| 单词 | Vp (Velocity of propagation) |
| 释义 | Vp (Velocity of propagation) The speed with which a signal travels through a medium or through free space, expressed as a percentage of the speed of light in a vacuum. All electromagnetic signals propagate, or travel, in a vacuum at the speed of light, which is 299,792.458 kilometers per second (km/s), or 186,282.397 miles per second. ( Note: This actually is a definition rather than a measurement, as the meter is formally defined as the distance traveled by light in a vacuum in 1⁄299,792,458 of a second.) The speed of light typically is expressed in nominal, or approximate, terms as 300,000 km/s, or 186,000 miles per second, for ease of expression and calculation. As signals propagate in a vacuum unimpeded by any physical matter, this speed is the base number for indexing the velocity of propagation in all media.Table V-1 (Electromagnetic Signal Propagation Velocity) compares nominal signal velocities in various media, some of which are relevant and some of which simply serve as points of reference.The signal velocity in a vacuum applies to the space segment of a microwave satellite transmission, while the velocity in air applies to the propagation of the signal in the few kilometers of atmosphere it encounters.Terrestrial microwave systems, cellular systems, and all other varieties of terrestrial radio systems all operate in the atmosphere, of course.The signal velocity in copper cable refers to the propagation of electricity in a copper conductor, such as those in a twisted pair cable or a coaxial cable.The signal velocity in polyethylene, polypropylene, and polyvinyl chloride insulating materials is significant as much of the electrical signal energy is in the form of an electromagnetic field that propagates through the insulation surrounding the metallic conductor, rather than traveling through it. The signal velocity in air also is significant as air is introduced into the insulation of some high speed, multi-pair data cables in order to compensate for delay skew, i.e., differences in propagation delay across pairs. Propagation delay is the fundamental factor impacting latency.The inverse of Vp is the index of refraction (IOR), which is used to describe the Vp in a given fiber optic medium relative to the speed of light in a vacuum. See also delay skew, IOR, latency, meter, and vacuum. 74570c22.qxd 9/12/07 12:48 AM Page 528 |
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