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Content for  TS 29.502  Word version:  18.7.0

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5.2.2.3.8 EPS to 5GS Handover using N26 interface  5.2.2.3.8.1 General  5.2.2.3.8.2 EPS to 5GS Handover Preparation  5.2.2.3.8.3 EPS to 5GS Handover Execution  5.2.2.3.8.4 EPS to 5GS Handover Cancellation  5.2.2.3.8.5 EPS to 5GS Handover Failure   5.2.2.3.9 5GS to EPS Handover using N26 interface  5.2.2.3.9.1 General   5.2.2.3.9.2 Data forwarding tunnels setup during 5GS to EPS handover   5.2.2.3.9.3 Indirect data forwarding tunnels removal for 5GS to EPS handover cancellation or failure  
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5.2.2.3.8  EPS to 5GS Handover using N26 interface  p. 43
5.2.2.3.8.1  General  p. 43
The NF Service Consumer (e.g. AMF) shall request the SMF to handover a UE EPS PDN connection to 5GS using N26 interface, following the same requirements as specified for N2 handover in clause 5.2.2.3.4 with the modifications specified in this clause.
5.2.2.3.8.2  EPS to 5GS Handover Preparation  p. 43
The requirements specified in clause 5.2.2.3.4.2 shall apply with the following modifications.
Reproduction of 3GPP TS 29.502, Fig. 5.2.2.3.8.2-1: EPS to 5GS Handover Preparation
Figure 5.2.2.3.8.2-1: EPS to 5GS Handover Preparation
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Step 1.
Same as step 1 of Figure 5.2.2.2.3-1.
Step 2a.
Same as step 2 of Figure 5.2.2.2.3-1.
Step 2b.
Same as step 2b of Figure 5.2.2.3.1-1.
Step 3.
Same as step 3 of Figure 5.2.2.3.4.2-1.
Step 4a.
Same as step 4 of Figure 5.2.2.3.4.2-1, with the following modifications:
The 200 OK response shall not include N2 SM information for DL forwarding tunnel setup, but shall additionally contain:
  • the epsBearerSetup IE(s), containing the list of EPS bearer context(s) successfully handed over to the 5GS and DL data forwarding information, containing either:
    • CN tunnel information generated based on the list of accepted QFI(s) received from the 5G-RAN, if indirect data forwarding applies; or
    • NG-RAN F-TEID per E-RAB accepted for direct data forwarding, as received from the target NG-RAN, if direct data forwarding applies.
Step 4b.
Same as step 2b of Figure 5.2.2.3.4.2-1.
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5.2.2.3.8.3  EPS to 5GS Handover Execution  p. 44
The requirements specified in clause 5.2.2.3.4.3 shall apply, with the following modifications.
In step 1 of Figure 5.2.2.3.4.3-1, the NF Service Consumer, i.e. the target AMF, shall include one or more SecondaryRatUsageDataReportContainer(s) in the SmContextUpdateData for the POST request if it received one or more Secondary RAT Usage Data Report(s) applicable for the PDU session from the source MME.
In step 2 of Figure 5.2.2.3.4.3-1, for a Home Routed PDU session, the SMF shall complete the execution of the handover by initiating an Update service operation towards the H-SMF in order to switch the PDU session towards the V-UPF (see clause 5.2.2.8.2.3).
If there are PDU sessions that failed to handover due to timeout of SMF responses in any step of the handover preparation phase (e.g. if the Update SM Context Response arrived too late or not at all during the handover preparation phase), then the AMF shall consider that the PDU session will be released by the MME and remove the PDU session context from the UE context. For a HR PDU session or a PDU session with I-SMF, the AMF shall also release the SM Context in the V-SMF or the I-SMF only.
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5.2.2.3.8.4  EPS to 5GS Handover Cancellation  p. 44
The requirements specified in clause 5.2.2.3.4.4 shall apply, with the following modifications.
In step 2 of Figure 5.2.2.3.4.4-1, for a Home Routed PDU session, the V-SMF shall cancel the handover by initiating an Update service operation towards the H-SMF in order to release resources at H-SMF and H-UPF reserved for handover (see clause 5.2.2.8.2.14).
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5.2.2.3.8.5  EPS to 5GS Handover Failure   p. 44
If the handover to 5GS failed, e.g. rejected by the target NG-RAN, the requirements specified in clause 5.2.2.3.4.4 shall apply, with the following modifications:
  • the hoState attribute set to "CANCELLED", to indicate the handover is cancelled;
  • the cause attribute set to "HO_FAILURE".
In step 2 of Figure 5.2.2.3.4.4-1, for a Home Routed PDU session, the V-SMF shall cancel the handover by initiating an Update service operation towards the H-SMF in order to release resources at H-SMF and H-UPF reserved for handover (see clause 5.2.2.8.2.17).
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5.2.2.3.9  5GS to EPS Handover using N26 interface  p. 44
5.2.2.3.9.1  General   p. 44
The NF Service Consumer (e.g. AMF) shall request the SMF to setup data forwarding tunnels if direct or indirect data forwarding applies to the 5GS to EPS handover using N26 interface, and to remove the indirect data forwarding tunnels previously established when the handover is cancelled or failed.
The AMF should initiate this procedure only if data forwarding is enabled and the MME returns data forwarding F-TEIDs for the related PDN connection context in the Forward Relocation response.
5.2.2.3.9.2  Data forwarding tunnels setup during 5GS to EPS handover   p. 45
If direct or indirect data forwarding applies to the 5GS to EPS handover, the NF Service Consumer (e.g. AMF) shall provide the SMF with the data forwarding information received from the MME, as specified in clause 4.11.1.2.1 of TS 23.502), as follows.
Reproduction of 3GPP TS 29.502, Fig. 5.2.2.3.9-1: 5GS to EPS Handover using N26 interface (data forwarding tunnels setup)
Figure 5.2.2.3.9-1: 5GS to EPS Handover using N26 interface (data forwarding tunnels setup)
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Step 1.
The NF Service Consumer shall send a POST request, as specified in clause 5.2.2.3.1, with the following information:
  • dataForwarding IE set to true;
  • EPS bearer contexts received from the MME in the Forward Relocation Response, including F-TEID(s) for DL data forwarding tunnel(s) towards the target eNB (for direct data forwarding) or towards the forwarding SGW (for indirect data forwarding).
Step 2a.
If indirect data forwarding applies, the SMF shall map the EPS bearers for Data Forwarding to the 5G QoS flows based on the association between the EPS bearer ID(s) and QFI(s) for the QoS flow(s).
The SMF shall return a 200 OK response including the following information:
  • N2 SM information (see Handover Command Transfer IE in clause 9.3.4.10 of TS 38.413) containing DL forwarding tunnel information to be sent to the source 5G-AN by the AMF if direct or indirect data forwarding applies (see step 11f of clause 4.9.1.3.2 of TS 23.502).
    If direct data forwarding applies, the DL forwarding tunnel information shall contain the E-UTRAN tunnel info for data forwarding per EPS bearer received from the MME.
    If indirect data forwarding applies, the DL forwarding tunnel information shall contain the CN transport layer address and tunnel endpoint (i.e. UPF's GTP-U F-TEID) for Data Forwarding and the QoS flows for Data Forwarding for this PDU session.
Step 2b.
If the SMF cannot proceed with the request, the SMF shall return an error response, as specified for step 2b of Figure 5.2.2.3.1-1.
If none of the EPS bearer contexts received in the POST request body includes an F-TEID for DL data forwarding, the SMF shall return a 403 Forbidden response including a ProblemDetails structure with the "cause" attribute set to "NO_DATA_FORWARDING". Upon receipt of this response, the AMF shall proceed with the handover procedure (as if data forwarding was disabled).
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5.2.2.3.9.3  Indirect data forwarding tunnels removal for 5GS to EPS handover cancellation or failure   p. 45
During 5GS to EPS handover, if indirect data forwarding tunnel(s) have been previously established during the preparation phase and the handover is cancelled, the AMF shall update the SMF of handover cancellation by sending a POST request with the cause attribute set to "HO_CANCEL" and dataForwarding IE set to false with an empty list of EPS bearer contexts. The SMF shall then release the resources prepared for the handover and proceed with the PDU session as if no handover procedure had taken place.
If no resources for EPS bearer(s) can be assigned for any PDU session attempted to be handed over, the AMF shall update the SMF with the information that the handover preparation failed by sending a POST request with the cause attribute set to "HO_FAILURE" and with an empty list of EPS bearer contexts (and without the dataForwarding IE). The SMF shall then release the resources prepared for the handover and proceed with the PDU session as if no handover procedure had taken place.
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