ocedure S1AP E-RAB Modification Indication 53: GTPv2-C Modify Bearer Request 54:Switch Path from MN-eNB to SN-gNB 55: GTPv2-C Modify Bearer Response 56: GTPv2 End Marker Packet 56: GTPv2 End Marker Packet 57: S1 AP E-RAB Modification Confirmation 58: GTP PDU Figure 10.11 - Option 3x call flow - part 4 20. The UE extracts NR information from the LTE RRC Connection Reconfiguration message. The UE also extracts the NR RACH configuration information and C-RNTI for 5G access. It then prepares the RRC Connection Reconfiguration Complete message and sends it to the master eNB. This message also contains the NR RRC Reconfiguration Complete message meant for the secondary gNB. 21. The master eNB then sends the NR RRC Reconfiguration message received from the UE to the secondary gNB via the X2AP SgNB Reconfiguration Complete message. At this point, the eNB starts copying data to the gNB. 22. The master eNB informs the gNB about the PDCP sequence number and the HFN for all the bearers, which are transferred to the 5G NR gNB. 23. The SGW can now send data to the MN-eNB. The master eNB keeps forwarding the data to the secondary NR gNB. 24. Since the UE has not yet established the data path, the gNB is buffering the data it receives from the packet core. 25. The master eNB then sends an SIAP E-RAB Modification Indication request to the MME, which notifies the MME that the data bearer is being switched from 4G-eNB to 5G-NR. 26. The MME updates this information with the SGW via a modify bearer request. The data path is now switched from the eNB to the gNB. Advantages and shortcomings 27. The response is then sent to the MME indicating a successful or an unsuccessful path switch. 28. The master eNB then sends an End Marker Packet to the eNB. This marks the end of the transmission to the secondary 4G-eNB. All packets henceforth will be handled by the 5G-NR-gNB. 29. MME sends an E-RAB Modification Confirmation message to the master eNB. 30. Data now flows to the secondary 5G-gNB. This concludes a typical option 3-based call flow. I