GSM, which stands for Global System for Mobile Communications (although originally Groupe Spécial Mobile), is a second generation (2G) Mobile Data Service for wireless telecommunications. Development of GSM was initiated and promoted in 1982 by the GSM Association, a group started by the European Conference of Postal and Telecommunications Administrations (CEPT), and then handled by the European Telecommunications Standards Institute (ETSI).
New innovations have been integrated into the system by the 3rd Generation Partnership Project (3GPP). Used by about 82% of the global mobile market, with 2 billion users over at least 212 countries, GSM is currently the most popular standard for mobile phones in the world.
GSM was initially developed as a standard for a mobile telephone system across Europe. In 1991, the first GSM network was launched by a Finnish telecommunications company, Radiolinja, assisted by Ericsson in technical infrastructure maintenance. About two years after the inception of the Radiolinja GSM network, there were GSM phone networks being operated by 70 carriers over 48 countries, catering to over a million subscribers.
International roaming is made possible because of the popularity of GSM, so subscribers are able to use their mobile phones on GSM networks all over the globe. GSM also offers digital call quality signaling and speech channels, and short message service (SMS)
There are four frequency ranges that GSM networks tend to operate in: 850 MHz, 900 MHz, 1800 MHz, and 1900 MHz. It is more common for a GSM network to operate in either the 900 MHz or 1800 MHz range, although there are some GSM networks in the Americas that operate in the other two ranges. Still, there are other countries like Scandinavia where GSM networks operate in 400 and 450 MHz frequency bands, previously used for first generation (1G) systems.
For its modulation scheme, GSM uses Gaussian minimum-shift keying (GMSK). GMSK, which is a type of continuous-phase frequency shift keying, operates by smoothing the signal to be modulated with a Gaussian low-pass filter before it is fed to a frequency modulator. This is done in order to minimize interference to neighboring channels.
The GSM system network structure consists of the following: the Base Station Subsystem, which includes the base stations and their controllers; the Network and Switching Subsystem or "core network," which is comparable to a fixed network; and the GPRS Core Network, an optional portion of the network structure which allows packet-based Internet connections.
GSM includes the Subscriber Identity Module (SIM), otherwise known as a SIM card, among its key features. The SIM card, which contains a user’s subscription information and phonebook, allows a GSM subscriber to use different mobile phone handsets while retaining his or her personal subscription, or change operators while using the same handset. To block this, there is an illegal practice known as "SIM locking" done by some operators where handsets are only allowed to use a single SIM.