How a GSM phone communicates with the network during a call

The voice, fax or data call contents pass between the base and the mobile as a series of data bursts. For more information about how voice is digitised, see the Digitising page.

Wrapped up in Packets

The signal consists of 122ms of data, divided into 26 frames (numbered from zero, of course). Frame 11 is the Slow Associated Control Channel (SACCH) which is a low-speed data channel for non-urgent system messages. Frame 25 is not used. The remainder are Traffic Channels (TCH) which carry the data representing the speech being carried.

Each frame is transmitted on a different frequency to minimise the effects of interference and fading. The mobile learns the way that the frequency-hopping will go from information in the BCCH channel, described in the In Idle Mode page.

Each frame contains eight burst periods, where each of the mobiles using that cell get their turn to communicate.

A TCH burst is 0.577 ms in duration, and consists of 156 bits of data sent in two blocks, of which two lots of 57bits are the information being passed.

Sorting the Data from the Noise

There is a standard 26bit training sequence in the middle of each TCH burst. The receiver compares this with the actual signal received, and uses the difference as the basis for processing the rest of the signal received. This reduces the effects of multipath reflections (where the signal bounces off objects, so arrives out of phase, distorting the direct signal).

BTS receivers also normally use diversity - basically two receiving antennas, so that the receiver can choose the best signal from both of them. This is why BTS often have pairs of antennas a few feet apart. Some "lamppost" style masts get the diversity effect by having the second antenna horizontally polarised.

How Far Can You Go?

Because of the need to send the data bursts to the base so that they arrive exactly on time, the mobile sends the burst early to allow for the transit time if it is some way from the base. This is called Timing Advance, and the maximum timing advance corresponds to a distance of up to 35km.

The joint limitations of the speed of light and the time available mean that special measures are needed to exceed this range, based on the idea of using the following slot as well. This increases the maximum theoretical range to something like 120km, but halves the cell's call carrying capacity.

Power Economy

To save battery power and to allow reuse of frequencies, both ends reduce the transmitter power wherever possible, so that the minimum power is used. This means that the mobile could reduce power when near the base. For GSM handheld units, the minimum average power level is 1.25 milliwatts, though Phase 1 of the GSM 900 standard minimum power was 2.50 milliwatts.

In addition, most networks use Discontinuous Transmission (DTX) which switches off the mobile's transmitter if there is no speech to send. The other end gets a faint hiss (comfort noise) to stop it sounding as if the line has dropped, and the mobile battery lasts longer.

[ Air Interface ] : [ Idle Mode ] : [ Digitising ]

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