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How does LTE handle the release of S1 connections when a UE departs from the network?

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When a User Equipment (UE) departs from the LTE (Long-Term Evolution) network, whether voluntarily or due to network decisions, the release of S1 connections is a critical process. The S1 interface connects the evolved NodeB (eNB) and the Serving Gateway (SGW) in LTE, and releasing the S1 connection involves several technical steps. Below is a detailed technical explanation of how LTE handles the release of S1 connections when a UE departs from the network:

  1. UE Detachment Trigger:
    • UE detachment can occur for various reasons, including:
      • User-initiated detach (e.g., when the user turns off the device).
      • Network-initiated detach due to inactivity or other network policies.
      • Handover to another technology (3G, 2G) or RAT (Radio Access Technology).
  2. Initiation of S1 Release:
    • When the UE initiates detachment or the network decides to release the UE, the eNB initiates the S1 Release procedure by sending an S1 Release Request message to the SGW.
  3. Release Cause Indication:
    • The S1 Release Request message includes a release cause indicating why the S1 connection is being released. Common causes include user equipment detach, handover, or other network-related decisions.
  4. S1 Release Request Signaling:
    • The S1 Release Request message is sent over the S1 interface using the S1 Application Part (S1AP) signaling protocol. This message carries information about the UE and the context to be released.
  5. Acknowledgment from SGW:
    • Upon receiving the S1 Release Request, the SGW processes the request and sends an S1 Release Command to the eNB, confirming its acknowledgment and readiness to release the S1 context.
  6. Context Release Timer:
    • A context release timer may be started upon sending the S1 Release Request. If the SGW fails to respond within a specified time, the eNB may take appropriate actions, such as retransmitting the request or initiating alternative procedures.
  7. Data Path Release:
    • Once the S1 Release Command is received, the eNB initiates the release of the data path, discontinuing the forwarding of user data between the SGW and the eNB.
  8. Signaling Path Release:
    • Simultaneously, the signaling path between the eNB and the SGW is released. This ensures that control plane signaling, including mobility-related messages, is terminated.
  9. S1 Release Complete:
    • The SGW sends an S1 Release Complete message to the eNB, confirming the successful release of the S1 context. This message finalizes the S1 Release procedure.
  10. UE State Transition:
    • The UE transitions to an idle state or connects to another network technology if a handover to a different RAT is involved.
  11. Clean-Up Procedures:
    • The network may perform clean-up procedures, including the release of resources associated with the UE, updating subscriber databases, and updating mobility management information.
  12. Security Context Handling:
    • If the UE is involved in a security context, the release procedure ensures that security keys and related parameters are handled appropriately, maintaining security during the release process.

In summary, the release of S1 connections when a UE departs from the LTE network involves a series of technical steps to ensure a controlled and orderly termination of the communication session. The signaling procedures, acknowledgment mechanisms, and context release timers contribute to the reliability and efficiency of the S1 Release procedure in LTE.