24.80 28.11 As an example of a trunking gain calculation, the difference is to be found in trunking gains between the cases of 7 and 30 traffic channels with a 2 % blocking probability. The first, seven traffic channels, corresponds to one TRX case when one of the eight time slots is reserved for signalling. The second one corresponds to three TRX cases, when two time slots of 32 are reserved for signalling. The trunking gains can be calculated with the Erlang B formula and in this case the required traffic figures can be found from Table 2.13: Trunking gain with 7 channels = 2.93 Erl/7Erl = 0.42 Trunking gain with 30 channels = 21.93Erl/32Erl=0.69 another example, calculate the number of required channels when the traffic per user is 20 mErl and the number of subscribers is 500. The blocking probability is less than 2 %. Firstly, the 500 users create traffic of 500 X 20 mErl = 10 Erl. The Erlang B table is used to calculate the number of required channels. The column is selected based on the blocking probability. Table 2.14 is a snapshot of the Erlang B table for a 2 % blocking probability. The required number of channels is 17 to be able to serve 10 Erl of traffic. The second step would be to calculate the total number of needed TRXs. The general dimensioning rule with a noncombined SDCCH channel is: 7 timeslots, 1 TRX; 14 timeslots, 2 TRX; and 22 timeslots, 3 TRX. Thus the required number of TRXs is 3. Channel Structure Due to the fact that the GSM frequency band is limited, the system needs to be able to carry out intelligent frequency band division. The GSM system combines time division multiple access (TDMA) and frequency division multiple access (FDMA) for effective use of the frequency band. Frequency Table 2.14 Erlang B table for a 2 % blocking probability Traffic [Erl] 10.67 11.49 Channels Radio Network Planning and Optimisation TDMA Frame 4.6ms 01234567 26-Frame Multiframe 120ms 51-Frame Multiframe 235ms 01234567 01234567 51-Multiframe-Superframe 6.12s 26-MultiframeSuperframe6.12s 01234567 01234567 8-S