sic GPRS throughput: since the system has been designed for a sufficient margin to permit a low blocking level, some of the spare instantaneous capacity can be utilised for packet data transmission. As long as the packet traffic can be temporarily interrupted to accommodate the peaks in circuit switched traffic, no degradation in the circuit switched services will result. From the example above, a cell offering a circuit switched load of 14.9 Erlangs with 22 circuits will, on average, have 7.1 spare circuits. These could carry packet data on an on-demand basis, relinquishing the channels for circuit switched traffic when required. In this way, the blocking probability of the circuit switched facility is not degraded, even though the traffic channels are subject to a higher utilisation. However, there is a certain overhead associated with the division of the circuit switched area and the Capacity Planning Table 2.15 Mean number of timeslots available Number of GSM traffic at 1 % GSM traffic at 2 % Mean free TCH for Mean free TCH for (TCH) carrier blocking (Erl) blocking (Erl) GPRS (1 % blocking) GPRS (2% blocking) 1 (7) 2 (14) 3 (22) 4 (30) 5 (38) 6 (46) 7 (54) 8 (62) GPRS area. This is described in the following sections, and results in a reduction in the number of channels available to GPRS, in this case from 7.1 channels to 5.6 channels. Table 2.15 shows the mean number of timeslots (TCHs) available for GPRS for different numbers of carriers per cell and for circuit switched blocking probabilities of 1 and %. It takes account of the channel overhead (see Table 2.16 and the associated description later). It would be generally expected that the number of timeslots available to the GPRS would increase as the number of carriers increases, but there are several examples in Table 2.15 where this does not occur. This is a result of the channel overhead taking on increasing values with increasing numbers of carriers, in a step like manner. Theoretically, 5.6 timeslots are, on average, available for the GPRS in the prev