This question refers to the following excerpt from Computer Networks by Andrew Tanenbaum:
The amount of information that an electromagnetic wave can carry is related to its bandwidth. With current technology, it is possible to encode a few bits per Hertz at low frequencies, but often as many as 8 at high frequencies, so a coaxial cable with a 750 MHz bandwidth can carry several gigabits/sec. From Fig. 2-11 it should now be obvious why networking people like fiber optics so much.
Fig. 2-11 basically just shows the electromagnetic spectrum with "Fiber optics" spanning from \$10^{14}\$ to \$10^{15}\$ Hz, i.e. at substantially higher frequencies than other media (like twisted pair, coax or FM radio).
I understand that a signal's bandwidth corresponds to the "speed" at which it can be modulated, thereby influencing the amount of information that it can transmit. But why does the absolute value of the carrier frequency matter in this case? Why, for example, is a frequency band of 1 kHz more "valuable" if it is located around 100 MHz rather than around 500 Hz (i.e. the baseband)? Wouldn't this also mean that, for example, using a blue instead of a red laser as optical transmitter results in an increased transmission rate?
Best Answer
Tannenbaum is alluding to secondary effects, such as intersymbol interference, that reduce the effective signal-to-noise ratio.
Intersymbol interference is created by nonlinearities in the phase/frequency response of the channel. These irregularities are generally proportional to the carrier frequency — for example, if a channel centered at 1 MHz has a quality factor (Q) of 50, it will have a 3-dB bandwidth of 20 kHz. But a similar channel centered at 100 MHz will have a bandwidth of 2 MHz.
If you need a bandwidth of 20 kHz for your signal, the 100-MHz channel will have a flatter response over any 20 kHz segment than the 1-MHz channel, reducing the phase distortion.
This is just one way of thinking about it. In reality, there are many factors that affect the flatness of any given communication channel — and the analog circuitry used to interface to it. It's just that in general, it's easier to keep distortions of this type low if the bandwidth is a smaller fraction of the carrier frequency.