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  • GSM


    The Global System for Mobile Communications GSM is a standard developed by the European Telecommunications Standards Institute ETSI to describe the protocols for second-generation 2G digital cellular networks used by mobile devices such as mobile phones and tablets. It was first deployed in Finland in December 1991. By the mid-2010s, it became a global standard for mobile communications achieving over 90% market share, and operating in over 193 countries and territories.

    2G networks developed as a replacement for first generation 1G analog cellular networks. The GSM standard originally described a digital, circuit-switched network optimized for full duplex voice telephony. This expanded over time to include data communications, first by circuit-switched transport, then by packet data transport via General Packet Radio Service GPRS, and Enhanced Data Rates for GSM Evolution EDGE.

    Subsequently, the 3GPP developed third-generation 3G UMTS standards, followed by fourth-generation 4G LTE Advanced standards, which do not form part of the ETSI GSM standard.

    "GSM" is a trade mark owned by the GSM Association. It may also refer to the initially most common voice codec used, Full Rate.

    History


    In 1983, work began to develop a European standard for digital cellular voice telecommunications when the European Conference of Postal and Telecommunications Administrations CEPT set up the Groupe Sp├ęcial Mobile GSM committee and later provided a permanent technical-support group based in Paris. Five years later, in 1987, 15 representatives from 13 European countries signed a memorandum of understanding in Copenhagen 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 very first agreed GSM Technical Specification. Ministers from the four big EU countries 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 France, and Renzo Failli Italy. In 1989 the Groupe Sp├ęcial Mobile committee was transferred from CEPT to the European Telecommunications Standards Institute ETSI.

    In parallel France and Germany signed a joint development agreement in 1984 and were joined by Italy and the UK in 1986. In 1986, the European Commission proposed reserving the 900 MHz spectrum band for GSM. The former Finnish prime minister Harri Holkeri made the world's first GSM call on July 1, 1991, calling Kaarina Suonio deputy mayor of the city of Tampere using a network built by Nokia and Siemens and operated by Radiolinja. The following year saw the sending of the first short messaging service SMS or "text message" message, and Vodafone UK and Telecom Finland signed the first international roaming agreement.

    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 and DCS 1800. Also that year, Telecom Australia became the first network operator to deploy a GSM network outside Europe and the first practical hand-held GSM mobile phone 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 formed. Pre-paid GSM SIM cards were launched in 1996 and worldwide GSM subscribers passed 100 million in 1998.

    In 2000 the first commercial 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 MMS was introduced and the first GSM network in the 800 MHz frequency band became operational. 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-capable network also became operational. The first HSUPA network launched in 2007. High-Speed Packet Access HSPA and its uplink and downlink versions are 3G technologies, not part of GSM. Worldwide GSM subscribers exceeded three billion in 2008.

    The GSM Association 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 UMTS code division multiple access CDMA technology nor the 4G LTE orthogonal frequency-division multiple access 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.

    Telstra in Australia shut down its 2G GSM network on December 1, 2016, the first mobile network operator to decommission a GSM network. The second mobile provider to shut down its GSM network on January 1, 2017 was AT&T Mobility from the United States. Optus in Australia completed the shut down its 2G GSM network on August 1, 2017, part of the Optus GSM network covering Western Australia and the Northern Territory had earlier in the year been shut down in April 2017. Singapore shut down 2G services entirely in April 2017.

    Technical details


    The network is structured into several discrete sections:

    GSM utilizes a cellular network, meaning that cell phones connect to it by searching for cells in the immediate vicinity. There are five different cell sizes in a GSM network:

    The coverage area of each cell varies according to the implementation environment. Macro cells can be regarded[] as cells where the ] for use in residential or ] 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, 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 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.

    GSM supports indoor coverage - achievable by using an indoor picocell base station, or an ] 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 networks operate in a number of different carrier frequency ranges separated into GSM frequency ranges 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.

    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. These eight radio timeslots or burst periods are grouped into 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 GSM 850/900 and 1 watt in GSM 1800/1900.

    GSM has used a variety of voice codecs 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 6.5 kbit/s and Full Rate 13 kbit/s. These used a system based on 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 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, or less robust but still relatively high quality when used in good radio conditions on half-rate channel.

    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 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.

    Sometimes mobile network operators restrict handsets that they sell for exclusive use in their own network. This is called SIM locking 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 e.g., Bangladesh, Belgium, Brazil, Canada, Chile, Germany, Hong Kong, India, Iran, Lebanon, Malaysia, Nepal, Norway, Pakistan, Poland, Singapore, South Africa, Sri Lanka, Thailand 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 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.

    The development of UMTS introduced an optional Universal Subscriber Identity Module 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 The Hacker's Choice started the A5/1 cracking project with plans to use FPGAs that allow A5/1 to be broken with a rainbow table 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 has been revealed in the literature. As an example, Karsten Nohl developed a number of rainbow tables 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, make calls, or send text messages using a seven-year-old Motorola cellphone and decryption software available for free online.

    GSM uses General Packet Radio Service 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 ciphers and published the open-source "gprsdecode" software for fake base stations and downgrade attacks, users will be protected in the medium term, though migration to 128-bit GEA/4 is still recommended.

    Standards information


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

    GSM open-source software


    Several open source software projects exist that provide certain GSM features:

    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 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 ]