The paper deals with the state estimation problem for nonlinear dynamical systems via communication channels with limited data rate. We introduce several minimum data-rate limits associated with various types of observability. A notion of the restoration entropy (RE) is also introduced and its relevance to the problem is outlined by a corresponding Data Rate Theorem. Theoretical lower and upper estimates for the RE are proposed in the spirit of the first and second Lyapunov methods, respectively. For three classic chaotic multi-dimensional systems, it is demonstrated that the lower and upper estimates for the RE coincide for one of them and are nearly the same for the others.