Global System for Mobile communications (GSM) is a digital cellular phone technology based on Time Division Multiple Access (TDMA). GSM defines the entire cellular system, not just the TDMA air interface. In the early 1990s, GSM enabled roaming across European nations for the first time, and today, more than 1.5 billion GSM customers worldwide can phone each other via roaming agreements between carriers. AT&T and T-Mobile offer GSM services in the U.S., while Rogers Wireless uses GSM in Canada.

GSM operates in several frequency bands, including 900 MHz and 1.8 GHz in Europe and 850 MHz and 1.9 GHz in the U.S. and Canada. GSM's TDMA technology is based on a circuit-switched system that divides each 200 kHz channel into eight 25 kHz time slots.