This tutorial covers the discussion of Nyquist capacity equation. This equation is often used to develop a capacity number for digital signals. Nyquist equation was later modified to include the multi-signalling factor, M. Nyquist capacity is often a achievable starting point.
Then came Shannon, who gave us the expression for the bound on capacity beyond which communication transfer is not possible. For uncoded signals we can come within 7-10 dB of this limit and with coding, we can pretty darn close.
Here is the full tutorial in pdf format.