GSM
The most popular standard for mobile phones in the world.
GSM service is used by over 2 billion people across more than 212 countries and territories.
The ubiquity of the GSM standard makes international roaming very common between mobile phone operators, enabling subscribers to use their phones in many parts of the world.
GSM differs significantly from its predecessors in that both signaling and speech channels are Digital call quality, which means that it is considered a second generation (2G) mobile phone system.
This fact has also meant that data communication was built into the system from the 3rd Generation Partnership Project (3GPP).