A method for remotely detecting electromagnetic interference on a data transmission line of a communication network includes measuring at a client site a noise power spectrum on the data transmission line as a function of frequency, for a frequency range. The noise is present when the data transmission line is not carrying a data signal. The method further includes forwarding the measured noise power spectrum to a remote computer at a network operator's site, determining periodicities in the measured power spectrum by the remote computer, and determining the switching frequency of a power supply unit at the client site by the remote computer based on periodicities in the power spectrum.