A SIM card is a smart card that is used in mobile phones, to identify the client. SIM stands for subscriber identity module. A SIM card has a microchip, and its use is protected by a PIN. When the phone is powered on, an international mobile subscriber identity number (IMSI) is broadcast. The microchip is also needed for some encryption and decryption. The SIM card and the device that it is used in (the mobile phone) are often referred to as the "Mobile station".