A method of measuring the performance of a DSL line provides an indication of potential faults in the line. Analysis of broadband DSL is less straightforward than for other technologies, since “normal” behavior depends on a number of factors, including line length and line quality, where normal behavior for a long line could be regarded as poor for a short line. The data transmission characteristics of a DSL line under test are measured with reference to its upstream line attenuation, then the results are compared with the model for the corresponding data transmission characteristic. A performance measure is calculated for each characteristic indicative of the difference between the measured characteristic and the observed ‘average’ for other lines having the same upstream line attenuation. A final line performance indicator is calculated based on the combination of all the individual performance measures, which provides a quantitative measure of a line's performance.