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5G Standalone Mode Registration-Attach Call Flow

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The 5G Standalone (5G SA) mode registration and attach procedure involves the interaction between the User Equipment (UE), the 5G Core (5GC), and the Radio Access Network (RAN). The following is a detailed technical explanation of the 5G SA registration and attach call flow:

  1. UE Power On:
    • The UE powers on and performs initial procedures like cell selection and PLMN (Public Land Mobile Network) selection.
  2. Initial Access:
    • The UE initiates initial access by sending a Random Access Preamble to the gNB (gNodeB), indicating its presence and readiness to establish a connection.
  3. Random Access Response:
    • The gNB responds with a Random Access Response, providing the Timing Advance for the UE and indicating that it has received the UE's preamble successfully.
  4. RRC Connection Establishment:
    • The UE proceeds to establish an RRC (Radio Resource Control) connection with the gNB by sending an RRC Connection Request.
  5. RRC Connection Setup:
    • The gNB responds with an RRC Connection Setup message, confirming the establishment of the RRC connection.
  6. Registration Request:
    • The UE initiates the registration process by sending a Registration Request to the 5GC, indicating its intent to register and attach to the network.
  7. Access Authentication and Authorization:
    • The 5GC performs access authentication and authorization procedures to verify the UE's credentials and whether it is allowed to access the network.
  8. Authentication Request:
    • If necessary, the 5GC sends an Authentication Request to the UE, prompting the UE to provide authentication information.
  9. Authentication Response:
    • The UE responds to the Authentication Request with an Authentication Response, providing the necessary authentication information.
  10. Security Mode Command:
    • The 5GC sends a Security Mode Command to the UE, instructing it to establish a secure connection using specific security algorithms and keys.
  11. Security Mode Complete:
    • The UE responds with a Security Mode Complete message, indicating that it has successfully established the requested security measures.
  12. Registration Accept:
    • The 5GC sends a Registration Accept message to the UE, indicating that the registration and attach procedures are successful.
  13. RRC Reconfiguration:
    • The 5GC instructs the gNB to perform RRC reconfiguration, updating the radio resources and parameters for the established connection.
  14. RRC Connection Reconfiguration Complete:
    • The gNB sends an RRC Connection Reconfiguration Complete message to the UE, confirming the completion of the reconfiguration.
  15. Attach Accept:
    • The 5GC sends an Attach Accept message to the UE, indicating that the attach procedure is complete, and the UE is now successfully attached to the 5G network.
  16. Data Transfer:
    • With the successful attach, the UE is now ready to exchange data with the network, and normal user data transfer can begin.

This sequence of steps outlines the technical details of the 5G Standalone Mode Registration-Attach call flow. Each step involves the exchange of specific messages between the UE, gNB, and 5GC to establish a secure and authenticated connection for data transfer.