Description
The Tracking Area Identifier (TAI) is a structured code that uniquely identifies a Tracking Area within a Public Land Mobile Network (PLMN). A Tracking Area is a logical grouping of one or more cells, defined by the network operator for the purpose of tracking and paging User Equipment (UE) in idle or inactive states. The TAI is constructed from the Mobile Country Code (MCC), Mobile Network Code (MNC), and a Tracking Area Code (TAC). The MCC and MNC identify the PLMN, while the TAC, typically 16 bits in length, is assigned by the operator to differentiate Tracking Areas within that PLMN. This hierarchical structure ensures global uniqueness and efficient routing of location management messages.
Architecturally, the TAI is a key parameter used by both the Radio Access Network (RAN) and the Core Network (CN), particularly the Access and Mobility Management Function (AMF) in 5G. When a UE moves, it performs Tracking Area Updates (TAU) to inform the network when it enters a new TA, as identified by a change in TAI. The network maintains a list of TAIs for each UE, known as the Tracking Area Identity List (TAI List), which allows the UE to move within multiple TAs without frequent updates, optimizing signaling. The AMF uses the TAI to determine the appropriate serving area and to manage paging; when a downlink message arrives for an idle UE, the network pages the UE across all cells belonging to the TAIs in its registered list.
In operation, the TAI is broadcast by each cell in the System Information Blocks (SIBs), allowing the UE to detect its current TA. During initial registration or attach procedures, the UE reports the TAI of the cell it is camped on. The AMF validates this TAI against its configured areas and may assign a new TAI List. For mobility, if a UE in idle mode detects a TAI not in its assigned list, it triggers a TAU procedure. The TAI also plays a role in network slicing and policy enforcement, as certain slices or services may be restricted to specific Tracking Areas. Its design balances the need for precise location knowledge with the imperative to minimize signaling, a critical consideration for network scalability and UE battery life.
Purpose & Motivation
The TAI was introduced to address the challenges of location management and paging efficiency in cellular networks, particularly as networks evolved to support packet-switched services and increased mobility. Prior to the concept of Tracking Areas, location management was often cell-based, leading to excessive signaling overhead as UEs moved, especially in idle mode. This was inefficient for both network resources and UE battery consumption. The creation of Tracking Areas, identified by TAIs, allowed networks to group cells into larger logical areas, enabling the network to track UEs at a coarser granularity during idle periods, thereby reducing the frequency of location update signaling.
Historically, in 2G/GSM, location areas served a similar purpose, but with the advent of 3G/UMTS and especially 4G/LTE (where TAI was formally standardized in Release 8), the need for a more streamlined, IP-centric architecture became apparent. The TAI provides a standardized, scalable method for mobility management that is independent of the underlying radio technology, supporting seamless evolution from LTE to 5G. It solves the problem of efficiently locating a UE for incoming calls or data sessions without requiring constant, fine-grained location reporting. By optimizing the trade-off between location accuracy and signaling load, the TAI enables networks to support massive numbers of devices, a necessity for the Internet of Things (IoT) and widespread mobile broadband.
Furthermore, the TAI facilitates network operations such as load balancing, emergency service routing, and regulatory compliance for location reporting. It is a foundational element for features like Registration Areas in 5G, which extend the concept to include multiple types of areas (e.g., Tracking Areas, Registration Areas). The TAI's design ensures backward compatibility and forward flexibility, allowing operators to reconfigure their TA layouts as network topology and traffic patterns change without impacting core protocols.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (392 CRs across 5 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 TAI (Tracking Area Identity) function was clarified for operation within the 5G System (5GS), including its usage during interworking with non-3GPP access. The release also provided specific clarification on the assignment of TAI lists for different 5G radio access technologies (RATs) and corrected the procedures for TAI list generation during mobility and registration, particularly within equivalent PLMNs.
- Tracking area update request from S1 mode to N1 mode TS 24.301CR3106
- Storing Configured NSSAI when the PLMN is changed TS 24.501CR0203
- Support for 15 bearers in PGWs across PLMN TS 23.401CR3430
- Use of identifiers for mobility between GERAN/UTRAN and 5GS TS 23.501CR0017
- Network sharing prioritised PLMN handling TS 23.501CR0056
- Inter-PLMN mobility when N26 is not used TS 23.501CR0070
+ 47 more changes
In Release 16, enhancements for the Tracking Area Identifier function were made to support integration with Standalone Non-Public Networks (SNPNs). Specifically, when the serving cell is operated by an SNPN, the TAI now includes the Network Identifier (NID) together with the PLMN ID for unique identification. Furthermore, procedures were adapted to replace the term PLMN with SNPN across relevant architectures and mobility scenarios, such as during inter-PLMN handover and access control.
- Handling of forbidden PLMNs, forbidden PLMN for GPRS service and equivalent PLMNs list on ATTACH ACCEPT and TRACKING AREA ACCEPT in RLOS TS 24.301CR3294
- Serving PLMN rate control, general description TS 24.501CR0972
- Serving PLMN rate control, activation TS 24.501CR0973
- PLMN ID and NID provided to the lower layers TS 24.501CR1055
- Abnormal case handling for receipt of 5GMM cause value #11 from a PLMN TS 24.501CR1229
- "S-NSSAI not available in the current PLMN" when non NSSAA supported UE requesting the S-NSSAI subjects to NSSAA TS 24.501CR1598
+ 83 more changes
In Release 17, specific enhancements were made for the Tracking Area Identifier (TAI) function to support Non-Terrestrial Networks (NTN) and new network types. This included defining a **last visited registered TAI for 5GSat** (5G Satellite) and IoT NTN in EPS, and extending procedures to handle **forbidden TAI list IEs in ATTACH and TAU REJECT messages**. Furthermore, updates were made to support SNPNs (Standalone Non-Public Networks), where the TAI includes both the PLMN ID and a Network Identifier (NID) when the serving cell is operated by an SNPN.
- Identifiers for Layer-2 UE-to-Network Relay discovery TS 23.304CR0020
- Informative guideline on supporting session/service continuity between SNPN and PLMN when using N3IWF TS 23.501CR2563
- User Plane Remote Provisioning of UEs if PLMN as ON TS 23.501CR2802
- Mobility support between SNPNs and between SNPN and PLMN TS 23.501CR2826
- Indicating a last visited TAI in a Registration for NR Satellite Access TS 23.501CR3379
- New reject cause #78 PLMN not allowed to operate at the present UE location TS 24.301CR3621
+ 104 more changes
In Release 18, the TAI (Tracking Area Identifier) function was extended to support Standalone Non-Public Networks (SNPNs), requiring the TAI to include the Network Identifier (NID) together with the PLMN ID when the serving cell is operated by an SNPN. Additionally, enhancements were made to manage tracking areas for equivalent SNPNs, including the handling of 5GS forbidden tracking areas and the last visited TAI within such networks.
- Identifiers for 5G ProSe UE-to-UE Relay Discovery TS 23.304CR0174
- Identifiers for Discovery integrated into PC5 unicast link establishment TS 23.304CR0175
- Identifier availability for Lawful Interception during Inter-MME/ MME-5GS handover TS 23.401CR3720
- Edge Relocation within the same hosting PLMN's EHEs TS 23.501CR3820
- KI#4: Support for Centralized NSACF in a PLMN with multi-service areas TS 23.501CR3822
- PIN identifiers TS 23.501CR4287
+ 65 more changes
In Release 19, the TAI (Tracking Area Identifier) function was enhanced to support Store and Forward (S&F) satellite operation, specifically within the tracking area update procedure. Furthermore, the specifications were adapted for SNPNs (Standalone Non-Public Networks), where the TAI now includes the Network Identifier (NID) together with the PLMN ID when the serving cell is operated by an SNPN. These updates facilitate operation in new satellite and non-public network deployment scenarios.
- Identifiers for 5G ProSe Multi-hop UE-to-UE Relay for non-IP PDU type TS 23.304CR0508
- NF discovery and selection by target PLMN TS 23.501CR5399
- UDR enhancement supporting Device Identifier of non-3GPP Devices connecting behind a UE/5G-RG TS 23.501CR5547
- Definition of identifiers of N3GPP device behind UE/5G-RG TS 23.501CR5749
- No TAI to FTAI list due to RAT restriction TS 24.301CR4270
- Support of Store and Forward (S&F) satellite operation - tracking area update procedure TS 24.301CR4209
+ 63 more changes
Explore further
Broader topics and technologies where TAI plays a role.
Defining Specifications
3GPP specifications that define or reference TAI, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.304 vk00 | 5G Proximity Services (ProSe) Stage 2 | Rel-20 |
| TS 23.401 vj50 | Evolved Packet System (EPS) Stage 2 Description | Rel-19 |
| TS 23.501 vk00 | 5G System Architecture Stage 2 | Rel-20 |
| TS 23.558 vk00 | Architecture for Edge Applications | Rel-20 |
| TS 23.700 vk00 | XR Services Application Enablement Layer | Rel-20 |
| TS 23.712 vd00 | Enhancements to Warning Status Reporting Mechanisms | Rel-13 |
| TR 23.758 vh00 | Study on Edge Application Architecture | Rel-17 |
| TS 24.301 vj60 | NAS protocol for Evolved Packet System | Rel-19 |
| TS 24.501 vj50 | 5G NAS Protocols Specification | Rel-19 |
| TS 24.801 v810 | CT1 SAE NAS Aspects for EPC | Rel-8 |
| TS 24.890 vg00 | 5G NAS Protocol for 5GS Stage 3 | Rel-16 |
| TR 26.917 vj00 | TV Service Enhancements over 3GPP | Rel-19 |
| TR 28.840 vi10 | Technical Report | Rel-18 |
| TS 28.875 vj00 | Study on IAB Node Management | Rel-19 |
| TS 29.122 vj40 | T8 Reference Point for Northbound APIs | Rel-19 |
| TS 29.303 vj10 | DNS Procedures for Evolved Packet System | Rel-19 |
| TS 29.518 vj50 | AMF Service Based Interface Protocol | Rel-19 |
| TS 29.522 vj40 | 5G NEF Northbound APIs Stage 3 | Rel-19 |
| TS 29.543 vj20 | 5G Data Transfer Policy Control Services Stage 3 | Rel-19 |
| TS 29.554 vj10 | 5G Background Data Transfer Policy Control Service | Rel-19 |
| TS 29.558 vj40 | Enabling Edge Applications | Rel-19 |
| TS 29.890 vg00 | CT3 5G System Technical Report | Rel-16 |
| TS 31.117 vj10 | USIM Application Toolkit Test for Non-Removable UICC | Rel-19 |
| TS 31.127 vi40 | UICC-terminal interaction testing specification | Rel-18 |
| TS 32.299 vj00 | Diameter Charging Applications for 3GPP | Rel-19 |
| TS 33.127 vj50 | Lawful Interception Architecture and Functions | Rel-19 |
| TS 33.401 vj10 | EPS Security Architecture | Rel-19 |
| TS 33.701 vj00 | Study on mitigations against bidding down attacks | Rel-19 |
| TS 33.897 vd10 | Security for Isolated E-UTRAN Operation (IOPS) | Rel-13 |
| TS 36.413 vj10 | S1 Application Protocol (S1AP) | Rel-19 |
| TS 36.875 vd10 | Dual Connectivity Extension Requirements | Rel-13 |
| TS 37.473 vj00 | W1 Application Protocol (W1AP) Specification | Rel-19 |
| TS 37.822 vc10 | SON Enhancements for UE Types and Active Antennas | Rel-12 |
| TS 38.413 vj10 | NG Application Protocol (NGAP) | Rel-19 |
| TS 38.423 vj10 | Xn Application Protocol (XnAP) specification | Rel-19 |
| TS 38.473 vj10 | 5G F1 Application Protocol (F1AP) | Rel-19 |
| TS 43.318 vj00 | Generic Access Network (GAN) Stage 2 | Rel-19 |
| TS 44.318 vj00 | Generic Access Network (GAN) Interface Procedures | Rel-19 |
| TS 48.018 vj00 | BSS-SGSN Interface for GPRS Control | Rel-19 |