| 单词 | Utopia |
| 释义 | Utopia From the Greek ou, meaning not, and topos, meaning place, and translating literally as no place.The word was first used by Sir Thomas More (1516) in his book Utopia as the name of an imaginary island that was the home of a perfect political and social system. In contemporary usage, utopia refers to an ideal place, state of being, or situation. ( Note: Utopia sounds like no place I’ve ever been. If there were such a place, someone surely would foul it up. If not it would get so crowded that nobody would go there any more.) UTP (Unshielded Twisted Pair) A pair of copper conductors, separately insulated by a dielectric material and smoothly twisted in a helix with a constant pitch or distance to make a 360° twist, as illustrated in Figure U-3.The conductors generally are solid core, although stranded wires are employed where additional flex strength is required.The insulating material is polyethylene, polyvinyl chloride, flouropolymer resin, Teflon®, or some other low-smoke, fire-retardant substance. The insulation separates the conductors so that the electrical circuit is not shorted, and protects the conductors from physical damage. Twisted pair is known as a balanced medium as both conductors serve for signal transmission and reception, and as each conductor carries a similar electrical signal with identical direct and return current paths. At any given point in the cable, the signals are equal in voltage to ground but opposite in polarity, which has the effect of reducing radiated energy and, therefore, reducing attenuation.The result is increased signal strength over a distance. Reducing the radiated energy also serves to minimize the impact on adjacent pairs in a multipair cable configuration. Generally speaking, the tighter the twist, i.e., the more twists per foot, the better the performance of the circuit. Unlike shielded twisted pair (STP) and screened twisted pair (ScTP), UTP has no shield to protect transmissions from electromagnetic interference (EMI). See also dielectric, EMI, flex strength, ScTP, shield, STP, and UTP. 74570c21.qxd 9/11/07 12:36 PM Page 516 |
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