GSM (Global System for Mobile Communications) is a standard originally developed by the European Telecommunications Standards Institute (ETSI) to describe technologies for second generation (2G) digital cellular networks.
The GSM standard was developed as a replacement for first generation(1G) analog cellular networks, and originally described a digital, circuit switched network optimized for full duplex voice telephony. Over time, the standard has been expanded to include first circuit switched data transport and packet data transport via GPRS (General Packet Radio services). Packet data transmission speeds were later increased via EDGE (Enhanced Data rates for GSM Evolution). The GSM standard was improved further after the development of the third generation (3G) UMTS standard developed by the 3GPP. GSM networks will continue to evolve as they incorporate fourth generation (4G) LTE Advanced standards. The "GSM" trademark is owned by the GSM Association.
In the early days of the mobile / cell phone industry, European analogue cellular networks employed an uncoordinated mix of technologies and protocols that varied from country to country. This prevented interoperability of subscriber equipment and increased the complexity for equipment manufacturers who were having to deal with varying standards from a fragmented market. The work to develop a European standard for digital cellular voice telephony began in 1982 when the European Conference of Postal and Telecommunications Administrations (CEPT) created the Groupe Spécial Mobile (which was the original meaning of "GSM") committee and provided a permanent group of technical support personnel, based in Paris. In 1987, 15 representatives from 13 European countries signed a memorandum of understanding to develop and deploy a common cellular telephone system across Europe. The foresight of deciding to develop a continental standard paid off, eventually resulting in a unified, open, standard-based network larger than that in the United States.
France and Germany signed a joint development agreement in 1984 and were later joined (in 1986) by the UK and Italy. In 1986, the European Commission proposed to reserve the 900 MHz spectrum band for GSM. By 1987, basic parameters of the GSM standard had been agreed upon and 15 representatives from 13 European nations signed a memorandum of understanding in Copenhagen, committing to deploy GSM. In 1989, the Groupe Spécial Mobile committee was transferred from CEPT to the European Telecommunications Standards Institute (ETSI).
Phase I of the GSM specifications were published in 1990. The world's first GSM call was made by the Finnish prime minister Harri Holkeri to Kaarina Suonio (mayor of the city of Tampere) on July 1, 1991. The first network was built by Telenokia and Siemens and operated by Radiolinja. In 1992 the first short messaging service (SMS or "text message") message was sent and Vodafone UK and Telecom Finland signed the first international roaming agreement.
By 1991, work had begun to expand the GSM standard to the 1800 MHz frequency band and the first 1800 MHz network became operational in the UK in 1993. Also in 1993, Telecom Australia became the first network operator to deploy a GSM network outside of Europe and the first practical hand-held GSM mobile phone became available.
In 1995, fax, data and SMS messaging services became commercially operational, the first 1900 MHz GSM network in the world became operational in the United States, and GSM subscribers worldwide exceeded 10 million. In the same year, the GSM Association was formed. Pre-paid GSM SIM cards were launched in 1996 and worldwide GSM subscribers passed 100 million in 1998.
In 2000, the first commercial GPRS services were launched and the first GPRS compatible handsets became available for sale. In 2001 the first UMTS (W-CDMA) network was launched and worldwide GSM subscribers exceeded 500 million. In 2002, the first multimedia messaging services (MMS) were introduced and the first GSM network in the 800 MHz frequency band became operational. EDGE services first became operational in a network in 2003, and the number of worldwide GSM subscribers exceeded one billion in 2004.
By 2005, GSM networks accounted for more than 75% of the worldwide cellular network market, serving 1.5 billion subscribers. Also in 2005, the first HSDPA capable network also became operational. The first HSUPA network was launched in 2007 and, by 2008, worldwide GSM subscribers had exceeded two billion.
As of January 2012, the GSM Association estimates that technologies defined in the GSM standard serve 80% of the global mobile market, encompassing more than five billion people across more than 212 countries and territories, making GSM the most ubiquitous of the many standards for cellular networks.
GSM is a cellular network, which means that cell phones connect to it by searching for cells in the immediate vicinity. There are five different cell sizes in a GSM network: macro, micro, pico, femto and umbrella cells. The coverage area of each cell varies according to the implementation environment. Macro cells can be regarded as cells where the base station antenna is installed on a mast or a building above average roof top level. Micro cells are cells whose antenna height is under average roof top level; they are typically used in urban areas. Picocells are small cells whose coverage diameter is a few dozen metres; they are mainly used indoors. Femtocells are cells designed for use in residential or small business environments and connect to the service provider's network via a broadband internet connection. Umbrella cells are used to cover shadowed regions of smaller cells and fill in gaps in coverage between those cells.
Cell horizontal radius varies depending on antenna height, antenna gain and propagation conditions from a couple of hundred metres to several tens of kilometres. The longest distance the GSM specification supports in practical use is 35 kilometres (22 miles). There are also several implementations of the concept of an extended cell, where the cell radius could be double or even more, depending on the antenna system, the type of terrain and the timing advance.
Indoor coverage is also supported by GSM and can be achieved by using an indoor picocell base station, or an indoor repeater with distributed indoor antennas fed through power splitters, to deliver the radio signals from an antenna outdoors to the separate indoor distributed antenna system. This setup is often used when a lot of call capacity is needed indoors, for example, in shopping centers or airports. However, this is not a prerequisite, since indoor coverage is also provided by in-building penetration of the radio signals from any nearby cell.
The modulation used in GSM is Gaussian minimum-shift keying (GMSK) - a kind of continuous-phase frequency shift keying. In GMSK, the signal to be modulated onto the carrier is first smoothed with a Gaussian low-pass filter prior to being fed to a frequency modulator, which greatly reduces the interference to neighboring channels (adjacent-channel interference).
You can find out more about GSM and its history here: http://en.wikipedia.org/wiki/GSM.