Global System for Mobile communications (GSM) is an international standard for mobile communication. Its promoter, the GSM Association, estimates that 80% of the global mobile market uses this standard. GSM is used by over 3 billion people across more than 212 countries. Another advantage is that the standard includes one worldwide Emergency telephone number “112” which is very useful in case of some emergency when local emergency help center number is unknown.

If you travel to any country of the world, GSM is the only type of cellular (mobile phone) service available everywhere. Originally, the full form of GSM is “Groupe Spécial Mobile”, which is name of a group formed by the Conference of European Posts and Telegraphs (CEPT) in 1982 for research purpose of the merits of a European standard for mobile telecommunications. This technology standard, GSM, also enabled mobile making companies to make such mobiles which are able to be operated anywhere in the world using the standard SIM. Until 1991, GSM was not in use commercially. Instead of using analog service, GSM was developed for digital system using Time Division Multiple Access (TDMA) technology. TDMA is a technology for shared medium networks, such as radio. It allows several users to share the same frequency by diving it into different time slots.

TDMA uses a narrow band of 30 kHz wide and of 6.7 milliseconds long which is divided time-wise into three time slots. Narrow band, sometimes, mean “channels” in the traditional sense. Each conversation gets the radio for one-third (1/3) of the time. This feature is possible because of voice data that has been converted to digital information. This digital information is compressed so that it takes up appreciably small transmission space. TDMA has the capacity three times greater than an analog system using the same number of channels (narrow bands).

GSM operates in the 1900 MHz band, (1.9 GHz) in United States of America (USA) and in 900 MHz band (actually, 890 MHz - 960 MHz) in Europe and Asia. It is used in digital cellular and PCS-based systems. PCS is abbreviation of Personal Communications Service.

GSM is also the foundation for Integrated Digital Enhanced Network (iDEN). iDEN is a Motorola copyrighted version of TDMA with a only one of its kind "push-to-talk" two-way radio capability. Nextel Communications is the largest iDEN operator in the United States. The unbelievable growth of GSM is a big part of why the acronym is now commonly considered to be standard for the Global System for Mobile communications.

GSM technology provides a number of useful features some of which are listed below.

• Data networking
• Uses encryption to make phone calls which is more secure
• Call forwarding
• Call waiting
• Short Message Service (SMS) for text messages and paging
• Caller ID
• Call conferencing (more than two callers in same call at a time).