GSM: The cellular phone is a radio

( Part of Henrik Kaare Poulsen's GSM pages )

The cellular phone is basically a radio. The radio makes a radio connection to the nearest radio station (also called Base Transceiver Station or BTS) in the cellular network.

Frequencies

As with any radio, the cellular phone operates in a given frequency band. The GSM-900 system operates at frequencies around 900 MHz (mega-hertz), the GSM-1800 system (also called DCS) operates around 1800 MHz. The american GSM-1900 system (also called PCS) operates around - you already guessed it - 1900 MHz.

The available frequency band is divided into individual frequencies, also called channels. This technique is called FDMA: Frequency divided multiple access. This means that more than one radio can operate at the same time, each radio operating at a different frequency (ie. different channel).

Time-slots

In order to increase the efficiency of the communication, each frequency is further divided into so-called time-slots or TS. The duration of a time-slot is approximatly 0.05 second, and there is eight timeslots.

In GSM-900 there is 124 channels, each divided into 8 timeslots. As time-slot number 0 is usually used to communicate control information, this gives the possibility to have 868 calls at the same time.

In the figure below, each star represents one call, and each cross represents one control channel.

	TS 0	TS 1	TS 2	TS 3	TS 4	TS 5	TS 6	TS 7
Channel 1	X	*	*	*	*	*	*	*
Channel 2	X	*	*	*	*	*	*	*
Channel 3	X	*	*	*	*	*	*	*
…	…	…	…	…	…	…	…	…
Channel 123	X	*	*	*	*	*	*	*
Channel 124	X	*	*	*	*	*	*	*

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