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The last number of the IMEI is a check digit, calculated using the Luhn algorithm, as defined in the IMEI Allocation and Approval Guidelines: The Check Digit shall be calculated according to Luhn formula (ISO/IEC 7812). (See GSM 02.16 / 3GPP 22.016). The Check Digit is a function of all other digits in the IMEI.
To avoid confusion the database of GSM Association was renamed to IMEI Database - IMEI DB [1] (it was in 2003-2008, see “Document History” at IMEI Database File Format Specification [2]). Also sometimes a common IMEI database for several EIRs is called SEIR (Shared EIR). In each country, the CEIR can interact with IMEI DB differently.
The Type Allocation Code (TAC) is the initial eight-digit portion of the 15-digit IMEI and 16-digit IMEISV codes used to uniquely identify wireless devices.. The Type Allocation Code identifies a particular model (and often revision) of wireless telephone for use on a GSM, UMTS, LTE, 5G NR, iDEN, Iridium or other IMEI-employing wireless network.
It consists of three fields, including an 8-bit regional code (RR), a 24-bit manufacturer code, and a 24-bit manufacturer-assigned serial number. The check digit (CD) is not considered part of the MEID. The MEID was created to replace electronic serial numbers (ESNs), whose virgin form was exhausted in November 2008. [1]
The check digit is computed as follows: Drop the check digit from the number (if it's already present). This leaves the payload. Start with the payload digits. Moving from right to left, double every second digit, starting from the last digit. If doubling a digit results in a value > 9, subtract 9 from it (or sum its digits).
Add the digits (up to but not including the check digit) in the even-numbered positions (second, fourth, sixth, etc.) to the result. Take the remainder of the result divided by 10 (i.e. the modulo 10 operation). If the remainder is equal to 0 then use 0 as the check digit, and if not 0 subtract the remainder from 10 to derive the check digit.
The "Reporting Body Identifier" is also known as the Regional Code in the CDMA context. The International Mobile Station Equipment Identity and Mobile Equipment Identifier (MEID) structures are superficially the same, except that the first two digits must be decimal for an IMEI, and must be hexadecimal for an MEID.
ESNs are currently mainly used with CDMA phones (and were previously used by AMPS and TDMA phones), compared to International Mobile Equipment Identity (IMEI) numbers used by all GSM phones. [1] The first eight bits of the ESN were originally the manufacturer code, leaving 24 bits for the manufacturer to assign up to 16,777,215 codes to mobiles.