Global System for Mobile Communications (GSM)

Definition of Global System for Mobile Communications (GSM) in The Network Encyclopedia.

What is GSM (Global System for Mobile Communications)?

A digital cellular phone technology popular in Europe, Asia, and other parts of the world. Global System for Mobile Communications (GSM) supports voice, data, Group 3 fax, and paging services for both vehicle-mounted and handheld mobile use. In addition, its speech quality equals that of the analog Advanced Mobile Phone Service (AMPS) and can interface with packet-switched networks.

How it works

The GSM Phase 1 implementation uses a combination of Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) media access control methods to provide full-duplex communication over two frequency bands within the 862-to-960-MHz World Association of Radio Communications (WARC) portion of the electromagnetic spectrum. These two frequency bands are

  • The 890-to-915-MHz band for mobile-to-base (uplink) communication
  • The 935-to-960-MHz band for base-to-mobile (downlink) communication

Carrier signals are spaced 200 kHz apart within these bands to provide 124 pairs of superchannels based on frequency-division multiplexing (FDM), each of which is then subdivided into eight traffic channels using time-division multiplexing (TDM). Each channel carries voice communication at 13 Kbps (or 9.6 Kbps for data transmission). GSM thus provides 992 full-duplex channels for voice communication. Power classes for GSM mobile units range from 0.8 through 2.0 watts transmission power for handsets to 8 through 20 watts for vehicle-mounted units. Approximately half of a GSM transmission consists of overhead for signaling, such as synchronization and error handling. Such high overhead is typical in cellular phone systems, and is necessary - not so much because of external interference of buildings and other structures, but because of internal interference due to crosstalk between channels and across cell boundaries.

GSM is a secure system that uses key-based encryption for authentication and, optionally, for data transfer. The diagram shows the process that occurs when a mobile user wants to place a call. When the user dials a number, the mobile unit connects with the base station requesting authorization. The base station generates a random number and transmits it to the mobile unit, which then combines the random number with the owner’s secret key stored in the phone’s standard Subscriber Identity Module (SIM) card by using a ciphering algorithm called A3. The result of this process is transmitted to the base station. Meanwhile, the base station, which has the private keys for all its subscribers stored in a database, follows the same steps, using the A3 algorithm to combine the generated random number with the caller’s private key. The result is compared with the result transmitted by the user. If the two results agree, the user is logged on to the system.

Graphic G-5. The authentication process for GSM.

GSM Phase 1 supports call forwarding, global roaming, call barring, and other features. GSM Phase 2 adds additional features such as the following:

  • Short message service for sending and receiving short text messages using phones
  • Call holding, call waiting, and caller ID
  • Multiparty calling supporting up to five parties per call
  • Mobile fax and data services

GSM Phase 2+ (just being implemented at the time of this writing) includes support for data transmission at 64 Kbps and higher, packet radio, virtual private networks, enhancements to the SIM card, higher spectral efficiency, integration with satellite links, and even GSM services in the local loop.

NOTE

GSM has a counterpart service called Digital Communication Service (DCS) that works in essentially the same way as GSM, except at a higher 1.8-GHz frequency band. DCS provides a total of 2992 channels for voice communication. One advantage DCS has over GSM is that it uses much lower power levels for mobile units, ranging from 0.25 to 1.0 watts transmission power.

The SIM card is a small device about the size of a stamp that is issued when a user subscribes to the GSM service. It contains the user’s phone number, private key, billing information, and other information. When users visit a locale at which the GSM system is different, they can simply remove the SIM card from their phone and install it in a rented phone that can function in that locale.

Encryption of messages is similar to the encrypted authentication process, except that each transmitted frame is encrypted using a different random number. This makes encrypted GSM messages extremely difficult to crack, so much so that some countries prohibit GSM providers from encrypting user messages!