What is TAU (tracking area update)

Within the realm of mobile communication networks, particularly LTE (Long-Term Evolution) and 5G, the Tracking Area Update (TAU) procedure serves as a critical mechanism for managing user equipment (UE) mobility and maintaining efficient network operation. It ensures the network core is aware of the UE's location within the network as it moves between different geographical areas.

Understanding Tracking Areas (TAs):

• Cellular networks are divided into smaller geographical regions called Tracking Areas (TAs). Each TA is identified by a unique Tracking Area Identifier (TAI).
• UEs connect to the network through cells, and a group of neighboring cells typically forms a single TA.

The Need for Tracking Area Updates:

• As UEs move around the network, they transition between different TAs.
• Without informing the network core about these changes, efficient communication and handover procedures (when the UE switches between serving cells) would become challenging.

The TAU Procedure:

1. UE Detects TA Change: When a UE moves to a new cell belonging to a different TA, it decodes the broadcasted TAI from the serving cell. It compares this TAI with the list of TAs it is currently registered in (often referred to as the TAI List).
2. Initiating TAU Request: If the UE detects it has entered a new TA not included in its TAI List, it initiates a TAU procedure by sending a TAU Request message to the network. This message typically includes:
   • The old TAI (the TA the UE was previously connected to).
   • Other relevant UE information like its identifier (GUTI - Globally Unique Temporary Identifier).
3. Network Processing:
   • The network core (specifically, the Mobility Management Entity or MME) receives the TAU Request and identifies the old TA using the provided information.
   • The MME retrieves context information about the UE associated with the old TA.
   • Depending on network configuration, the MME might initiate procedures to verify the UE's legitimacy before proceeding.
4. Handover or Context Transfer:
   • If the new TA belongs to the same network operator as the old TA (intra-network mobility), the MME initiates a handover procedure to seamlessly transfer the ongoing communication to the new serving cell within the new TA.
   • If the new TA belongs to a different network operator (inter-network mobility), the MME performs a context transfer procedure, involving communication with the new operator's core network to establish connectivity for the UE.
5. TAU Acceptance and Update:
   • Upon successful processing, the MME sends a TAU Accept message back to the UE, potentially including an updated TAI List reflecting the new location.
   • The UE receives the TAU Accept message and updates its internal information about the current TA.

Benefits of TAU:

• Efficient Network Management: TAU procedures allow the network to track UE locations and optimize resource allocation for ongoing communication.
• Seamless Handovers: By informing the network about TA changes, TAU facilitates smooth handover procedures, minimizing call drops or service disruptions during UE movement.
• Security Considerations: TAU procedures can be integrated with network security measures to verify UE identity and prevent unauthorized access.

Additional Considerations:

• The frequency of TAU updates can be configured based on network requirements and UE mobility patterns. More frequent updates might be needed in areas with high user density or frequent cell transitions.
• In some network architectures, UEs might be assigned a default TAI List containing all TAs within the network operator's coverage area. However, more granular control is often implemented through customized TAI Lists for specific UEs.

Future of TAU:

• As cellular network technologies evolve, the concept of TAU is likely to remain relevant for managing UE mobility and ensuring efficient communication.
• Advancements in network intelligence and UE positioning techniques might influence how TAU procedures are triggered and optimized in the future.

Conclusion:

The Tracking Area Update (TAU) procedure serves as a cornerstone for managing UE mobility in cellular networks. By enabling the network to track UE locations and facilitate seamless handovers, TAU paves the way for efficient network operation and a smooth user experience. As mobile communication technologies continue to develop, TAU will likely remain a crucial mechanism for ensuring network performance and user satisfaction.