The SIM (subscriber identity module) is a fundamental component of cellular phones. It also known as an integrated circuit card (ICC), which is a microcontroller-based access module. It is a physical entity and can be either a subscriber identity module (SIM) or a universal integrated circuit card (UICC). A SIM can be removed from a cellular handset and inserted into another; it allows users to port identity, personal
information, and service between devices. All cell phones are expected to incorporate some type of identity module eventually, in part because of this useful property. Basically, the ICC deployed for 2G networks was called a SIM and the UICC smart card running the universal subscriber identity module(USIM) application. The UICC card accepts only 3G universal mobile telecommunications service (UMTS) commands. USIMs are enhanced versions of present-day SIMs, containing backward-compatible information. A USIM has a unique feature in that it allows one phone to have multiple numbers. If the SIM and USIM application are running on the same UICC, then they cannot be working simultaneously.
What is a microsim card?
Simply put, a Micro SIM is really just the same as a standard SIM card, just a bit smaller in size. It was developed by the European Telecommunications Standards Institute who settled on the 12 x 15mm size. So, what is its purpose? When you want to use a mobile phone you need some way of communicating which mobile network you are subscribed to. Essentially, this is what the SIM card is for: to contain network specific info that enables you to be identified by whatever mobile phone network you are signed up to. If you don’t have a SIM your phone simply will not work. You can also save lots of mobile phone contacts and other information on a SIM.
A regular SIM card is actually quite a large item when you consider how the current gadget world is focused on smaller and smaller scales, or more compact forms, for everything. Its size occupies a relatively large amount of internal space within the actual handset, which is often a frustration for the top designers. This is where the micro-SIM comes in. It actually made its appearance in the iPhone 4S and iPad, which are some of the thinnest gadgets seen so far in the industry. The iPhone 5 features a nano sim, what is even smaller.
What we consider today to be a conventional SIM, which we use today in normal phones, is in fact a more compact version of the credit card sized SIMs used in the old mobile bricks people carried around back in the 1990s. The micro-SIM is more like a micro micro-SIM in reality.
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File System in Sim:-
A SIM card contains a processor and operating system with between 16 and 256 KB of persistent, electronically erasable, programmable read-only memory (EEPROM). It also contains RAM (random access memory) and ROM (read-only memory). RAM controls the program execution flow and the ROM controls the operating system work flow, user authentication, data encryption algorithm, and other applications. The hierarchically organized file system of a SIM resides in persistent memory and stores data as names and phone number entries, text messages, and network service settings. Depending on the phone used, some information on the SIM may coexist in the memory of the phone. Alternatively, information may reside entirely in the memory of the phone instead of available memory on the SIM.
The hierarchical file system resides in EEPROM. The file system consists of three types of files: master file(MF), dedicated files, and elementary files. The master file is the root of the file system. Dedicated files are the subordinate directories of master files. Elementary files contain various types of data, structured as either a sequence of data bytes, a sequence of fixed-size records, or a fixed set of fixed-size records used cyclically.
As can be seen in the above figure, dedicated files are subordinate directories under the MF, their contents and functions being defined by the GSM11.11 standards. Three are usually present: DF (DCS1800), DF (GSM), and DF (Telecom). Also present under the MF are EFs (ICCID). Subordinate to each of the DFs are supporting EFs, which contain the actual data. The EFs under DF (DCS1800) and DF (GSM) contain network-related information and the EFs under DF (Telecom) contain the service-related information.
All the files have headers, but only EFs contain data. The first byte of every header identifies the file type and the header contains the information related to the structure of the files. The body of an EF contains information related to the application. Files can be either administrative- or application-specific and access to stored data is controlled by the operating system.