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The Global System for Mobile Communications (GSM) is a family of standards to describe the protocols for second-generation ( 2G) digital
cellular network A cellular network or mobile network is a telecommunications network where the link to and from end nodes is wireless network, wireless and the network is distributed over land areas called ''cells'', each served by at least one fixed-locatio ...
s, as used by mobile devices such as
mobile phones A mobile phone or cell phone is a portable telephone that allows users to make and receive calls over a radio frequency link while moving within a designated telephone service area, unlike fixed-location phones ( landline phones). This radio ...
and mobile broadband modems. GSM is also a
trade mark A trademark (also written trade mark or trade-mark) is a form of intellectual property that consists of a word, phrase, symbol, design, or a combination that identifies a product or service from a particular source and distinguishes it from oth ...
owned by the
GSM Association The GSM Association (GSMA) is a non-profit trade association that represents the interests of mobile network operators worldwide. More than 750 mobile operators are full GSMA members and a further 400 companies in the broader mobile ecosystem ar ...
. "
GSM The Global System for Mobile Communications (GSM) is a family of standards to describe the protocols for second-generation (2G) digital cellular networks, as used by mobile devices such as mobile phones and Mobile broadband modem, mobile broadba ...
" may also refer to the voice codec initially used in GSM. 2G networks developed as a replacement for first generation ( 1G) analog cellular networks. The original GSM standard, which was developed by the
European Telecommunications Standards Institute The European Telecommunications Standards Institute (ETSI) is an independent, not-for-profit, standardization organization operating in the field of information and communications. ETSI supports the development and testing of global technical ...
(ETSI), originally described a digital, circuit-switched network optimized for
full duplex A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow ...
voice
telephony Telephony ( ) is the field of technology involving the development, application, and deployment of telecommunications services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is ...
, employing
time division multiple access Time-division multiple access (TDMA) is a channel access method for shared-medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession ...
(TDMA) between stations. This expanded over time to include
data communication Data communication, including data transmission and data reception, is the transfer of data, transmitted and received over a point-to-point or point-to-multipoint communication channel. Examples of such channels are copper wires, optic ...
s, first by circuit-switched transport, then by packet data transport via its upgraded standards,
GPRS General Packet Radio Service (GPRS), also called 2.5G, is a mobile data standard on the 2G cellular communication network's Global System for Mobile Communications, global system for mobile communications (GSM). Networks and mobile devices wit ...
and then
EDGE Edge or EDGE may refer to: Technology Computing * Edge computing, a network load-balancing system * Edge device, an entry point to a computer network * Adobe Edge, a graphical development application * Microsoft Edge, a web browser developed by ...
. GSM exists in various versions based on the frequency bands used. GSM was first implemented in
Finland Finland, officially the Republic of Finland, is a Nordic country in Northern Europe. It borders Sweden to the northwest, Norway to the north, and Russia to the east, with the Gulf of Bothnia to the west and the Gulf of Finland to the south, ...
in December 1991. It became the global standard for mobile cellular communications, with over 2 billion GSM subscribers globally in 2006, far above its competing standard,
CDMA Code-division multiple access (CDMA) is a channel access method used by various radio communication technologies. CDMA is an example of multiple access, where several transmitters can send information simultaneously over a single communicatio ...
. Its share reached over 90% market share by the mid-2010s, and operating in over 219 countries and territories. The specifications and maintenance of GSM passed over to the
3GPP The 3rd Generation Partnership Project (3GPP) is an umbrella term for a number of standards organizations which develop protocols for mobile telecommunications. Its best known work is the development and maintenance of: * GSM and related 2G and ...
body in 2000, which at the time developed third-generation ( 3G)
UMTS The Universal Mobile Telecommunications System (UMTS) is a 3G mobile cellular system for networks based on the GSM standard. UMTS uses Wideband Code Division Multiple Access, wideband code-division multiple access (W-CDMA) radio access technolog ...
standards, followed by the fourth-generation ( 4G) LTE Advanced and the fifth-generation 5G standards, which do not form part of the GSM standard. Beginning in the late 2010s, various carriers worldwide started to shut down their GSM networks; nevertheless, as a result of the network's widespread use, the acronym "GSM" is still used as a generic term for the plethora of ''G'' mobile phone technologies evolved from it or mobile phones itself.


History


Initial European development

In 1983, work began to develop a European standard for digital cellular voice telecommunications when the
European Conference of Postal and Telecommunications Administrations The European Conference of Postal and Telecommunications Administrations (CEPT) was established on 26 June 1959 by nineteen European states in Montreux, Switzerland, as a coordinating body for European state telecommunications and postal orga ...
(CEPT) set up the ''Groupe Spécial Mobile'' (GSM) committee and later provided a permanent technical-support group based in
Paris Paris () is the Capital city, capital and List of communes in France with over 20,000 inhabitants, largest city of France. With an estimated population of 2,048,472 residents in January 2025 in an area of more than , Paris is the List of ci ...
. Five years later, in 1987, 15 representatives from 13 European countries signed a memorandum of understanding in
Copenhagen Copenhagen ( ) is the capital and most populous city of Denmark, with a population of 1.4 million in the Urban area of Copenhagen, urban area. The city is situated on the islands of Zealand and Amager, separated from Malmö, Sweden, by the ...
to develop and deploy a common cellular telephone system across Europe, and EU rules were passed to make GSM a mandatory standard. The decision to develop a continental standard eventually resulted in a unified, open, standard-based network which was larger than that in the United States. In February 1987 Europe produced the first agreed GSM Technical Specification. Ministers from cemented their political support for GSM with the Bonn Declaration on Global Information Networks in May and the GSM MoU was tabled for signature in September. The MoU drew in mobile operators from across Europe to pledge to invest in new GSM networks to an ambitious common date. In this short 38-week period the whole of Europe (countries and industries) had been brought behind GSM in a rare unity and speed guided by four public officials: Armin Silberhorn (Germany), Stephen Temple (UK),
Philippe Dupuis Philippe Dupuis (; born April 24, 1985) is a Canadian former professional ice hockey player who played in the National Hockey League (NHL) with the Colorado Avalanche and the Toronto Maple Leafs. Playing career Dupuis was drafted 104th overall ...
(France), and Renzo Failli (Italy). In 1989 the Groupe Spécial Mobile committee was transferred from CEPT to the
European Telecommunications Standards Institute The European Telecommunications Standards Institute (ETSI) is an independent, not-for-profit, standardization organization operating in the field of information and communications. ETSI supports the development and testing of global technical ...
(ETSI). The IEEE/RSE awarded to
Thomas Haug Thomas Haug (26 April 1927 – 9 December 2023) was a Norwegian-Swedish electrical engineer known for developing the Nordic Mobile Telephone, cellular telephone networks. Early life and education Thomas Haug was born in Norway on 26 April 1927. ...
and
Philippe Dupuis Philippe Dupuis (; born April 24, 1985) is a Canadian former professional ice hockey player who played in the National Hockey League (NHL) with the Colorado Avalanche and the Toronto Maple Leafs. Playing career Dupuis was drafted 104th overall ...
the 2018 James Clerk Maxwell medal for their "leadership in the development of the first international mobile communications standard with subsequent evolution into worldwide smartphone data communication". The GSM (2G) has evolved into 3G, 4G and 5G.


First networks

In parallel
France France, officially the French Republic, is a country located primarily in Western Europe. Overseas France, Its overseas regions and territories include French Guiana in South America, Saint Pierre and Miquelon in the Atlantic Ocean#North Atlan ...
and
Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...
signed a joint development agreement in 1984 and were joined by
Italy Italy, officially the Italian Republic, is a country in Southern Europe, Southern and Western Europe, Western Europe. It consists of Italian Peninsula, a peninsula that extends into the Mediterranean Sea, with the Alps on its northern land b ...
and the UK in 1986. In 1986, the
European Commission The European Commission (EC) is the primary Executive (government), executive arm of the European Union (EU). It operates as a cabinet government, with a number of European Commissioner, members of the Commission (directorial system, informall ...
proposed reserving the 900 MHz spectrum band for GSM. It was long believed that the former Finnish prime minister
Harri Holkeri Harri Hermanni Holkeri (; 6 January 1937 – 7 August 2011) was a Finnish statesman representing the National Coalition Party of Finland (Kokoomus / Samlingspartiet). He was the Prime Minister of Finland 1987–1991, president of the UN General ...
made the world's first GSM call on 1 July 1991, calling Kaarina Suonio (deputy mayor of the city of
Tampere Tampere is a city in Finland and the regional capital of Pirkanmaa. It is located in the Finnish Lakeland. The population of Tampere is approximately , while the metropolitan area has a population of approximately . It is the most populous mu ...
) using a network built by Nokia and Siemens and operated by
Radiolinja Radiolinja was a Finnish GSM operator founded on September 19, 1988. On March 27, 1991, the world's first GSM phone call was made on Radiolinja's network. The network was opened for commercial use on July 1, 1991. Radiolinja's slogan was ''So tha ...
. In 2021 a former Nokia engineer Pekka Lonka revealed to making a test call just a couple of hours earlier. "World's first GSM call was actually made by me. I called Marjo Jousinen, in Salo.", Lonka informed. The following year saw the sending of the first
short messaging service Short Message Service, commonly abbreviated as SMS, is a text messaging service component of most telephone, Internet and mobile device systems. It uses standardized communication protocols that let mobile phones exchange short text messages, ty ...
(SMS or "text message") message, and
Vodafone UK Vodafone Limited, trading as Vodafone UK (stylised as vodafone), is a British telecommunications company, owned since May 2025 by VodafoneThree, a joint venture between Vodafone Group (51%) and Three owner CK Hutchison Holdings (49%). The ...
and Telecom Finland signed the first international
roaming Roaming is a wireless Wireless communication (or just wireless, when the context allows) is the transfer of information (''telecommunication'') between two or more points without the use of an electrical conductor, optical fiber or oth ...
agreement.


Enhancements

Work began in 1991 to expand the GSM standard to the 1800 MHz frequency band and the first 1800 MHz network became operational in the UK by 1993, called the DCS 1800. Also that year,
Telstra Telstra Group Limited is an Australian telecommunications company that builds and operates telecommunications networks and markets related products and services. It is a member of the S&P/ASX 20 stock index, and is Australia's largest telecomm ...
became the first network operator to deploy a GSM network outside Europe and the first practical hand-held GSM
mobile phone A mobile phone or cell phone is a portable telephone that allows users to make and receive calls over a radio frequency link while moving within a designated telephone service area, unlike fixed-location phones ( landline phones). This rad ...
became available. In 1995 fax, data and SMS messaging services were launched commercially, the first 1900 MHz GSM network became operational in the United States and GSM subscribers worldwide exceeded 10 million. In the same year, the
GSM Association The GSM Association (GSMA) is a non-profit trade association that represents the interests of mobile network operators worldwide. More than 750 mobile operators are full GSMA members and a further 400 companies in the broader mobile ecosystem ar ...
formed. Pre-paid GSM SIM cards were launched in 1996 and worldwide GSM subscribers passed 100 million in 1998. In 2000 the first commercial
General Packet Radio Service General Packet Radio Service (GPRS), also called 2.5G, is a mobile data standard on the 2G cellular communication network's global system for mobile communications (GSM). Networks and mobile devices with GPRS started to roll out around the ...
(GPRS) services were launched and the first GPRS-compatible handsets became available for sale. In 2001, the first UMTS (W-CDMA) network was launched, a 3G technology that is not part of GSM. Worldwide GSM subscribers exceeded 500 million. In 2002, the first
Multimedia Messaging Service Multimedia Messaging Service (MMS) is a standard way to send messages that include multimedia content to and from a mobile phone over a cellular network. Users and providers may refer to such a message as a PXT, a picture message, or a multimedia ...
(MMS) was introduced and the first GSM network in the 800 MHz frequency band became operational.
Enhanced Data rates for GSM Evolution Enhanced Data rates for GSM Evolution (EDGE), also known as 2.75G and under various other names, is a 2G digital mobile phone technology for packet switched data transmission. It is a subset of General Packet Radio Service (GPRS) on the G ...
(EDGE) services first became operational in a network in 2003, and the number of worldwide GSM subscribers exceeded 1 billion in 2004. By 2005 GSM networks accounted for more than 75% of the worldwide cellular network market, serving 1.5 billion subscribers. In 2005, the first
HSDPA High Speed Packet Access (HSPA) is an amalgamation of two mobile protocols—High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA)—that extends and improves the performance of existing 3G mobile telecommunic ...
-capable network also became operational. The first
HSUPA High Speed Packet Access (HSPA) is an amalgamation of two mobile protocols—High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA)—that extends and improves the performance of existing 3G mobile telecommunic ...
network launched in 2007. (
High Speed Packet Access High Speed Packet Access (HSPA) is an amalgamation of two mobile telephony, mobile Communications protocol, protocols—High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA)—that extends and improves the perfor ...
(HSPA) and its uplink and downlink versions are 3G technologies, not part of GSM.) Worldwide GSM subscribers exceeded three billion in 2008.


Adoption

The
GSM Association The GSM Association (GSMA) is a non-profit trade association that represents the interests of mobile network operators worldwide. More than 750 mobile operators are full GSMA members and a further 400 companies in the broader mobile ecosystem ar ...
estimated in 2011 that technologies defined in the GSM standard served 80% of the mobile market, encompassing more than 5 billion people across more than 212 countries and territories, making GSM the most ubiquitous of the many standards for cellular networks. GSM is a second-generation (2G) standard employing time-division multiple-access (TDMA) spectrum-sharing, issued by the European Telecommunications Standards Institute (ETSI). The GSM standard does not include the 3G
Universal Mobile Telecommunications System The Universal Mobile Telecommunications System (UMTS) is a 3G mobile cellular system for networks based on the GSM standard. UMTS uses wideband code-division multiple access (W-CDMA) radio access technology to offer greater spectral efficiency ...
(UMTS),
code-division multiple access Code-division multiple access (CDMA) is a channel access method used by various radio communication technologies. CDMA is an example of channel access method, multiple access, where several transmitters can send information simultaneously over ...
(CDMA) technology, nor the 4G LTE
orthogonal frequency-division multiple access Orthogonal frequency-division multiple access (OFDMA) is a multi-user version of the popular orthogonal frequency-division multiplexing (OFDM) digital modulation scheme. Multiple access is achieved in OFDMA by assigning subsets of subcarriers ...
(OFDMA) technology standards issued by the 3GPP. GSM, for the first time, set a common standard for Europe for wireless networks. It was also adopted by many countries outside Europe. This allowed subscribers to use other GSM networks that have roaming agreements with each other. The common standard reduced research and development costs, since hardware and software could be sold with only minor adaptations for the local market.


Discontinuation

Telstra Telstra Group Limited is an Australian telecommunications company that builds and operates telecommunications networks and markets related products and services. It is a member of the S&P/ASX 20 stock index, and is Australia's largest telecomm ...
in
Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
shut down its 2G GSM network on 1 December 2016, the first mobile network operator to decommission a GSM network. The second mobile provider to shut down its GSM network (on 1 January 2017) was AT&T Mobility from the
United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
.
Optus Singtel Optus Pty Limited is an Australian Telecommunications in Australia, telecommunications company headquartered in Macquarie Park, a suburb in the Northern Sydney region of Sydney, New South Wales, Australia. It is a wholly owned subsidiar ...
in
Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
completed the shut down of its 2G GSM network on 1 August 2017, part of the Optus GSM network covering
Western Australia Western Australia (WA) is the westernmost state of Australia. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to the south, the Northern Territory to the north-east, and South Australia to the south-east. Western Aust ...
and the
Northern Territory The Northern Territory (abbreviated as NT; known formally as the Northern Territory of Australia and informally as the Territory) is an states and territories of Australia, Australian internal territory in the central and central-northern regi ...
had earlier in the year been shut down in April 2017.
Singapore Singapore, officially the Republic of Singapore, is an island country and city-state in Southeast Asia. The country's territory comprises one main island, 63 satellite islands and islets, and one outlying islet. It is about one degree ...
shut down 2G services entirely in April 2017.


Technical details


Network structure

The network is structured into several discrete sections: * Base station subsystem – the base stations and their controllers * Network and Switching Subsystem – the part of the network most similar to a fixed network, sometimes just called the "core network" *
GPRS Core Network The GPRS core network is the central part of the general packet radio service (GPRS) which allows 2G, 3G and WCDMA mobile networks to transmit Internet Protocol (IP) packets to external networks such as the Internet. The GPRS system is an integ ...
– the optional part which allows packet-based Internet connections *
Operations support system Operations support systems (OSS), operational support systems in British usage, or Operation System (OpS) in Nippon Telegraph and Telephone, NTT are computer systems used by telecommunications service providers to manage their networks (e.g., tele ...
(OSS) – network maintenance


Base-station subsystem

GSM utilizes a
cellular network A cellular network or mobile network is a telecommunications network where the link to and from end nodes is wireless network, wireless and the network is distributed over land areas called ''cells'', each served by at least one fixed-locatio ...
, meaning that
cell phone A mobile phone or cell phone is a portable telephone that allows users to make and receive calls over a radio frequency link while moving within a designated telephone service area, unlike fixed-location phones ( landline phones). This radio ...
s connect to it by searching for cells in the immediate vicinity. There are five different cell sizes in a GSM network: * macro *
micro Micro may refer to: Measurement * micro- (μ), a metric prefix denoting a factor of 10−6 Places * Micro, North Carolina, town in U.S. People * DJ Micro, (born Michael Marsicano) an American trance DJ and producer * Chii Tomiya (都宮 � ...
*
pico Pico may refer to: Places The Moon * Mons Pico, a lunar mountain in the northern part of the Mare Imbrium basin Portugal * Pico, a civil parish in the municipality of Vila Verde * Pico da Pedra, a civil parish in the municipality of Ribe ...
*
femto A metric prefix is a unit prefix that precedes a basic unit of measure to indicate a multiple (mathematics), multiple or submultiple of the unit. All metric prefixes used today are decimal, decadic. Each prefix has a unique symbol that is prepen ...
, 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 rooftop level. Micro cells are cells whose antenna height is under average rooftop level; they are typically deployed 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 a
telecommunications service provider Telecommunication, often used in its plural form or abbreviated as telecom, is the transmission of information over a distance using electronic means, typically through cables, radio waves, or other communication technologies. These means of ...
's network via a broadband-internet connection. Umbrella cells are used to cover shadowed regions of smaller cells and to fill in gaps in coverage between those cells. Cell horizontal radius varies – depending on antenna height,
antenna gain In electromagnetics, an antenna's gain is a key performance parameter which combines the antenna's directivity and radiation efficiency. The term ''power gain'' has been deprecated by IEEE. In a transmitting antenna, the gain describes how ...
, 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 . 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 In the GSM cellular mobile phone standard, timing advance (TA) value corresponds to the length of time a signal takes to reach the base station from a mobile phone. GSM uses TDMA technology in the radio interface to share a single frequency between ...
. GSM supports indoor coverage – achievable 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. Picocells are typically deployed when significant call capacity is needed indoors, as in shopping centers or airports. However, this is not a prerequisite, since indoor coverage is also provided by in-building penetration of radio signals from any nearby cell.


GSM carrier frequencies

GSM networks operate in a number of different
carrier frequency In telecommunications, a carrier wave, carrier signal, or just carrier, is a periodic waveform (usually sinusoidal) that conveys information through a process called ''modulation''. One or more of the wave's properties, such as amplitude or fre ...
ranges (separated into
GSM frequency ranges GSM frequency bands or frequency ranges are the cellular frequencies designated by the ITU for the operation of GSM mobile phones and other mobile devices. Frequency bands GSM frequency usage around the world A dual-band 900/1800 device ...
for 2G and UMTS frequency bands for 3G), with most 2G GSM networks operating in the 900 MHz or 1800 MHz bands. Where these bands were already allocated, the 850 MHz and 1900 MHz bands were used instead (for example in Canada and the United States). In rare cases the 400 and 450 MHz frequency bands are assigned in some countries because they were previously used for first-generation systems. For comparison, most 3G networks in Europe operate in the 2100 MHz frequency band. For more information on worldwide GSM frequency usage, see
GSM frequency bands GSM frequency bands or frequency ranges are the cellular frequencies designated by the International Telecommunication Union, ITU for the operation of GSM mobile phones and other mobile devices. Frequency bands GSM frequency usage around th ...
. Regardless of the frequency selected by an operator, it is divided into timeslots for individual phones. This allows eight full-rate or sixteen half-rate speech channels per
radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the u ...
. These eight radio timeslots (or
burst Burst may refer to: *Burst mode (disambiguation), a mode of operation where events occur in rapid succession **Burst transmission, a term in telecommunications **Burst switching, a feature of some packet-switched networks **Bursting, a signaling mo ...
periods) are grouped into a TDMA frame. Half-rate channels use alternate frames in the same timeslot. The channel data rate for all is and the frame duration is TDMA noise is interference that can be heard on speakers near a GSM phone using TDMA, audible as a buzzing sound. The transmission power in the handset is limited to a maximum of 2 watts in and in .


Voice codecs

GSM has used a variety of voice
codec A codec is a computer hardware or software component that encodes or decodes a data stream or signal. ''Codec'' is a portmanteau of coder/decoder. In electronic communications, an endec is a device that acts as both an encoder and a decoder o ...
s to squeeze 3.1 kHz audio into between 7 and 13 kbit/s. Originally, two codecs, named after the types of data channel they were allocated, were used, called
Half Rate Half Rate (HR or GSM-HR or GSM 06.20) is a speech coding system for GSM, developed in the early 1990s. Since the codec, operating at 5.6 kbit/s, requires half the Bandwidth (computing), bandwidth of the Full Rate codec, network capacity for v ...
(6.5 kbit/s) and Full Rate (13 kbit/s). These used a system based on
linear predictive coding Linear predictive coding (LPC) is a method used mostly in audio signal processing and speech processing for representing the spectral envelope of a digital signal of speech in compressed form, using the information of a linear predictive model ...
(LPC). In addition to being efficient with
bitrate In telecommunications and computing, bit rate (bitrate or as a variable ''R'') is the number of bits that are conveyed or processed per unit of time. The bit rate is expressed in the unit bit per second (symbol: bit/s), often in conjunction ...
s, 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 The Universal Mobile Telecommunications System (UMTS) is a 3G mobile cellular system for networks based on the GSM standard. UMTS uses Wideband Code Division Multiple Access, wideband code-division multiple access (W-CDMA) radio access technolog ...
, 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, or less robust but still relatively high quality when used in good radio conditions on half-rate channel.


Subscriber Identity Module (SIM)

One of the key features of GSM is the
Subscriber Identity Module A typical SIM card (mini-SIM with micro-SIM cutout)A SIM card or SIM (subscriber identity module) is an integrated circuit (IC) intended to securely store an international mobile subscriber identity (IMSI) number and its related key, which are u ...
, commonly known as a SIM card. The SIM is a detachable
smart card A smart card (SC), chip card, or integrated circuit card (ICC or IC card), is a card used to control access to a resource. It is typically a plastic credit card-sized card with an Embedded system, embedded integrated circuit (IC) chip. Many smart ...
containing a user's subscription information and phone book. This allows users to retain their information after switching handsets. Alternatively, users can change networks or network identities without switching handsets - simply by changing the SIM.


Phone locking

Sometimes
mobile network operator A mobile network operator (MNO), also known as a mobile network provider, mobile network carrier, mobile , wireless service provider, wireless carrier, wireless operator, wireless telco, or cellular company, is a telecommunications provider of se ...
s restrict handsets that they sell for exclusive use in their own network. This is called
SIM lock A SIM lock, simlock, network lock, carrier lock or (master) subsidy lock is a technical restriction built into GSM and CDMA mobile phones by mobile phone manufacturers for use by service providers to restrict the use of these phones to specific co ...
ing and is implemented by a software feature of the phone. A subscriber may usually contact the provider to remove the lock for a fee, utilize private services to remove the lock, or use software and websites to unlock the handset themselves. It is possible to hack past a phone locked by a network operator. In some countries and regions (e.g.
Brazil Brazil, officially the Federative Republic of Brazil, is the largest country in South America. It is the world's List of countries and dependencies by area, fifth-largest country by area and the List of countries and dependencies by population ...
and
Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...
) all phones are sold unlocked due to the abundance of dual-SIM handsets and operators.


GSM security

GSM was intended to be a secure wireless system. It has considered the user authentication using a
pre-shared key In cryptography, a pre-shared key (PSK) is a shared secret which was previously shared between the two parties using some secure channel before it needs to be used. Key To build a key from shared secret, the key derivation function is typically us ...
and challenge–response, and over-the-air encryption. However, GSM is vulnerable to different types of attack, each of them aimed at a different part of the network. Research findings indicate that GSM faces susceptibility to hacking by script kiddies, a term referring to inexperienced individuals utilizing readily available hardware and software. The vulnerability arises from the accessibility of tools such as a DVB-T TV tuner, posing a threat to both mobile and network users. Despite the term "script kiddies" implying a lack of sophisticated skills, the consequences of their attacks on GSM can be severe, impacting the functionality of
cellular networks A cellular network or mobile network is a telecommunications network where the link to and from end nodes is wireless and the network is distributed over land areas called ''cells'', each served by at least one fixed-location transceiver (suc ...
. Given that GSM continues to be the main source of cellular technology in numerous countries, its susceptibility to potential threats from malicious attacks is one that needs to be addressed. The development of
UMTS The Universal Mobile Telecommunications System (UMTS) is a 3G mobile cellular system for networks based on the GSM standard. UMTS uses Wideband Code Division Multiple Access, wideband code-division multiple access (W-CDMA) radio access technolog ...
introduced an optional
Universal Subscriber Identity Module A typical SIM card (mini-SIM with micro-SIM cutout)A SIM card or SIM (subscriber identity module) is an integrated circuit (IC) intended to securely store an international mobile subscriber identity (IMSI) number and its related key, which are us ...
(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, A5/2, and A5/3
stream cipher stream cipher is a symmetric key cipher where plaintext digits are combined with a pseudorandom cipher digit stream ( keystream). In a stream cipher, each plaintext digit is encrypted one at a time with the corresponding digit of the keystrea ...
s 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 In cryptography, a ciphertext-only attack (COA) or known ciphertext attack is an attack model for cryptanalysis where the attacker is assumed to have access only to a set of ciphertexts. While the attacker has no channel providing access to the p ...
, and in January 2007, The Hacker's Choice started the A5/1 cracking project with plans to use
FPGA A field-programmable gate array (FPGA) is a type of configurable integrated circuit that can be repeatedly programmed after manufacturing. FPGAs are a subset of logic devices referred to as programmable logic devices (PLDs). They consist of a ...
s that allow A5/1 to be broken with a
rainbow table A rainbow table is a precomputed table for caching the outputs of a cryptographic hash function, usually for cracking password hashes. Passwords are typically stored not in plain text form, but as hash values. If such a database of hashed passw ...
attack. The system supports multiple algorithms so operators may replace that cipher with a stronger one. Since 2000, different efforts have been made in order to crack the A5 encryption algorithms. Both A5/1 and A5/2 algorithms have been broken, and their
cryptanalysis Cryptanalysis (from the Greek ''kryptós'', "hidden", and ''analýein'', "to analyze") refers to the process of analyzing information systems in order to understand hidden aspects of the systems. Cryptanalysis is used to breach cryptographic se ...
has been revealed in the literature. As an example, Karsten Nohl developed a number of
rainbow table A rainbow table is a precomputed table for caching the outputs of a cryptographic hash function, usually for cracking password hashes. Passwords are typically stored not in plain text form, but as hash values. If such a database of hashed passw ...
s (static values which reduce the time needed to carry out an attack) and have found new sources for known plaintext attacks. He said that it is possible to build "a full GSM interceptor... from open-source components" but that they had not done so because of legal concerns. Nohl claimed that he was able to intercept voice and text conversations by impersonating another user to listen to
voicemail A voicemail system (also known as voice message or voice bank) is a computer-based system that allows callers to leave a recorded message when the recipient has been unable (or unwilling) to answer the phone. Calls may be directed to voicemail m ...
, make calls, or send text messages using a seven-year-old
Motorola Motorola, Inc. () was an American multinational telecommunications company based in Schaumburg, Illinois. It was founded by brothers Paul and Joseph Galvin in 1928 and had been named Motorola since 1947. Many of Motorola's products had been ...
cellphone and decryption software available for free online. GSM uses
General Packet Radio Service General Packet Radio Service (GPRS), also called 2.5G, is a mobile data standard on the 2G cellular communication network's global system for mobile communications (GSM). Networks and mobile devices with GPRS started to roll out around the ...
(GPRS) for data transmissions like browsing the web. The most commonly deployed GPRS ciphers were publicly broken in 2011. The researchers revealed flaws in the commonly used GEA/1 and GEA/2 (standing for GPRS Encryption Algorithms 1 and 2) ciphers and published the open-source "gprsdecode" software for sniffing GPRS networks. They also noted that some carriers do not encrypt the data (i.e., using GEA/0) in order to detect the use of traffic or protocols they do not like (e.g.,
Skype Skype () was a proprietary telecommunications application operated by Skype Technologies, a division of Microsoft, best known for IP-based videotelephony, videoconferencing and voice calls. It also had instant messaging, file transfer, ...
), leaving customers unprotected. GEA/3 seems to remain relatively hard to break and is said to be in use on some more modern networks. If used with USIM to prevent connections to fake base stations and downgrade attacks, users will be protected in the medium term, though migration to 128-bit GEA/4 is still recommended. The first public cryptanalysis of GEA/1 and GEA/2 (also written GEA-1 and GEA-2) was done in 2021. It concluded that although using a 64-bit key, the GEA-1 algorithm actually provides only 40 bits of security, due to a relationship between two parts of the algorithm. The researchers found that this relationship was very unlikely to have happened if it was not intentional. This may have been done in order to satisfy European controls on export of cryptographic programs.


Standards information

The GSM systems and services are described in a set of standards governed by
ETSI The European Telecommunications Standards Institute (ETSI) is an independent, not-for-profit, standardization organization operating in the field of Information and communications technology, information and communications. ETSI supports the de ...
, where a full list is maintained.


Patents and open source

Several
open-source software Open-source software (OSS) is Software, computer software that is released under a Open-source license, license in which the copyright holder grants users the rights to use, study, change, and Software distribution, distribute the software an ...
projects exist that provide certain GSM features, such as a
base transceiver station A base transceiver station (BTS) or a baseband unit (BBU) is a piece of equipment that facilitates wireless communication between user equipment (UE) and a network. UEs are devices like mobile phone A mobile phone or cell phone is a portab ...
by
OpenBTS OpenBTS (Open Base Transceiver Station) is a software-based GSM access point, allowing standard GSM-compatible mobile phones to be used as SIP endpoints in Voice over IP (VoIP) networks. OpenBTS is open-source software developed and maintained by ...
and the Osmocom stack providing various parts. Patents remain a problem for any open-source GSM implementation, because it is not possible for GNU or any other free software distributor to guarantee immunity from all lawsuits by the patent holders against the users. Furthermore, new features are being added to the standard all the time which means they have patent protection for a number of years. The original GSM implementations from 1991 may now be entirely free of patent encumbrances, however patent freedom is not certain due to the United States' "first to invent" system that was in place until 2012. The "first to invent" system, coupled with "
patent A patent is a type of intellectual property that gives its owner the legal right to exclude others from making, using, or selling an invention for a limited period of time in exchange for publishing an sufficiency of disclosure, enabling discl ...
term adjustment" can extend the life of a U.S. patent far beyond 20 years from its priority date. It is unclear at this time whether
OpenBTS OpenBTS (Open Base Transceiver Station) is a software-based GSM access point, allowing standard GSM-compatible mobile phones to be used as SIP endpoints in Voice over IP (VoIP) networks. OpenBTS is open-source software developed and maintained by ...
will be able to implement features of that initial specification without limit. As patents subsequently expire, however, those features can be added into the open-source version. , there have been no lawsuits against users of OpenBTS over GSM use.


See also

*
Cellular network A cellular network or mobile network is a telecommunications network where the link to and from end nodes is wireless network, wireless and the network is distributed over land areas called ''cells'', each served by at least one fixed-locatio ...
*
Enhanced Data Rates for GSM Evolution Enhanced Data rates for GSM Evolution (EDGE), also known as 2.75G and under various other names, is a 2G digital mobile phone technology for packet switched data transmission. It is a subset of General Packet Radio Service (GPRS) on the G ...
(EDGE) * GSM forwarding standard features codes – list of call forward codes working with all operators and phones *
GSM frequency bands GSM frequency bands or frequency ranges are the cellular frequencies designated by the International Telecommunication Union, ITU for the operation of GSM mobile phones and other mobile devices. Frequency bands GSM frequency usage around th ...
*
GSM modem The Global System for Mobile Communications (GSM) is a family of standards to describe the protocols for second-generation ( 2G) digital cellular networks, as used by mobile devices such as mobile phones and mobile broadband modems. GSM is als ...
*
GSM services GSM services are a standard collection of applications and features available over the Global System for Mobile Communications (GSM) to mobile phone subscribers all over the world. The GSM standards are defined by the 3GPP collaboration and imp ...
**
Cell Broadcast Cell Broadcast (CB) is a method of simultaneously sending Short Message Service, short messages to multiple mobile telephone users in a defined area. It is defined by the ETSI's GSM committee and 3GPP and is part of the 2G, 3G, 4G and 5G standa ...
** GSM localization **
Multimedia Messaging Service Multimedia Messaging Service (MMS) is a standard way to send messages that include multimedia content to and from a mobile phone over a cellular network. Users and providers may refer to such a message as a PXT, a picture message, or a multimedia ...
(MMS) ** NITZ Network Identity and Time Zone **
Wireless Application Protocol Wireless Application Protocol (WAP) is an obsolete technical standard for accessing information over a mobile cellular network. Introduced in 1999, WAP allowed users with compatible mobile devices to browse content such as news, weather and sp ...
(WAP) *
GSM-R GSM-R, Global System for Mobile Communications – Railway or GSM-Railway is an international wireless communications standard for railway communication and applications. A sub-system of European Rail Traffic Management System (ERTMS), it is use ...
(GSM-Railway) * GSM USSD codes – Unstructured Supplementary Service Data: list of all standard GSM codes for network and SIM related functions *
Handoff In cellular telecommunications, handover, or handoff, is the process of transferring an ongoing call or data session from one channel connected to the core network to another channel. In satellite communications it is the process of transfe ...
*
High-Speed Downlink Packet Access High Speed Packet Access (HSPA) is an amalgamation of two mobile protocols—High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA)—that extends and improves the performance of existing 3G mobile telecommunica ...
(HSDPA) *
International Mobile Equipment Identity The International Mobile Equipment Identity (IMEI) is a numeric identifier, usually unique, for 3GPP and iDEN mobile phones, as well as some satellite phones. It is usually found printed inside the battery compartment of the phone but can also b ...
(IMEI) *
International Mobile Subscriber Identity The international mobile subscriber identity (IMSI; ) is a number that uniquely identifies every user of a cellular network. It is stored as a field and is sent by the mobile device to the network. It is also used for acquiring other details of t ...
(IMSI) *
Long Term Evolution In telecommunications, long-term evolution (LTE) is a standard for wireless broadband communication for cellular mobile devices and data terminals. It is considered to be a "transitional" 4G technology, and is therefore also referred to as 3.9 ...
(LTE) *
MSISDN MSISDN ( ) is a number uniquely identifying a subscription in a Global System for Mobile communications or a Universal Mobile Telecommunications System mobile network. It is the mapping of the telephone number to the subscriber identity module in ...
Mobile Subscriber ISDN Number *
Nordic Mobile Telephone NMT (''Nordic Mobile Telephony'') is an automatic cellular phone system specified by Nordic countries, Nordic telecommunications administrations (Postal Telephone and Telegraph, PTTs) and opened for service on 1 October 1981. NMT is based on ana ...
(NMT) * ORFS *
Personal communications network Personal communications network (PCN) is the European digital cellular mobile telephone network. The underlying standard is known as Digital Cellular System, which defines a variant of GSM operating at 1.7–1.88 GHz. GSM-1800 has since been adopt ...
(PCN) * RTP audio video profile * Simulation of GSM networks * Standards ** Comparison of mobile phone standards **
GEO-Mobile Radio Interface GEO-Mobile Radio Interface (GEO stands for Geostationary Earth Orbit), better known as GMR, is an ETSI standard for satellite phones. The GMR standard is derived from the 3GPP-family terrestrial digital cellular standards and supports access to GSM ...
** GSM 02.07 – Cellphone features ** GSM 03.48 – Security mechanisms for the SIM application toolkit **
Intelligent Network The Intelligent Network (IN) is the standard network architecture specified in the ITU-T Q.1200 series recommendations. It is intended for fixed as well as mobile telecom networks. It allows operators to differentiate themselves by providing ...
** Parlay X ** RRLP – Radio Resource Location Protocol *
Um interface The Um interface is the air interface for the GSM mobile telephone standard. It is the interface between the mobile station (MS) and the Base transceiver station (BTS). It is called Um because it is the mobile analog to the U interface of IS ...
* Visitors Location Register (VLR)


References


Further reading

* * * * *


External links


GSM Association
��Official industry trade group representing GSM network operators worldwide
3GPP
��3G GSM standards development group
LTE-3GPP.info: online GSM messages decoder fully supporting all 3GPP releases from early GSM to latest 5G
{{DEFAULTSORT:Gsm ETSI GSM standard Telecommunications-related introductions in 1991