SIM card is the abbreviation of the initials of each word of Subscriber Identity Module. \ xd \ xd \ SIM card is also called subscriber identity card and smart card. GSM digital mobile phone can only be used with this card. \ xd \ xd \ Subscriber Identity Module (SIM), commonly called "SIM card", is a smart card mainly used to store user identification data, SMS data and telephone numbers. \ xd \ xd \ SIM card is mainly used in GSM network, W-CDMA network and TD-SCDMA network, but compatible modules can also be used in IDEN telephone. Some people refer to the RUIM card and UIM card of CDMA2 and cdmaOne, which are also called SIM cards. Although they have similar functions and abide by all the same mechanical and electrical standards and some software standards, the upper applications are not necessarily compatible. \ xd \ xd \ sim consists of CPU, ROM, RAM, EEPROM and I/O circuit. When users use SIM, in fact, the mobile phone sends commands to the SIM card, and the SIM card should be executed or rejected according to the standard specification; SIM card is not a simple information memory.