Transmission mediaData can be transmitted over various different media. The most common are cable, ra-dio, satellite links, and fiberoptics. Cable, radio/TV, and satellite communications usethe radio-frequency spectrum. Fiberoptics uses infrared or visible light energy.CableThe earliest cables were wires that carried dc. Nowadays, data-transmission cablesmore often carry ac at radio frequencies. One advantage of using RF is that the signalscan be amplified at intervals on a long span. This greatly increases the distances overwhich data can be sent by cable. Another advantage of using RF is that numerous sig-nals can be carried over a single cable, with each signal on a different frequency.Cables can consist of pairs of wires, somewhat akin to lamp cords. But more oftencoaxial cable is used. This has a center conductor surrounded by a cylindrical shield.The shield is grounded, and the center conductor carries the signals (Fig. 26-16). Thecenter conductor is kept in place by an insulating dielectric, usually made of polyeth-ylene. The shield keeps signals confined to the cable, and also keeps external electro-magnetic fields from interfering with the signals.Transmission media49326-16A coaxial cable has a center conductor surrounded by a cylindrical shield.Cable signals can be modulated using any of the techniques outlined earlier in thischapter. The most familiar example is cable television.RadioAll radio and TV signals are electromagnetic waves. The radio or TV transmitter outputis coupled into an antenna system located at some distance from the transmitter. Theenergy follows a transmission line, also called a feed line, from the transmitter PAoutput to the antenna itself.Most transmission lines are coaxial cables. There are other types, used in specialapplications. At microwaves, hollow tubes called waveguides are used to transfer theenergy from a transmitter to the antenna. A waveguide is more efficient than coaxial ca-ble at the shortest radio wavelengths.