In computer networks, propagation delay is the amount of time it takes for the head of the signal to travel from the sender to the receiver over a medium.  It can be computed as the ratio between the link length and the propagation speed over the specific medium. Propagation delay = d / s where d is the distance and s is the wave propagation speed.  In wireless communication, s=c, i.e. the speed of light. In copper wire, the speed s generally ranges from .59c to .77c.This delay is the major obstacle in the development of high-speed computers and is called the interconnect bottleneck in IC systems.