Abstract: A telephone line test system that may be used to compute line length of subscriber lines connected to a central office or other distribution point. When initially installed, the test system may automatically compute CO limits. The CO limits are computed from single-ended measurements on subscriber lines and represent electrical effects of the path between a test head and the distribution point. These CO limits are used to more accurately compute the length of lines connected to the distribution point using measurements made by the test system. Because the process uses single-ended measurements, it may be run in background mode during the initial installation of the test system or at any subsequent time it is desired to update the CO limits.