In physics, the measure of resistance to the flow of electric current. Impedance to signal flow within a transmission line has three components: a resistive component, a capacitative component, and an inductive component.
The value for each component varies with the frequency of the current, which means that the overall impedance of a transmission line also varies with frequency. A perfect transmission would have an impedance that does not change with frequency.
Characteristic impedance is the measure of resistance of a transmission line (such as a cable) calculated with the assumption that the cable is of infinite length. It is represented by the symbol Z0 . Each type of network cabling has its own characteristic impedance. Twisted-pair cabling can have a relatively constant impedance by virtue of its design and dimensional characteristics. The EIA/TIA wiring standards, specifically EIA/TIA 568-A (Commercial Building Telecommunications Cabling Standard), mandate that category 5 cabling should have an impedance of 100 ohms, plus or minus 15 percent, up to a frequency of 100 MHz.
It is important that cabling meet these standards because networking equipment, such as hubs, switches, and routers, is designed to match this impedance value.
If wiring with a different impedance is used with such equipment, reflections of signals can occur that can distort signals, create signal loss, and degrade network communications - or even render them impossible.
Impedance is an issue only with copper cabling and is not a relevant physical characteristic for fiber-optic cabling.