How the GSM system works?.

In various countries the frequency bandwidths specified for the GSM services are GSM-400, GSM-800, GSM-900, GSM-1800 and GSM-R.The GSM-900 and the GSM-1800 are the most widely used frequency bandwidths in different parts of the globe.

The GSM-900 has a down link frequency range of 935-960 MHz and an up link frequency of range of 895-915 MHz. This frequency band is partitioned into 124 pairs of simplex channels with separation of 200KHz.A particular range of simplex channels is given to a particular network provider.

The type of interface used in GSM is digital air interface.The analogue voice signals are converted to digital signals before transmission.Up to 8 MS subscribers can be handled by the GSM RF carrier at a time.The rate of transmission is 270 Kbps.

The Gaussian minimum shift keying (GMSK) is used for transmitting the digital signals.In GMSK , a phase change represents the change from a digital “1 ” or a “0 “,occurs over a period of time.The addition of high frequency components to the spectrum is reduced.In GSMK ,the phase change is not constant and it is spread- out.

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Schematic of a GSM system.

SIM – Subscriber Identification Module.
MS – Mobile Station.
ME – Mobile Equipment.
BTS – Bare Transceiver System.
MSC – Mobile Services switching Center.
NSS – Network & Switching Subsystem.
PSTN – Public Switched Telephone Network.