IMEI vs IMSI vs TMSI-difference between IMEI,IMSI,TMSI

This page on IMEI vs IMSI vs TMSI mentions difference between IMEI, IMSI and TMSI with respect to GSM system.

All these terms are associated with mobile subscriber in GSM network. The figure below depicts GSM network architecture consisting of various system elements.

IMEI-International Mobile Equipment Identifier

IMEI of all the mobile stations are stored in the database known as EIR (Equipment Identification Register). The network can check this list in order to confirm whether mobile has been stolen or not.

IMEI is the unique number. It is 15 digit in size. The number is assigned to the mobile station at the time of manufacturing.

IMEI (15 digits) = {
TAC-Type Allocation Code ( 8 digits )
SNR-Serial Number ( 6 digits )
Spare-Set to zero (1 digit )

gsm network architecture

IMSI-International Mobile Subscriber Identity

The IMSI number is assigned to the mobile user by the telecom service provider or telecom carrier. This telephone number is stored on SIM card.

It is 15-digit in size. This number takes into account specific country codes in order to use international subscriber service.

TMSI-Temporary Mobile Subscriber Identity

The TMSI identifier is assigned by VLR entity after GSM mobile station establishes communication with the GSM network(i.e. Base station or BTS). The network then uses TMSI instead of IMSI during call processing and call management tasks.

This identifier is shorter compare to IMSI number. Hence it is more efficient to transmit. The purpose to use TMSI inplace of IMSI is to provide security to the mobile subscriber, as IMSI need not have to be transmitted continuously.

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RF and Wireless Terminologies