Description
The Tracking Area Update (TAU) is a fundamental mobility management procedure in 3GPP EPS (Evolved Packet System) for LTE and continued in 5GS (5G System). It is initiated by the User Equipment (UE) when it detects that it has entered a new Tracking Area (TA) that is not in its registered TA list, or periodically based on a timer. A Tracking Area is a logical grouping of cells defined for location tracking and paging efficiency. The primary purpose of TAU is to inform the core network—specifically the Mobility Management Entity (MME) in LTE or the Access and Mobility Management Function (AMF) in 5G—of the UE's current location at the TA level, allowing the network to efficiently page the UE when there is downlink data or an incoming call.
The procedure involves signaling messages between the UE and the network. When a UE in idle mode (ECM-IDLE in LTE, CM-IDLE in 5G) moves and selects a new cell, it reads the Tracking Area Identity (TAI) broadcast in the system information. If this TAI is not in the list of TAIs the UE received during its last Attach or TAU procedure (the registered TAI list), the UE initiates a TAU Request message to the network via the selected base station (eNB in LTE, gNB in 5G). This message is forwarded to the MME/AMF. The MME/AMF then updates its location context for the UE, may perform authentication and security procedures, and returns a TAU Accept message containing a new registered TA list and possibly a new temporary identifier (GUTI in LTE, 5G-GUTI in 5G). If the UE is moving to a TA served by a different MME/AMF, an inter-MME or inter-AMF TAU occurs, involving context transfer between the old and new core nodes.
TAU is crucial for balancing signaling load and paging overhead. By grouping cells into TAs, the network does not need to know the UE's exact cell; it only needs to page the UE in all cells of its last known TA(s). The TAU procedure ensures this TA-level location is kept updated. The network can assign a UE a list of multiple TAs (a TA List) to reduce the frequency of TAU signaling—the UE can move within all TAs in the list without performing an update. The parameters controlling TAU, like the periodic TAU timer (T3412), are broadcast by the network and configured in the UE's subscription. TAU also serves to re-establish network connectivity, update security keys, and can trigger other procedures like bearer modification.
Purpose & Motivation
The TAU procedure was introduced with the System Architecture Evolution (SAE) in 3GPP Release 8 for LTE to address the limitations of location management in previous 3GPP systems. In 2G/3G networks, location updates were performed at the Location Area (LA) or Routing Area (RA) level, which were relatively static and could lead to inefficient paging or excessive signaling in high-mobility scenarios. TAU, with the concept of configurable Tracking Areas and TA Lists, provides a more flexible and scalable mechanism optimized for the all-IP, flatter architecture of EPS.
It solves the critical problem of how to efficiently locate idle-mode UEs in a packet-switched network without a permanent connection. Efficient location tracking is essential for mobile-terminated services like voice calls (VoLTE) and push notifications. TAU reduces unnecessary signaling compared to cell-level updates while keeping paging areas optimally sized. The ability to assign a TA List allows the network to tailor the mobility management to user mobility patterns—for example, giving a frequently moving UE a larger TA List to minimize updates. This design was motivated by the need to support always-on connectivity with minimal battery drain on the UE and reduced signaling load on the network, which are key requirements for high-capacity LTE and 5G networks.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (32 CRs across 6 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-8, normative work from Rel-15.
In Release 15, the Tracking Area Update (TAU) procedure was enhanced to support the negotiation of a long periodic TAU timer value and to enable UE Radio Capability updates. The procedure was also modified to handle a Modify Bearer Request during TAU and to manage an SGW change with data forwarding. Furthermore, it was specified that a UE's CIoT capability can be indicated in the RRC Connection Establishment request initiated for a TAU.
- Updated scope for Dual Connectivity with other RATs TS 23.401CR3255
- Correction on the update of RAN in subscription change TS 23.401CR3342
- Modify Bearer Request during TAU TS 23.401CR3353
- Negotiation of long periodic TAU timer value TS 23.401CR3443
- UE Radio Capability Update using TAU procedure TS 23.401CR3444
- Update of TAU procedure with SGW change and data forwarding TS 23.401CR3324
+ 1 more changes
In Release 16, the TAU procedure was updated to support Enhanced EPS Location Update Reporting for UEs using Dual Connectivity. It also introduced a mechanism for the Deletion of a UE's stored Radio Capability ID using GUTI reallocation and included updates related to Mobile-Terminated Early Data Transmission (MT-EDT). Furthermore, the release provided clarifications on UE capability updates and included various editorial updates.
- Enhanced EPS Location Update Reporting with Dual Connectivity TS 33.108CR0419
- Clarification on UE capability update TS 23.401CR3541
- Deletion of UE Radio Capability ID using GUTI reallocation TS 23.401CR3587
- MT-EDT related update TS 23.401CR3589
- Update to Rel-16 version (MCC) TS 29.305
- Editorial updates in RACS clauses TS 23.401CR3588
In Release 17, the TAU procedure was enhanced to support MUSIM capabilities for emergency attached UEs and to better handle paging restrictions during a mobility TAU performed outside the current registration area. The release also included updates to align GUTI allocation with best practices for generating unpredictable identifiers and to update the paging cause feature within EPS.
- Update Paging Cause Feature in EPS TS 23.401CR3647
- On Connection Release and Paging Restriction during a mobility TAU in a TA outside the current Registration Area TS 23.401CR3665
- MUSIM capabilities in TAU for Emergency attached UE TS 23.401CR3679
- Align GUTI allocation to best practices of unpredictable identifier generation TS 33.401CR0702
- Update to Rel-17 version (MCC) TS 29.305
In Release 18, the TAU procedure was enhanced to support Cellular IoT and to accommodate discontinuous coverage scenarios, particularly for Non-Terrestrial Networks (NTN) like satellite access. Updates were made to verify UE location for regulatory requirements and to align the core network procedures for these new access types. Furthermore, the specifications underwent editorial updates to correct references and to reflect the Release 18 version.
- Verification of UE location update in 23.401 TS 23.401CR3718
- Discontinuous coverage update to remove EN and alignments TS 23.401CR3744
- Correcting the reference for UE radio capability update procedure TS 23.401CR3746
- Update on support of discontinuous coverage for satellite access TS 23.401CR3755
- Update of UE location verification for NTN for regulatory requirements TS 23.401CR3779
- Support of TAU in Cellular IoT TS 23.401CR3774
+ 2 more changes
In Release 19, the Tracking Area Update (TAU) procedure was enhanced with specific updates for the S&F (Steering and Forcing) operation, including location procedure enhancements for this function. The release also introduced updates to the table detailing CIoT functionalities, specifically for UE-specific DRX in NB-IoT, within the context of TAU. Furthermore, the Mobility and Connection Control (MCC) aspects of the procedure were updated to the Release 19 version.
In Release 20, the update to the Tracking Area Update (TAU) function was an editorial update for publication following TSG SA approval. This change, documented as an MCC editorial update from SA#109, did not introduce new technical procedures or capabilities. It focused solely on the final editorial preparation and publication of the existing specification.
- MCC Editorial update for publication after TSG SA approval (SA#109) TS 23.700
Explore further
Broader topics and technologies where TAU plays a role.
Defining Specifications
3GPP specifications that define or reference TAU, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.271 vj00 | LCS Stage 2 Specification | Rel-19 |
| TS 23.401 vj50 | Evolved Packet System (EPS) Stage 2 Description | Rel-19 |
| TS 23.700 vk00 | XR Services Application Enablement Layer | Rel-20 |
| TS 29.303 vj10 | DNS Procedures for Evolved Packet System | Rel-19 |
| TS 29.305 vj00 | Interworking Functions for EPS-Legacy Systems | Rel-19 |
| TS 29.805 v800 | IWF for MAP-Diameter Interworking | Rel-8 |
| TS 32.422 vk00 | Telecom Management: Trace Control & Configuration | Rel-20 |
| TS 33.108 vj00 | LI Handover Interface Specification | Rel-19 |
| TS 33.401 vj10 | EPS Security Architecture | Rel-19 |
| TS 33.859 vb10 | UTRAN Key Hierarchy Enhancement Study | Rel-11 |
| TR 36.763 vh00 | NB-IoT/eMTC Support for Non-Terrestrial Networks | Rel-17 |
| TS 45.820 vd10 | CIoT for Internet of Things | Rel-13 |