When signals are transmitted over a physical transmission medium, the signal propag ation delay is longer than that of light and depends on the nature, structure and transmission parameters of the medium. The Nominal Velocity of Propagation (NVP) indicates the delayed propagation time in physical media compared to the speed of light ( c) in vacuum.
The NVP value is a dimensionless characteristic value for the signal propagation speed. It is also referred to as the shortening factor (VKF) and corresponds to the signal velocity relative to the speed of light in a vacuum. The NVP value is calculated from the ratio of the signal propagation time in the transmission medium to the speed of light in free space and is given as a percentage. Typically, phase velocities in cables and optical fibers are between 60 % and 80 % of the speed of light (c). For example, coaxial cables have a shortening factor of 0.77, TP cables have a shortening factor between 0.65 and 0.80, and optical fibers(FOC) have a shortening factor of 0.67. Thus, the transit time in coaxial cables is only 77% of the speed of light, and that of optical fibers is only 67%. The signals therefore take correspondingly longer to travel from the beginning to the end of the cable.
In twisted pair cables, the signal propagation speed is influenced by various factors such as the lay length and the stranding, the dielectric, the length of the cable sheath and the copper core. Metrologically, the shortening factor can be determined using time domain reflectometry(TDR). For this purpose, the TDR device sends a short pulse to the cable to be measured, which is used to measure the transit time in the cable. The quotient of the measured transit time and the speed of light is the shortening factor.
The nominal propagation velocity influences the limit values for cable lengths in runtime-critical cable systems such as Ethernet. If the NVP value is too low or the cables are too long, the signals are delayed and collisions are not detected in time. It is also critical if pairs of wires in a TP cable have different propagation speeds, resulting in minimal differences in skew.