GSM

The GSM logo is used to identify compatible handsets and equipment

GSM (Global System for Mobile Communications: originally from Groupe Spécial Mobile) is the most popular standard for mobile telephony systems in the world. The GSM Association, its promoting industry trade organization of mobile phone carriers and manufacturers, estimates that 80% of the global mobile market uses the standard.^[1] GSM is used by over 1.5 billion people^[2] across more than 212 countries and territories.^[3] Its ubiquity enables international roaming arrangements between mobile phone operators, providing subscribers the use of their phones in many parts of the world. GSM differs from its predecessor technologies in that both signaling and speech channels are digital, and thus GSM is considered a second generation (2G) mobile phone system. This also facilitates the wide-spread implementation of data communication applications into the system.

The ubiquity of implementation of the GSM (Global System Market) standard has been an advantage to both consumers, who may benefit from the ability to roam and switch carriers without replacing phones, and also to network operators, who can choose equipment from many GSM equipment vendors.^[4] GSM also pioneered low-cost implementation of the short message service (SMS), also called text messaging, which has since been supported on other mobile phone standards as well. The standard includes a worldwide emergency telephone number feature (112).^[5]

Newer versions of the standard were backward-compatible with the original GSM system. 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).

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 led 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]

Technical details

GSM cell site antennas in the Deutsches Museum, Munich, Germany

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 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 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,^[12] 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 any 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).

GSM carrier frequencies

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.

Most 3G networks in Europe operate in the 2100 MHz frequency band.

Regardless of the frequency selected by an operator, it is divided into timeslots for individual phones to use. This allows eight full-rate or sixteen half-rate speech channels per radio frequency. These eight radio timeslots (or eight 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 GSM850/900 and 1 watt in GSM1800/1900.

Voice codecs

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 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^[13] 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.

Network structure

The structure of a GSM network

The network is structured into a number of discrete sections:

• The Base Station Subsystem (the base stations and their controllers).
• the Network and Switching Subsystem (the part of the network most similar to a fixed network). This is sometimes also just called the core network.
• The GPRS Core Network (the optional part which allows packet based Internet connections).
• The Operations support system (OSS) for maintenance of the network..

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.

Phone locking

Sometimes mobile phone 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. Because the purchase price of the mobile phone to the consumer is typically subsidised with revenue from subscriptions, operators must recoup this investment before a subscriber terminates service. A subscriber may usually contact the provider to remove the lock for a fee, utilize private services to remove the lock, or make use of free or fee-based software and websites to unlock the handset themselves.

In some territories (e.g., Bangladesh, Hong Kong, Pakistan, India) all phones are sold unlocked. In others (e.g., Belgium, Finland, Germany) it is unlawful for operators to offer any form of subsidy on a phone's price.

GSM service security

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 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.^[14] 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.^[15] 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.^[16]

In 2010, threatpost.com reported that "A group of cryptographers has developed a new attack that has broken Kasumi, the encryption algorithm used to secure traffic on 3G GSM wireless networks. The technique enables them to recover a full key by using a tactic known as a related-key attack, but experts say it is not the end of the world for Kasumi."^[17] Kasumi is the name for the A5/3 algorithm, used to secure most 3G traffic.

Although security issues remain for GSM newer standards and algorithms may address this. New attacks are growing in the wild which take advantage of poor security implementations, architecture and development for smart phone applications. Some wiretapping and eavesdropping techniques hijack^[18] the audio input and output providing an opportunity for a 3rd party to listen in to the conversation. Although this threat is mitigated by the fact the attack has to come in the form of a Trojan, malware or a virus and might be detected by security software.

Standards information

The GSM systems and services are described in a set of standards governed by ETSI, where a full list is maintained.^[19]

GSM open-source software

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

• gsmd daemon by Openmoko^[20]
• OpenBTS develops a Base transceiver station
• OpenBSC is developing a minimalistic, self-contained GSM network^[21][22]
• The GSM Software Project aims to build a GSM analyzer for less than $1000^[23]
• OsmocomBB developers intend to replace the proprietary baseband GSM stack with a free software implementation^[24]

Issues with patents and open source

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 are now entirely free of patent encumbrances and it is expected that OpenBTS will be able to implement features of that initial specification without limit and that as patents subsequently expire, those features can be added into the open source version. There have been to date no suits against users of OpenBTS over GSM use.

See also

• Wireless electronic devices and health
• Mobile network operator
• International Mobile Subscriber Identity (IMSI)
• Mobile Subscriber ISDN Number (MSISDN)
• Handoff
• Visitors Location Register (VLR)
• Um Interface
• GSM frequency ranges
• GSM-R (GSM-Railway)
• GSM services
  • GSM localization
  • Multimedia Messaging Service (MMS)
  • Wireless Application Protocol (WAP)
  • Cell Broadcast
  • Network Identity and Time Zone (NITZ)
• Standards:
  • Comparison of mobile phone standards
  • GEO-Mobile Radio Interface
  • Intelligent network
  • Parlay
  • RRLP - Radio Resource Location Protocol
• RTP audio video profile
• Enhanced Network Selection (ENS)

References

External links

• GSM Association – the group representing GSM operators (official site) GSM Association
• 3GPP - The current standardization body for GSM with free standards available
• Spectrum Frequency Chart
• GSM900 Frequency and Provider Chart
• GSM1800 Frequency and Provider Chart