The Subscriber Identity Module (SIM), also known as a SIM Card, is a small smart card which contains both programming and subscriber information.
The A3 and A8 algorithms are implemented in the Subscriber Identity Module (SIM). Subscriber information, such as the International Mobile Subscriber Identity (IMSI), is stored in the SIM. It can also store user-defined information such as phonebook entries, contacts, addresses...etc.
One of the advantages of the GSM architecture is that the SIM may be moved from one Mobile Station to another. This makes upgrades very simple for the GSM telephone user.