GSM (Groupe Special Mobile) - Global System for Mobile communications -
most popular standard for mobile phones in the world.
GSM 900 / GSM 1800 MHz are used in most parts of the world: Europe, Asia, Australia, Middle East, Africa.
GSM 850 / GSM 1900 MHz are used in the United States, Canada, Mexico and most countries of S. America.
5G Field testing / trials / research / development by country (March 2017):
More information: World Time Zone: http://www.worldtimezone.com/gsm.html