Global System for Mobile Communications
The Global Systems for Mobile Communications (GSM), is a radio-based technology that allows mobile phones to communicate. GSM is considered to be the world's most ubiquitous radio-based cellular technology, with 1.7 billion users as of March 2006. In the United States, carriers such as T-Mobile and Cingular utilize GSM technology for their cellular networks.
U.S. GSM Cell Phone Providers
Other GSM Cell Phone Providers
GSM is considered to be 2G and uses digital data for both signaling and voice purposes. GSM uses a time division multiple access (TDMA) protocol to transmit information. Aside from voice, GPRS and EDGE allow for data transmissions across a GSM network.
In terms of frequencies used, GSM networks often use the 900MHz and 1800MHz bands, but carriers in the United States use 850MHz and 1900MHz bands. As a result, "dual-band" phones, which only support two of the four major bands, can only be used in certain parts of the world. More common are "tri-band" phones, which usually operate on 900/1800MHz and 1900MHz. "Quad-band" phones support all four frequencies and can be used in all parts of the world.