What is GSM?

GSM (Global System for Mobile) is the second generation digital technology originally developed for Europe, but now covers over 71% of the world market.
It was initially developed to operate in the 900 MHz band and later modified for the 850, 1800 and 1900MHz bands. Originally, GSM stood for Groupe Speciale Mobile, i.e., the CEPT committee that started the GSM standardization process.