is. That is, mobile devices take turns to use a frequency resource one at a time. In this way, the mobile devices do not inter- fere with each other despite using the same carrier frequency. Note that FDMA operation still exists as the allocated bandwidth is divided into a number of carriers as specified by the standard. The operation of a GSM system based on TDMA and FDMA is illustrated in Figure 10.16. As noted, GSM uses TDMA in combination with FDMA in that several voice channels are supported on the same frequency carrier (TDMA) and a number of carriers supporting differ- ent voice channel clusters. The GSM standard specifies that each carrier frequency has a band- width of 200 kHz, which is then shared among 8 users, each of which is assigned one of eight independent time slots every 577 us. Figure 10.17 shows how a voice channel resource is allocated to an active subscriber. Effectively a voice channel is given a one-eighth share of a 200 kHz carrier on a time-shared basis. Each cell may be assigned several such carrier frequencies, the number of which determines the total number of available voice channels in the cell. To increase the number of users that can simultaneously communicate with the base station in the cell, it is necessary to increase the number of assigned carrier frequencies. Terrestrial Broadband Wireless Telecommunications Spectrum given one operator: e.g. 4.2 MHz (downlink) 1.4 MHz (Frequency reuse factor = 3) 200 kHz (one GSM carrier) One time slot, one user (0.577 ms) Figure 10.17 Resource allocation in a GSM system An important feature of 2G systems was usage of digital technologies. Voice was digitised using several different technologies. GSM uses a voice coder with a transmission rate of 12.2 kbps. The data bits thus produced were packed in frames at intervals of 20 ms. GSM was designed to tolerate a 1% frame error rate, that is, of every 100 voice frames with duration of 20 ms that are transmitted, one frame may be lost without significant impact on the voice quality. To achieve th