uperframe Hyperframe 3h 28min 53s 760ms Figure 2.23 GSM channel structure division means the division of the bandwidth to carrier frequencies, which are spaced 200 kHz apart. The frequency ranges for the GSM frequency bands are specified in ETSI specifications. The GSM 900 frequency ranges are 890-915 MHz for uplink and 935-960 MHz for downlink. The GSM 1800 frequency ranges are 1710-1785 MHz for uplink and 1805-1880 MHz for downlink. The GSM 850 frequency ranges are 824-849 MHz for uplink and 869-894 MHz for downlink. The GSM 1900 frequency ranges are 1850-1910 MHz for uplink and 1930-1990 MHz for downlink. Finally, The GSM 400 frequency ranges are 450.4-457.6 MHz for uplink and 460.4-467.6 MHz for downlink or 478.8-486 MHz for uplink and 488.8-496 MHz for downlink. For example, the GSM 900 sub-bands are divided into 124 carrier frequencies. The time division divides each carrier frequency into timeslots numbered from 0 to 7 (see Figure 2.23). The length of one timeslot is 0.577 ms. As stated above, the GSM system divides the carrier frequencies into timeslots. The TDMA frame consists of eight timeslots and therefore the length is 4.6 ms. To identify the TDMA frames, each has a 22 bit part used for identification information. The structure of a traffic channel (TCH) is the following: one hyperframe, which is consists of 2048 superframes each having 51 multiframes and 26 TDMA frames. The structure of a control channel (CCH) is the following: 2048 superframes each consisting of 26 multiframes and 51 TDMA frames. One burst in the TDMA frame, one timeslot, is equal to a physical channel. Two concepts are good to separate at this point, physical and logical channels. The first are the actual channels for carrying the information between the BSS and MS. The timeslots are in other words the physical channels. The logical channels structure the transferred information, data and signalling and which specific type. Traffic channels are for user data and control channels are for signalling. The user data can be either voic