The Global Standard for Mobile communications is a digital standard used by mobile networks across the globe. The most common frequency used is 900mhz, followed by 1800mhz. In the Americas, 1900mhz and 850mhz are also used. These bandings are what is referred to by single, dual, tri or quad band phones. A quad band phone will operate on any of the GSM frequencies, offering a larger range of roaming options.