IMEI, commonly known as "serial number", is stored in EEPROM (commonly known as chip) of mobile phone, and each mobile device corresponds to a unique IMEI. Its composition structure is TAC(6 bits) +FAC (2 bits) +SNR(6 bits) +SP (1 bit). TAC (Equipment Type Approval Number), designated by European Type Approval Center. If we are familiar with and understand this number, it will play a very important role in identifying mobile phones in the future. First of all, the first three digits of TAC code will change in different periods, and the first three digits of TAC code in the past will not appear on the current mobile phone. In the past, some old mobile phones basically started with 446/448, such as moto v328 Nokia 7 1 10, and even the same mobile phone will have different TAC codes at different times, such as the most familiar 82 10 upgrade 8250. If the first three digits are 448902, it cannot be upgraded due to different CPUs. Therefore, in the past, it was said that as long as it is the same model of mobile phone, the TAC code must be the same, which is not accurate. For example, the T39MC of Ericsson's mobile phone of the same model will have different numbers in different regions. The licensed product in Chinese mainland is 520406, and that in Hongkong is 520407. Knowing these differences, we can use TAC code for "equipment identification" without having to bother to remember the place of origin represented by 7.8 digits of IMEI.