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Global system for mobile communications

The global system for mobile communications (GSM), formerly known as "Groupe Spécial Mobile" (French), is a world-wide standard for digital wireless mobile phones. The standard was originated by CEPT and further developed by ETSI as a standard for European mobile phones, with the intention of developing a standard for adoption world-wide.

The standard is open, non-proprietary and still evolving. It has obtained wide support especially in Europe, where it is the major mobile standard.

The ubiquity of the GSM standard makes intra-nation roaming very common, with international roaming frequently enabled by "roaming agreements" between operators.

GSM differs from its predecessors most significantly in that both signalling and speech channels are digital. It has also been designed for a moderate level of security.

GSM employs time division multiple access between stations on a frequency duplex pair of radio channels, with slow frequency hopping between channels. GSM uses also SDMA and FDMA

Table of contents
1 GSM versions
2 Data Calls
5 GSM Services
6 The GSM Radio Network
7 See also
8 External links

GSM versions

GSM exists in four main versions, based on the band used: GSM-900, GSM-1800, GSM-850 and GSM-1900. GSM-900 (900 MHz) and GSM-1800 (1.8 GHz) are used in most of the world, excluding the United States and Canada. The United States and Canada use GSM-850 and GSM-1900 (1.9 GHz) instead, since in the U.S. the 900 and 1800 bands were already allocated.

In some countries the GSM-900 band has been extended to cover a larger frequency range.

In Europe and other areas outside North America the GSM system initially used a frequency of 900 MHz, shortly afterwards the PCN network used the 1800 MHz frequency, nowadays the PCN networks are considered part of the GSM system and many phones are dual-band operating on 900/1800 MHz.

In GSM, a call is dedicated either as voice or data. A voice call uses a GSM specific codec to transmit the audio over a 9600 bit/s digital link to the base station.

Data Calls

A data call lets the user use the phone as a modem with 9600 bit/s bandwidth (some networks may also handle 14400 bit/s). All newer GSM phones can be controlled by a standardised hayes AT command set through a serial cable or a wireless link (irDA or bluetooth). The AT commands can control everything in the phone from ring tones to data compression algorithms. An extension to the GSM data capabilities, high-speed circuit-switched data (HSCSD), allows data transmission speeds up to 43.3 kbit/s by allocating several data channels into one logical link. Realistic bandwidth is usually about 30 kbit/s when standing still. Expect 10 kbit/s when moving.


A GSM extension, called GPRS, allows packet switched data transmission. GPRS has been called 2.5G as it is viewed as a stepping stone toward pure 3G systems like UMTS, WCDMA or similar.

GPRS is backward compatible with GSM. This eases the migration path for a GSM operator, who can gradually upgrade the infrastructure to GPRS as the market expands.

Packet switched data under GPRS is achieved by allocating unused cell bandwidth to transmit data. As dedicated voice (or data) channels are setup by phones, the bandwidth available for packet switched data shrinks. A consequence of this is that packet switched data has a poor bitrate in busy cells. The theoretical limit for packet switched data is approx. 170 kbit/s. A realistic bitrate is 30-70 kbit/s. A change to the radio part of GPRS called EDGE will allow higher bit rates of between 20 and 200 kbit/s.

GPRS packet switched data is IP-based. Each phone has one (or more?) IP addresses allocated. GPRS will store and forward the IP packets to the phone during cell handover (when you move from one cell to another). TCP's inability to differ between radio noise induced pauses and network congestion makes the protocol unsuitable for GPRS (or any radio based IP traffic). A radio noise induced pause will make TCP (unnecessarily) throttle back its transmission speed.

From the user's point of view, GPRS is a wireless extension of data networks. It can access multiple types of data networks, such as IP based networks like the public Internet, private intranet, both IPv4 and IPv6 protocols, and X.25 based networks.

GPRS upgrades GSM data services providing:


WAP and its transmission layer protocol, WTP, use UDP/IP to solve the problem of TCP's inadequacy for high packet loss networks . Application developers creating a new mobile IP based protocol can

GSM Services

GSM has been defined with the main purpose of voice services. Operators offer also data services at speeds of 9,6 and 14,4 kbit/s. Although a cellular network can never be regarded as complete, today the operators can less and less compete on coverage area or quality of the network. However, in these days the data services start to play a big role in operator business. The High Speed Circuit Switch Data Service (HSCSD) offers higher rate data services (see above), and GPRS offers yet higher bit rates for these services. Short message service, real-time messaging (USSD) and a number of value added services are also specified in the GSM system.

The GSM Radio Network

The GSM network consists of cells. The coverage area of each cell is different in different environments. Macro cells can be regarded as cells where the base station antenna is installed in a mast or a building above the average roof top level. However, small cells or micro cells are cells where the antenna height is under the average roof top level. Thus the cell radius can vary depending on the antenna height, antenna gain and propagation conditions from couple of hundred meters to several tens of kilometers. Officially 35 km is the longest distance GSM specification supports, though the specifications define an extended cell, where the cell radius could be double. Indoor coverage is also supported by GSM.

Indoor coverage can be built by using power splitters to deliver an RF signal from the antenna outdoors to a separate indoor antenna distribution system. When all the capacity of the cell is needed indoors, e.g. in shopping centers or airports etc., the indoor coverage can be built by using antennas only inside the building. In suburban areas the indoor coverage is usually provided by the inbuilding penetration of radio signal, not by a separate indoor antenna system.

See also

External links