What Is a SIM Card?

A SIM (Subscriber Identity Modula) card is an IC card held by a mobile user of the GSM system, and is called a subscriber identity card. The GSM system uses a SIM card to identify GSM users. The same SIM card can be used on different mobile phones. GSM mobile phones can only be used in the network after inserting a SIM card ^[1] .

SIM card is a subscriber identity card for digital cellular mobile phones. Its full name is "Subscriber Identity Module". In the GSM digital mobile phone system, the SIM card plays an extremely important role. It is not only an important part of the mobile phone, but also an identity card equipped for each GSM mobile phone user. Therefore, the SIM card can be recognized Machine conception ^[3]

1.Structure of SIM card

The SIM card is a smart card containing a microprocessor. It consists of a CPU (8 bits), RAM (work memory, 6 to 16 kB), ROM (program memory, 3 to 8 kB), EPROM or EEPROM (data memory, 128 to 256 kB). ) And serial communication unit. These 5 modules are integrated in an integrated circuit to prevent illegal access and theft, because the connection between the chips may become an important clue to illegal access and theft of the SIM card. ^5]

A SIM card is a "smart" card that complies with the GSM specification. It contains information about the user that is stored on the user's side, including ^[6]

There are two working states of the SM card: working mode and idle mode. When in the working mode, complete the information transmission with the mobile phone; when in the idle mode, all relevant data will be maintained, and the internal sleep mode, instruction sleep or clock sleep mode will be supported to effectively save power ^{[8 ]}

1. Do not twist the card, especially the metal contact points on the card. ^[9]

Legal SIM cards must pass strict management steps before they can be used. The SIM card has been written with specific data and secret keys of the card at the factory for personalization. Users can read or modify some of the information stored in the SM card, which is an attractive improvement. Reading out or modifying personal information can be done with the keyboard of a mobile phone, or by connecting a SIM card to a personal computer and using related software ^[10] .

The SIM card is the key to the GSM mobile communication terminal. Once the SIM card is removed from the mobile terminal, the terminal cannot be used except for emergency calls. In addition to the SIM card as a key, it also provides users with a lot of convenience. Users only need to insert or embed the SIM card into any GSM terminal to achieve communication ^[10] .

The SIM card also takes care of a lot of information provided to the user's business, which can be used to store short messages, especially those received when the user is not turned on or away. A more specialized application is to store a priority list for users to choose when several networks are accessible. When real-time charging notices appear online, the SIM card can remember this information so that users can know their expenses at any time ^[10] .