Propagation delay

Propagation delay is a technical term that can have a different meaning depending on the context. It can relate to networking, electronics or physics. In general it is the length of time taken for the quantity of interest to reach its destination.

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 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 wires, the speed s is typically about 67% av of speed of light. This delay is the major obstacle in the development of high-speed computers and called interconnect bottleneck in IC systems.

Electronics

A full adder has an overall gate delay of 3 logic gates from thie inputs A and B to the carry output C_out shown in red

In electronics, digital circuits and digital electronics, the propagation delay, or gate delay, is the length of time starting from when the input to a logic gate becomes stable and valid, to the time that the output of that logic gate is stable and valid. Often this refers to the time required for the output to reach 80% of its final output level when the input changes. Reducing gate delays in digital circuits allows them to process data at a faster rate and improve overall performance.

The difference in propagation delays of logic elements is the major contributor to glitches in asynchronous circuits as a result of race conditions.

The principle of logical effort utilizes propagation delays to compare designs implementing the same logical statement.

Propagation delay increases with operating temperature, marginal supply voltage as well as an increased output load capacitance. The latter is the largest contributor to the increase of propagation delay. 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.

Wires have an approximate propagation delay of 1 ns for every 6 in of length.^[1] Logic gates can have propagation delays ranging from more than 10 ns down to the picosecond range, depending on the technology being used.^[1]

Physics

In physics, particularly in the electromagnetism field, the propagation delay is the length of time it takes for a signal to travel to its destination. For example, in the case of an electric signal, it is the time taken for the signal to travel through a wire. See also, velocity of propagation.

See also

• Delay calculation
• Contamination delay
• Transmission delay

References