Method and system for determining data transfer margins and/or bit rates for network connections, the physical length of a network connection between a transmitter and receiver being known. A power spectrum is measured depending upon transfer frequency for possible modem types. Attenuation, and thus actual signal strength, is determined for different physical lengths and cable wire thicknesses of the network connection. Noise level is determined at least depending upon crosstalk parameters, number of interference sources, and equalizer factors based on the power spectrum. The data transfer margins are determined for different data transmission modulations and/or modulation codings for a pre-defined bit rate, based on the actual signal strengths and corresponding noise levels, by a Gaussian transformation module. The data transfer margins are corrected by a correction factor containing average deviation of the stored data transfer margin with respect to the actual data transfer margin. The data transfer margin for the network connection is determined based on the stored actual data transfer margins with reference to the known physical length.