The Subscriber Identity Module (SIM) is a smart card chip, of the size of a postage stamp, which when inserted in your mobile phone, forms the intelligent source of the various types of communications. A Smart Card is an integrated circuit card (ICC) and is defined as a miniature card with embedded integrated circuit. Though there are many applications, ICCs are catagorised into two broad segments. One is the non-volatile memory card which contains only the storage components, and the other being a kind of a microprocessor circuitry consisting of memory or microprocessor components.

The SIM is a microprocessor unit with memory storage capability, which stores all the unique information about your account. It holds your identity in the form of your account number with the service provider and identifies you to the network. The currently available SIM card has 16 to 64 kb of memory, which stores your personal telephone numbers that you call, text messages that you receive or send and other value added services. There are often options given as value added services where you can download a picture or MP3 songs and even a ring tome for your mobile. The memory storage available in your SIM card stores these data or information for you to avail them as and when you require. There is a demand of larger memory spaces to store high resolution pictures, large MP3 files and also enable live video streaming.

One of the third generation mobile phone technologies is called Universal Mobile Telecommunications System (UMTS). A Universal Subscriber Identity Module (USIM) is an application for UMTS mobile telephony running on Universal Integrated Circuit Card (UICC). UICC is another type of chip smart card used in GSM and UMTs. Unlike SIM, this card has a huge memory space and holds a few kilobytes of data. With the coming of more services the storage area in the smart chip need to be larger. In a GSM environment, the UICC has an application of that of SIM where-as in a UTMS environment it is the USIM application. The SIM that you have is uniquely identified by its International Circuit Card ID (ICCID).

SIM cards are available on the basis of a subscription on signing a contract with the service provider. They are available either on a pay-as-you-go basis, where you get your monthly bills or as a pre-paid subscriber, where you pay a certain charge in advance for the service that you want. This depends on the type of contract that you sign.

Two SIM cards may also be used in your mobile phone. All you need to do is to buy dual SIM cover from your dealer for the model of the mobile phone that you have. This has many advantages viz. you could hold two SIM cards in a single mobile phone, featuring pay-as-you-go and pre-paid as well, for your office and personal use respectively.

In most of the cases, the SIM can be removed from the mobile phone, enabling you to carry your mobile subscription and data through different types of mobile phones that you may choose to have along the line of GSM phones available. It may so happen that the mobile phone with the SIM card in it is misplaced somewhere, stolen or lost. You may, at the earliest, request the service provider to deactivate that SIM, giving them the reasons as to why, and acquire another replacement card for your new phone. You may be liable to pay a charge to the service provider for issuing the duplicate SIM card to you.

Generally the mobile phones available are 'SIM unlocked' and as such it may be used with any SIM card from any carrier in the world. Guided by the contract you sign, many mobile phones are 'SIM locked' by the service provider, and in most of the circumstances, till a certain contract period. These phones will not work with another SIM card as in the former case.

The embedded circuitry of the SIM card holds information about the service plan that you have chosen as a subscriber. The information about your current area location (LAI) in the network is also stored by the SIM as network information. You will find that when you switch on your mobile phone, the software within, searches for the network that you are registered with and lock you on to that service provider. It does so by taking the necessary data from your SIM and search for the LAI it was in.

The SIM card in your mobile phone identifies you, by holding an International Mobile Subscriber Identity (IMSI). All GSM and Universal Mobile Telecommunications System (UMTS) network users are uniquely associated with IMSI, and this number is stored in the SIM. When a call is made, the mobile phone sends this number to the network which is used to acquire other details from the Home Location Register (HLR) or as available in the Visitor Location Register (VLR). The VLR contains all the data of the subscriber, which are required for any call handling and mobility management in the area controlled by the VLR. Due to security reasons, the IMSI is sent out very rarely and TMSI is sent instead, which is the Temporary Mobile Subscriber Identity and is quite often used.

For security purposes, the SIM card features storage of Personal Identification Number (PIN) code, which is a 4-8 digit password chosen by you, and entered each time you wish to gain access to your mobile phone. There is also a PIN2, used to gain access to more advanced features in the mobile and is available for GSM II applications. On error, the system allows you to enter the password three times before the SIM card get locked and you would need a Personal Unblocking Key (PUK) from your network operator to unblock your mobile. PUK2 is used for similar purpose as PUK, but for PIN2.

In addition to the mobile phone smart cards mentioned above, viz., SIM and USIM, the following are the other types which have application in different network technology:

ISIM - IP International Services Identity Module is a smart card like the SIM, which is applicable in a UICC smart card environment in a 3G (Third Generation Technology) mobile telephone in the IP Multimedia Subsystem (IMS) environment. IMS is architecture for telecom operators applied to a standarised Next Generation Networking (NGN), a network that usually encompasses data and voice communication, as well as the optionally available media, such as video. ISIM can co-exist with SIM and USIM, provided that they are on the same UICC.

R-UIM - The Removable User-identity Module is like a SIM but capable of working with both, in CDMA and GSM phones and networks. The R-UIM fits into any existing GSM phone and is compatible with any GSM SIM.

W-SIM - The Wilcom SIM has been developed by Willcom, which, in addition to the functions of a SIM, provides radio receiver/transmitter facilities as its built-in feature.