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GSM stands for Global System for Mobile Communications and is the most popular standard for mobile phones in the world. GSM phones are used by over a billion people across more than 200 countries. The ubiquity of the GSM standard makes international roaming very common with "roaming agreements" between operators. GSM differs from its predecessors most significantly in that both signalling and speech channels are digital, which means that it is seen as a second generation (2G) mobile phone system. This fact has also meant that data communication was built into the system very early. GSM is an open standard which is developed by the 3GPP.
The key advantage of GSM systems from the point of view of the consumer has been early delivery of new services at low costs, for example text messaging was developed first for GSM, whilst the advantage for network operators has been the low infrastructure cost which is caused by open competition. The primary disadvantage has been that GSM's radio network is based on TDMA technology, which is considered less "technologically advanced" than competing CDMA based systems, though practical performance figures are rather similar.
GSM has remained backward compatible with the original GSM phones, at the same time, the GSM standard continues to develop and packet data capabilities were added in the Release '97 version of the standard with GPRS. Higher speed data transmission has been introduced by providing a new modulation scheme with EDGE.
The GSM group ("Groupe Spécial Mobile" (French) 1, 2, 3 and 4) was founded during the year 1982. The name of the system comes from the name of this group, though later the decision was made to keep the initials but to change what they stood for. Originally the group was hosted by CEPT. The technical fundamentals of the GSM system were defined 1987. In 1989 ETSI took over control and in 1990 the first GSM specification was born (over 6000 pages of text). Commercial operation starts in 1991 with Radiolinja in Finland.
In 1998 the (3GPP) was formed. Originally it was only supposed to produce the specifications of the next (third, 3G) generation of mobile networks. However, 3GPP also took over the maintenance and development of the GSM specification. ETSI is a partner in 3GPP.
As of 2004 more than one billion people are using GSM phones.
GSM was also been designed for a moderate level of security. The system is designed to authenticate the subscriber using shared secret cryptography. Communications between the subscriber and the base station can be encrypted.
GSM's original encryption algorithm has been broken, but, in principle at least, the system supports multiple algorithms so operators may replace that cipher with a stronger one.
GSM employs TDMA between stations on a frequency duplex pair of radio channels, with slow frequency hopping between channels. GSM uses also SDMA and FDMA. It uses a modified Gaussian shift-key modulation. This modulation scheme inherently gives mobile units better battery life because it encodes the data by varying the frequency of the signal, not the amplitude. This allows amplifiers to be run at high power levels without distorting the transmitted data(good power efficiency). However, the tradeoff is that each user consumes more bandwidth, which means that more spectrum is necessary to serve the same number of users than with other modulation schemes (poor spectral efficiency).
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. GSM-850 is also sometimes called GSM-800.
Another less common GSM version is GSM-450 (450 MHz), sometimes also called GSM-400. It uses the same frequency as and can co-exist with old analog NMT systems. NMT is a first generation (1G) mobile phone system which was in some widespread usage in Europe and other areas. Widespread is of course a relative term since the boom of mobile phones started later with GSM.
Please note, however, that GSM-1800 is known as DCS (Digital Cellular System) and it is commonly used in PCN-networks (Personal Communications Network). The term 'DCS' has long been abandoned by ETSI/3GPP. Also, GSM-1900 is known as PCS1900 (Personal Communications Service). All the different systems fall into the general category called GSM. PCS-1900 standards have been integrated and harmonized with that set of GSM/DCS Specifications, resulting in a specification for GSM/DCS/PCS based on the Release 98 Series of GSM Specifications.
In some countries the GSM-900 band has been extended to cover a larger frequency range. The extended GSM, E-GSM, uses frequency range 880 - 915 MHz (uplink) and 925 - 960 MHz (downlink), adding 50 traffic channels to the original GSM-900 band. The GSM specifications also describe railways GSM, R-GSM, which uses frequency range 876 - 915 MHz (uplink) and 921 - 960 MHz (downlink). All these specifications are known as GSM-900.
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 or tri-band adding the 1900 MHz frequency. There are even a few quad-band phones, which add GSM-850 support, allowing the phone to work on almost every GSM network built to date (that would require GSM-450 support, too).
Nowadays most phones support multiple frequencies used in different countries. These are typically referred to as dual-band or triband phones. Dual band phones can cover GSM networks in pairs such as 900 and 1800 MHz frequencies (good operation in Europe) or 900 and 1900 (Europe and America). European triband phones (and the ones offered by T-Mobile USA) typically cover the 900, 1800 and 1900 bands giving good coverage in Europe and allowing use in North America as well. American triband phones (used by Cingular Wireless and AT&T Wireless) cover the 850, 1800, and 1900 bands giving good coverage in North America but limited coverage in Europe.
Countries requiring phones which cover the 1900 MHz frequency:
The network behind the GSM system seen by the customer is large and complicated in order to provide all of the services which are required. It is divided into a number of sections and these are each covered in separate articles in BambooWeb.
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. At first, the voice is sampled at 8 kHz and uniformly quantised into 13 bits. The network changes this quantisation to 8-bits using A-law compression (standard PCM), and this is converted to 13-bit linear quantisation.
For further compression to maximise use of the available spectrum, the speech is transmitted in terms of filter coefficients of the vocal tract and parameters of an excitation sequence. The error is constantly measured so that the output of the decoder bears similar - but not representative - characteristics to the speech input. This system employs both long term prediction (LTP) and short term prediction (STP).
The speech codecs used in GSM are called Half-Rate (HR), Full-Rate (FR), Enhanced Full-Rate (EFR) and Adaptive Multirate (AMR). All codecs except AMR operate with a fixed data rate and error correction level.
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 stationary and 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/W-CDMA or similar. 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.
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.
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.
GSM has been defined with the main purpose of voice services. Operators also offer 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 network consists of cells and cells can be named after their size. Basically there are 4 different cell sizes - Macro, micro, pico and umbrellacells. 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, micro cells are cells where the antenna height is under the average roof top level and they are typically used in urban areas. The picocells are small cells whose diameter is a few dozen metres and are mainly used indoors. On the other hand, umbrellacells are used to cover shadow regions of smaller cells and fill in gaps in coverage between those cells. These cells are usually built on top of tall buildings or in other high places.
The cell radius can vary depending on the antenna height, antenna gain and propagation conditions from couple of hundred meters to several tens of kilometres. Because of the timeslot (time period allocated to one call) overlap that occurs when calls are maintained at large Handset-Basestation separations, practically 35 km is the longest distance GSM specification supports, though the specifications define an extended cell, where the cell radius could be double or even more. This is done by utilizing 2 timeslots per user, so the call has a better chance hitting the right timeslot. 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 centres 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.
One of GSM's key features is the Subscriber Identity Module, commonly known as a SIM card. The SIM is a detachable smartcard containing the user's subscription information and phonebook. This allows the user to retain his information while switching handsets. Alternatively, the user can also change operators while retaining the handset simply by changing the SIM. Some operators will block this by allowing the phone to use only a single SIM; this practice is known as SIM locking, and is illegal in some countries. In the US most operators do lock the mobiles they sell. This is done because the price of the mobile phone is usually subsidised with revenue from subscriptions and try to avoid subsidizing competitor's mobiles. A subscriber can usually contact the provider to remove the lock for a fee (which the providers try to claim to be ignorant of), utilize private services to remove the lock, or make use of ample software and websites available on the Internet to unlock the handset themselves. Some providers in the US, such as T-Mobile, will unlock the phone for free if the customer has held an account for a certain period. In most countries, however, removing the lock is not illegal.
GSM is by far the dominant mobile phone system worldwide with about 70% of the worlds market. GSM's main competitor, CDMA2000, is used primarily in the United States, although it was seeing increased, but limited, worldwide adoption as a stepping stone to a 3G standard when WCDMA did not appear to be fully functional. As WCDMA networks have begun to take off, at least in high density markets, GSM's rate of expansion may slow, this seems likely to take some time, however.
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