Provided is a method of determining a spectrum management of digital communication systems having a plurality of communication lines by determination of the power levels within each band, for each user, assuming a predetermined maximum interference from other users. The spectral management center has a power allocation determinator for receiving a modelled power level and a noise weight from each user communication line and is able to determine allocated power of its respective communication line based on the optimised determined power needs of the plurality of communication lines of the digital communication systems. In one form the calculations are undertaken in the SMC. In another form the master is undertaken in the SMC while the slave is undertaken at the user's modem and the power level of an individual communication line and its interference by adjacent lines is determined at the user's modem and communicated to the spectral management center.