3 burst periods later than reception. 200 kHz TDMA frame Reception 012345670123 Burst period Transmission 45670123456701 =0.577 ms Bidirectional radio transmission A mobile station receives, shifts the has fixed duplex distance: frequency by 45 or 95 MHz, and 45 MHz (900 MHz band) and emits a moment later. 95 MHz (1,800 MHz band). Figure 5.10 Multiple-access scheme of GSM. Introduction to Telecommunications Network Engineering 900-MHz band) and of 75 MHz (in the 1,800-MHz band) and this duplex principle is called frequency-division duplex (FDD). The separation in time is three time slots, as shown in Figure 5.10. This principle makes the imple- mentation of mobile equipment efficient because there is no need to transmit and receive simultaneously. Two bursts after reception on the downlink or forward frequency, the mobile equipment sends on the uplink or reverse fre- quency as shown in Figure 5.10. One time slot in each eight-slot TDMA frame represents one physical channel. Each call typically occupies one of the eight physical channels at one carrier frequency. 5.5.3 Logical Channels The physical channels at the GSM radio interface are divided into logical channels. They fall into two main categories, dedicated channels and com- mon control channels. There are many different logical channels and the dis- tinction between them is based on the purpose and the information transmitted via a channel. These logical channels are mapped onto one physical channel defined as one slot (usually TSO) in each TDMA frame and transmitted as regular radio bursts. 5.5.3.1 Traffic Channel and Associated Slow-Rate Control Channel When the call is connected, two channels on the radio path are dedicated to it: the traffic channel (TCH) and the slow associated control channel (SACCH). The SACCH is used, for example, to transmit power control information to the MS and measurement results from the mobile stations to the network. These two channels belong to the dedicated channels because they are allocated for one user. 5.5.3.2 Common C