What is GSM:

Which stands for Global System for Mobile Communications, is a digital cellular radio network operating in all over Countries world-wide. It provides almost complete coverage in western Europe, and growing coverage in the Americas, Asia and elsewhere.In 1980, the European Conference of Postal and Telecommunications Administrations (CEPT) created the Groupe Spécial Mobile (GSM) to develop a standard for a mobile telephone system that could be used across Europe. The Europeans realized this early on, and in 1982 the Conference of European Posts and Telegraphs (CEPT) formed a study group called the Group Special Mobile (GSM) to study and develop a pan-European public land mobile system. The GSM facility can be used by many kinds of mobiles.
In 1989, GSM responsibility was transferred to the European Telecommunication Standards Institute (ETSI), and phase I of the GSM specifications were published in 1990. The first GSM network was launched in 1991 by Radiolinja in Finland with joint technical infrastructure maintenance from Ericsson.Commercial service was started in mid-1991, and by 1993 there were 36 GSM networks in 22 countries. By the end of 1993, over a million subscribers were using GSM phone networks being operated by 70 carriers across 48 countries. which had grown to more than 55 million by October 1997. With North America making a delayed entry into the GSM field with a derivative of GSM called PCS1900, GSM systems exist on every continent, and the acronym GSM now aptly stands for Global System for Mobile communications.

The GSM frequency are three types:

1. GSM 900: It works in According to the frequency of DCS 900 MHZ
2. GSM 1800: It works in According to the frequency of DCS 1800 MHZ
3. GSM 1900: It works in ACcording to the frequency of DCS 1900 MHZ.

This is the main frequency used by the GSM mobiles all over the world.