A smart card, also known as an Integrated Circuit Card (ICC), is a micro-controller based access module. It is a physical/logical entity and can be either a Subscriber Identity Module (SIM) or a Universal Integrated Circuit Card (UICC). Originally, the ICC defined for 2G networks was the SIM. In 3G networks, the SIM may also be a logical entity (application) on a 3G UICC thereby making it functionally the same as a 2G SIM. The Universal Subscriber Identity Module (USIM) is a logical application running on a UICC smart card, which normally only accepts 3G Universal Mobile Telecommunications Service (UMTS) commands. A USIM can have multiple phone numbers assigned to it, thus allowing one phone to have multiple numbers. If the USIM and SIM applications reside on the same UICC, they cannot be active at the same time.

SIM Technology and Functionality
SIMs are found in GSM, iDEN, and Blackberry handsets and are also used by satellite phone networks such as Iridium, Thuraya, and Inmarsat. Under the GSM framework, a cell phone is termed a Mobile Station, consisting of a SIM card and a handset (Mobile Equipment–ME). One very important and functional feature of a SIM card is that it can be moved from one GSM compatible phone to another, thereby transferring all of the subscriber’s information.

The first SIM cards were about the size of a credit card. As cell phones began to shrink in size, the mini-SIM (about one-third the size of a credit card) was developed. Today an even smaller version, the micro-SIM, is available. Each of these three iterations varies in physical size and the functionality supported. Normally, a SIM card provides functionality for both the identification and authentication of the subscriber’s phone to its network; contains storage for phone numbers, SMS, and other information; and allows for the creation of applications on the card itself. The basic functions are illustrated in Figure 1.

SIM Structure
SIMs contain both a processor (CPU) and an operating system which is either native (proprietary, vendor specific) or Java Card (a subset of the Java programming language). SIMs also have Electrically Erasable Programmable Read Only Memory (EEPROM), Random Access Memory (RAM) for controlling program execution, and persistent Read Only Memory (ROM) which stores user authentication, data encryption algorithms, the operating system, and other applications. Communication between the SIM card and the handset is via a serial interface.