A system for measuring the transmission characteristic of an xDSL system that implements high-speed data communication over existing copper telephone wires connecting an office and a subscriber includes a polling device included in the outside line of an xDSL circuit, which is installed in an office, for polling a subscriber line. A noise level measuring circuit measures the existing level of cross-talk noise on the subscriber line. A decision circuit determines, based on the existing level of cross-talk noise measured, whether or not the subscriber line is usable.