1. In a multicarrier data transmission system that transmits data continuously over dispersive, noisy subscriber loops for digital service from a telephone central office to a user location and corresponding digital service between transmitters and receivers at said user location and said central office, a method for implementing variable transmission bandwidth as a function of line frequency-variable attenuation, noise power spectrum, a set of programmed and possibly variable carrier target bit-error-rates, and a programmed and possibly frequency-variable transmit power mask to improve data transmission speed or to improve performance margin at any given transmission speed subject to a power or power-spectral-density constraint, said method using subcarrier-indexed measurements of channel gains, channel noises, desired carrier bit-error-rates, and the programmed power mask to provide subcarrier-indexed estimates of transmission quality and to maintain the transmission quality at high levels, comprising the steps of: (a) sorting the subcarrier-indexed estimates of the transmission quality, scaled by the desired subcarrier bit-error-rates, into an invertible ordering for assessment of the relative data-carrying capabilities of the subcarriers at initialization and/or during data transmission; (b) calculating bit and energy allocation tables for said multicarrier data transmission system based on the sorted subcarrier-indexed estimates for either improving aggregate transmitted data rate at fixed performance margin with said power or power-spectral-density constraint or improving performance margin at a fixed data rate with said power or power-spectral-density constraint; (c) communicating said bit and energy allocation tables between the transmitters and the receivers of said multicarrier transmission system; and (d) implementing said bit and energy allocation tables in coordination between the transmitters and the receivers during initialization of said multicarrier transmission system and/or during simultaneous transmission of said digital data by said multicarrier transmission system.