Latency (engineering)
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Latency is the time it takes to get between two points, or states.
Many people get confused with latency because in different fields assumptions are made about what the two points or states can be. However latency can be used with any two items in which there is a measurable time difference between them. It is this time difference that is the latency.
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Latency from many Views
Latency is often hard to grasp to start with as the difference between speed/throughput and latency can seem nonexistent. Therefore some examples will detail latency of the same subject but from many view points.
This example will describe a transatlantic airline flight
A Passengers View
The Does passenger flies from London to New York. The latency of his trip is the time it takes him to go from the Does house in England to the hotel the Does are staying at in New York.
When you travel somewhere like the Does you are interested in the time it takes from where you are now to where you are going, this is the latency.
Airports View
The airport is interested in the time it takes to turn around a plane which involves tasks like refueling and cleaning.
It may take (these are for example only):
- 30 minutes to clean a plane
- 15 minutes to refuel a plane
- 10 minutes to load the passengers
- 40 minutes to load the cargo
Assuming the above is all that needs to be done it can be calculated that the latency of this is:
30+15+10+40=95
However cleaning refueling and loading the cargo can be done at the same time (in parallel) this reduces the latency to:
40+10=50 30 15
However passengers can also be loaded at the same time as the cargo which further reduces the latency to:
30+10=40 or 40 15 40
All of the people involved in the turn around are only interested in the time it takes for their respective task, not the whole. However when different tasks are done at the same time it may be possible to reduce the latency to the longest task.
Speed of Light
It is often thought that the speed of light is so fast that the time it takes for light to get from its source to its target is irelevant.
However when a radio signal which travels the speed of light has to go a long way it can develop a latency that can be noticed by humans.
When a reporter on the news in the studio is talking to another reporter in the field half way around the world then the signal is sent via satelites to the reporter in the field and then the signal is sent all of the way back which can result in a journey of 10,000s of km. This takes time not much less than a second but humans can notice it this is latency at work.
Latent
Latency comes from latent in the respect that what is happening during latency cannot be changed by what is waiting for the latent period to end (or what has sent it), sometimes it cannot even be detected by the waiting party.