I am reading Computer Networks by Tanenbaum and the the chapter about the physical link and there is an example that I don't understand.
The author states that given a bit rate of b bits/sec the time required to send 8 bits is 8/b bits/sec. thus the frequency of the first harmonic is 8/b. Then there is an example of a telephone line which have a cutoff at 3000 Hz and claims that the highest harmonic passed is 3000 /(b/8) = 24000/b.
An Example is given for a 300 Bps, the transmission time is 26.67 msec, the first harmonic is 37.5 Hz and the number of harmonics sent is 80.
I understand that a real signal is finite and we can regard it as having a period of some T. I noted that the first harmonic is at 1/T.
I have a few questions:
When sending a stream of data, do we do modulation for some of the bits up to a point and this is the signal we calculate its period etc' ? we can't read all the data to transmit before transmission.
I get that we can reconstruct the signal from the Fourier coefficients. I didn't understand the relation between harmonics and coefficients (does sending k harmonics means sending k coefficients ?)
I don't understand the calculation that was made to get that we can send 80 harmonics in the above example, why does it taking the bandwidth and dividing it to the frequency of the first harmonic ?
Is it possible or wished for that the harmonics sent would not be the first harmonic and all of its multiples up to a certain point ? e.g sending the first harmonic and sending the third one but not the second one.
I'd appreciate an answer to any of those questions, I have studied Fourier analysis but with no relation to signals and real world applications so I'm having difficulties putting theory to use.