In a transmission line, an echo propagating in the same direction as the original wave and consisting of energy reflected back by one discontinuity and then forward again by another discontinuity. Note: Forward echoes can be supported by reflections caused by splices or other discontinuities in the transmission medium (e.g., optical fiber, twisted pair, or coaxial tube. ) In metallic lines, they may be supported by impedance mismatches between the source or load and the characteristic impedance of the transmission medium.