Understanding Tracking Area Update in LTE and 5G Networks

Introduction

Central to this system is the Tracking Area Update (TAU) procedure, which allows mobile devices, known as User Equipment (UE), to inform the network of their location. This process is vital for efficient call and data delivery without overwhelming the network or draining device batteries. As mobile technology evolves into 5G, TAU transforms into Registration Area Update (RAU), adapting to the demands of denser, faster networks.

What is a Tracking Area in LTE?

In LTE, a Tracking Area (TA) is a logical collection of cells served by one or more eNodeBs (base stations) with a distinct Tracking Area Code (TAC). The TAC, a 16-bit identifier, combines with the Public Land Mobile Network (PLMN) identifier to form the globally unique Tracking Area Identity (TAI). Each eNodeB broadcasts the TAI via System Information Block 1 (SIB1), enabling UEs, such as smartphones, to identify their current TA.

Role of MME in LTE Mobility Management

The Mobility Management Entity (MME) is a critical component of the LTE Evolved Packet Core (EPC), handling control plane signaling for mobility-related tasks. Specifically, the MME manages UE location tracking, attachment, detachment, and handovers. The MME updates its records with the current TA of the UE during a TAU to ensure that the network can effectively page the device.

What is Tracking Area Update (TAU) in LTE?

A UE can inform the network of its current TA using the Tracking Area Update (TAU) procedure in LTE. Maintaining connectivity as users move between regions necessitates this. For instance, when a UE enters a new TA that is not on its registered TAI list, a TAU to update the MME is started. This guarantees that the network will know where to page the UE when calls or data sessions come in.

Necessity of TAU

TAU is necessary because LTE networks track idle UEs at the TA level rather than per cell. This hierarchical approach reduces signaling compared to tracking every cell change, which would overwhelm the network and drain UE batteries. When a UE moves to a new TA, it performs a TAU to inform the MME, enabling efficient paging when there’s incoming traffic.

Types of Tracking Area Updates

• Normal TAU: Occurs when a UE enters a TA not in its registered TAI list, typically due to physical movement across regions.
• Periodic TAU: Triggered by the expiration of the T3412 timer, set by the network, to confirm the UE’s availability and location periodically.
• Combined TAU: Used for UEs supporting circuit-switched services, updating both LTE TA and 2G/3G Location/Routing Areas.
• Other Triggers: Include network load balancing, changes in UE capabilities, or reselection to LTE from other technologies.

The Tracking Area Update Procedure in LTE

1. UE Detects New TA: The UE reads the TAI from the serving cell’s SIB1 and compares it to its registered TAI list.
2. TAU Request: If the TAI is not in the list, the UE sends a TAU Request, including its old Globally Unique Temporary Identifier (GUTI), current TAI, and capabilities.
3. eNodeB Forwards Request: The eNodeB forwards the request to the MME, adding its identity and the cell’s identity.
4. MME Verifies Whether: It Serves the UE Before Processing the Request  If not, the context for the UE is retrieved from the previous MME.
5. Context Transfer: The old MME transfers the UE’s security and bearer context if a new MME is involved.
6. Authentication and Security: The MME may authenticate the UE and establish a security context.
7. TAU Accept: The MME sends a TAU Accept message, which may contain a new GUTI and TAI list.
8. TAU Complete: A TAU Complete message is sent to the UE as an acknowledgement.

Key Messages in TAU

• TAU Request: Starts the update and contains information about the UE’s identity and location.
• TAU Accept: Confirms the update, providing new parameters like GUTI or TAI list.
• TAU Complete: Concludes the procedure by acknowledging receipt of the TAU Accept.

Impact of TAU on Network Performance

• Signaling Overhead: Frequent TAUs increase signaling traffic, potentially causing congestion in busy networks.
• Paging Efficiency: Larger TAs reduce TAU frequency but increase paging load, as the network pages more cells to locate a UE.
• User Experience: Optimizing TA size and timers ensures quick call setup while minimizing battery drain.

Evolution to 5G: Registration Area Update (RAU)

Mobility management adapts to support increased device connectivity, lower latency, and higher data rates as mobile networks move to 5G. TAU was replaced by the Registration Area Update (RAU), which introduced a location tracking method that was more adaptable.

Introduction to 5G Mobility Management

The Access and Mobility Management Function (AMF), which functions similarly to the MME in LTE, is introduced in the 5G Core (5GC). A key innovation is the Registration Area, a list of TAs assigned to a UE. This allows the UE to move across multiple TAs without updating the network, significantly reducing signaling in dense or high-mobility environments.

Registration Area vs. Tracking Area

• Tracking Area (LTE): A single area defined by a TAC, requiring updates when a UE moves to a new TA.
• Registration Area (5G): A list of TAs that lets you move around the list without having to update.

RAU Procedure in 5G

1. UE checks: To see if the current TAI is on its list of Registration Areas before detecting a new area.
2. Registration Request: If not, the UE sends a Registration Request to the AMF via the Radio Access Network (RAN).
3. AMF Processing: The AMF verifies the UE’s identity, updates its location, and may assign a new Registration Area.
4. Registration Accept: The AMF responds with a Registration Accept message, including the updated Registration Area list.

Conclusion

Tracking Area Update (TAU) in LTE and Registration Area Update (RAU) in 5G are pivotal for efficient mobility management in mobile networks. By enabling the network to track UEs at a TA or Registration Area level, these procedures ensure seamless connectivity while minimizing signaling overhead. As 5G continues to evolve, RAU and RNAU enhance scalability and efficiency, supporting the demands of modern applications. Network professionals and enthusiasts should stay informed about these mechanisms to optimize network performance and user experience.

Meta Description:
Explore the intricacies of Tracking Area Update (TAU) in LTE and its evolution to Registration Area Update (RAU) in 5G. Learn about the procedures, their significance, and the ways in which they affect user experience and network performance.

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Citations:

• EventHelix – LTE Tracking Area Update
• 3GLTEInfo – LTE Tracking Area Update Call Flow
• IEEE Xplore – Tracking Area Update and Paging in 5G Networks
• Dr Moazzam Tiwana – 5G RAU

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