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?
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.
I would like to add one benefit with fiber connections. Consider a connection between two buildings with different ground potential. If you where to use copper in this situation you could end up with current leakage and possibly a dangerous situation. This is not the case with fiber because it isn't a conductor.
The propagation speed is often expressed as the velocity factor of a medium - the fraction of the speed of light in vacuum that you get.
On the physical side, light going through a medium is slowed by the medium depending on its refractive index. Fiber has the added 'problem' that the core requires a slightly higher refractive index (optical density) than the cladding to properly guide the wave. The effective propagation speed is the speed of light divided by the refractive index, or the velocity factor is the reciprocal of the refractive index. Most fibers have a velocity factor of or close to .67.
Copper is a bit more complicated. The actual electrons are not moving substantially, a high-frequency signal is rather an electric wave (field fluctuation) flowing through the cable - somewhat comparable to sound in air. The propagation speed of this wave surprisingly does not depend on the conductor alone but on the combination of the conductor and especially the insulator (its permittivity) because the wave needs to propagate through the latter as well. The effective propagation speed is the speed of light divided by the square root of the permittivity.
For copper, a velocity factor of close to 1.00 is possible by using air as insulation as with special coax cables or open ladder cables. Copper network cables range from .77 (RG-8 for ancient 10BASE5) to .585 (Cat-3 for 10BASE-T) with the common Cat-5e and Cat-6 at .65 (=slower than fiber).
As has been pointed out, in practice, there are lot of other factors contributing to the effective propagation delay such as transceiver technology, encoding overhead, forward error correction and possibly retransmissions. Especially the encoding overhead can vary significantly from any-length fiber (<.1 μs delay) to voice-grade twisted-pair copper using interleaved ADSL (~20 ms = 20,000 μs delay).
So, the propagation speed or the velocity factor isn't usually critical.
As to fiber "being better" - it's got higher performance for sure, but "better" depends on your requirements, including cost.
I believe another reason fiber propagates "slower" than copper is because the light is, by definition, refracting across the fiber along the distance of the cable. The physics stack exchange has a different take on this:
It depends on distance and number of transactions/users
Copper is better than fiber over short distances (under 10 meters) where bandwidth requirements are currently under 40 Gbit per second. At higher rates and distances the packet loss rate rises to over 50% quite rapidly. To correct this requires repeaters which rapidly raise the latency, and cost of connections.
Even a 10% loss will cause at least 1% of end users to perceive up to 10x latency increase.
Fiber is better than copper where the bandwidth is over 100G and distance over 1 Kilometer and number of users per network exceeds 1,000.
Both Fiber and Copper have massively lower cost expansion capability than wireless, but for many of the domains between copper and fiber, wireless is a rapidly deploy able, unreliable, ever degrading connection medium.
Any mechanism that can be used to expand wireless bandwidth, can almost always be used to immediately expand copper and fiber bandwidth.
The speed of light and the speed of the electromagnetic waves all travel at the speed of light in a vacuum. In cable, all of these waves travel at about 67 to 95% of the speed of light. If I could send something down some type of cable at twice the speed of light it would not make any difference to network speed because the rate at which data is sent is much slower than the speed of light. The reason that fiber is better is because the losses are less. In copper, the signal heats up the cable and the power level drops much more than the light in fiber gets dim. If you are out at sea in a really bad storm, would you rather look for a light house, or hope your gps picks up enough sattelites to know where the rocky shore is? Also, in copper, the current does not flow smooth, and electric noise is generated. When the signal strength drops down to the noise level, bits that should be a 1 start to look like 0's, and 0's start to look like 1's. This effect is much less in fiber. If either the data, or the error correction code that was associated with the data gets corrupted, the network requests the data to be re sent. This takes a really long time. Fiber is better because the signal strength is better and there are less errors.
