Solution#2.9
The Nyquist Theorem is a mathematical property and has nothing to do with technology. It says that if you have a function whose Fourier spectrum does not contain any harmonic component above f, then by sampling the function at 2f you capture all the information there is. Thus the Nyquist theorem is true for all media. So basically Nyquist theorem is for signals and not for the media.
Solution#2.14
Each telephone makes 4 calls / 8 hours means 0.5 call/hour. Since each call is for 6 minutes a telephone occupies a circuit for 3 minutes/hour. Twenty telephones can share a circuit. Since 10 % of the calls are long distance, it takes 200 telephones to occupy a long distance circuit full time. The interoffice trunk has 1,000,000/4000 = 250 circuits multiplexed onto it. With 200 telephones per circuit, an end office can support 200 x 250 = 50,000 telephones.
Solution#2.20
Ignoring speech compression, digital PCM telephone needs 8000 samples/sec that is 64 kbps. So the number of houses per cable = 10 x 109/64 x 103 = 156,250.
Solution#2.21
It's TDM as well as FDM. Each of the 100 channels is assigned its own fequency band (FDM) and on each channel the two logical streams are intermixed by TDM. But this alternation is irregular.
Solution#2.26
A coder accepts an arbitrary analog signal and generates a digital signal from it. A demodulator accepts a modulated sine wave only, and generates a digital signal.