GSM

Global System for Mobile communications (GSM: originally from Groupe Spécial Mobile) is the most popular standard for mobile phones in the world. Its promoter, the GSM ASSOCIATION, estimates that 82% of the global mobile market uses the standard. GSM is used by over 2 billion people across more than 212 countries and territories. Its ubiquity makes international roaming very common between mobile phone ops, enabling subscribers to use their phones in many parts of the world. GSM differs from its predecessors in that both signalling and speech channels are digital quality, and thus is considered a second generation mobile phone system. This has also meant that data communication was easy to build into the system.
The ubiquity of the GSM standard has been advantageous to both consumers (who benefit from the ability to roam and switch carriers without switching phones) and also to network operators (who can choose equipment from any of the many vendors implementing GSM. GSM also pioneered a low-cost alternative to voice calls, the short mess service (SMS, also called "text messaging"), which is now supported on other mobile standards as well.
Newer versions of the standard were backward-compatible with the original GSM phones. For example, release 97 of the standard added packet data capabilities, by means of general packet radio service (GPRS). Release '99 introduced higher speed data transmission using enhanced data rates for GSM (EDGE).