# signal propagation speed

Electrical signals propagate at the speed of light. Within metallic cables, the signals are subject to capacitive and inductive influences that counteract propagation at the speed of light, which means that the signal propagation speed is lower than that of light. The same applies to optical fibers, where dispersion reduces the propagation speed.

The reduction in propagation speed is caused by the shortening factor, Nominal Velocity of Propagation( NVP). This indicates by what percentage a distance is shortened that is traversed by a signal within a given time interval with respect to the vacuum. The shortening factor for coaxial cables is 0.77, for TP cables 0.6, and for fiber optic cables(FOC) 0.67. This means that the signal propagation time in coaxial cables is 77% of the speed of light, in TP cables it is 60%, and in fiber optic cables, where the propagation velocity for different wavelengths is expressed in group velocity, it is 67%.