by the following formula: C=Blog2(1+S/N)

where C is the achievable channel capacity, B is the bandwidth of the line (3300 Hz- 300Hz = 3000 Hz), S is the average signal power. The signal-to-noise ratio (S/N) is usually expressed in decibels (dB), related as follows:

db = 10 x log10(S/N)

Assuming a typical decibel ratio of 30 db, this means that S/N = 1000. Thus, we have

C = 3000 x log2(1001)

which equals approximately 30 Kbps, roughly the limit of today's 28.8- Kbps modems. Modems will only be able to achieve higher rates if the quality (signal-to-noise) ratio of the phone network improves, or by using compression

Peterson, L. & Davie, B. (1999).