GSM Mobile Network – mobile network of 2nd generation
Mobile Terminal and its Connection to Base Station

Two frequency bands with a width of 25 MHz with separation of 45 MHz are reserved for the transmission of signals in GSM 900. Signal in the direction from the BTS to the MS is transmitted in the frequency band from 935 MHz up to 960 MHz. The opposite direction is then carried in the band from 890 MHz up to 915 MHz. Access methods for the implementation of the radio circuit between the MS and BTS is based on a combination of FDMA and TDMA access methods. In both frequency bands, the band is divided into 124 sub-bands by FDMA (F1 through F124) with a nominal width of 200 kHz. In each frequency sub-band, the radio resources are then split into 8 time slots (T1 to T8) by using TDMA.

Principle block scheme of MS based on the standard GSM 900 and its relation to the BTS

Each channel frame (Burst) with a length of 156,25 bits and with duration of 0.577 ms contains bits including the user information (UI) and service information (SI). This means that the BTS can offer total capacity of up to 124 × 8 = 992 pairs of usable channels, it means, 992 radio circuits.

Synchronization is derived in the GSM from the clock generator with nominal frequency ft = 13 MHz. The basic transmission rate is vp = 13 000 000/48 = 270 833.33 bit/s. One TDMA frame (including 8 time slots) has duration equal to 4.615 ms (multi-frame is composed from 26 frames and its duration equals 120 ms).