ical link between the mobile and the network. Some of the key characteristics of the air Downlink and uplink power control interface are given in Table 47.3. As already described, GSM is a Discontinuous transmission and reception digital system employing time division multiple access (TDMA) Detailed description of GSM 47/11 1 hyperframe= 2048 superframes= 2715648 TDMA frames (3h 28mn 760ms) 1 superframe= 1326 TDMA frames (6.12s) (= 51 (26-frame) multiframes or 26 (51-frame) multiframes) 1 (26-frame) multiframe= 26 TDMA frames (120ms) 1 (51-frame) multiframe= 51 TDMA frames (3060/13ms) 1 TDMA frame = 8 time slots (120/26 approx. 4.615ms) 1 time slot 156.25 bit durations (15/26 approx. 0.577ms) (1 bit duration 48/13 approx 3.69 microsec) Encrypted bits Normal burst (NB) Training sequence Encrypted bits TB: Tail bits GP Guard period Frequency correction Fixed bits burst (FB) Synchronisation burst (SB) Encrypted bits Synchronisation sequence Encrypted bits Access burst (AB) Synchronisation sequence Encrypted bits 68.25 Figure 47.10 GSM timeframes, timeslots and bursts (Extract from GSM Recommendation 05.01) physical channel, such that each carrier can support eight physical These are further divided into channels with specific purposes and channels, in both the uplink and downlink directions. for a detailed description of these channels the reader is referred to The timeslot allocation in either direction is staggered SO that the the GSM Recommendations published by ETSI. mobile station does not need to transmit and receive at the same Each of these logical channels is mapped onto the physical chan- time. Data is transmitted in bursts within the timeslots and a number nels, using the appropriate burst type as shown in Figure 47.10. of different types of burst can be carried as shown in Figure 47.10. TDMA frames are built up into 26 or 51 frame multiframes, such The normal burst has a data structure as shown. It consists of 148 that individual timeslots can use either of the multiframe types, and bits of which 114 are