This is primarily used by the mobile to acquire the timing reference. The timing is very important as the base stations are kept in tight synchronism with each other by a signal derived from the Global Positioning System (GPS). Each base station has a fixed timing offset from the GPS signal to minimize interference between cells and the synchronization channel provides the mobile with the required timing offset information to access the cell. The third channel is the paging channel. This provides system parameters, voice pages, short message service and any other broadcast messages to users in the cell. Cellular telecommunications 271 The traffic (radio or dedicated) channels carry the digitized speech and data. There are two types of traffic channel, namely the fundamental channel and the supplemental channel. Speech data is always transmitted over the fundamental channel and then between one and seven supplemental ones can be used to provide the additional capacity needed for data transmissions. Power control information from the base station to the mobile is also transmitted over the traffic channel. It enables the mobile to accurately control its power SO that the signal received by the base station is within its required limits. If it becomes too strong, it will mask out the other mobile stations, preventing their signals from being decoded. If it is too weak, the base station will not be able to decode the data. The speech is encoded into a digital format using a similar encoder to that used for GSM, and once encoded, error correction information is applied. This brings the modulation symbol rate to 19.2 kbps. This is then multiplied by the orthogonal code, known as a Walsh code, to give a 1.228 Mbps signal that is then transmitted. The transmitted signal occupies a channel of 1.25 MHz. The uplink from the mobile to the base station is constructed slightly differently. Speech is generated in the same way, but more error correction is applied and this brings the modulation symbol rate up to 28.8 kbps. The si