Points on Transmission Media
- 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.
