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5g nsa option 3x call flow

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The 5G New Radio (5G NR) architecture provides various deployment options to integrate with existing 4G LTE networks, known as Non-Standalone (NSA) mode. Among the different NSA modes, Option 3x is one of the prominent configurations that utilize the LTE Core network (EPC) for signaling and control while employing the 5G NR for user data.

Here's a technical breakdown of the 5G NSA Option 3x call flow:

  1. Initial Attachment:
    • The UE (User Equipment) first attaches to the LTE eNodeB (eNB) as in a typical LTE attach procedure.
    • It acquires an LTE RRC connection with the eNB and performs the initial registration procedures, including the acquisition of an LTE Radio Bearer for signaling.
  2. 5G NR Registration Request:
    • After attaching to LTE, the UE detects 5G coverage and initiates the 5G NR registration by sending a Registration Request message to the eNB.
  3. RRC Reconfiguration (Handover to 5G):
    • Upon receiving the Registration Request from the UE, the eNB sends an RRC Reconfiguration message to the UE, instructing it to perform a handover to 5G NR.
    • This RRC Reconfiguration message includes necessary parameters for 5G NR connection setup.
  4. 5G NR Connection Setup:
    • The UE tunes to the specified 5G NR frequency and synchronizes with the 5G NR cell.
    • The UE then sends a Random Access Request to the 5G gNodeB (gNB) for initial access and synchronization.
    • After successful synchronization, the gNB sends a Random Access Response, and the UE transmits a RRC Connection Request to the gNB.
  5. Establishment of Data Radio Bearer:
    • After successful RRC connection establishment with the gNB, the UE and gNB set up a Data Radio Bearer (DRB) in the 5G NR for user data transfer.
  6. Dual Connectivity (DC):
    • In 5G NSA Option 3x, the UE maintains the LTE connection to the eNB for control plane signaling, while the user data is transmitted over the 5G NR connection with the gNB. This architecture allows for seamless mobility and efficient use of both LTE and 5G resources.
  7. Data Transfer:
    • For user data sessions, the UE sends and receives data through the 5G NR DRB established with the gNB.
    • Control plane signaling, such as mobility management, paging, and session management, continues to be handled by the LTE eNB.
  8. Mobility Handling:
    • As the UE moves, handover scenarios may arise. If the UE moves out of the 5G coverage area but remains within the LTE coverage area, the control plane signaling continues with LTE.
    • Conversely, if the UE moves back into 5G coverage, the gNB may instruct the UE to perform a handover back to 5G, and the process would be reversed.