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'''Propagation delay''' is a technical term that can have a different meaning depending on the context. It can relate to [[Computer network|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 network]]s, '''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 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.<ref name="Ethernet propagation delay">{{ cite web | url = http://stason.org/TULARC/networking/lans-ethernet/3-11-What-is-propagation-delay-Ethernet-Physical-Layer.html | title=What is propagation delay? (Ethernet Physical Layer) | accessdate = 2010-11-09 | date = 2010-10-21 | work = Ethernet FAQ}}</ref><ref name="Propagation Delay">{{ cite web | url = http://www.wildpackets.com/resources/compendium/ethernet/propagation_delay | title=Propagation Delay and Its Relationship to Maximum Cable Length | accessdate = 2010-11-09 | work = Networking Glossary}}</ref> This delay is the major obstacle in the development of high-speed computers and is called the [[interconnect bottleneck]] in IC systems.

==Electronics==
[[File:Full-Adder Propagation Delay.svg|thumb|400px|A [[full adder]] has an overall gate delay of 3 [[logic gate]]s from the inputs ''A'' and ''B'' to the carry output ''C''<sub>out</sub> shown in red]]

In [[electronics]], [[digital circuit]]s 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 from 10% to 90% of its final output level when the input changes. Reducing gate delays in [[digital circuit]]s 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 [[glitch]]es in [[asynchronous circuit]]s as a result of [[race condition]]s.

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.<ref name='Complete digital design'>{{Cite book | last = Balch | first = Mark | authorlink = | coauthors = | title = Mcgraw Hill - Complete Digital Design A Comprehensive Guide To Digital Electronics And Computer System Architecture | publisher = McGraw-Hill Professional | year = 2003 | location = | pages = 430 | url = http://books.google.co.uk/books?id=uFSRT-OIxyoC&printsec=frontcover&cad=0#v=onepage&q=&f=false | doi =  | isbn = 9780071409278 }}</ref> Logic gates can have propagation delays ranging from more than 10 ns down to the picosecond range, depending on the technology being used.<ref name='Complete digital design'/>

==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]]
* [[Latency (engineering)]]

==References==
{{Reflist}}

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[[Category:Digital circuits]]
[[Category:Digital electronics]]
[[Category:Timing in electronic circuits]]

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