ID. T-AMF allocates a 5G-GUTI valid for the UE in the AMF and target TAI. N2 MM Information includes, for example, security information and Mobility Restriction List if available in the T-AMF. N2 SM Information list includes N2 SM Information for the T-RAN in the Nsmf_PDUSession_UpdateSMContext Response messages received within allowed max delay supervised by the T-AMF in step 8. |
10 |
The T-RAN sends Handover Request Acknowledge to the T-AMF. The Acknowledge message includes Target to Source transparent container, List of PDU Sessions to Hand-over with N2 SM information, List of PDU Sessions that failed to be established with the failure cause given in the N2 SM information element. |
11a |
The AMF sends Nsmf_PDUSession_UpdateSMContext Request (PDU Session ID, N2 SM response) to the SMF. For each N2 SM response received from the T-RAN, the AMF sends the N2 SM response to the SMF indicated by the respective PDU Session ID. If no new T-UPF is selected, the SMF stores the N3 tunnel info of T-RAN from the N2 SM response if N2 handover is accepted by T-RAN. The SMF/UPF allocates the N3 UP address and Tunnel IDs for indirect data forwarding corresponding to the data forwarding tunnel endpoints established by T-RAN. If a PDU session is indicated as a rejected PDU session by the Target NG-RAN, the SMF triggers the release of this PDU session. In all other cases of PDU Session rejection, the SMF decides whether to release the PDU session or to deactivate the UP connection of this PDU session. If some of the QoS Flows of a PDU Session are not accepted by the Target NG-RAN, the SMF initiates the PDU Session Modification procedure to remove the non-accepted QoS Flows from the PDU Session(s) after the handover is completed. |
11b |
The SMF sends N4 Session Modification Request to the T-UPF. This request includes T-RAN SM N3 forwarding Information list, and indication to allocate DL forwarding tunnel(s) for indirect forwarding. |
11c |
The T-UPF allocates Tunnel Info and returns an N4 Session Modification Response message to the SMF. The T-UPF SM N3 forwarding info list includes T-UPF N3 address, T-UPF N3 Tunnel identifiers for forwarding data. |
11d |
The SMF sends N4 Session Modification Request to the S-UPF. This request includes T-RAN SM N3 forwarding Information list or T-UPF SM N3 forwarding Information list, and an indication to allocate DL forwarding tunnel(s) for indirect forwarding. |
11e |
The S-UPF allocates Tunnel Info and returns an N4 Session establishment Response message to the SMF. The S-UPF SM N3 forwarding Information list includes S-UPF N3 address and S-UPF N3 Tunnel identifiers for DL data forwarding. |
11f |
The SMF sends an Nsmf_PDUSession_UpdateSMContext Response message per PDU session to the T-AMF. |
12 |
The AMF supervises the Nsmf_PDUSession_UpdateSMContext Response message from the involved SMFs. At the expiry of maximum wait time or when all Nsmf_PDUSession_UpdateSMContext Response messages are received, the T-AMF sends the Namf_Communication_CreateUEContext Response to the S-AMF. |
The following figure depicts the call flow for the execution phase of the N2-based inter NG-RAN handover procedure.

Step |
Description |
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1 |
The Source AMF (S-AMF) sends the Handover Command to the Source NG-RAN (S-RAN). The Handover Command includes Target to Source transparent container, List Of PDU Sessions to be handed-over with N2 SM information containing information received from T-RAN during the handover preparation phase, and List Of PDU Sessions failed to be set up. The SM forwarding info list includes T-RAN SM N3 forwarding info list for direct forwarding or S-UPF SM N3 forwarding info list for indirect data forwarding. The S-RAN uses the PDU Sessions failed to be setup list and the indicated reason for failure to decide whether to proceed with the N2 handover procedure. |
2 |
The S-RAN sends Handover Command (UE container) to the UE. The UE container is a UE part of the Target to Source transparent container which is sent transparently from T-RAN via AMF to S-RAN and is provided to the UE by the S-RAN. |
2a - 2c |
The S-RAN sends the Uplink RAN Status Transfer message to the S-AMF. The S-RAN refrains from sending this message if none of the radio bearers of the UE are treated with Packet Data Convergence Protocol (PDCP) status preservation. |
3 |
The T-RAN sends the uplink packets to the T-UPF and UPF (PSA). The UPF (PSA) sends the downlink packets to the S-RAN via S-UPF. The S-RAN forwards the downlink data towards the T-RAN for QoS flows or Data Radio Bearers (DRBs) subject to data forwarding. The data forwarding path is either direct (step 3a) or indirect forwarding (step 3b). |
4 |
After the UE has successfully synchronized to the target cell, it sends a Handover Confirm message to the T-RAN. |
5 |
The T-RAN sends Handover Notify message to the T-AMF. This message is sent to indicate that the handover is successful. |
6a. |
The T-AMF notifies to the S-AMF about the N2 handover notify received from the T-RAN by invoking the Namf_Communication_N2InfoNotify. The S-AMF uses a timer to supervise the release of resources in S-RAN. |
6b |
The S-AMF acknowledges by sending the Namf_Communication_N2InfoNotify ACK to the T-AMF. |
6c |
The S-AMF sends Nsmf_PDUSession_ReleaseSMContext Request to the SMF. This request includes SUPI, PDU Session ID, and N2 SM Information (Secondary RAT Usage Data). If the PDU Session(s) is not accepted by the T-AMF, the S-AMF triggers PDU Session Release procedure after the reception of N2 Handover Notify. |
7 |
The T-AMF sends Nsmf_PDUSession_UpdateSMContext Request to the SMF. This request includes Handover Complete indication for PDU Session ID, UE presence in LADN service area, and N2 SM Information (Secondary RAT usage data). The T-AMF sends Handover Complete indication per each PDU Session to the corresponding SMF to indicate the success of the N2 handover. |
8a |
If a new T-UPF is inserted or an existing intermediate S-UPF is reallocated, the SMF sends N4 Session Modification Request indicating DL AN Tunnel Info of T-RAN to the T-UPF. |
8b |
The T-UPF acknowledges by sending N4 Session Modification Response message to the SMF. |
9a |
If the UPF is not reallocated, the SMF sends N4 Session Modification Request indicating DL AN Tunnel Info of T-RAN to the S-UPF. |
9b |
The S-UPF acknowledges by sending N4 Session Modification Response message to SMF. |
10a |
For non-roaming or local breakout roaming scenario, the SMF sends N4 Session Modification Request message to PDU Session Anchor UPF, UPF (PSA). If a new T-UPF is inserted or an existing intermediate S-UPF is reallocated, the SMF provides N3 AN Tunnel Info of T-RAN or the DL CN Tunnel Info of T-UPF. If the T-UPF is not inserted or an existing intermediate S-UPF is not reallocated, skip the step 10a and step 10b. |
10b |
The UPF (PSA) sends N4 Session Modification Response message to the SMF. When there are multiple UPFs (PSA), perform step 10a and step 10b for each UPF (PSA). |
11 |
The SMF sends Nsmf_PDUSession_UpdateSMContext Response (PDU Session ID) to the T-AMF. The SMF confirms reception of Handover Complete. |
12 |
The UE initiates Mobility Registration Update procedure as defined in 3GPP TS 23.502. |
13a |
If there is a source intermediate UPF, the SMF initiates resource release by sending an N4 Session Release Request (Release Cause) to the source UPF. This message is also used to release the indirect data forwarding resource in the S-UPF. |
13b |
The S-UPF acknowledges with an N4 Session Release Response message to confirm the release of resources. In case of indirect data forwarding, the resource of indirect data forwarding is also released. |
14a |
After the expiry of timer (defined in step 6a), the AMF sends UE Context Release Command. |
14b |
The source NG-RAN releases its resources related to the UE and responds with a UE Context Release Complete () message. |
15a |
If indirect forwarding applies and the UPF is reallocated, after the timer of indirect data forwarding expires, the SMF sends N4 Session Modification Request to the T-UPF. Then, the T-UPF releases the indirect data forwarding resources. |
15b |
The T-UPF acknowledges with an N4 Session Modification Response message to confirm the release of indirect data forwarding resources. |