le because other users require some of the frequencies in the band. Other similar systems, namely PCS 1800 and DCS 1900, use bands around 1800 MHz and 1900 MHz but adopt the same basic formats as GSM in other respects. In order to make the optimum use of the channels available, GSM uses a combination of time and frequency division multiple access techniques. To enable the multiple frequencies to be used, the 25 MHz bandwidth is split into 124 carriers each of 200 kHz bandwidth. One or more of these can then be allocated to each base station, dependent upon the expected traffic. Then further capacity is gained using time division techniques. Each carrier is split into eight 'bursts' each of which lasts approximately 0.58 ms and each one is allocated to a user. The group of eight bursts is called a frame and this lasts 4.62 ms after which the sequence repeats, allowing an almost constant flow of data for each user. As GSM is a digital system the speech has to be sampled and digitized. If this were linearly digitized it would require a data rate of around 64 kbps which is far too high as it would occupy too much of the available bandwidth. Accordingly the system that is used with GSM uses a form prediction to help reduce the data rate. The actual system used is known as Regular Pulse Excited Linear Predictive Coder, and this gives a good balance between speech quality, data rate reduction, bandwidth usage, and also the complexity of processing required as this has an impact on battery usage. More complicated systems require more processing which in turn requires more circuitry and more current. In the system, the speech is sampled and encoded as 260 bits and this gives a total data rate of 13 kbps. The data stream has error redundancy added to reduce bit errors. This data is then modulated onto the carrier using Gaussian filtered Minimum Shift Keying (GMSK). This was used because it gave a good compromise between transmitter complexity, efficient use of the available spectrum and good performance in terms of spuriou