 and it can be very effective. The power used by the system is tightly controlled. There are five classes of mobile station, and these are related to their output power. The highest is rated at 20 watts, but others are 8, 5, 2 and 0.8 watts. To minimize the levels of interference and also to conserve battery power both base stations and mobiles are able to step the power down to 20 milliwatts (13 dBm) SO that the lowest power that can maintain acceptable communications can be used. To assess the required levels the mobile checks the signal quality and passes the information to the base station, which also assesses its received transmission quality. The base station alters its level and sends control data to the mobile SO that it can set its output level accordingly. The short message service (SMS) is particularly useful. Allowing text only messages of up to 160 characters it provides additional flexibility to the service and its unprecedented growth and popularity can be judged by the fact that in the year 2000 over six billion messages were sent every month. Although originally associated with GSM, other networks including CDMA and TDMA now support the service meaning that it is an almost universal mobile data service. 268 Newnes Guide to Radio and Communications Technology At the centre of the SMS system is the Short Message Centre (SMC). This is the entity that stores and forwards the messages for the networks. Each network has a gateway mobile switching centre (GMSC), and this is the mobile network's point of contact with the other networks. Messages are passed to and from the SMC from each of the network's GMSCs. When a message is received by the SMC it determines to which network it should be routed, and it enters the gateway MSC for that network. From here the network detects its location in the normal way and routes the message accordingly. Data for the SMS messages is passed over the signalling channels using a header to identify it as a short message. In this way messages can be received even when a voi