Propagation delay is the time duration taken for a signal to reach its destination, for example in the electromagnetic field, a wire, gas, fluid or solid body.

Physics

  • An electromagnetic wave travelling through a medium has a propagation delay determined by the speed of light in that particular medium, or ca.(circa/approx) 1 nanosecond per in a vacuum.
  • An electric signal travelling through a wire has a propagation delay of ca. 1 nanosecond per .

See also radio propagation, velocity factor, signal velocity and mechanical wave.

Electronics

thumb|400px|Propagation delay [[Digital timing diagram|timing diagram of a NOT gate]]

thumb|400px|A [[full adder has an overall gate delay of 3 logic gates from the inputs A and B to the carry output C<sub>out</sub> shown in red.]]

Logic gates can have a gate delay ranging from picoseconds to more than 10 nanoseconds, depending on the technology being used.

  • Increases in output load capacitance, often from placing increased fan-out loads on a wire, will also increase propagation delay.

All of these factors influence each other through an RC time constant: any increase in load capacitance increases C, heat-induced resistance the R factor, and supply threshold voltage increases will affect whether more than one time constants are required to reach the threshold. If the output of a logic gate is connected to a long trace or used to drive many other gates (high fanout) the propagation delay increases substantially.

Networking

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. It can be computed as the ratio between the link length and the propagation speed over the specific medium.

Propagation delay is equal to 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.

See also

  • Contamination delay
  • Delay calculation
  • Latency (engineering)
  • Time of flight
  • Transmission delay

References