* Transmission media lie below the physical layer. * A guided medium provides a physical conduit from one device to another. * Twisted-pair cable, coaxial cable, and optical fiber are the most popular types of guided media. * Twisted-pair cable consists of two insulated copper wires twisted together. Twisting allows each wire to have approximately the same noise environment. * Twisted-pair cable is used in telephone lines for voice and data communications. * Coaxial cable has the following layers (starting from the center): a metallic rod-shaped inner conductor, an insulator covering the rod, a metallic outer conductor (shield), an insulator covering the shield, and a plastic cover. * Coaxial cable can carry signals of higher frequency ranges than twisted-pair cable. * Coaxial cable is used in cable TV networks and traditional Ethernet LANs. * Fiber-optic cables are composed of a glass or plastic inner core surrounded by cladding, all encased in an outside jacket. * Fiber-optic cables carry data signals in the form of light. The signal is propagated along the inner core by reflection. * Fiber-optic transmission is becoming increasingly popular due to its noise resistance, low attenuation, and high-bandwith capabilities. * Signal propagation in optical fibers can be multimode (multiple beams from a light source) or single-mode (essentially one beam from a light source). * In multimode step-index propagation, the core density is constant and the light beam changes direction suddenly at the interface between the core and the cladding. * In multimode graded-index propagation, the core density decreases with distance from the center. This causes a curving of the light beams. * Fiber-optic cable is used in backbone networks, cable TV networks, and Fast Ethernet networks. * Unguided media (usually air) transport electromagnetic waves without the use of a physical conductor. * Wireless data is transmitted through ground propagation, sky propagation, and line-of-sight propagation. * Wireless data can be classifed as radio waves, microwaves, or infrared waves. * Radio waves are omnidirectional. The radio wave band is under government regulation. * Microwaves are unidirectional; propagation is line of sight. Microwaves are used for cellular phone, satellite, and wireless LAN communications. * The parabolic dish antenna and the horn antenna are used for transmission and reception of microwaves. * Infrared waves are used for short-range communications such as those between a PC and a peripheral device.