One of the leading digital cellular telephone systems. GSM uses a variation of TOMA that employs eight time slots in a 200-kHz channel. GSM was originally developed as a pan-European digital cellular system, and has become the de facto digital cellular standard in much of the world. GSM operates in the 900 MHz and 1.8 GHz bands in Europe, and 800 MHz and 1.9 GHz bands in the United States.