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Whenever the bandwidth of a medium linking two devices is greater than the bandwidth needs of the devices, the link can be shared. Multiplexing is the set of techniques ...

Forouzan, Data Communications, p. 156

I always knew that bandwidth is always of a signal. Composite signals (made of many simple sine signals) are made up of different frequencies. The difference between the highest and the lowest frequency is the bandwidth of that composite signal. Now, in the above quote, the author talks about the bandwidth of a medium. What does that even mean? How can a medium have a bandwidth? I also do not think that the author is talking about the bandwidth that is measured in bps.

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In the version of the book I found, Forouzan gives a good example:

Modulation is needed if the medium is bandpass in nature or if only a bandpass channel is available to us. An example is radio. The government assigns a narrow bandwidth to each radio station. The analog signal produced by each station is a low-pass signal, all in the same range. To be able to listen to different stations, the low-pass signals need to be shifted, each to a different range.

Can't signals of any bandwidth/frequency pass through any medium?

Well, you can try... but it might not work.

Suppose you have a baseband signal that has a spectrum indicated in the figure below by the orange triangle, $\color{orange}{X(f)}$. Also suppose you want to pass it through the bandpass channel indicated by the light tan rectangles, $\color{brown}{H(f)}$.

The output of trying to do that without modulation is:

$$ Y(f) = \color{orange}{X(f)} \cdot \color{brown}{H(f)} = 0 $$

so nothing sensible comes out of the channel.

That's why we need modulation: to match the bandwidth of the signal with the bandwidth of the channel.

Baseband signal and a bandpass channel.

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  • $\begingroup$ So, different mediums/links/channels also have a bandwidth associated with them? I am just a bit foggy about the mechanism of how a medium/link/channel has a bandwidth. This might be a stupid question, but can't signals of any bandwidth/frequency pass through any medium? $\endgroup$
    – tryingtobeastoic
    Commented Dec 4, 2023 at 12:41
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    $\begingroup$ You should edit your question to reflect this confusion. In the mean time, reflect on the frequency range of signals you might be able to get from point A to point B with two tin cans and a string, with a length of twisted-pair telephone cable, and with a length of high-quality coax cable. $\endgroup$
    – TimWescott
    Commented Dec 4, 2023 at 23:19
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    $\begingroup$ @tryingtobeastoic I've illustrated the example given in the text book. Perhaps that makes things clearer. $\endgroup$
    – Peter K. ♦
    Commented Dec 4, 2023 at 23:23
  • $\begingroup$ I had an additional question. If the baseband signal modulates a carrier signal of a higher bandwidth range, then the bandpass channel will act as a filter to the signal, right? The width of the channel is less than the width of the signal. $\endgroup$
    – tryingtobeastoic
    Commented Dec 6, 2023 at 5:54

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