Signal Propagation: Is copper faster than fiber?
Network World Fusion Focus on High Speed LANs
By John Curtis
Network World, 01/11/99
As new technologies further define high performance networking, engineers and network administrators alike are taking a fresh look at cabling to further enhance overall network performance. Today's cables carry data signals at more than 50% of the speed of light in a vacuum, but there are measurable speed differences among cable types. Do such differences have a significant impact in overall network performance? Should they be considered when choosing between fiber and copper cable? Let's take a closer look.
Today's leading fiber cables from vendors such as Corning, AT&T Network Cable Systems and Spectran Specialty, propagate signals along the cable at approximately two thirds the speed of light in a vacuum. (In technical jargon, they have an index of refraction, denoted by the Greek letter Eta, of approximately 1.5.) Today's Category 5 copper cables, by contrast, actually propagate signals faster than fiber -- approximately 72% of the speed of light in a vacuum (or 10% faster than fiber).
To determine the real impact of this difference on network traffic, we need to do a little math. If we compare the time required to send data across 100 meters of fiber vs. copper cable, the copper transmission will arrive less than one tenth of one microsecond sooner - virtually undetectable by even the best network analyzers. Even if we were to stretch the distance to 20 kilometers, well beyond what's allowed for Category 5 copper, the difference in cable speeds still amounts to less than one microsecond.
All of this means that, from the perspective of overall network performance, the propagation delays of fiber and copper cable are effectively identical. Clearly, issues such as cost and bandwidth remain key factors in choosing between the two technologies, but customers can comfortably ignore propagation delay as a performance metric in overall network performance. |
