How does LTE use TAU Reject Causes to provide specific rejection reasons to UEs?
In LTE (Long-Term Evolution) networks, the Tracking Area Update (TAU) Reject Causes are communicated to User Equipment (UE) to provide specific rejection reasons when the TAU procedure is unsuccessful. These causes play a crucial role in informing the UE about the reason for the rejection and guiding its subsequent actions. Here's a technical explanation of how LTE uses TAU Reject Causes to provide specific rejection reasons to UEs:
- TAU Reject Message:
- When the Mobility Management Entity (MME) in the LTE network decides to reject a TAU request from a UE, it sends a TAU Reject message back to the UE.
- Cause Value:
- Within the TAU Reject message, there is an information element called "Cause" that includes a numeric value known as the Cause Value. This value corresponds to a specific TAU Reject Cause, indicating the reason for the rejection.
- Cause Values and Specific Reasons:
- The Cause Values are standardized in the LTE specifications, and each value corresponds to a specific reason for rejecting the TAU. For example:
- Cause Value 2: "IMSI not known in HSS" may indicate that the International Mobile Subscriber Identity (IMSI) of the UE is not recognized by the Home Subscriber Server (HSS).
- Cause Value 3: "Illegal ME" may indicate that the Mobile Equipment (ME) attempting the TAU is considered illegal or not authorized.
- Cause Value 6: "PLMN not allowed" may indicate that the Public Land Mobile Network (PLMN) the UE is trying to access is not allowed.
- The Cause Values are standardized in the LTE specifications, and each value corresponds to a specific reason for rejecting the TAU. For example:
- Enhanced Connection Management:
- TAU Reject Causes enhance connection management by providing specific information to the UE about why the TAU was rejected. This allows the UE to take appropriate actions based on the rejection reason.
- Network Policies and Restrictions:
- TAU Reject Causes align with network policies and restrictions. The rejection reasons reflect the policies set by the network operator, such as barring certain UEs, restricting access to specific tracking areas, or enforcing security measures.
- Security and Authentication Failures:
- Some TAU Reject Causes are related to security and authentication failures. For example, Cause Value 8: "No suitable cells in the tracking area" may indicate that the UE failed to authenticate or establish a secure connection with the cells in the requested tracking area.
- Impact on Mobility Behavior:
- The UE interprets the Cause Value and adjusts its behavior accordingly. For instance, if the TAU is rejected due to a temporary issue like network overload (Cause Value 26: "Congestion"), the UE may implement retry strategies or adjust its mobility parameters before attempting the TAU again.
- Retry Strategies and Timer Adjustments:
- Based on the TAU Reject Cause, the UE may implement specific retry strategies. The Cause Value may also influence timer adjustments, determining how long the UE should wait before attempting another TAU to avoid overwhelming the network with repeated requests.
- Handling Temporary Network Conditions:
- TAU Reject Causes help the UE distinguish between permanent and temporary rejection reasons. Temporary conditions like network congestion or resource limitations (Cause Values 26-29) may prompt the UE to implement more aggressive retry strategies after a short delay.
- Logging and Diagnostics:
- TAU Reject Causes are valuable for logging and diagnostics. Network operators can analyze these causes to identify patterns, troubleshoot issues, and take corrective actions to improve the efficiency of the TAU procedure and overall network performance.
In summary, LTE uses TAU Reject Causes to provide specific rejection reasons to UEs through standardized Cause Values. These causes guide the UE's behavior, enabling it to adapt to network conditions, implement appropriate retry strategies, and ensure efficient mobility management within the LTE network.