GSM (Global System for Mobile Communications, originally Groupe Spécial Mobile), is a standard developed by the European Telecommunications Standards Institute (ETSI) to describe protocols for second generation (2G) digital cellular networks used by mobile phones. It is the de facto global standard for mobile communications with over 90% market share, and is available in over 219 countries and territories.
The GSM standard was developed as a replacement for first generation (1G) analog cellular networks, and originally described a digital, circuit-switched network optimized for full duplex voice telephony. This was expanded over time to include data communications, first by circuit-switched transport, then packet data transport via GPRS (General Packet Radio Services) and EDGE (Enhanced Data rates for GSM Evolution or EGPRS).
In 1981, work began to develop a European standard for digital cellular voice telephony when the European Conference of Postal and Telecommunications Administrations (CEPT) created the Groupe Spécial Mobile 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 1989, the Groupe Spécial Mobile committee was transferred from CEPT to the European Telecommunications Standards Institute (ETSI).
In 1987 Europe produced the very first agreed GSM Technical Specification in February. 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. It got GSM up and running fast.
In this short 37-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 Spécial 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.
Phase I of the GSM specifications were published in 1990. The world's first GSM call was made by the former Finnish prime minister Harri Holkeri to Kaarina Suonio (mayor in city of Tampere) on July 1, 1991, on a network built by Telenokia and Siemens and operated by Radiolinja. The following year in 1992, the first short messaging service (SMS or "text message") message was sent 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. 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. Also this year, the GSM Association was 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) were 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 was 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 2010 that technologies defined in the GSM standard serve 80% of the global 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.
It is important to note that 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 UMTS CDMA-based technology nor the 4G LTE OFDMA-based technology standards issued by the 3GPP.
The network is structured into a number of discrete sections:
- Base Station Subsystem – the base stations and their controllers explained
- 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 optional part which allows packet-based Internet connections
- Operations support system (OSS) – network maintenance
Base station subsystem[แก้]
GSM is a cellular network, which means that cell 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 rooftop level. Micro cells are cells whose antenna height is under average rooftop level; they are typically used in urban areas. Picocells are small cells whose coverage diameter is a few dozen metres; 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 metres to several tens of kilometres. The longest distance the GSM specification supports in practical use is 35 กิโลเมตร (22 ไมล์). 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 significant call capacity is needed indoors, like 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 any nearby cell.
GSM carrier frequencies[แก้]
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.
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 6.5 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.
As 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.
Subscriber Identity Module (SIM)[แก้]
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.
Sometimes mobile network operators restrict handsets that they sell for use with their own network. This is called 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.
In some countries (e.g., Bangladesh, Brazil, Chile, Hong Kong, India, Lebanon, Malaysia, Nepal, Pakistan, Singapore, South Africa) all phones are sold unlocked. In others (e.g., Singapore) it is unlawful for operators to offer any form of subsidy on a phone's price.
GSM service security[แก้]
GSM was designed with a moderate level of service 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 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 A5/1, A5/2, and A5/3 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 January 2007, 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.
On 28 December 2009 German computer engineer Karsten Nohl announced that he had cracked the A5/1 cipher. According to Nohl, he 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 also 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.
New attacks have been observed that take advantage of poor security implementations, architecture, and development for smartphone applications. Some wiretapping and eavesdropping techniques hijack the audio input and output providing an opportunity for a third party to listen in to the conversation.
The researchers revealed flaws in the commonly used GEA/1 and GEA/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), 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.
GSM open-source software[แก้]
Several open-source software projects exist that provide certain GSM features:
- gsmd daemon by Openmoko
- OpenBTS develops a Base transceiver station
- The GSM Software Project aims to build a GSM analyzer for less than $1,000
- OsmocomBB developers intend to replace the proprietary baseband GSM stack with a free software implementation
Issues with patents and open source[แก้]
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.[ต้องการอ้างอิง]
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 OpenBTS 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. As of 2011, there have been no lawsuits against users of OpenBTS over GSM use.[ต้องการอ้างอิง]
- Enhanced Data Rates for GSM Evolution (EDGE)
- High-Speed Downlink Packet Access (HSDPA)
- Long Term Evolution (LTE)
- Personal communications network (PCN)
- Nordic Mobile Telephone (NMT)
- International Mobile Subscriber Identity (IMSI)
- MSISDN Mobile Subscriber ISDN Number
- Visitors Location Register (VLR)
- Um interface
- GSM-R (GSM-Railway)
- GSM services
- Network simulation Simulation of GSM networks
- RTP audio video profile
- Enhanced Network Selection (ENS)
- GSM frequency bands
Sauter, Martin (21 November 2013). "The GSM Logo: The Mystery of the 4 Dots Solved". สืบค้นเมื่อ 23 November 2013.
[...] here's what [Yngve Zetterstrom, rapporteur of the Maketing and Planning (MP) group of the MoU (Memorandum of Understanding group, later to become the GSM Association (GSMA)) in 1989] had to say to solve the mystery: '[The dots symbolize] three [clients] in the home network and one roaming client.' There you go, an answer from the prime source!
- "GSM Global System for Mobile Communications". 4G Americas. สืบค้นเมื่อ 22 March 2014.
- EU Seeks To End Mandatory GSM for 900Mhz - Source
- Leader (7 September 2007). "Happy 20th Birthday, GSM". zdnet.co.uk. CBS Interactive. Archived from the original on 5 May 2011. สืบค้นเมื่อ 5 May 2011.
Before GSM, Europe had a disastrous mishmash of national analogue standards in phones and TV, designed to protect national industries but instead creating fragmented markets vulnerable to big guns from abroad.
- "GSM". etsi.org. European Telecommunications Standards Institute. 2011. Archived from the original on 5 May 2011. สืบค้นเมื่อ 5 May 2011.
GSM was designed principally for voice telephony, but a range of bearer services was defined...allowing circuit-switched data connections at up to 9600 bits/s.
- "History". gsmworld.com. GSM Association. 2001. Archived from the original on 5 May 2011. สืบค้นเมื่อ 5 May 2011.
1982 Groupe Speciale Mobile (GSM) is formed by the Confederation of European Posts and Telecommunications (CEPT) to design a pan-European mobile technology.
- "Cellular History". etsi.org. European Telecommunications Standards Institute. 2011. Archived from the original on 5 May 2011. สืบค้นเมื่อ 5 May 2011.
The task was entrusted to a committee known as Groupe Spécial Mobile (GSMTM), aided by a 'permanent nucleus' of technical support personnel, based in Paris.
- "Who created GSM?". Stephen Temple. สืบค้นเมื่อ 7 April 2013.
Before GSM, Europe had a disastrous mishmash of national analogue standards in phones and TV, designed to protect national industries but instead creating fragmented markets vulnerable to big guns from abroad.
- "Maailman ensimmäinen GSM-puhelu". yle.fi. Yelisradio OY. 22 February 2008. Archived from the original on 5 May 2011. สืบค้นเมื่อ 5 May 2011.
Harri Holkeri made the first call on the Radiolinja (Elisa's subsidiary) network, at the opening ceremony in Helsinki on 07.01.1991.Unknown parameter
- "GSM World statistics". gsmworld.com. GSM Association. 2010. สืบค้นเมื่อ 8 June 2010.[ลิงก์เสีย]
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- Kevin J. O'Brien (28 December 2009). "Cellphone Encryption Code Is Divulged". New York Times.
- "A5/1 Cracking Project". สืบค้นเมื่อ 30 December 2009.[ลิงก์เสีย]
Owano, Nancy (27 December 2011). "GSM phones -- call them unsafe, says security expert". Archived from the original on 27 December 2011. สืบค้นเมื่อ 27 December 2011.
Nohl said that he was able to intercept voice and text conversations by impersonating another user to listen to their voice mails or make calls or send text messages. Even more troubling was that he was able to pull this off using a seven-year-old Motorola cellphone and decryption software available free off the Internet.
- "cPanel". Infosecurityguard.com. สืบค้นเมื่อ 30 August 2010.
- "Codebreaker Karsten Nohl: Why Your Phone Is Insecure By Design". Forbes.com. 12 August 2011. สืบค้นเมื่อ 13 August 2011.
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- GSM Association—Official industry trade group representing GSM network operators worldwide
- 3GPP—3G GSM standards development group
- Anatomy of contemporary GSM cellphone hardware, Harald Welte, April 16, 2010
- Unstructured Supplementary Service Data (USSD)