  Ideal dense coding protocols allow one to use prior maximal entanglement to
send two bits of classical information by the physical transfer of a single
encoded qubit. We investigate the case when the prior entanglement is not
maximal and the initial state of the entangled pair of qubits being used for
the dense coding is a mixed state. We find upper and lower bounds on the
capability to do dense coding in terms of the various measures of entanglement
and in terms of the average mutual distinguishability of the signal states.
