Normal Velocity of Propagation (NVP)

NVP is the speed of a signal through a cable relative to the speed of light.  In a vacuum, electrical signals travel at the speed of light.  In a cable, signals travel slower than the speed of light.  Typically, the speed of an electrical signal in a cable is between 60% and 80% of the speed of light. 

NVP values affect the limits on cable length for Ethernet systems because Ethernet operation depends on the system's ability to detect collisions in a specified amount of time.  If a cable's NVP is too low or the cable is too long, signals are delayed and the system cannot detect collisions soon enough to prevent serious problems in the network.