ten accomplished by means of radio and microwave transmissions. Until recently, however, signals were transmitted back to ordinary (wired) telephones after their satellite processing. In recent years, radio transmissions and equipment have become more sophisticated, and the technology and applications which support these applications have also increased in sophistication.
Radio also uses a transmitter and receiver to process its signals. Radio waves carry information encoded directly on the wave (accomplished by periodically interrupting its transmission), or information may be impressed on the carrier frequency by modulation, e.g., amplitude modulation (AM) or frequency modulation (FM). In its most common form, radio transmits sounds (voice and music) and pictures (television). The sounds (or images) are converted into electrical signals by a microphone (or camera tube), amplified, and used to modulate a carrier wave that has been generated by a transmitter. The modulated carrier is also amplified, then applied to an antenna that converts the electrical signals to electromagnetic waves that radiate into space at the speed of light. Receiving antennas intercept part of this radiation, convert it back into electrical signals, and feed it to a receiver. Once the basic signals have been separated from the carrier wave, they are fed to a loudspeaker or cathode-ray tube, where they are converted into sound and visual images, respectively.
Perhaps the most visible application of wireless technology is the cellular phone, which enables individuals to send and receive calls without having to be at a wired telephone. This means that phones can be installed in cars or carried with people. Although the transmitter and receiver (the handsets themselves) resemble telephones, they are actually radios, using the radio spectrum to transmit their information instead of traditional cable. However, users view them essentially as telephones, ...