Subscriber Identity Module (SIM) cards are small electronic storage devices used by telephones and networks using the Global System for Mobile (GSM) technology. Networks and telephones based on the Code Division Multiple Access (CDMA) system, generally don't use SIM cards, instead "locking" each subscriber and telephone number to a specific cellphone. Despite this long-standing difference, there is an emerging technology for bringing SIM-like functions to CDMA phones.
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What's a SIM Card?
SIM cards are small (smaller than thumbnail-size) data storage cards used in GSM telephones. As their full name suggests, SIM cards hold information about a subscriber's unique identity, and are each linked to a single person or phone account.
What are SIM cards used for?
SIM cards can be removed from one telephone and inserted into another -- allowing a person to take their unique phone number (and sometimes contacts and text messages) with them onto any unlocked GSM cellphone they want. Once the SIM card is inserted into a cellphone, that cellphone is automatically assigned the subscriber's telephone number.
Every GSM network uses SIM cards. Some American GSM cellular networks are AT&T and T-Mobile. Overseas, companies such as Orange, Cellcom and Telstra are among the world's many GSM network operators.
What is CDMA?
CDMA is a radio technology invented by the Qualcomm company. It enables large numbers of cellphones to work simultaneously in the same area by assigning each telephone a unique radio transmission pattern defined by a preset code. Verizon and Sprint are the two largest CDMA operators in the U.S.
How do CDMA networks manage identity?
Naturally, CDMA networks also need to keep track of their subscribers' identities -- which phones carry which phone numbers and belong to which people. Instead of a removable card, CDMA networks use data carried in the circuits of each telephone. The Electronic Serial Number (ESN) and Mobile Equipment Identifier (MEID) are numbers unique to each CDMA telephone, and used by CDMA networks to manage service to phones and users.
Because CDMA networks rely on unique data in each telephone's non-removable circuitry, it's generally more difficult for CDMA subscribers to start using a different phone. CDMA cellphones don't have a slot for accepting a SIM card, and GSM and CDMA are mutually incompatible radio systems.
SIM (well, RUIM) comes to CDMA
The Telecommunications Industry Association (TIA) has developed a standard that will allow SIM-like cards known as removable user identity modules (RUIMs) to be used on CDMA networks. The RUIM would fill the same role for CDMA as SIM cards do for GSM. As of 2010, CDMA networks in China are the most prolific users of RUIM-equipped phones, while RUIM technology in America isn't widespread.
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