LTE Tracking Area Update (TAU) Procedure Explained

This document explains the Tracking Area Update (TAU) procedure used in LTE systems. TAU stands for Tracking Area Update. We’ll cover both scenarios: TAU without S-GW change and TAU with S-GW change.

A standalone TAU (tracking area update), whether it involves an S-GW change or not, occurs when a GPRS-attached UE or an E-UTRAN-attached UE experiences certain conditions. These conditions are detailed in section 5.3.3.0 of the 3GPP TS 23.401 document.

TAU Procedure without S-GW Change

The following diagram illustrates the message flow between E-UTRAN entities during a TAU procedure without a Serving Gateway (S-GW) change.

Figure 1: TAU Procedure without S-GW change

TAU Procedure with S-GW Change

Now, let’s look at the scenario where the TAU procedure does involve an S-GW change. The diagram below outlines the message flow between E-UTRAN entities in this case.

Figure 2: TAU Procedure with S-GW change

References

• 3GPP TS 23.401 version 11.3.0 Release 11
• 3GPP TS 24.301 V8.1.0