Global System for Mobile Communications

From Academic Kids

Template:Table Mobile phone standards The Global System for Mobile Communications (GSM) 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 mobile phone operators. GSM differs significantly from its predecessors 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 from very early on. GSM is an open standard which is currently developed by the 3GPP.

From the point of view of the consumer, the key advantage of GSM systems has been higher digital voice quality and low cost alternatives to making calls such as text messaging. The advantage for network operators has been the ability to deploy equipment from different vendors because the open standard allows easy inter-operability. Also, the standards have allowed network operators to offer roaming services which mean subscribers can use their phone all over the world.

GSM retained backward-compatibility with the original GSM phones as the GSM standard continued to develop, for example packet data capabilities were added in the Release '97 version of the standard, by means of GPRS. Higher speed data transmission have also been introduced with EDGE in the Release '99 version of the standard.



In spite of its current popularity, the history of mobile phones began long before GSM was conceived. The GSM group ("Groupe Spcial Mobile" (French) 1, 2, 3 and 4) was founded in 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 in 1987. In 1989, ETSI took over control and by 1990 the first GSM specification was completed, amounting to over 6,000 pages of text. Commercial operation began in 1991 with Radiolinja in Finland.

In 1998, the 3rd Generation Partnership Project (3GPP) was formed. Originally it was intended only 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.

Market situation

Missing image
GSM base station in Walbrzych, Poland

More than one billion people use GSM phones as of 2005, making GSM the dominant mobile phone system worldwide with about 70% of the world's market. GSM's main competitor, CDMA2000, is used primarily in North America, 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. It is important to note that there is no such thing as a standard CDMA handset, so roaming across CDMA networks from different operators is difficult or impossible, depending on the handset and operators concerned. 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.

A major reason for the growth in GSM usage, particularly between 1998 to 2002, was the availability of prepaid calling from mobile phone operators. This allows people who are either unable or unwilling to enter into a contract with an operator to have mobile phones. Prepaid also enabled the rapid expansion of GSM in many developing countries where large sections of the population do not have access to banks or bank accounts and countries where there are no effective credit rating agencies.

Radio interface

GSM is a cellular network, which means that mobile phones connect to it by searching for cells in the immediate vicinity. GSM networks operate at various different radio frequencies. Most GSM networks operates at 900MHz and/or 1800MHz. The exception to the rule is networks in the USA and Canada who operates at 800MHz and/or 1900MHz.

There are four different cell sizes in a GSM network - 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 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 diameter is a few dozen metres; they are mainly used indoors. On the other hand, umbrellacells are used to cover shadowed regions of smaller cells and fill in gaps in coverage between those cells.

Cell radius varies depending on antenna height, antenna gain and propagation conditions from a couple of hundred meters to several tens of kilometres. The longest distance the GSM specification supports in practical use is 35 km. There is also a concept of an extended cell, where the cell radius could be double or even more.

Indoor coverage is also supported by GSM and is achieved by using power splitters to deliver the radio signal from the antenna outdoors to a separate indoor antenna distribution system. This is typically deployed when a lot of call capacity is needed indoors, for example in shopping centres or airports. However, this is not a pre-requisite, since indoor coverage is also provided by in-building penetration of the radio signal.

The modulation used in GSM is derived from Quadrature phase-shift keying (QPSK). Each byte of data is split into four pairs of bits. To each pair of bits corresponds a particular phase for I(t) and Q(t). The modulated signal is the sum of the two sinusoidal waves, with a phase shift depending on the logic symbol. Smooth transitions in the Gaussian Modulated shift prevent the spread of harmonics in the emission spectrum.

Network structure

The structure of a GSM network
The structure of a GSM network

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.

Subscriber Identity Module

One of the key features of GSM is the Subscriber Identity Module (SIM), 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 after 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, or only a SIM issued by them; this practice is known as SIM locking, and is illegal in some countries.

In the USA and Europe, most operators lock the mobiles they sell. This is done because the price of the mobile phone is usually subsidised with revenue from subscriptions and operators want to try to avoid subsidising competitor's mobiles. A subscriber can usually contact the provider to remove the lock for a fee (which operators sometimes 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 USA, such as T-Mobile and Cingular, will unlock the phone for free if the customer has held an account for a certain period. Third party unlocking services exist that are often quicker and lower cost than that of the operator. In most countries removing the lock is legal.

GSM security

GSM was designed with a moderate level of security. The system was designed to authenticate the subscriber using shared-secret cryptography. Communications between the subscriber and the base station can be encrypted. The development of UMTS introduces an optional USIM, that uses a longer authentication key to give greater security, as well as mutually authenticating the network and the user - whereas GSM only authenticated the user to the network (and not vica versa). The security model therefore offers confidentiality and authentication, but limited authorization capabilities, and no non-repudiation.

GSM uses several cryptographic algorithms for security. The A5/1 and A5/2 stream ciphers are used for ensuring over-the-air voice privacy. A5/1 is a stronger algorithm used within Europe; A5/2 is weaker and used in other countries. Serious weaknesses have been found in both algorithms, and it is possible to break A5/2 in real-time in a ciphertext-only attack. The system supports multiple algorithms so operators may replace that cipher with a stronger one.

Related topics

  • Related technologies: GSM-R (GSM-Railroads)

External links

es:GSM fr:Global System for Mobile Communications id:GSM it:Global System for Mobile Communications nl:GSM ja:GSM no:GSM pl:GSM pt:GSM fi:GSM ru:GSM sv:Globalt system fr mobil kommunikation zh:全球移动通信系统


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