Impedance
Impedance is the AC resistance of the link. Unlike DC resistance, impedance is a frequency-dependant parameter. Read on to find out more.
Impedance is the frequency-dependent AC resistance of a link. It is a measure of the resistance an electromagnetic wave (the data signal) experiences when travelling along the cable. Depending on the tester, impedance is measured only on one point of the cable (e.g. 4 m after the jack in the patch panel). Impedance is also called pair impedance.
Impedance should be as close as possible to the 100 Ohms specified by the cabling standards and should be the same for all four pairs.

Impedance testing is only rarely called for. Where impedance suddenly changes, e.g. where the wires are connected to a plug or jack or where there is a kink in the cable, a part of the data signal is reflected. This is recorded much better by return loss testing, so impedance testing is of small importance.
Trouble-shooting tips if impedance tests fail
If return loss tests also fail, start with the trouble-shooting of return loss. Very often this solves the impedance problems as well.
Typical cause of the problem | This might help (without engagement) |
Cable has become wet | Let the cable dray and test agein, replace the cable |
Maximum tensile load of the cable was exceeded when the cable was pulled in | Replace the cable |
The cable runs near a powerful heat or cold source | Re-route the cable |
Category of the cable is too low | Select appropriate test program, replace the cable |
Cable defective (material defect) | Replace the cable |