US Patent 7355416 Measurement of cable quality by power over ethernet

A method of determining impedance comprising: supplying power to a powered device from a power sourcing equipment at a first current limited level, Ilim1; measuring, at a plurality of times a voltage associated with the output of the power sourcing equipment; determining a minimum voltage, Vmin1, of the measured plurality of voltages; determining an associated time of the determined Vmin1; removing the supplied power from the powered device; subsequent to the removing, supplying power to the powered device from the power sourcing equipment at a second current limited level, Ilim2, the Ilim2 being different than the Ilim1; measuring, at the determined associated time in relation to the beginning of the supplying power at the Ilim2, a voltage associated with the output of the power sourcing equipment, Vmin2; and determining an impedance responsive to the Vmin1, Vmin2, Ilim1 and Ilim2.