 from the chain of ground- based networks, of which they are a vital link. The role of satellites is growing steadily. Their altitude above the equator enables them to cover a full one-third of our globe. The possibilities they offer are also expanding because of their growing transmission capabilities and the anticipated development of intersatellite links. The latter will provide direct communication between geostationary satellites (do- mestic and international) or between geostationary satellites and data-relay satellite systems, thus setting up other communication net- works in space. Figure 7-1 shows a simplified satellite channel and its components. The channel starts at the host computer, which is connected by way of a local communications loop (using traditional telephone com- pany facilities) to the central office of the satellite communications vendor. The data from the local loop is then multiplexed with other data received from separate sources into a microwave signal that is sent to the satellite vendor's earth station. This signal becomes part of a composite signal that is sent by the earth station to the satellite and is transmitted by the satellite to the receiving earth stations. The satellite uses a transponder (a device that receives RF signals at one frequency and converts them to another frequency for transmission) 108 TELECOMMUNICATIONS Satellite Composite signal Earth Microwave Earth station station Earth station Microwave link Satellite Local central office Local Satellite central office computer Fig. 7-1. Satellite channel components. to transfer the composite signal from one earth station to another. At the receiving earth station, the data is transferred by a microwave link to the satellite vendor's central office; from there, it is broken down into separate communications channels and transmitted over local telephone company facilities to the receiving terminal. The distance between the host computer and the receiving terminal is essentially the same for all points within the area ser