I'm reading Computer Networks – A Systems Approach 5th ed., and I came across the following statistics for the speed of light through different mediums:
Copper – 2.3 × 108 m/s
Fiber – 2.0 × 108 m/s
So, are these figures wrong, or is there another reason to explain why copper is worse than fiber? Does fiber have better bandwidth (per volume) or something?
Best Answer
No, the numbers are right (Page 46). If I can reword your question, it's "Why should I use fiber if the propagation delay is worse than copper?" You are assuming that propagation delay is an important characteristic. In fact (as you'll see a few pages later), it rarely is.
Fiber has three characteristics that make it superior to copper in many (but not all) scenarios.
Higher bandwidth. Because fiber uses light, it can be modulated at a much higher frequency than electrical signals on copper wire, giving you a much higher bandwidth. Also the maximum modulation frequency on copper wire is highly dependent on the length -- inductance and capacitance increase with length, reducing the maximum modulation frequency.
Longer distance. Light over fiber can travel tens of kilometers with little attenuation, which makes it ideal for long distance connections.
Less interference. Because fiber uses light, it is impervious to electromagnetic interference. That makes it best for "noisy" electromagnetic environments.
Electrical isolation. Fiber does not conduct electricity, so it can electrically isolate devices.
But fiber has drawbacks too.
Expense. The optical transmitters and receivers can be expensive ($100's) and have more stringent environmental requirements than copper wire.
Fiber optic cable is more fragile than wire. If you bend it too sharply, it will fracture. Copper wire is much more tolerant of movement and bending.
Difficult to terminate. Placing a connector on a optical fiber strand requires precision tools, technique, and expertise. Fiber cables are usually terminated by trained specialists. In comparison, you can terminate a copper cable in seconds with little or no training.