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5g attach call flow

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The 5G attach procedure is a fundamental process that allows a User Equipment (UE), like a smartphone or any other device, to connect and register with the 5G network. This call flow involves several steps to establish a connection between the UE and the 5G Core (5GC).

Here's a technical breakdown of the 5G attach call flow:

  1. Initial Access:
    • The UE performs initial access to the 5G network by searching for available 5G cells and synchronizing with them. This step is similar to the initial cell selection and cell reselection procedures in previous generations of mobile networks.
  2. Random Access Procedure (RACH):
    • After initial access, the UE uses the Random Access Channel (RACH) to initiate a connection request with the network.
    • The UE sends a Random Access Preamble to the gNB (5G New Radio NodeB), indicating its intention to attach to the network.
  3. gNB Processing:
    • Upon receiving the Random Access Preamble, the gNB decodes it and identifies the UE.
    • The gNB then sends a Random Access Response (RAR) back to the UE, which includes necessary parameters like the Temporary C-RNTI (Cell Radio Network Temporary Identifier) and the physical layer resources for the UE to use for further communication.
  4. UE Attach Request:
    • Using the resources allocated by the gNB in the RAR, the UE sends an InitialUEMessage to the gNB.
    • This InitialUEMessage contains essential information such as the UE's capabilities, identity, and other necessary parameters.
  5. 5G Core Network (5GC) Interaction:
    • After the gNB receives the InitialUEMessage, it forwards this message to the 5G Core (5GC) network for further processing.
    • The 5GC processes the InitialUEMessage and performs functions such as security setup, mobility management, and session management.
  6. Security Setup:
    • The 5GC initiates the security setup by sending a Security Mode Command to the UE through the gNB.
    • The UE responds with a Security Mode Complete message after completing the security procedures, ensuring a secure communication link.
  7. Registration:
    • Once security is established, the 5GC performs the UE registration process.
    • This involves creating a context for the UE in the 5GC, allocating necessary resources, and updating the location information of the UE.
  8. Attach Acceptance:
    • After successful registration, the 5GC sends an Attach Accept message to the gNB, indicating that the UE has been successfully attached to the network.
    • The gNB then forwards this message to the UE, confirming its attachment to the 5G network.
  9. Data Transfer:
    • With the attachment procedure completed, the UE can now initiate data transfer sessions, make calls, or access various services offered by the 5G network.