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The GSM logo is used to identify compatible handsets and equipment
GSM world coverage as of 2008

GSM (Global System for Mobile communications: originally from Groupe Spécial Mobile) is the most popular standard for mobile phones in the world. Its promoter, the GSM Association, estimates that 80% of the global mobile market uses the standard.[1] GSM is used by over 3 billion people across more than 212 countries and territories.[2][3] Its ubiquity makes international roaming very common between mobile phone operators, enabling subscribers to use their phones in many parts of the world. GSM differs from its predecessors in that both signaling and speech channels are digital, and thus is considered a second generation (2G) 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 an advantage 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[4]). GSM also pioneered a low-cost (to the network carrier) alternative to voice calls, the Short message service (SMS, also called "text messaging"), which is now supported on other mobile standards as well. Another advantage is that the standard includes one worldwide Emergency telephone number, 112[5]. This makes it easier for international travellers to connect to emergency services without knowing the local emergency number.

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 Evolution (EDGE).


[edit] History

In 1982, the European Conference of Postal and Telecommunications Administrations (CEPT) created the Groupe Spécial Mobile (GSM) to develop a standard for a mobile telephone system that could be used across Europe.[6] In 1987, a memorandum of understanding was signed by 13 countries to develop a common cellular telephone system across Europe.[7][8] Finally the system created by SINTEF lead by Torleiv Maseng was selected.[9]

In 1989, GSM responsibility was transferred to the European Telecommunications Standards Institute (ETSI) and phase I of the GSM specifications were published in 1990. The first GSM network was launched in 1991 by Radiolinja in Finland with joint technical infrastructure maintenance from Ericsson.[10] By the end of 1993, over a million subscribers were using GSM phone networks being operated by 70 carriers across 48 countries.[11]

[edit] Technical details

[edit] Cellular Radio Network

GSM is a cellular network, which means that mobile phones connect to it by searching for cells in the immediate vicinity.

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. Femtocells are cells designed for use in residential or small business environments and connect to the service provider’s network via a broadband internet connection. 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 kilometres. The longest distance the GSM specification supports in practical use is 35 kilometres (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 repeater 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 cell.

The modulation used in GSM is Gaussian minimum-shift keying (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 interference to neighboring channels (adjacent channel interference).

[edit] GSM Frequencies

GSM networks operate in a number of different frequency ranges (separated into GSM frequency ranges for 2G and UMTS frequency bands for 3G). Most 2G 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. Most 3G GSM networks in Europe operate in the 2100 MHz frequency band.

The rarer 400 and 450 MHz frequency bands are assigned in some countries where these frequencies were previously used for first-generation systems.

GSM-900 uses 890–915 MHz to send information from the mobile station to the base station (uplink) and 935–960 MHz for the other direction (downlink), providing 124 RF channels (channel numbers 1 to 124) spaced at 200 kHz. Duplex spacing of 45 MHz is used.

In some countries the GSM-900 band has been extended to cover a larger frequency range. This 'extended GSM', E-GSM, uses 880–915 MHz (uplink) and 925–960 MHz (downlink), adding 50 channels (channel numbers 975 to 1023 and 0) to the original GSM-900 band. Time division multiplexing is used to allow eight full-rate or sixteen half-rate speech channels per radio frequency 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 for all 8 channels 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.

[edit] Voice Codecs

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[12] 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.

[edit] Network structure

The structure of a GSM network

The network behind the GSM 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.

[edit] Subscriber Identity Module (SIM)

One of the key features of GSM is the Subscriber Identity Module, commonly known as a SIM card. The SIM is a detachable smart card containing the user's subscription information and phone book. This allows the user to retain his or her 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 Australia, North America and Europe many operators lock the mobiles they sell. This is done because the price of the mobile phone is typically 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, utilize private services to remove the lock, or make use of ample software and websites available on the Internet to unlock the handset themselves. While most web sites offer the unlocking for a fee, some do it for free. The locking applies to the handset, identified by its International Mobile Equipment Identity (IMEI) number, not to the account (which is identified by the SIM card).

In some countries such as Bangladesh, Belgium, Costa Rica, India, Indonesia, Malaysia, and Pakistan, all phones are sold unlocked. However, in Belgium, it is unlawful for operators there to offer any form of subsidy on the phone's price. This was also the case in Finland until April 1, 2006, when selling subsidized combinations of handsets and accounts became legal, though operators have to unlock phones free of charge after a certain period (at most 24 months).

[edit] GSM security

GSM was designed with a moderate level of security. The system was designed to authenticate the subscriber using a pre-shared key and challenge-response. 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 authenticates the user to the network (and not vice 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 was developed first and is a stronger algorithm used within Europe and the United States; A5/2 is weaker and used in other countries. Serious weaknesses have been found in both algorithms: it is possible to break A5/2 in real-time with a ciphertext-only attack, and in February 2008, Pico Computing, Inc revealed its ability and plans to commercialize FPGAs that allow A5/1 to be broken with a rainbow table attack.[13] The system supports multiple algorithms so operators may replace that cipher with a stronger one.

[edit] Standards information

The GSM systems and services are described in a set of standards governed by ETSI, where a full list is maintained.[14]

[edit] Example specifications

  • GSM 07.07 "AT command set for GSM Mobile Equipment (ME)" describes the Main AT commands to communicate via a serial interface with the GSM subsystem of the phone.[15] For more, see Hayes command set.
  • 3GPP TS 27.007 - AT command set for User Equipment (UE).[16]
  • GSM 07.05 has additional AT commands for SMS and CBS.[17][18]

[edit] See also

[edit] Literature

  • Siegmund M. Redl, Matthias K. Weber, Malcolm W. Oliphant (March 1995): "An Introduction to GSM", Artech House, ISBN 978-0890067857
  • Siegmund M. Redl, Matthias K. Weber, Malcolm W. Oliphant (May 1998): "GSM and Personal Communications Handbook", Artech House, ISBN 978-0890069578
  • Friedhelm Hillebrand, ed. (2002): "GSM and UMTS, The Creation of Global Mobile Communications", John Wiley & Sons, ISBN 0470 84322 5
  • Michel Mouly, Marie-Bernardette Pautet (June 1992): "The GSM System for Mobile Communications", ISBN 0945592159.

[edit] References

  1. ^ "GSM World statistics". GSM Association. 2007. Retrieved on 2009-01-10. 
  2. ^ "About GSM Association". GSM Association. Retrieved on 2007-01-08. 
  3. ^ "Two Billion GSM Customers Worldwide". 3G Americas. June 13, 2006. Retrieved on 2007-01-08. 
  4. ^ "Texas Instruments Executive Meets with India Government Official to outline Benefits of Open Standards to drive mobile phone penetration". Texas Instruments. July 12, 2006. Retrieved on 2007-01-08. 
  5. ^ Australian Communications and Media Authority (ACMA)
  6. ^ "Brief History of GSM & GSMA". GSM World. Retrieved on 2007-01-08. 
  7. ^ "Happy 20th birthday, GSM". ZDNet. 2007-09-07.,1000002982,39289154,00.htm. Retrieved on 2007-09-07. 
  8. ^ GSM Association (2007-09-06). Global Mobile Communications is 20 years old. Press release. Retrieved on 2007-09-07. 
  9. ^ "Inventor of the GSM system". Gemini. Retrieved on 2008-10-31. 
  10. ^ "Nokia delivers first phase GPRS core network solution to Radiolinja, Finland". Nokia. January 24, 2000. Retrieved on 2006-01-08. 
  11. ^ "History and Timeline of GSM". Emory University. Retrieved on 2006-01-09. 
  12. ^ "GSM 06.51 version 4.0.1" (ZIP). ETSI. December 1997. Retrieved on 2007-09-05. 
  13. ^
  14. ^ this ETSI site
  15. ^ TS 100 916 - V07.04.00 - Digital cellular telecommunications system (Phase 2+); AT command set for GSM Mobile Equipment (ME) (GSM 07.07 version 7.4.0 Release 1998)
  16. ^ 3GPP specification: 27.007
  17. ^ GTS 07.05 - Version 5.5.0 - Digital cellular telecommunications system (Phase 2+); Use of Data Terminal Equipment - Data Circuit terminating; Equipment (DTE - DCE) interface for Short Message Service ...
  18. ^ Short Message Service / SMS Tutorial

[edit] External links

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