International Mobile Equipment Identity Type Allocation Code

Description

The International Mobile Equipment Identity Type Allocation Code (IMEI/TAC) is a globally unique identifier assigned to mobile equipment. It forms the first eight digits of the complete 15-digit IMEI. The TAC itself is allocated by the Global System for Mobile Communications Association (GSMA) or its designated bodies to device manufacturers. The structure is standardized: the first two digits are the Reporting Body Identifier, which indicates the GSMA-approved group that allocated the TAC, and the following six digits are the Type Allocation Code proper, uniquely identifying the model and origin. This code is permanently embedded in the device hardware, typically in non-volatile memory, and is transmitted to the network during initial registration and attachment procedures.

In network operations, the IMEI/TAC is used by the core network, specifically the Mobility Management Entity (MME) in 4G or the Access and Mobility Management Function (AMF) in 5G, as detailed in 3GPP specifications 23.401 and 23.501. When a User Equipment (UE) attempts to attach to the network, it provides its International Mobile Subscriber Identity (IMSI) and its IMEI. The network can extract the TAC from the IMEI. This TAC is then used for Equipment Identity Register (EIR) queries. The EIR is a database that contains lists of device identifiers, typically categorized as white (approved), grey (monitored), or black (barred). The network consults the EIR to determine if the specific device type or model is permitted to access network services.

The role of the IMEI/TAC extends beyond simple access control. It is fundamental for device management, fraud prevention, and regulatory compliance. Operators use it to track device types on their network, which aids in capacity planning, service tailoring, and ensuring compatibility with network features. For security, it helps identify and block devices that are reported stolen or are of a type known to be vulnerable or non-compliant with standards. In the context of 5G and network slicing, the TAC can be a parameter in policy decisions, potentially influencing which network slice a device is allowed to access based on its capabilities. The TAC is a static, hardware-based identifier that provides a reliable anchor for these network functions, independent of the user's SIM card or subscription.

Purpose & Motivation

The IMEI/TAC was created to provide a standardized, globally unique method for identifying the type and origin of mobile equipment. Prior to its standardization, identifying device models across different networks and manufacturers was inconsistent, hindering interoperability, fraud management, and regulatory oversight. The TAC solves the problem of device model ambiguity, allowing networks, regulators, and industry bodies to accurately categorize and manage the vast array of mobile devices entering the market.

Its introduction addresses critical needs in network security and operations. From a security perspective, it enables the implementation of Equipment Identity Registers (EIRs), which are essential for barring stolen phones and preventing the use of counterfeit or non-type-approved devices that could harm network integrity or user safety. Operationally, it allows mobile network operators to profile the device population on their network. This profiling is vital for troubleshooting device-specific issues, planning for the introduction of new services (like VoLTE or 5G NR), and ensuring that network resources are compatible with the capabilities of the attached devices. The TAC provides a clear link between a physical device and its certified technical specifications.

The evolution into 5G (from Rel-16 onwards) reinforced its purpose within a more complex, service-based architecture. In 5G systems, with the proliferation of Internet of Things (IoT) devices and diverse use cases, accurate device identification becomes even more critical for policy enforcement, slice selection, and security. The TAC remains a foundational element for these advanced functions, ensuring backward compatibility and a consistent identifier across generations of mobile technology.

Key Features

• Globally unique eight-digit code identifying device model and manufacturer
• Forms the initial segment of the full 15-digit IMEI
• Allocated by GSMA or its designated reporting bodies
• Used for EIR (Equipment Identity Register) queries and network access control
• Enables device population management and type approval tracking
• Hardware-embedded, non-changeable identifier for reliable device profiling

Evolution Across Releases

Defining Specifications