LAU vs RAU vs TAU: Location Update Procedures Explained

This article explains the differences between LAU (Location Area Update), RAU (Routing Area Update), and TAU (Tracking Area Update). These are important update procedures used by multimode User Equipment (UE) in GERAN, UTRAN, and EUTRAN networks. Multimode UEs are designed to work across multiple network technologies like GSM, UMTS, and LTE.

These update procedures become relevant when GERAN, UTRAN, and EUTRAN networks are interconnected using Gn interfaces. Let’s break down each one:

LAU - Location Area Update

• Initiation: A UE initiates a Location Area Update (LAU) procedure when it detects a new location area while roaming.
• Network: This occurs in GSM (2G) networks.
• Trigger: When a mobile device moves from one location area to another, it detects a change in the Location Area Code (LAC).
• Process: The GSM-compliant mobile device is responsible for detecting the LAC. If it finds a different LAC compared to the last update, it sends a Location Area Update Request to the GSM network (GERAN).

RAU - Routing Area Update

• Initiation: A UE initiates a Routing Area Update (RAU) procedure when it detects a new routing area while roaming.
• Network: This occurs in UMTS (3G) networks.

TAU - Tracking Area Update

• Initiation: A UE initiates a Tracking Area Update (TAU) procedure when it detects a new tracking area while roaming.
• Network: This occurs in LTE (4G) networks.

In essence, each update procedure serves a similar purpose – to inform the network of the UE’s new location – but they operate within different cellular technologies (GSM, UMTS, and LTE) and use different location identifiers (Location Area, Routing Area, and Tracking Area, respectively). As UEs move between these areas, they trigger the corresponding update procedure to maintain connectivity and allow the network to efficiently route calls and data.