SIM Full Form

SIM is the abbreviation for Subscriber Identity Module or Subscriber Identification Module.

You can think of the Subscriber Identity Card as a smart card that is essentially used for communication when installed on a mobile phone. This card being an IC(integrated chip) keeps the IMSI(international mobile subscriber dialling) number secure within itself which is later utilized for the identification of the subscribers on the mobile communications system and to further authenticate them.

Moreover, a Subscriber Identity Card also stores a memory so as to customized information such as text messages and telephone logs.

This is what a SIM consists of.

It has a unique number called ICCID number which acts as a serial for it, IMSI number or International Mobile and Subscriber Identity, features for identification and authentication,  data for the network which is stored temporarily, and ciphering info. A SIM also has a record of the services utilized by the users, and it is also protected with a PUC (personal unblocking code) for the purpose of PIN unlock. PIN unblocking key (PUK) and PIN are 4-8 passwords for the purpose of safeguarding the SIM from any unwarranted use. However, a PIN is not necessarily used.

A user can use a SIM in any phone which belongs to them just by transferring the card to the phone he is using. These cards are also utilized in smartwatches, satellite devices and others.

Structure of SIM and memory hierarchy

Have you seen the hard disk drives traditionally used to store information? The memory structure of the Subscriber Identity Module is comparable to the directory of such a device.

SIM has a file system that includes a master file abbreviated as MF, an elementary file referred to as EF and a directory file or DF.

Master file

The file system has its basis in the master file (MF), or you can call MF its roots. The master file is quite similar to the basic directory used in any other OS (operating systems). This file does not necessarily need to be EF (elementary files). There can exist a single or even more dedicated file in the file system of SIM.

Dedicated file

DF or dedicated files refer to the sub-directory or subordinate directories of the MF. The functions and the consistent data of dedicated files are specified by the GSM11.11 standards.

The dedicated files can be identified by a two-byte identifier. The phonebook elementary files also come in the same class.

Elementary file

Elementary files lie on the end of the SIM memory hierarchy settings, below the dedicated files. Understand the elementary files as the leaf node of the file system.

It is important to note here that all the files in the file system have headers but only the elementary files store data. The file type is recognized by the first byte of the header which further contains the data related to the structure of the file system. 

Data related to the application is stored in the body of the elementary files. 

The file structure can correspond to being application-specific or administration specific. Accessing the information stored is decided by the operating system.

Security of SIM

The Subscriber Identity Module card comes with security measures that are in-built added to it. These security features are contained in the 3 file types in the file system we just discussed- the memory files, the dedicated files and the elementary files. 

As all the tasks carried out have to go through these in-built security features and the executions that have gone standard authorization are granted access to the function intended. There are several levels of conditions for granting access to dedicated files and elementary files.

The ‘always’ condition enables access to the files without additional limitations.

• CHV1 or cardholder verification 1 condition comes in handy when the PIN verification is not enabled or only after PIN verification is successful
• CHV2 or cardholder verification 2 condition works in a similar manner as CHV1 and it allows access to the file system only when the PIN2 is successfully verified or the verification of the PIN2 is disabled
• The ADM or administrative access is given to the card issuer who avails the card to the user, can access the files only by undergoing specified requirements for the ADM access
• The NEV or ‘never’ condition means that accessing is the file over the subscriber identity module interface is not possible and it has been forbidden

The type of execution being attempted and the condition of access applicable are the basis on which the subscriber identity module OS regulates the access to the individual units of the file system.

Also, the number of trials being undertaken by the user is controlled by OS. For example, if the user doesn’t enter the correct cardholder verification (CHV) three times (usually), any further attempts are restricted.

To remove the restrictions, a PIN Unblocking Key-PUK is required and it resets the cardholder verification. A service provider can give the CHV1/CHV2 unblocking access to a known subscriber.

Benefits of SIM

Here are the benefits of a subscriber identity module card:

• This little smart card has your contact number, the phone number of your connections in a phonebook, the plan subscribed and many other elements
• This tiny card is transferable from one phone to another and easily accessible
• The SIM card has files that are secured with the use of a PIN lock often given by the provider. But the PIN change can be accessed by you
  

  Conclusion

  SIM cards are an inevitable part of the communication infrastructure. The best part is that the users can transfer their calling plan and the contact number to another phone.