Comprehensive reference for undergraduate electrical engineering students
Global System for Mobile Communication (GSM) - Architecture, Components & Functionalities
The Global System for Mobile Communication (GSM) is a standard developed by the European Telecommunications Standards Institute (ETSI) to describe protocols for second-generation (2G) digital cellular networks. It became the de facto global standard for mobile communications, with over 90% market share at its peak.
Key Facts: GSM was first deployed in Finland in 1991. It uses digital technology and Time Division Multiple Access (TDMA) for communication. The system operates in multiple frequency bands (900 MHz, 1800 MHz, 850 MHz, 1900 MHz).
GSM was created to provide a unified, pan-European mobile system that would:
GSM architecture is based on a cellular structure where geographic areas are divided into cells, each served by at least one transceiver. The architecture separates the radio subsystem from the network switching subsystem, allowing for modularity and easier expansion.
The GSM architecture is divided into four main subsystems, each with specific functions and components:
The GSM architecture follows a hierarchical structure with clear interfaces between subsystems:
The user equipment that connects to the network. It consists of the physical device (Mobile Equipment) and the SIM card containing subscriber information.
Manages radio resources and handles radio communication with mobile stations. It includes all radio equipment (antennas, signal processing) needed to service each cell.
Performs call switching and mobility management. It manages communications between mobile users and other fixed or mobile network users.
Provides network operation and maintenance capabilities. It monitors and controls the entire GSM network to ensure optimal performance.
Interfaces: Key interfaces in GSM architecture include Um (MS-BTS), Abis (BTS-BSC), A (BSC-MSC), and various signaling interfaces between NSS components (SS7 signaling network).
The physical terminal such as a phone or mobile device. It contains hardware and software for radio communication with the base station.
A smart card that stores subscriber-specific information and enables personal mobility.
Contains radio transceivers that define a cell and handles radio link protocols with the MS.
Manages radio resources for one or more BTSs. Handles handovers, radio channel allocation, and power control.
Converts speech coding between GSM-specific coding (13 kbps) and standard PCM (64 kbps) used in terrestrial networks.
The central component of the NSS, performing telephony switching functions and mobility management.
A central database that contains details of each subscriber authorized to use the GSM network.
A temporary database that contains subscriber information for users currently located in the geographical area controlled by the associated MSC.
Provides authentication and encryption parameters to verify user identity and ensure call privacy.
Database that tracks mobile equipment using IMEI to prevent calls from stolen or unauthorized devices.
GSM uses a combination of TDMA and FDMA. The radio spectrum is divided into 200 kHz frequency bands, each further divided into 8 time slots (TDMA frames).
GSM distinguishes between physical channels (specific time slots on specific frequencies) and logical channels (types of information carried by physical channels).
Carry user information (voice or data).
Carry signaling and synchronization information. Divided into three categories:
TDMA Frame Structure: Each TDMA frame has 8 time slots (0-7) of 577 μs each. 26 TDMA frames form a 120 ms multiframe for traffic channels, while 51 TDMA frames form a 235 ms multiframe for control channels.
GSM employs several techniques to improve quality and capacity:
Test your understanding of GSM architecture with this 5-question quiz. Select your answer for each question, then check your score at the end.
1. Which component of the GSM architecture is responsible for managing handovers between BTSs and allocating radio channels?
2. What is the primary function of the AUC (Authentication Center) in GSM?
3. Which GSM logical channel is used by the mobile station to request access to the network?
4. What does the SIM card in a GSM mobile station contain?
5. Which interface connects the BTS to the BSC in GSM architecture?
Study Tip: After completing the quiz, review the questions you got wrong by revisiting the relevant sections in this study guide. Pay special attention to the component functions and channel types.