iber. The length of one timeslot is 0.577 ms and the length of the TDMA frame consisting of eight timeslots is 4.6 ms. The frequency hopping method implemented in the GSM system is slow frequency hopping; after every TDMA frame the frequency can be changed. By changing the frequency after each 4.615 ms, in one second the TDMA frame makes 217 changes. The frequency hopping types of the GSM system are base band (BB) frequency hopping and synthesised frequency (RF) hopping. The broadcast control channel (BCCH) in the first timeslot of the TRX and the traffic channels (TCHs) are treated differently. The BCCH frequency transmits continuously whereas the TCH slots are based on traffic. In BB hopping the TRXs actually have fixed frequencies and the frequency hopping operates SO that the bursts are shifted from one TRX to another according to the hopping sequence. The number of hopping frequencies therefore comes from the number of TRXs. The length of the mobile allocation (MA) list is the number of TRXs. The BCCH TRX does not hop and therefore is excluded from the hopping groups. With BB hopping there are two actual hopping groups, one for the first timeslots of each TRX (in one cell) excluding the BCCH TRX and another for all the other timeslots. The first hopping group has TRX-1 frequencies, due to the dedicated BCCH frequency. See Figure 2.29 for an illustration of base band hopping. In RF hopping the frequency of the TRX changes but a single call uses only one TRX. This does not have the same limitation to the number of frequencies as in BB hopping. The RF hopping pattern can be either random or cyclic. In random frequency hopping the frequencies are randomly selected from the available frequencies. Cyclic hopping has a defined cycle for the frequencies in the MA list. Figure 2.30 shows the basic functionality of RF hopping. Frequency Planning BCCH timeslot 01234567 TRX 1 TRX 2 TRX n Figure 2.29 BB hopping principle BCCH timeslot 01234567 TRX 1, no hopping TRX 2 MA list frequencies TRX n Figure 2.30 RF hopping princ