A measure of the ratio of signal power transmitted into a system to the power reflected or returned. It can be thought of as an echo that is reflected back by impedance changes in the system. Any variation in impedance from the source results in some returned signal. Real-life cabling systems do not have perfect impedance structure and matching, and therefore have a measurable return loss. Twisted pairs are not completely uniform in impedance. Changes in twist, distance between conductors, cabling handling, cable structure, length of link, patch cord variation, varying copper diameter, dielectric composition and thickness variations, and other factors all contribute to slight variations in cable impedance. In addition, not all connecting hardware components in a link may have equal impedance. At every connection point there is the potential for a change in impedance. Each change in the impedance of the link causes part of the signal to be reflected back to the source. Return loss is a measure of all the reflected energy caused by variations in impedance of a link relative to a source impedance of 100 ohms. Each impedance change contributes to signal loss (attenuation) and directly causes return loss.