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What is the Global System for Mobile Communications (GSM)?

The Global System for Mobile Communications (GSM) is a digital cellular technology for mobile telecommunication systems. This technology provides a framework for wireless communication by utilizing a combination of time division multiple access (TDMA) and frequency division multiple access (FDMA) techniques, and allows for voice and data transmission over wireless networks, operating in various frequency bands globally.

Dissecting Global System for Mobile Communications (GSM)

In the early 1980s, faced with a patchwork of incompatible analog mobile systems, European countries urgently sought a unified standard for seamless cross-border communications. The Conference of European Posts and Telecommunications (CEPT) initiated the ‘Groupe Spécial Mobile’ in 1982, from which GSM gets its name, to develop this standard. In 1986, the European Telecommunication Standards Institute (ETSI) took over the development, culminating in the historic first GSM call made by Finnish Prime Minister Harri Holkeri on July 1, 1991, using Nokia equipment on the 900 MHz band.

Through GSM, a single, high-quality standard with enhanced features was established, resolving compatibility issues and enabling international roaming across Europe.

Global System for Mobile Communications (GSM) Standards

GSM standards encompass a wide range of specifications and protocols that ensure interoperability, reliability, and functionality of mobile networks. These standards cover various aspects, including frequency bands, modulation techniques, network architecture, and protocols for communication and security. Some of the key standards within GSM are:

  1. Frequency Bands: GSM operates in several frequency bands, including 850 MHz, 900 MHz, 1800 MHz, and 1900 MHz. The 900 MHz and 1800 MHz bands are widely used in Europe and Asia, while the 850 MHz and 1900 MHz bands are prevalent in the Americas.
  2. Multiple Access Scheme: GSM uses a combination of Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA). FDMA divides the frequency bands into separate channels, while TDMA further divides these channels into time slots. This combination allows multiple users to share the same frequency channel without interference.
  3. Modulation: GSM uses Gaussian Minimum Shift Keying (GMSK) modulation to minimize the use of bandwidth and reduce interference.
  4. Voice Coding: The GSM standard includes specifications for voice encoding, the most common being the Regular Pulse Excitation – Long Term Prediction (RPE-LTP) coder, which compresses voice signals into data rates of approximately 13 kbps.
  5. Data Services: General Packet Radio Service (GPRS) and Enhanced Data Rates for GSM Evolution (EDGE) are standards within GSM for packet-switched data transmission. GPRS provides data rates of up to 114 kbps, while EDGE offers data rates up to 384 kbps.
  6. Network Architecture:GSM defines a hierarchical network architecture including the Mobile Station (MS), Base Station Subsystem (BSS), Network and Switching Subsystem (NSS), and Operation and Support Subsystem (OSS).
  7. Communication Protocols: GSM standards define protocols for communication between different network elements. These include protocols such as the Base Station System Application Part (BSSAP) for communication between the BSC and MSC, and the Mobile Application Part (MAP) for communication within the NSS.
  8. Security: GSM standards include specifications for authentication and encryption to secure communications. The A3 algorithm is used for authentication, while the A5 family of algorithms is used for encrypting communication between the mobile station and the network.
  9. International Roaming: GSM specifies standards for international roaming, allowing subscribers to use their mobile devices in different countries with compatible GSM networks. This is facilitated through agreements between mobile network operators and the use of the International Mobile Subscriber Identity (IMSI).
  10. Signaling and Call Control: The signaling within GSM is largely based on Signaling System No. 7 (SS7), a set of telephony signaling protocols used to set up and tear down telephone calls, as well as perform other functions such as billing and local number portability.

Composition of the Network

The Global System for Mobile Communications (GSM) comprises several components and subsystems, each playing a crucial role in enabling mobile communication.

Mobile Station (MS): This refers to the mobile device used by the subscriber. It can be a mobile phone, tablet, or any other device with GSM capabilities. The MS includes a SIM (Subscriber Identity Module) card, which contains the International Mobile Subscriber Identity (IMSI) and authentication information. It identifies the user to the network and stores user-specific information.

Base Station Subsystem (BSS): This is the section of a GSM network that communicates directly with mobile stations (MSs). The BSS is responsible for handling traffic and signaling between the MS and the rest of the network. The BSS includes:

  • Base Transceiver Station (BTS): Contains the radio transceivers and antennas to communicate with the mobile stations. It handles the radio link to the mobile phones.
  • Base Station Controller (BSC): Manages multiple BTSs. It handles call setup, frequency hopping, handovers, and power control in the BTSs under its control.

Network and Switching Subsystem (NSS): This is the backbone of the GSM system and manages communications between GSM mobile devices and other networks (e.g., PSTN, other mobile networks). It includes:

  • Mobile Switching Center (MSC): The central component that performs call switching and manages mobile services such as registration, authentication, location updating, handovers, and call routing.
  • Home Location Register (HLR): A database that contains detailed information about each subscriber, including the IMSI, services the subscriber is entitled to, and the current location area of the mobile device.
  • Visitor Location Register (VLR): Typically integrated with the MSC, this database temporarily stores information about mobile devices roaming within the area that the MSC serves.
  • Authentication Center (AuC): A protected database that stores the encryption and authentication keys for securing communications and verifying the user's identity.
  • Equipment Identity Register (EIR): A database that contains a list of mobile equipment identifiers (IMEIs) to check if the devices trying to access the network are authorized and not blacklisted.

GPRS Core Network: For data services, GSM includes components for the General Packet Radio Service (GPRS), which allows packet-based data transmission. This includes:

  • Serving GPRS Support Node (SGSN): It processes and sends data packets to and from the mobile stations within its service area. It also keeps track of the location of the mobile stations and performs security functions and access control.
  • Gateway GPRS Support Node (GGSN): Acts as an interface between the GPRS network and the external packet data networks. It routes data packets from the SGSN to the appropriate data network and vice versa.

Operation and Support Subsystem (OSS):This subsystem is used by the network operator for monitoring and controlling the GSM system. It includes functions for network configuration, performance management, security management, and subscriber billing.

Radio Frequency (RF) Spectrum: The GSM network operates in specific frequency bands, and the radio spectrum is divided into frequency channels. GSM originally operated in the 900 MHz band and was later expanded to include 850 MHz, 1800 MHz, and 1900 MHz bands.

Communication Channels: GSM uses a combination of Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) to divide the RF spectrum into channels that can be used for communication. There are various logical channels in GSM, including traffic channels for voice and data communication and control channels for signaling and call setup.

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