GSM is a cellular networkwhich means that mobile phones connect to it by searching for cells in the immediate vicinity. GSM networks operate in four different frequency ranges. Most GSM networks operate in the 900 MHz or 1800 MHz bands. Some countries in the Americas (including Canada and the United States) use the 850 MHz and 1900 MHz bands because the 900 and 1800 MHz frequency bands were already allocated.
The rarer 400 and 450 MHz frequency bands are assigned in some countries, notably Scandinavia, where these frequencies were previously used for first-generation systems.
In the 900 MHz band the uplink frequency band is 890–915 MHz, and the down link frequency band is 935–960 MHz. This 25 MHz bandwidth is subdivided into 124 carrier frequency channels, each spaced 200 kHz apart. TDM is used to allow eight full-rate or sixteen half-rate speech channels per radio channel. There are eight radio timeslots (giving eight burst periods) grouped into what is called a TDMA frame. Half rate channels use alternate frames in the same timeslot. The channel data rate is 270.833 kbit/s, and the frame duration is 4.615 ms.
The transmission power in the handset is limited to a maximum of 2 watts in GSM850/900 and 1 watt in GSM1800/1900.
GSM has used a variety of voice codecs to squeeze 3.1 kHz audio into between 5.6 and 13 kbit/s. Originally, two codecs, named after the types of data channel they were allocated, were used, called half rate (5.6 kbit/s) and full rate (13 kbit/s). These used a system based upon linear predictive coding (LPC). In addition to being efficient with bitrates, these codecs also made it easier to identify more important parts of the audio, allowing the air interface layer to prioritize and better protect these parts of the signal.
GSM was further enhanced in 1997 with the enhanced full rate (EFR) codec, a 12.2 kbit/s codec that uses a full rate channel. Finally, with the development of umts, EFR was refactored into a variable-rate codec called AMR-narrowband, which is high quality and robust against interference when used on full rate channels, and less robust but still relatively high quality when used in good radio conditions on half-rate channels.
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 meters; they are mainly used indoors. 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 meters to several tens of kilometers. The longest distance the GSM specification supports in practical use is 35 KM (22 mi). 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 may be achieved by using an indoor picocell base station, or an indoor repeator with distributed indoor antennas fed through power splitters, to deliver the radio signals from an antenna outdoors to the separate indoor distributed antenna system. These are typically deployed 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 nearby cells.
The modulation used in GSM is (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 interferance to neighboring channels (adjacent channel interference).
Interference with audio devices
This is a form of RFI, and could be mitigated or eliminated by use of additional shielding and/or bypass capacitors in these audio devices. However, the increased cost of doing so is difficult for a designer to justify.
It is a common occurrence for a nearby GSM handset to induce a "dit, dit di-dit, dit di-dit, dit di-dit" output on PAs, wireless microphones, home stereo systems, televisions, computers, cordless phones, and personal music devices. When these audio devices are in the near feild of the GSM handset, the radio signal is strong enough that the solid state amplifiers in the audio chain act as a detector. The clicking noise itself represents the power bursts that carry the TDMA signal. These signals have been known to interfere with other electronic devices, such as car stereos and portable audio players. This also depends on the handset's design, and its conformance to strict rules and regulations allocated by the FCC in part 15 of FCC rules and regulations pertaining to interference with electronic devices.

Global System for Mobile communications (GSM: originally from Groupe Spécial Mobile) is the most popular standard for mobile phones in the world. Its promoter, the GSM ASSOCIATION, estimates that 82% of the global mobile market uses the standard. GSM is used by over 2 billion people across more than 212 countries and territories. Its ubiquity makes international roaming very common between mobile phone ops, enabling subscribers to use their phones in many parts of the world. GSM differs from its predecessors in that both signalling and speech channels are digital quality, and thus is considered a second generation mobile phone system. This has also meant that data communication was easy to build into the system.
The ubiquity of the GSM standard has been advantageous to both consumers (who benefit from the ability to roam and switch carriers without switching phones) and also to network operators (who can choose equipment from any of the many vendors implementing GSM. GSM also pioneered a low-cost alternative to voice calls, the short mess service (SMS, also called "text messaging"), which is now supported on other mobile standards as well.
Newer versions of the standard were backward-compatible with the original GSM phones. For example, release 97 of the standard added packet data capabilities, by means of general packet radio service (GPRS). Release '99 introduced higher speed data transmission using enhanced data rates for GSM (EDGE).