What is a SIM?

As the name Subscriber Identity Module (SIM) implies, a SIM card holds the identity of the subscriber, which enables users to be registered to the telecommunication network of a Mobile Networt Operator (MNO).
In addition to

• identification and authentication
  

the SIM card can also store the subscriber’s information on

• contacts
• messages
• calls
• location

and other subscriber-specific data.

The components of a SIM card include

• a central processor unit (CPU)
• an operating system (OS)
• Random Access Memory (RAM) that controls the program execution flow
• FLASH memory that can be electrically erased and reprogrammed which controls the operating system workflow, user authentication, data encryption algorithm, and other applications.
  

SIM card file system

The SIM card file system is organized in a hierarchal tree structure and resides in the EEPROM for storing data such as names and phone number entries, text messages, and network service settings.

Fig.: SIM file system

The anatomy of the file system includes three types of files:

• Master File (MF)
• Dedicated Files (DF)
• Elementary Files (EF)
  

The Master File is the root of the file system.
Dedicated files are the child directories of the master files such as the DF (DCS1800) and DF (GSM), which contain network-related information, and DF (Telecom), which holds service/carrier- related information.
Furthermore, elementary files contain the actual data in various types, structured as either a sequence of data bytes, a sequence of fixed-size records, or a fixed set of fixed-size records used cyclically. It is important to note that all the files have headers, but only EFs contain data