What is GSM: Primer

Although only a second generation, 2G mobile phone system, GSM is still widely used and relied upon today around the globe.


GSM primer includes:
GSM introduction     Network architecture     Network interfaces     RF interface / slot & burst     GSM frames     Power classes & control     Channels     Audio codecs / vocoders     Handover    


GSM is a second generation or 2G mobile phone system. Originally introduced in 1991, it is still used for many voice calls and some low data rate services that require its use.

The acronym for GSM originally meant Groupe Speciale Mobile, but this changed to Global System for Mobile Communications as the system spread on a global basis.

The use of GSM grew beyond all the original expectations. Initially it had been conceived as a European system, but its use spread globally and by 2004 there were over a billion subscriptions - a milestone achievement as it had taken over 100 years for landlines to reach this point.

A selection of old cellphones that were used with the 2G GSM cellular system

Although technology has moved on significantly since the introduction of GSM as a 2G technology, it is still in widespread use as phones are low cost, and battery usage small enabling battery charge intervals to be often a week or more.

What is GSM - system overview

The GSM system was designed as a second generation (2G) cellular phone technology. One of the basic aims was to provide a system that would enable greater capacity to be achieved than the previous first generation analogue systems. GSM achieved this by using a digital TDMA (time division multiple access approach). By adopting this technique more users could be accommodated within the available bandwidth. In addition to this, ciphering of the digitally encoded speech was adopted to retain privacy. Using the earlier analogue cellular technologies it was possible for anyone with a scanner receiver to listen to calls and a number of famous personalities had been "eavesdropped" with embarrassing consequences.

Note on the History of Mobile Phone Technology:

Mobile phone technology has developed every year. From the first introductions of cellular phone systems in the 1980s to current day, new technology development has steadily improved the systems available. Starting with the first generation analogues systems, new 2G digital systems were introduced and these have moved on so that 5G technology is now being developed.

Read more about Mobile Phone History.

GSM services

Speech or voice calls are obviously the primary function for the GSM cellular system. To achieve this the speech is digitally encoded and later decoded using a vocoder. A variety of vocoders are available for use, being aimed at different scenarios.

In addition to the voice services, GSM cellular technology supports a variety of other data services. Although their performance is nowhere near the level of those provided by 3G, they are nevertheless still important and useful. A variety of data services are supported with user data rates up to 9.6 kbps. Services including Group 3 facsimile, videotext and teletex can be supported.

One service that has grown enormously is the short message service. Developed as part of the GSM specification, it has also been incorporated into other cellular technologies. It can be thought of as being similar to the paging service but is far more comprehensive allowing bi-directional messaging, store and forward delivery, and it also allows alphanumeric messages of a reasonable length. This service has become particularly popular, initially with the young as it provided a simple, low fixed cost.


GSM basics

The GSM cellular technology had a number of design aims when the development started:

  • It should offer good subjective speech quality
  • It should have a low phone or terminal cost
  • Terminals should be able to be handheld
  • The system should support international roaming
  • It should offer good spectral efficiency
  • The system should offer ISDN compatibility

The resulting GSM cellular technology that was developed provided for all of these. The overall system definition for GSM describes not only the air interface but also the network or infrastructure technology. By adopting this approach it is possible to define the operation of the whole network to enable international roaming as well as enabling network elements from different manufacturers to operate alongside each other, although this last feature is not completely true, especially with older items.

GSM cellular technology uses 200 kHz RF channels. These are time division multiplexed to enable up to eight users to access each carrier. In this way it is a TDMA / FDMA system.

The base transceiver stations (BTS) are organised into small groups, controlled by a base station controller (BSC) which is typically co-located with one of the BTSs. The BSC with its associated BTSs is termed the base station subsystem (BSS).

Further into the core network is the main switching area. This is known as the mobile switching centre (MSC). Associated with it is the location registers, namely the home location register (HLR) and the visitor location register (VLR) which track the location of mobiles and enable calls to be routed to them. Additionally there is the Authentication Centre (AuC), and the Equipment Identify Register (EIR) that are used in authenticating the mobile before it is allowed onto the network and for billing. The operation of these are explained in the following pages.

Last but not least is the mobile itself. Often termed the ME or mobile equipment, this is the item that the end user sees. One important feature that was first implemented on GSM was the use of a Subscriber Identity Module. This card carried with it the users identity and other information to allow the user to upgrade a phone very easily, while retaining the same identity on the network. It was also used to store other information such as "phone book" and other items. This item alone has allowed people to change phones very easily, and this has fuelled the phone manufacturing industry and enabled new phones with additional features to be launched. This has allowed mobile operators to increase their average revenue per user (ARPU) by ensuring that users are able to access any new features that may be launched on the network requiring more sophisticated phones.

GSM system overview

The table below summarises the main points of the GSM system specification, showing some of the highlight features of technical interest.

Specification Summary for GSM Cellular System
 
Multiple access technology FDMA / TDMA
Duplex technique FDD
Uplink frequency band 890 - 915 MHz
(basic 900 MHz band only)
Downlink frequency band 933 -960 MHz
(basic 900 MHz band only)
Channel spacing 200 kHz
Modulation GMSK
Speech coding Various - original was RPE-LTP/13
Speech channels per RF channel 8
Channel data rate 270.833 kbps
Frame duration 4.615 ms

Further developments of GSM

GSM was a particularly successful mobile telecommunications system. Initially it had been intended for use within Europe, but within a relatively short while the system was being used well beyond the borders of Europe, becoming an internationally accepted system.

In addition to its success as a voice communications system, it was developed beyond the basic voice capability to be able to carry data. With the Internet becoming more widely used, GSM was developed to start to meet these needs.

  • GPRS:   GPRS, the General Packet Radio Service was an evolution of the GSM 2G cellular telecommunications system. Using packet data rather than circuit switched data circuits, it enabled speeds similar to those experienced using dial up networking services to be achieved under ideal conditions. However data speeds were very slow by comparison with the systems used today.

    Note on 2G GPRS:

    GPRS - General Packet Radio System introduced packet data to the GSM cell phone system. Although data was slow by current standards, it enabled data to become a main part of the system.

    Read more about 2G GPRS.

  • GSM EDGE :   GSM EDGE, Enhanced Data rates for GSM Evolution, was the evolution of GSM, & GPRS which used 8PSK modulation to achieve data transfer rates up to 384 kbps. Although this provided a significant increase over GPRS, data speeds were still very slow.

    Note on 2G GSM EDGE:

    GSM EDGE - Enhanced Data rates for GSM Evolution provided for an increase in the data rates achievable by the 2G GSM based system. By using packet data (already used on GPRS) and changing the modulation format to 8PSK, considerable increases in data performance were obtained.

    Read more about 2G GSM EDGE.

Although it took some years, GSM steadily became the dominant 2G mobile phone system. Ground-breaking attributes like the SIM card enabling the same number and account to be retained when the phone was upgraded along with aspects like international roaming gave it a significant lead over other systems. Even many years later the basic GSM network and the GSM upgrade technologies and architecture are being used to provide legacy service for many systems that relay on a basic technology. Although operators have reduced the spectrum available to it, GSM is still likely to be maintained for quite a number of years to come.



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