5G to EPS Handover with IDFT Timer Call Flow
This section describes the 5G to EPS handover with IDFT timer call flow.

Step | Description |
---|---|
1 | NG-RAN determines to handover UE to E-UTRAN. If NG-RAN is configured to perform inter-RAT mobility due to the IMS voice fallback that is triggered by QoS flow setup and request to set up QoS flow for IMS voice is received, then NG-RAN indicates the rejection of the QoS flow establishment. This indication is because of mobility due to fallback for the IMS voice through N2 SM information and triggers the handover to E-UTRAN. The NG-RAN sends a Handover Required message to the AMF. This message includes the details on target eNB ID, direct forwarding path availability, source to target transparent container, and inter-system handover indication. NG-RAN uses the source to target transparent container to indicate bearers for the corresponding 5G QoS flows for data forwarding. |
2a | AMF sends the NSMF PDU Session Context Request to the SMF+PGW-C to provide SM Context. |
2b |
SMF+PGW-C sends the N4 session modification to PGW-U+UPF to establish the CN tunnel for each EPS bearer. The bearer mapping to the 5G QoS and PCC rules, which PCC sends, are available in the SMF. The SMF also has the bearer IDs that are received from the bearer ID allocation procedure. The SMF+PGW-C creates new PDRs for the N4 session and gets the TEID allocated for each bearer as required by the 4G system. The timer in SMF+PGW-C starts in this step. This timer monitors the resources for indirect data forwarding in UPF that are to be released. Following are the cases for the IDFT timer expiry:
|
2c |
SMF+PGW-C sends the EPS bearer contexts to AMF. The bearer context is a string with the byte format, which is the base64-encoded characters, encoding the UE EPS PDN Connection IE. The SMF+PGW-C also provides the CN tunnel information to AMF for all the bearers for the uplink traffic from E-UTRAN. |
3 | AMF sends a Forward Relocation Request to MME. The AMF includes the mapped SM EPS UE Contexts for the PDU Sessions with and without active UP connections. |
4 | MME sends the Create Session Request to SGW. See S1-based handover in the normal case section in 3GPP TS 23.401, clause 5.5.1.2.2 for details. |
5 | SGW sends the Create Session Response to MME. See S1-based handover in the normal case section in 3GPP TS 23.401, clause 5.5.1.2.2 for details. |
6 | MME sends the handover request to E-UTRAN. |
7 | E-UTRAN sends the handover request acknowledgment to MME. |
8 | MME and SGW send and receive the indirect data forwarding request and response to each other. |
9 | MME sends the Relocation Response to AMF. |
10a | In case of indirect data forwarding, AMF sends the NSMF PDU Session Update SM Context Request to PGW-C+SMF. This request is for SGW addresses and SGW DL TEIDs for data forwarding and for creating the indirect data forwarding tunnel. |
10b |
PGW-C+SMF sends the N4 Modification Request to UPF+PGW-U to create more PDRs and FARs. The PDRs and FARS are created to receive the redirected DL data over the indirect tunnel from NG RAN and to forward them to eNodeB. The UL PDRs have the QFI to match the forwarded DL data from NG RAN. The associated QER has the QFI to forward the data to eNodeB. Also, the FAR redirects the received data to eNodeB over the appropriate tunnel based on the QFI. |
10c |
PGW-C+SMF sends the NSMF PDU Session Update SM Context Response to AMF. This response includes details on cause, CN tunnel information for data forwarding, and QoS flows for data forwarding. PGW-C+SMF sends this response to create indirect data forwarding. Based on the correlation between QFIs and SGW addresses and TEIDs for data forwarding, the PGW-U+UPF maps the QoS flows into the data forwarding tunnels in EPC. |
11a | AMF sends the Handover Command to the source NG-RAN. |
11b |
The source NG-RAN sends the Handover Command to UE to handover to the target access network. The UE correlates the ongoing QoS Flows with the indicated EPS Bearer IDs (EBI) that are to be set up in the handover command. If the QoS Flow associated with the default QoS rule in the PDU Session has an unassigned EBI, the UE deletes the PDU Session locally. If the QoS Flow that is associatedthat is with the default QoS rule has an assigned EBI, the UE retains the PDU session. For the QoS Flow with unassigned EBIs, the UE deletes the QoS rules and the QoS Flow level QoS parameters locally if any associated with those QoS Flows. Then, the UE notifies the impacted applications that the dedicated QoS resource has been released. The UE deletes any UE-derived QoS rules. The EBI that was assigned for the QoS Flow of the default QoS rule in the PDU Session becomes the EBI of the default bearer in the corresponding PDN connection. |
12a |
UE sends the notification of handover completion to E-UTRAN. |
12b |
E-UTRAN sends the Handover Notify request to MME. |
12c |
MME sends the Relocation Complete Notification to AMF. |
12d |
AMF sends the Relocation Complete Notification acknowledgment to MME. |
13 |
MME sends the Modify Bearer Request to SGW. |
14a |
SGW sends the Modify Bearer Request to SMF+PGW-C. This request includes the information on DL TEIDs on SMF for the bearers. |
15 |
PGW-C+SMF initiates a N4 Session Modification procedure toward the UPF+PGW-U to update the User Plane path, which implies that DL User Plane for the indicated PDU Session is switched to E-UTRAN. The PGW-C+SMF releases the resource of the CN tunnel for PDU Session in UPF+PGW-U. |
16 |
PGW-C+SMF sends Modify Bearer Response to SGW. At this stage, the User Plane path is established for the default bearer and the dedicated EPS bearers between the UE, target eNodeB, SGW, and the PGW-U+UPF. The PGW-C+SMF uses the EPS QoS parameters as assigned for the dedicated EPS bearers during the QoS Flow establishment. PGW-C+SMF maps all the other IP flows to the default EPS bearer. If indirect forwarding tunnels are established, the PGW-C+SMF starts a timer to release the resources that are used for indirect data forwarding. |
17 |
SGW sends Modify Bearer Response to MME. |
18 |
UE initiates a Tracking Area Update procedure. See the S1-based handover in the normal case section in 3GPP TS 23.401, clause 5.5.1.2.2 for details. This procedure deregisters the old AMF for 3GPP access from HSS+UDM. Any registration that is associated with the non-3GPP access in the old AMF is not removed. It implies that an AMF that is serving the UE over both the 3GPP and non-3GPP accesses does not consider the UE as deregistered over non-3GPP access and remains registered and subscribed to subscription data updates in UDM. |
19 |
If PCC is deployed, then PCF determines to provide the earlier removed PCC rules to the PGW-C+SMF again. With these PCC rules, the PGW-C+SMF initiates the dedicated bearer activation procedure. |
20 |
SGW sends the Delete Indirect Data Forwarding Tunnel Request to MME. The MME sends the Delete Indirect Data Forwarding Tunnel Response to SGW. |
21a |
AMF initiates NSMF PDU Session Update SM Context Request service operation with an indication to release the forwarding tunnels. |
21b |
SMF sends the N4 Modification Request to UPF+PGW-U to delete the PDRs and FARs for the indirect tunnels. The PDRs and FARs for the 5G session which are not required are also removed. The IDFT timer that started in Step 2b stops. |