A method and system for transmitting data through a transmission medium in a wellbore. A surface transceiver and a subsurface transceiver are each configured to selectively transmit over a plurality of transmission channels. The surface transceiver is adapted to initialize a channel to determine at least one parameter affecting the current maximum data rate for transmission of information through the channel using a given transmission methodology. At least one of the channels uses carrierless phase/amplitude (CAP) modulation for its transmission methodology. The surface transceiver and the subsurface transceiver adapt to cooperatively periodically test a channel by transmission of a calibration signal from the subsurface transceiver to the surface transceiver. Control circuitry compares the received calibration signal with the calibration signal, responding to the result of the comparison to adjust at least one parameter as necessary to achieve an optimum rate of data transmission through said channel.