GSM is short for Global System for Mobile communications, which is one of the leading digital cellular system.

The GSM standard for digital cell phones was established in Europe in the mid 1980s.

GSM has now become the international standard in Europe, Australia, and much of Asia and Africa.

In covered areas, cell-phone users can buy one phone that will work anywhere where the standard is supported. To connect to the specific service providers in these different countries, GSM users simply switch Subscriber Identification Module (SIM) cards.

SIM cards are small removable disks that slip in and out of GSM cell phones. They store all the connection data and identification numbers you need to access a particular wireless service provider.

Unfortunately, the 1900-MHz GSM phone used in the United States (US) are not compatible with the international system.

GSM uses narrowband TDMA, which allows eight simultaneous calls on the same radio frequency.

TDMA stands for Time Division Multiple Access, which is a technology for delivering digital wireless service using Time-Division Multiplexing (TDM).

TDMA works by dividing a radio frequency into time slots and then allocating slots to multiple calls. In this way, a single frequency can support multiple simultaneous data channels.

GSM digitizes and compresses voice data, then sends it down a channel with other streams of user data, each in its own time slot.

GSM systems use encryption to make phone calls more secure.

GSM operates in the 900-MHz and 1800-MHz bands in Europe and Asia, and in the 1900-MHz, sometimes referred to as 1.9-GHz, based in the United States.

GSM is used in digital cellular and PCS based systems.

GSM is also the basis for the Integrated Digital Enhanced Network (IDEN), which is a popular system introduced by Motorola and used by Nextel.

SIM Card

The SIM (Subscriber Identity Module) is a chip card, the size of a first class postage stamp. It is a key element in over 600 million GSM (Global System for Mobile) mobile phones, representing about more than 70 percent of the mobile handset market.

A SIM is actually a tiny computer chip that gives a cellular device its unique phone number. It has memory, for data and applications, a processor and the ability to interact with the user. Current SIMs typically have 16 to 64 kb of memory, which provides plenty of room for storing hundreds of personal phone numbers, text messages and other data.

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