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GSM, short for Global Systems for Mobile Communication is one of the leading network technology deployed by cellular service providers world over. It is a 2nd Generation (2G) Communications Protocol or a Standard Set used by wireless mobile phones to exchange data from one geographical location to another.

GSM was first introduced in the year 1991 as a replacement to the 1st Generation (1G) Cellular Network. The 1G networks were analog cellular networks and comprised of a mix of technologies and protocols that varied across countries. In order to bring uniformity in cellular networks, the Groupe Special Mobile committee (now European Telecommunications Standards Institute) was formed to develop a uniform Standard Set to overcome the drawbacks of the 1G networks and thus the GSM network was developed.

The first GSM phone call was made on 1st July 1991 on a network which was built jointly by Telenokia and Siemens and operated by Radiolinja. In 1993, with further development in the GSM network, the world’s first SMS (Short Messaging Service) from person to person was sent successfully.

GSM Network Structure

The GSM network structure consists of:
1. Base Station Subsystem
2. Network and Switching Subsystem
3. GPRS Core Network
4. Operations Support System

GSM Network Structure Explained

1. Base Station Subsystem: In a GSM network, the cell phones connect by searching for cells in the immediate surrounding areas. A cell is the coverage range and each cell differs according to its size and environment. The GSM specification covers upto 35 kilometers in practical use. The Base station Subsystem is responsible for handling traffic and signaling between a cell phone and the Network Switching Subsystem.

2. Network Switching Subsystem: The Network Switching Subsystem carries out Automatic Telephone Exchange and Mobility Management functions for mobile phones that are roaming on the network of different base stations. It is owned and deployed by various mobile phone operators. The architecture contains various features and functions which are required because the phones are not fixed in one location.

3. GPRS Core Network: The General Packet Radio Service is an optional part of the GSM network structure which allows the transmission of data from a Cellular Network to an external network such as the Internet.

4. Operations Support System: The term Operations Support System most frequently describes the various network systems dealing with the telecom network itself supporting processes such as configuring network components and managing faults.

GSM Operating Frequencies and Voice Codecs used in GSM: GSM Operating frequencies differ from operators and countries. Most GSM networks are allocated and operated in 900 MHz bands or 1800 MHz bands. Irrespective of the frequency selected, it is divided into Timeslots for every individual mobile phone to use at a Channel Data Rate of 270 kBits/s. GSM uses Enhanced Full Rate codec, a 12.20 kBit/s codec that utilizes a full rate channel which provides a high quality audio sound.

Features of a GSM Network

SIM Card: A SIM (Subscriber Identification Module) Card is a removable smart card that stores the user’s and the network operator’s data like subscription information, phone book entries and other important data. This enables a person to switch to other cell phones without changing the network operator.

Phone Locking: It is a feature used by the network operators to restrict the users to change their mobile phones with different service providers. If Phone Locking is enabled, then the user cannot change his service provider with the same mobile phone. Some network operators usually charge a fee to disable this feature.

GSM Service Security: In GSM Networks, the data transmitted between the user and the Base Station is encrypted in order to ensure authencity and the safe transmission of data. GSM uses various cryptographic algorithms for security of data like A5/1, A5/2 and A5/3 stream ciphers to ensure over-the-air voice privacy.

Standards Information: The GSM Standards are maintained by the European Telecommunications Standards Institute which has been successful in standardizing the GSM cell phone system over the years. Several Open Source softwares are also available for the GSM development. Over the years, there were several issues with the initial GSM implementations since it is not possible for free software distributors like GNU to guarantee protection from lawsuits by the various patent holders. Though, over the years with the development in the GSM network, several new features like MMS (Multimedia Messaging Services), GPRS (General Packet Radio Service) and EDGE (Enhanced Data rates for GSM Evolution) were introduced.

GSM Frequency Bands


In North America, GSM operates on the primary mobile communication bands 850 MHz and 1,900 MHz. In Canada, GSM-1900 is the primary band used in urban areas with 850 as a backup, and GSM-850 being the primary rural band. In the United States, regulatory requirements determine which area can use which band.

GSM-1900 and GSM-850 are also used in most of South and Central America, and both Ecuador and Panama use GSM-850 exclusively (Note: Since November 2008, a Panamanian operator has begun to offer GSM-1900 service). Venezuela and Brazil use GSM-850 and GSM-900/1800 mixing the European and American bands. Some countries in the Americas use GSM-900 or GSM-1800, some others use three: GSM-850/900/1900, GSM-850/1800/1900, GSM-900/1800/1900 or GSM-850/900/1800. Soon some countries will use GSM-850/900/1800/1900 MHz like the Dominican Republic, Trinidad & Tobago and Venezuela.

In Brazil, the 1,900 MHz band is paired with 2,100 MHz to form the IMT-compliant 2,100 MHz band for 3G services.

The result is a mixture of usage in the Americas that requires travelers to confirm that the phones they have are compatible with the band of the networks at their destinations. Frequency compatibility problems can be avoided through the use of multi-band (tri-band or, especially, quad-band) phones.

System Band Uplink (MHz) Downlink (MHz) Channel number
T-GSM-380 380 380.2–389.8 390.2–399.8 dynamic
T-GSM-410 410 410.2–419.8 420.2–429.8 dynamic
GSM-450 450 450.6–457.6 460.6–467.6 259–293
GSM-480 480 479.0–486.0 489.0–496.0 306–340
GSM-710 710 698.2–716.2 728.2–746.2 dynamic
GSM-750 750 747.2–762.2 777.2–792.2 438–511
T-GSM-810 810 806.2–821.2 851.2–866.2 dynamic
GSM-850 850 824.2–849.2 869.2–894.2 128–251
P-GSM-900 900 890.0–915.0 935.0–960.0 1–124
E-GSM-900 900 880.0–915.0 925.0–960.0 975–1023, 0-124
R-GSM-900 900 876.0–915.0 921.0–960.0 955–1023, 0-124
T-GSM-900 900 870.4–876.0 915.4–921.0 dynamic
DCS-1800 1800 1,710.2–1,784.8 1,805.2–1,879.8 512–885
PCS-1900 1900 1,850.2–1,909.8 1,930.2–1,989.8 512–810