Twisted Pair - The oldest and still most common transmission medium is twisted pair. A twisted pair consists of two insulated copper wires, typically about 1 mm thick. The wires are twisted together in a helical form. The purpose of twisting the wires is to reduce electrical interference from similar pairs close by.
The most common application of the twisted pair is the telephone systems. Twisted pairs can be used for either analog or digital transmission. The bandwidth depends on the thickness of the wire and the distance travelled, but several megabytes/second can be achieved for a few kilometers in many cases. Due to their adequate performance and low cost, twisted pairs are widely used and are likely to remain so for years to come.
Coaxial Cable - Another communication transmission medium is the coaxial cable. It has better shielding than twisted pairs, so it can span longer distances at higher speeds. Two kinds of coaxial cable are widely used. One kind, 50-ohm cable is commonly used for digital transmission. The other kind, 75-ohm cable, is commonly used for analog transmission.
A coaxial cable consists of a stiff copper wire as the core, surrounded by an insulating material. The construction and shielding of the coaxial cable give it a bandwidth possible depends on the cable length. For 1 km cables, a data rate 1 or 2 Gbps is feasible. Coaxial cables are still widely used for cable television and some local area networks.
Optical Fiber - Optical fiber also known as fiber optic cable is made of fine strands of silicon glass fiber (thinner than a human hair), and is coated with a refractive surface. The signals are converted to light pulses before being sent. The coating helps protect the fibers from heat, cold, electromagnetic interference from other types of wiring, as well as some protection from ultraviolet rays from the sun. Fiber optics allow for a much faster data transmission than standard copper wires, because they have a much higher bandwidth.
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