An ETSI standard for second generation digital cellular wireless voice and data communications using time division multiple access transmission methods. Formerly known as Groupe Spécial Mobile, which was the study group of the Conference of European Posts and Telegraphs (CEPT) set up to develop a pan-European system for terrestrial mobiles.
Having such a standard ensures compatibility, so one can use the same cell phone for travel between countries. Today more than 105 countries, including the United States and Japan, have existing or planned GSM networks. These networks account for a large percentage of the world's cellular market.