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5G Call Flow (ENB – GNB Dual Connectivity)

ENDC -Dual Connectivity (EN-DC) which enables 5G services and 4G network.

  • UEs supporting EN-DC can connect simultaneously to LTE Master Node eNB and 5G-NR Secondary Node gNB.
  • UE first register with 4G EPC through ENDC.
  • Based on UE supportability it starts reporting measurement on 5G Frequencies and LTE nodeB communicate with g-NB For 5G resource assignment.
  • The 5G-NR resource assignment is then signaled to the UE via an LTE RRC Connection Reconfiguration message.
  • After RRC Connection Reconfiguration procedure completion, the UE simultaneously connects to the 4G and 5G networks.
  • RRC Connection setup complete carries the NAS Attach Request.
  • The DCNR bit in the “UE Network Capability” IE is set.
  • This signals is confirmation to the 4G Core Network that the UE supports dual connectivity with LTE and 5G-NR.
  • The NAS messages from the UE are signaled to the Core Network via the Initial UE message.
  • MME initiates the authentication procedure.     
  • Authentication is successfully completed.
  • MME initiates NAS level security procedure.
  • NAS level security procedure is completed.

MME responds eNB with

  1. S1AP Initial Context Setup Request.
  2. NAS Attach Accept.
  3. Activate Default Bearer Request.
  • UE capabilities are requested for 4G-LTE (utra), EN-DC (eutra-nr) and 5G (nr).
  • EUTRA-NR specific capabilities are specified in the UE-MRDC-Capability container. The message also contains information about the supported 5G frequency bands.
  • UE capabilities are transferred to the MME.
  • security set between the eNB and the UE.
  • Ciphering is enabled in both directions.
  • RRC Connection reconfiguration is sent for activation of default radio bearer.
  • It also contains attach accept message.
  • It contains measurement objects for 5G-NR frequency.
  • Initial Context Setup message contains the GTP TEID that used for sending downlink data to the eNB
  • Attach Complete is sent by UE to MME
  • MME modifies the bearer and sends the TEID to use for downlink data.
  • Respond back to MME.
  • UE acquires the 5G-NR PSS and SSS.
  • 5G signal is then reported to LTE.
  • The LTE eNB decides to add the 5G-NR as a secondary node.
  • The gNB provide with information about the radio resources and bearers admitted with the 5G network.
  • The NR RRC configuration message is included in the message.
  • UE acquires the 5G-NR PSS & SSS.
  • UE acquires the 5G-NR Broadcast Channel.
  • The UE initiates the RACH procedure with the 5G gNB.
  • NR PDCCH signals downlink resource allocation for the RA Response.
  • The 5G secondary node gNB responds with an RAR.
  • UE reports the Power Headroom (PHR) to eNB periodically.
  • The PHR MAC CE contains the power headroom for the cells on the eNB and gNB cells.
  • UE reports measurements  to e-NB include results from 5G NR cells.
  • gNB reports the usage data for 5G NR bearers to the eNB.
  • gNB also reports any overload information to the eNB.