Guide to GSM - What's the fuss about ?

GSM is a world standard for digital cellular communications using a variation (narrowband) of TDMA (Time Division Multiple Access), which allows up to eight calls at a time. Introduced in 1991, GSM has over 120 million users worldwide and is available in 120 countries, according to the GSM MoU Association. While it is still most commonly used in Europe and Asia, USA is beginning to adopt it as well. AT&T Wireless and Cingular Wireless have both recently announced plans to completely transition their networks to the GSM standard. Both providers have begun offering GSM services in limited areas in anticipation to convert their existing TDMA networks to the GSM protocol within the next couple of years.

GSM handsets use a Subscriber Identity Module (SIM) smart card, which contains user account information as well as additional storage for personalized user data (phonebooks, etc.). A GSM phone is immediately programmed upon insertion of a SIM card, thus allowing GSM phones to be easily rented, borrowed or exchanged. SIM cards can be programmed to display custom menus for personalized services. GSM provides a short messaging service (SMS) that enables text messages up to 160 characters in length to be sent to and from a GSM phone.

GSM together with other technologies is part of an evolution of wireless mobile telecommunication that includes High-Speed Circuit-Switched Data (HCSD), General Packet Radio System (GPRS), Enhanced Data GSM Environment (EDGE), and Universal Mobile Telecommunications Service (UMTS)

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The concepts of cell-based mobile radio systems were born at Bell Laboratories (USA) in the early 1970s. However, mobile cellular systems were not introduced for commercial and consumer use until the 1980s. During early 1980s, analog cellular telephone systems experienced a very rapid growth in Europe, particularly in Scandinavia (Finland, Sweden, Norway) and the United Kingdom. Today's cellular systems still represent one of the fastest growing areas of the telecommunications industry. At that time each country developed its own system, which was incompatible with everyone else's equipment. This was an undesirable situation - not only was the mobile equipment limited to operation within national boundaries, but there was a very limited market for each type of equipment. These factors would impede the cell-based mobile communications evolution especially considering Europe's unified state.
Keeping all these considerations (an many others) in mind, the Conference of European Posts and Telegraphs (CEPT) formed a study group in 1982 called the Groupe Spécial Mobile (GSM) to study and develop a pan European public land mobile system. The proposed system had to meet many criteria, most important of which were:

- Spectrum efficiency
- International roaming
- Low mobile and base stations costs
- Good subjective voice quality
- Compatibility with other systems such as ISDN (Integrated Services Digital Network)
- Ability to support new services
- Ability to support handheld terminals,

A technological departure from existing cellular systems, which were developed using an analog technology, the GSM system was developed using a digital technology.

In 1989, GSM responsibility was transferred to the European Telecommunication Standards Institute (ETSI), and phase I of the GSM specifications was published in 1990. At that time, the United Kingdom requested a specification based on GSM but for higher user densities with low-power mobile stations, and operating at 1.8 GHz. The specifications for this system, called Digital Cellular System (DCS1800) were published 1991. Commercial service was started in mid­1991, and by 1993 there were 36 GSM networks in 22 countries, with 25 additional countries having already selected or considering GSM. It was not only a European standard - South Africa, Australasia, and many Middle and Far East countries have elected GSM. According to statistics by the beginning of 1994, there were 1.3 million subscribers worldwide. Nowadays the acronym GSM aptly stands for Global System for Mobile telecommunications.

Below is a timeline which outlines the progress of GSM and other mobile systems (*) development:

Year Event
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*1981 Nordic Mobile Telephony (NMT), 450
1982 CEPT establishes a GSM study group in order to develop the standards for a pan-European cellular mobile system
*1983 American Mobile Phone System (AMPS)
1985 Adoption of a list of recommendations to be generated by the group
*1985 Total Access Communication System (TACS) Radiocom 2000 C-Netz
1986 Field tests were performed in order to test the different radio techniques proposed for the air interface
*1986 Nordic Mobile Telephony (NMT), 900
1988 Validation of the GSM system
1989 The responsibility of the GSM specifications is passed to the ETSI
1990 Phase 1 of the GSM specifications
1991 Commercial launch GSM services
1992 More countries express interest in GSM
1993 Coverage of main roads GSM services starts outside Europe
*1994 Personal Digital Cellular (PDC) or Japanese Digital Cellular (JDC)
1995 Phase 2 of the GSM specifications Coverage of rural areas
*1995 Personal Communications Systems (PCS) 1900- Canada
*1996 Personal Communications Systems (PCS) 1900- USA

Cellular Structure

In all cellular systems, the covering area of an operator is divided into cells. A cell corresponds to the covering area of one transmitter or a small collection of transmitters. The size of a cell is determined by the transmitter's power.
The concept of cellular systems is the use of low power transmitters in order to enable the efficient reuse of the frequencies. In fact, if the transmitters used are very powerful, the frequencies cannot be reused for hundred of kilometers as they are limited to the covering area of the transmitter.

The frequency band allocated to a cellular mobile radio system is distributed over a group of cells and this distribution is repeated in all the covering area of an operator. The whole number of radio channels available can then be used in each group of cells that form the covering area of an operator. Frequencies used in a cell will be reused several cells away. The distance between the cells using the same frequency must be sufficient to avoid interference. The frequency reuse will increase considerably the capacity in number of users.

GSM Network Architecture

The GSM technical specifications define the different entities that form the GSM network by defining their functions and interface requirements. The functional architecture of a GSM system can be broadly divided into the mobile station, the base station subsystem, and the network subsystem. Each subsystem is comprised of functional entities, which communicate through the various interfaces using specified protocols.

Mobile Station

The mobile station in GSM is really two distinct entities. The actual hardware is the mobile equipment, which is anonymous. The subscriber information, which includes a unique identifier called the International Mobile Subscriber Identity (IMSI), is stored in the Subscriber Identity Module (SIM), implemented as a smart card. By inserting the SIM card in any GSM mobile equipment, the user is able to make and receive calls at that terminal and receive
other subscribed services. By decoupling subscriber information from a specific terminal, personal mobility is provided to GSM users.

Base Station Subsystem

The Base Station Subsystem is composed of two parts, the Base Transceiver Station (BTS) and the Base Station Controller (BCS). The BTS houses the radio transceivers that define a cell and handles the radio (Um) interface protocols with the mobile station. Due to the potentially large number of BTSs, the requirements for a BTS are ruggedness, reliability, portability, and minimum cost.
The Base Station Controller (BSC) manages the radio resources for one or more BTSs, across the Abis interface. It manages the radio interface channels (setup, tear down, frequency hopping, etc.) as well as hand-overs.

Network Subsystem

The central component of the Network Subsystem is the Mobile services Switching Center (MSC). It acts like a normal switching node of the PSTN or ISDN, and in addition provides all the functionality needed to handle a mobile subscriber, including registration, authentication, location updating, inter-MSC hand-overs, and call routing to a roaming subscriber. These services are provided in conjunction with four intelligent databases, which together with the MSC form the Network Subsystem. The MSC also provides the connection to the public fixed networks.

The Home Location Register (HLR) contains all the administrative information of each subscriber registered in the corresponding GSM network, along with the current location of the subscriber. The location assists in routing incoming calls to the mobile, and is typically the SS7 address of the visited MSC. There is logically one HLR per GSM network, although it may be implemented as a distributed database.

The Visitor Location Register contains selected administrative information from the HLR, necessary for call control and provision of the subscribed services, for each mobile currently located in the geographical area controlled by the VLR. Although the VLR can be implemented as an independent unit, to date all manufacturers of switching equipment implement the VLR together with the MSC, so that the geographical area controlled by the MSC corresponds to that controlled by the VLR. The proximity of the VLR information to the MSC speeds up access to information that the MSC requires during a call. The other two registers are used for authentication and security purposes. The Equipment Identity Register (EIR) is a database that contains a list of all valid mobile equipment on the network, where each mobile equipment is identified by its International Mobile Equipment Identity (IMEI). An IMEI is marked as invalid if it has been reported stolen or is not type approved. The AuthenticationCenter (AuC) is a protected database that stores a copy of the secret key stored in each subscriber's SIM card, used for authentication and ciphering on the radio channel.

Speech Coding

The transmission of speech is, at the moment, the most important service of a mobile cellular system. Speech in GSM is digitally coded at a rate of 13 kbps, so-called full-rate speech coding. This is quite efficient compared with the standard ISDN rate of 64 kbps. One of the most important Phase 2 additions was the introduction of a half-rate speech code operating at around 7 kbps, effectively doubling the capacity of a network.

This 13 kbps digital stream (260 bits every 20 ms) has forward error correction added by a convolution encoder. The gross bit rate after channel coding is 22.8 kbps (or 456 bits every 20 ms). These 456 bits are divided into 8 57-bit blocks, and the result is interleaved amongst eight successive time slot bursts for protection against burst transmission errors.

The digital TDMA nature of the signal allows several processes intended to improve transmission quality, increase the mobile's battery life, and improve spectrum efficiency. These include discontinuous transmission, frequency hopping and discontinuous reception when monitoring the paging channel. Another feature used by GSM is power control, which attempts to minimize the radio transmission power of the mobiles and the BTS, and thus minimize the amount of co-channel interference generated.

Ciphering

Ciphering is used to protect signaling and user data. A ciphering key is computed using the algorithm A8 stored on the SIM card, the subscriber key and a random number delivered by the network (this random number is the same as the one used for the authentication procedure). Secondly, a 114-bit sequence is produced using the ciphering key, an algorithm called A5 and the burst numbers. This bit sequence is then XORed with the two 57 bit blocks of data included in a normal burst. In order to decipher correctly, the receiver has to use the same algorithm A5 for the deciphering procedure

What will the future bring?

GSM development has not stopped yet. Work on the standard is now in the third major phase, generally referred to as Phase 2 Plus. A key element of the work concerns data transmission, including bearer services and packet switched data at 115kbit/s.
What is more, GSM standards developers have recognized that an increase in the capabilities of mobile systems brings a demand from users for ubiquitous coverage and for increased mobility within their fixed network infrastructures. So GSM is to form the basis of the most comprehensive mobile phone systems yet - satellite-based systems that will cover the entire planet.

GSM is continuing to enter new application areas, such as the indoor business environment where the deployment of micro and pico base stations will provide seamless wireless access to advanced business communications services. GSM can also provide an alternative to fixed line public telephony through wireless local loop (WLL) services.

Conclusion

The core standard is now well tried and tested, but is also constantly developing. Since GSM first entered commercial service in 1992, it has been adapted to work at 1800MHz, and at 1900MHz for the American PCS operators.

GSM is the only international standard mobile phone system, which is being seriously developed at this level. Developers of other mobile standards may share the same vision, but none can claim the same level of global availability and high-speed data specification that GSM has today.

GSM systems were designed from the beginning with a digital future in mind. Operators will only need to carry out an upgrade in order to bring exciting new services on stream. There will be no need to install new infrastructure, or need to go through a large network re-planning project. With the help of leading suppliers such as Ericsson, Nokia, Motorola, Lucent, it can be done quickly.

In the new information age, the mobile phone will deliver much more than just voice calls. It will become a multi-media communications device, capable of sending and receiving graphic images and video. The mobile terminal of the future will connect to corporate LANs, enabling business people to share information using workgroup-computing applications. It will also allow them to talk to traveling colleagues via a wireless videoconference link.