What is a sim card?

SIM card is (Subscriber ldentity Module customer identification module) abbreviation, also known as smart card, user identification card, GSM digital mobile phone must be installed this card can be used. It stores the information of digital mobile phone customers, encrypted keys and the user's phone book on a computer chip, which can be used to identify GSM network customers and encrypt the voice information when the customer calls.
SIM cards are mainly used for GSM systems, but compatible modules are also used for UMTS UE (USIM) and IDEN phones. Some people refer to the RUIM and UIM cards of CDMA2000 and cdmaOne, also known as SIM cards, although the two have similar functions and are subject to the same mechanical and electrical standards and some software standards, but the upper application is not necessarily compatible. The SIM consists of CPU, ROM, RAM, EEPROM and /0 circuits. When the user uses the SIM, the phone actually issues commands to the SIM card, and the SIM card should be executed or rejected according to the standard specifications, and the SIM card is not a simple information memory.
The application of SIM card in GSM system separates the card from the mobile phone, and the SIM card uniquely identifies a customer. A SIM card can be inserted into any GSM mobile phone for use, and the communication fee generated by the use of the mobile phone is recorded in the customer account uniquely identified by the SIM card. The SIM card capacity is 8K, 16K, 32K, 64K, among which the large-capacity SIM cards above 512k are collectively referred to as STK cards.