Feature Description
3GPP specifies provisions for UEs capable of supporting both 5G and 4G NAS to connect to E-UTRAN and 5G core network.
To forward data (G-PDUs and End Marker packets) during an EPS to 5GS handover, the SMF:
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Provisions one PDR per E-RAB (that supports data forwarding for at least one QoS flow).
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Creates and associate one QER with each PDR, including the QFI IE set to the QFI value of one of the QoS flows mapped to the E-RAB, to request the UPF to insert a GTP-U PDU Session Container extension header including the QFI.
Data forwarding during handovers between 5GS and EPS is supported as follows (see, 3GPP TS 38.300):
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For 5G to 4G handover, the source NG-RAN node sends one or several end-markers including one QFI of those QoS flows mapped to the same E-RAB and sends the end-marker packets to the UPF over the PDU session tunnel. UPF removes the QFI and maps to an appropriate E-RAB tunnel towards SGW.
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For 4G to 5G handover, the source eNB forwards the received end markers in the EPS bearer tunnel to the SGW, which forwards them to the UPF. The UPF adds one QFI among the QoS flows mapped to that E-RAB to the end-markers and sends those end-markers to the target NG-RAN node in the per PDU session tunnel.