propagation delay

1) Propagation delay, symbolized t_pd, is the time required for a digital signal to travel from the input(s) of a logic gate to the output. It is measured in microseconds (µs), nanoseconds (ns), or picoseconds (ps), where 1 µs = 10^-6 s, 1 ns = 10^-9 s, and 1 ps = 10^-12 s.

The propagation delay for an integrated circuit (IC) logic gate may differ for each of the inputs. If all other factors are held constant, the average propagation delay in a logic gate IC increases as the complexity of the internal circuitry increases. Some IC technologies have interently longer t_pd values than others, and are considered "slower." Propagation delay is important because it has a direct effect on the speed at which a digital device, such as a computer, can operate. This is true of memory chips as well as microprocessors.

2) In a communications system, propagation delay refers to the time lag between the departure of a signal from the source and the arrival of the signal at the destination. This can range from a few nanoseconds or microseconds in local area networks (LANs) up to about 0.25 s in geostationary-satellite communications systems. Additional propagation delays can occur as a result of the time required for packets to make their way through land-based cables and nodes of the Internet.