The SIM card is a small, plastic chip that cell phones use to identify themselves on a network. The card contains information the cellular network interprets for cell phone companies to properly track what numbers the caller is dialing, how many text messages they send and receive, how long they are talking and what, if any, Internet data they are sending and receiving. They also allow phone companies to restrict usage of their SIM cards to a certain part of the world.
History
SIM stands for "Subscriber Identity Module." The cards were first introduced in 1991. They were manufactured by Munich-based Giesecke Devrient and sold to Finnish mobile network operator Elisa Oyj.
Function
A phone's SIM usually contains five numbers used to identify itself on the network: ICCID, IMSI, Authentication Key (Ki), Local Area Identity (LAI), and Operator-Specific Emergency Number. The ICCID (Integrated Circuit Card ID) is an 18- or 19-digit identification number used to identify the SIM itself. The IMSI (International Mobile Subscriber Identity) is a code used by the phone company to identify the SIM on the mobile network. The Authentication Key is a security key used by the network to ensure the SIM is valid. The LAI contains information about the location where the SIM originated; the OSEN is a location-specific emergency number.
Who Uses SIM cards?
All GSM carriers around the world are required to use SIM cards. American GSM carriers such as T-Mobile and ATTT use SIM cards in all of their phones. Most wireless carriers in Europe and Asia are GSM-GSM and SIM-compatible phones dominate the world market. Satellite phone carriers also use their version of a SIM card, sometimes compatible with wireless carriers' SIM cards.
Benefits
SIM cards allow users to own multiple phones and swap out the cards, giving them added flexibility with what devices they carry. Most SIM-based phones use GSM. GSM coverage exists nearly everywhere in the world-those traveling abroad benefit from its extended coverage. Many phones, especially those purchased in Europe and Asia, come unlocked and can be used with any SIM card.
Disadvantages
Many wireless phone companies "SIM lock" their SIM cards to particular areas of the world. For example, ATT may lock their SIMs to America-to travel abroad, you may have to call or use other means to get it unlocked. Most carriers allow this; some may charge a fee or require a contract adjustment, depending on their rules. SIM cards are also small and relatively flimsy. Getting a replacement for a damaged one is usually not a problem but can be a hassle.
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