Mobile Communication Before GSM: Analog 1G
Before GSM, mobile communication used national first-generation systems with incompatible technical and regulatory arrangements. These 1G systems were analog. A user travelling across Europe could encounter different standards and require another handset or subscription arrangement. Voice quality degraded with noise and coverage conditions, and unprotected radio transmission allowed passive interception. The result was a fragmented collection of national networks.
Examples included AMPS (Advanced Mobile Phone System) in North America, TACS (Total Access Communication System) in the UK, NMT (Nordic Mobile Telephone) in Scandinavia, and C-Netz in Germany. These analog systems had several technical limitations:
- Incompatibility and Limited Roaming: Each country, and sometimes different regions within a country, used its own analog standard. A phone bought in Germany could therefore be incompatible with networks in France or the UK. A single handset could not be assumed to work across national borders.
- Poor Security: Analog signals were transmitted without the GSM-style authentication and ciphering mechanisms. A radio receiver could capture the voice signal, and weak protection of identity information enabled phone cloning. A cloned device could generate charges for the legitimate subscriber.
- Low Capacity and Inefficient Spectrum Use: Analog technology supported a limited number of users within a given frequency band. As demand grew, urban networks experienced congestion, dropped calls, and blocked call attempts. The finite radio spectrum was used with lower spectral efficiency than a digital system using coding and time sharing.
- Poor Voice Quality: Analog calls were prone to static, hissing, and interference. The quality changed with received signal level and propagation conditions, especially as the user moved away from a base station.
- No Integrated Data Services: The 1G service model was built for voice. Text messaging and packet data services were absent from the GSM-style service architecture.
The Need for a Unified Standard: Formation of GSM
By the early 1980s, European administrations recognized that incompatible mobile networks limited cross-border service and equipment economies. In 1982, the European Conference of Postal and Telecommunications Administrations (CEPT) initiated work on a common pan-European standard for digital mobile telephony. It formed a committee named Groupe Spécial Mobile, and the abbreviation GSM later became the name of the standard. The project aimed to let a subscriber use one phone and number across the continent. ETSI was established in 1988, and responsibility for GSM standardization moved to ETSI in 1989. The committee also had to define common radio parameters, signalling procedures, and interfaces between network elements so that equipment from different suppliers could interoperate.
GSM: A Digital 2G Standard
GSM, now expanded as Global System for Mobile Communications, is a family of standards that defined a major part of second-generation (2G) cellular technology. It was among the first widely deployed cellular systems with a fully digital radio interface and digital core services. The first GSM call was made in Finland in 1991. Deployment then spread across Europe and other regions, and GSM became one of the most widely used cellular standards. The family covers radio access, mobility management, signalling, services, and interworking, so the name refers to a system architecture rather than one isolated modulation method.
Technical Foundations of GSM
GSM combined several technical mechanisms that addressed the limitations of 1G systems. The following features describe the main changes in the radio interface, subscriber identity, security, mobility, services, and access method.
Unlike 1G, which transmitted voice as a continuous analog waveform, GSM sampled and encoded speech as digital data before transmission. A analyzes speech, extracts relevant characteristics, and encodes them in a compact format. Channel coding and interleaving then protect the bit stream against radio errors. The result is more consistent quality at the target signal level and more efficient sharing of the radio spectrum.
GSM separated the subscriber identity from the physical handset through the . The module stores the IMSI and authentication-related data used by the network. Operator-specific files can also store service parameters and contacts. Moving the card to a compatible handset lets the network identify the subscription independently of the handset, subject to operator and device restrictions. The authentication exchange uses secrets held by the subscriber module and the operator's authentication centre; the secret value is not sent as ordinary user data over the radio link.
GSM introduced subscriber authentication and optional ciphering for the radio interface. The network challenges the subscriber module and verifies the response before granting service. A5-family algorithms, including A5/1 in classic deployments, protect traffic between the mobile station and the base station when ciphering is enabled. This protection covers the radio link; it does not by itself provide end-to-end encryption beyond the operator network. Ciphering is negotiated as part of radio-resource control, and the selected algorithm and implementation policy affect the protection actually available.
GSM uses a distributed subscriber database model for mobility. The "Home Location Register" (HLR) stores the subscription profile in the home operator's network. When a user registers in another area, the visited network creates a temporary record in a "Visitor Location Register" (VLR) and exchanges signalling with the HLR for authentication, location management, and call routing. Location areas let the network page an idle mobile station without tracking every cell continuously. Service abroad also requires compatible radio bands, operator agreements, and applicable roaming permissions.
GSM also defined services beside circuit-switched voice. The Short Message Service (SMS) transported short text messages through signalling procedures and initially supported network notifications. It later became a widely used person-to-person service. GSM specifications also provided a foundation for mobile data through services such as:
- CSD (Circuit Switched Data): An early circuit-switched bearer for data transfer, comparable to a dial-up connection. A common bearer example is around ; the exact rate depends on the negotiated channel and coding.
- GPRS (General Packet Radio Service): A packet-switched enhancement of GSM, often called 2.5G. The phrase "always-on" describes packet-service availability and session continuity, while radio resources remain scheduled as needed. GPRS allowed higher data rates than basic CSD.
- EDGE (Enhanced Data rates for GSM Evolution): A radio and coding enhancement for GSM packet data that can provide higher rates than GPRS under suitable signal and channel conditions. It served as an evolutionary step toward 3G.
GSM combines two multiple access techniques to organize the radio spectrum:
- FDMA (Frequency Division Multiple Access): The available spectrum is divided into narrow radio channels. The original GSM-900 plan uses paired frequencies in the band, with a 200 kHz channel raster. One frequency is assigned to communication from the mobile station to the base station (uplink), and the paired frequency serves the reverse direction (downlink).
- TDMA (Time Division Multiple Access): Each FDMA radio channel is divided into eight repeating time slots. A mobile station receives a slot assignment for its bursts, with the mapping depending on the logical channel and service. The eight-slot frame permits several traffic or signalling allocations on one carrier; the exact number of simultaneous calls also depends on channel configuration, control channels, and half-rate operation. One slot lasts about 577 microseconds and the complete TDMA frame about 4.615 milliseconds.
The Global Impact and Technical Legacy of GSM
GSM spread far beyond Europe because the standard supported interoperable equipment, international mobility, and large production volumes. By the early 2000s it was a dominant global mobile standard and served a very large subscriber base. Digital radio access, SIM-based identity, packet data evolution, and roaming procedures from the GSM family influenced later mobile systems, including 3G, 4G, and 5G architectures.