Transmission of Signals

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Transmission media lie below the physical layer. We can evaluate transmission media by throughput, propagation speed, and propagation time.

* 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...
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