IMEI vs IMSI vs TMSI: Key Differences Explained

This article explains the differences between IMEI, IMSI, and TMSI in the context of the GSM (Global System for Mobile communications) system. These terms are all related to identifying mobile subscribers within a GSM network.

The following diagram illustrates a typical GSM network architecture, which comprises various system elements that utilize these identifiers:

Image alt: gsm network architecture

IMEI - International Mobile Equipment Identifier

The IMEI is a unique identification number for each mobile device. All IMEIs are stored in a database called the EIR (Equipment Identification Register). The network uses this database to verify if a mobile device has been reported stolen or is otherwise unauthorized.

Key characteristics of the IMEI:

• It’s a 15-digit number.
• It’s assigned to the mobile station during manufacturing.
• The structure of the IMEI is as follows:
  • TAC (Type Allocation Code): 8 digits
  • SNR (Serial Number): 6 digits
  • Spare: Set to zero (1 digit)

IMSI - International Mobile Subscriber Identity

The IMSI is a unique number assigned to a mobile subscriber by their telecom service provider.

Important points about the IMSI:

• This number is stored on the SIM card.
• It’s a 15-digit number.
• It incorporates specific country codes to enable international subscriber services.

TMSI - Temporary Mobile Subscriber Identity

The TMSI is a temporary identifier assigned by the VLR (Visitor Location Register) after a GSM mobile station establishes communication with the GSM network (e.g., through a Base Transceiver Station or BTS).

Here’s what you need to know about the TMSI:

• The network uses the TMSI instead of the IMSI during call processing and call management tasks.
• It’s shorter than the IMSI, making it more efficient to transmit.
• The primary purpose of using the TMSI in place of the IMSI is to enhance security for the mobile subscriber. This avoids the need to continuously transmit the IMSI, which could be intercepted.