In 1944 Claude Shannon introduce a formula to determine the
theoretical highest data rate for a a channel

C=B log 2(1+S/n)

In this formula, B is the bandwidth of the channel. S/N is
the signal to noice ratio and C is the capacity (Shannon Capacity) in BPS.

We can calculate the theoretical highest bit rate of regular
telephone.

A Telephone line normally has a bandwidth of 3000Hz. The signal
to noise ratio is usually 3162-(35db) for this channel capacity is

C=B log

_{2}(1+S/N)
C=3000 log

_{2}^{ (}(1+3162)
C=3000 log

_{2}(3163)
C=3000*1162

C=34860 BPS