SIM card is the abbreviation of subscriber identity module, also known as smart card and subscriber identity card. GSM digital mobile phone can only use this card. It stores the information and encryption keys of digital mobile phone customers on a computer chip, which can be used to authenticate the identity of GSM network customers and encrypt the voice information when customers talk. The use of SIM card completely prevents parallel lines and telephones from being eavesdropped, and the SIM card is made in strict accordance with GSM international standards and specifications, thus reliably ensuring the normal communication of customers. The SIM card I got when I first entered the network was a big card with a small card embedded in it. At present, small cards are widely used in mobile phones, and large cards can be taken off for use. SIM cards come in different sizes. The size of a large card is 54mm×85mm (the standard size of a credit card), and the size of a small card is 25mm× 15mm (smaller than ordinary stamps). In fact, a small card is the kind with a chip on a big card. What really works for the big card is the small card above, which can be removed from the big card. At present, the popular style in China is "small card", which can also be changed into "big card" (as long as there is a Cato). "Big Card" and "Small Card" are suitable for different types of GSM mobile phones. Early models such as Motorola GC87C and 308C used "big cards", but now new models basically use "small cards". The application of SIM card in GSM system separates the card from the mobile phone, and the SIM card uniquely identifies a customer. The SIM card can be inserted into any GSM mobile phone for use, and the communication expenses generated by using the mobile phone are recorded in the customer account uniquely identified by the SIM card. SIM cards have four capacities: 8K, 16K, 32K and 64K, among which large-capacity SIM cards above 16K are collectively called multifunctional STK cards.