GSM (Global System for Mobile communication) is a digital mobile network that is widely used by mobile phone users in Europe and other parts of the world. GSM uses a variation of time division multiple access (TDMA) and is the most widely used of the three digital wireless telephony technologies: TDMA, GSM and code-division multiple access (CDMA). GSM digitizes and compresses data, then sends it down a channel with two other streams of user data, each in its own time slot. It operates at either the 900 megahertz (MHz) or 1,800 MHz frequency band.
GSM, together with other technologies, is part of the evolution of wireless mobile telecommunications that includes High-Speed Circuit-Switched Data (HSCSD), General Packet Radio Service (GPRS), Enhanced Data GSM Environment (EDGE) and Universal Mobile Telecommunications Service (UMTS).
Predecessors to GSM, including Advanced Mobile Phone System (AMPS) in the United States and Total Access Communication System (TACS) in the United Kingdom, were built with analog technology. However, these telecommunications systems were unable to scale with the adoption of more users. The shortcomings of these systems pointed to a need for a more efficient cellular technology that could also be used internationally.
To achieve that goal, the European Conference of Postal and Telecommunications Administrations (CEPT) set up a committee to develop a European standard for digital telecommunications in 1983. CEPT decided on several criteria that the new system must meet: international roaming support, high speech quality, support for handheld devices, low service cost, support for new services and Integrated Services Digital Network (ISDN) capability.
In 1987, representatives from 13 European countries signed a contract to deploy a telecommunications standard. The European Union (EU) then passed laws to require GSM as a standard in Europe. In 1989, the responsibility of the GSM project was transferred from CEPT to the European Telecommunications Standards Institute (ETSI).
Mobile services based on GSM were first launched in Finland in 1991. That same year, the GSM standard frequency band was expanded from 900 MHz to 1,800 MHz. In 2010, GSM represented 80% of the global mobile market. However, several telecommunications carriers have decommissioned their GSM networks, including Telstra in Australia. In 2017, Singapore retired its 2G GSM network.
Composition of the network
The GSM network has four separate parts that work together to function as a whole: the mobile device itself, the base station subsystem (BSS), the network switching subsystem (NSS) and the operation and support subsystem (OSS).
The mobile device connects to the network via hardware. The subscriber identity module (SIM) card provides the network with identifying information about the mobile user.
The BSS handles traffic between the cell phone and the NSS. It consists of two main components: the base transceiver station (BTS) and the base station controller (BSC). The BTS contains the equipment that communicates with the mobile phones, largely the radio transmitter receivers and antennas, while the BSC, is the intelligence behind it. The BSC communicates with and controls a group of base transceiver stations.
The NSS portion of the GSM network architecture, often called the core network, tracks the location of callers to enable the delivery of cellular services. Mobile carriers own the NSS. The NSS has a variety of parts, including mobile switching center (MSC) and home location register (HLN). These components perform different functions, such as routing calls and Short Message Service (SMS) and authenticating and storing caller account information via SIM cards.
Since many GSM network operators have roaming agreements with foreign operators, users can often continue to use their phones when they travel to other countries. SIM cards that hold home network access configurations may be switched to those with metered local access, significantly reducing roaming costs, while experiencing no reductions in service.
Although GSM was designed as a secure wireless system, it can still experience attacks. It uses authentication measures, such as challenge-response authentication, which prompts a user to provide a valid answer to a question, and a preshared key that can come in the form of a password or passphrase.
There are a few cryptographic security algorithms that GMS employs, including stream ciphers that encrypt plaintext digits. A5/1, A5/2 and A5/3 are three stream ciphers that ensure that a user's conversation is private. However, the algorithms for both A5/1 and A5/2 have been broken and published and are therefore susceptible to plaintext attacks.
GSM uses GPRS, a packet-based communication service, to transmit data, such as through web browsing. However, the ciphers that GPRS uses, GEA/1 and GEA/2, were broken and published as well in 2011. Researchers published open source software to sniff packets in the GPRS network.