GSM is a world standard for digital cellular communications using a variation (narrowband) of TDMA (Time Division Multiple Access), which allows up to eight calls at a time. Introduced in 1991, GSM has over 120 million users worldwide and is available in 120 countries, according to the GSM MoU Association. While it is still most commonly used in Europe and Asia, USA is beginning to adopt it as well. AT&T Wireless and Cingular Wireless have both recently announced plans to completely transition their networks to the GSM standard. Both providers have begun offering GSM services in limited areas in anticipation to convert their existing TDMA networks to the GSM protocol within the next couple of years.
GSM handsets use a Subscriber Identity Module (SIM) smart card, which contains user account information as well as additional storage for personalized user data (phonebooks, etc.). A GSM phone is immediately programmed upon insertion of a SIM card, thus allowing GSM phones to be easily rented, borrowed or exchanged. SIM cards can be programmed to display custom menus for personalized services. GSM provides a short messaging service (SMS) that enables text messages up to 160 characters in length to be sent to and from a GSM phone.
GSM together with other technologies is part of an evolution of wireless mobile telecommunication that includes High-Speed Circuit-Switched Data (HCSD), General Packet Radio System (GPRS), Enhanced Data GSM Environment (EDGE), and Universal Mobile Telecommunications Service (UMTS)
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