How to determine the capacity of a digital communication channel?

Question

Computer networks

Hello! There is a task:
A telephone conversation requires transmission in the 4 kHz frequency band. What is the capacity of a digital communication channel at 256 signal quantization levels?
How to calculate it? At best, I found the most similar formulas on the Internet I = B * t (Hartley's law and I = B * log2 (1 + S / N) (Shannon's formula), but why then is the number of signal levels given in the task?

Answer 1

256 quantization levels is a hint that 8 bits are needed for one signal sample, since 2^8=256.
Next, we are asked to keep the 4 kHz bandwidth. According to the audiophile theorem (Kotelnikov), we need a sampling rate of 8 kHz.
We multiply the sample rate by the length of one sample in bits, we get 8 [bits] * 8000 [1/s] = 64,000 bps.