The First Generation (1G) mobile communications technologies had limited capacity, serving only small markets for the military, certain government agencies and very few users. In the 1960s and 1970s, this service was geographically limited and the mobile device was big in size, so it was usually mounted in cars or trucks. This form of mobile communication was not ready for mass development, because of

$\quad$ (1) The limited capacity to service the general population,

$\quad$ (2) Limited technology capability to cover large areas,

$\quad$ (3) Large size of the mobile device, and

$\quad$ (4) High prices of mobile devices and tariffs.

In the 1970s, countries were still focused on building nation-wide landline communications network rather than mobile networks for a few customers. At least in developed countries, mobile service providers that were government-owned for e.g: PTT (Post, Telegraphy, and Telephony) bureaus or monopoly companies like AT&T developed or adopted any available technology to provide mobile services during 1960s and 1970s. Technology standardization for potential future markets was not a concern.And then the era of Global System for Mobile (GSM) started. GSM is a second generation cellular standard developed to cater voice services and data delivery using digital modulation. Before GSM, European countries used different cellular standards throughout the continent, and it was not possible for a customer to use a single subscriber unit through out Europe. GSM was originally developed to serve as the pan-European cellular service and promised a wide range of network services through the use of ISDN.

GSM is the world's first cellular system to specify digital modulation and network level architectures and services.

History of GSM

GSM was developed by Group Spéciale Mobile (founded 1982) which was an initiative of CEPT(The European Conference of Postal and Telecommunications Administrations). After a few years, the task of GSM standardization was given to the committee, European Telecommunication Standards Institute (ETSI), found the Groupe Speciale Mobile, with the mandate to develop proposals for a PAN-European digital mobile communication system. Two goals were supposed to be achieved:

• First, a better and more efficient technical solution for wireless communications – it had become evident at that time that digital systems would be superior in respect to user capacity, ease of use, and number of possible additional services compared with the then-prevalent analog systems.
• Second, a single standard was to be realized all over Europe, enabling roaming across borders. This was not possible before, as incompatible analog systems were employed in different countries.

Under ETSI, GSM is named as “ Global System for Mobile communication". Full set of specifications phase-I became available in 1990.

The success of GSM exceeded all expectations. Though, it was originally developed as a European system, it has spread all over the world in the meantime. Australia was the first non European country that signed the basic agreement (Memorandum of Understanding (MoU)). Since then,GSM became the worldwide mobile communication standard, till the onset of 3G technologies. A few exceptions remain in Japan and Korea, where GSM was never implemented. In the U.S.A., GSM was competing with the CDMA-based Interim Standard-95 (IS-95) system. In most of the countries, spectral licenses were provided on condition that the network operator would use GSM.

There are three versions of GSM, each using different carrier frequencies.

• The original GSM system uses carrier frequencies around 900 MHz.
• GSM 1800, which is also called Digital Cellular System at the 1800-MHz band (DCS 1800), was added later to support the increasing numbers of subscribers. Its carrier frequencies are around 1,800 MHz, the total available bandwidth is roughly three times larger than the one around 900 MHz, and the maximal transmission power of Mobile stations is reduced. Apart from this, GSM1800 is identical to the original GSM. Thus, signal processing, switching technology, etc. can be reused without changes. The higher carrier frequency, which implies a smaller path gain, and reduced transmission power reduce the sizes of the cells significantly. This fact, combined with the bigger available bandwidth, leads to a considerable increase in network capacity.
• A third system, known as GSM1900 or PCS-1900 (Personal Communication System) operates on the 1900 MHz carrier frequency, and is mainly used in the United States of America.

2) Services of GSM

GSM services can be categorized into three types:

• Tele-services
• Bearer or Data Services
• Supplementary services

Teleservices: Teleservices include standard mobile telephony and mobile-originated or base originated traffic. Telephone services, including emergency calling and facsimile. GSM also supports Videotex and Teletex, though they are not integral parts of the GSM standard.

Bearer services or data services: Data services include computer-to-computer communication and packet-switched traffic. Data services are limited to layers 1, 2, and 3 of the open system interconnection (OSI) reference model. Supported services include packet switched protocols and data rates from 300 bps to 9.6 Kbps. Data may be transmitted using either a transparent mode (where GSM provides standard channel coding for the user data) or non-transparent mode (where GSM offers special coding efficiencies based on the particular data interface). Data services also include the short messaging service (SMS) which allows GSM subscribers and Base Stations to transmit alphanumeric pages of limited length (160 7 bit ASCII characters) while simultaneously carrying normal voice traffic. SMS also provides cell broadcast, which allows GSM base stations to repetitively transmit ASCII messages with as many as fifteen 93- character strings in concatenated fashion. SMS may be used for safety and advisory applications, such as the broadcast of highway or weather information to all GSM subscribers. GSM offers this data service on a circuit switching mode i.e. a dedicated channel is allotted even for a small amount of data.

Supplementary ISDN Services: Are digital in nature, and include all added services that can be given to the users. These services were not offered in 1G. Some of them are:

Call related services:

• Call Waiting: Notification of an incoming call while a call is in progress
• Call Hold: Put a caller on hold to take another call
• Call Barring: All calls, outgoing calls, or incoming calls are blocked
• Call Forwarding: Calls can be sent to various numbers defined by the user
• Multi Party Call Conferencing: Link multiple calls together
• CLIP: Caller line identification presentation( helps the user to identify the caller: phonebook entries)
• CLIR: Caller line identification restriction( helps the caller to restrict his/her identity display to the called party)
• CUG: Closed user group