Latency, from a general point of view, is a time delay between the
cause and the effect of some physical change in the
system being observed.
Lag, as it is known in
gaming circles, refers to the latency between the input to a simulation and the visual or auditory response, often occurring because of
network delay in online games.
Latency is physically a consequence of the limited velocity at which any
physical interaction can propagate. The magnitude of this velocity is always less than or equal to the
speed of light
The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit fo ...
. Therefore, every physical system with any physical separation (distance) between cause and effect will experience some sort of latency, regardless of the nature of the stimulation at which it has been exposed to.
The precise definition of latency depends on the system being observed or the nature of the simulation. In
communications
Communication (from la, communicare, meaning "to share" or "to be in relation with") is usually defined as the transmission of information. The term may also refer to the message communicated through such transmissions or the field of inqui ...
, the lower limit of latency is determined by the
medium being used to transfer information. In
reliable two-way communication systems, latency limits the maximum rate that information can be transmitted, as there is often a limit on the amount of information that is "in-flight" at any given moment. Perceptible latency has a strong effect on
user satisfaction
Customer satisfaction (often abbreviated as CSAT) is a term frequently used in marketing. It is a measure of how products and services supplied by a company meet or surpass customer expectation. Customer satisfaction is defined as "the number of c ...
and
usability in the field of
human–machine interaction.
Communications
Online games are sensitive to latency (or "
lag"), since fast response times to new events occurring during a game session are rewarded while slow response times may carry penalties. Due to a delay in transmission of game events, a player with a high latency internet connection may show slow responses in spite of appropriate
reaction time. This gives players with low latency connections a technical advantage.
Capital markets
Minimizing latency is of interest in the
capital markets, particularly where
algorithmic trading is used to process market updates and turn around orders within milliseconds. Low-latency trading occurs on the networks used by financial institutions to connect to stock exchanges and electronic communication networks (ECNs) to execute financial transactions. Joel Hasbrouck and Gideon Saar (2011) measure latency based on three components: the time it takes for information to reach the trader, execution of the trader's algorithms to analyze the information and decide a course of action, and the generated action to reach the exchange and get implemented. Hasbrouck and Saar contrast this with the way in which latencies are measured by many trading venues who use much more narrow definitions, such as, the processing delay measured from the entry of the order (at the vendor's computer) to the transmission of an acknowledgement (from the vendor's computer). Electronic trading now makes up 60% to 70% of the daily volume on the
New York Stock Exchange
The New York Stock Exchange (NYSE, nicknamed "The Big Board") is an American stock exchange in the Financial District of Lower Manhattan in New York City. It is by far the world's largest stock exchange by market capitalization of its listed ...
and algorithmic trading close to 35%. Trading using computers has developed to the point where millisecond improvements in network speeds offer a competitive advantage for financial institutions.
Packet-switched networks
Network latency in a
packet-switched network is measured as either
one-way
One-way or one way may refer to:
*One-way traffic, a street either facilitating only one-way traffic, or designed to direct vehicles to move in one direction
*One-way travel, a trip that does not return to its origin
Music
*One Way (American ban ...
(the time from the source sending a packet to the destination receiving it), or
round-trip delay time (the one-way latency from source to destination plus the one-way latency from the destination back to the source). Round-trip latency is more often quoted, because it can be measured from a single point. Note that round trip latency excludes the amount of time that a destination system spends processing the packet. Many software platforms provide a service called
ping that can be used to measure round-trip latency. Ping uses the
Internet Control Message Protocol
The Internet Control Message Protocol (ICMP) is a supporting protocol in the Internet protocol suite. It is used by network devices, including routers, to send error messages and operational information indicating success or failure when commu ...
(ICMP) ''echo request'' which causes the recipient to send the received packet as an immediate response, thus it provides a rough way of measuring round-trip delay time. Ping cannot perform accurate measurements, principally because ICMP is intended only for diagnostic or control purposes, and differs from real communication protocols such as
TCP
TCP may refer to:
Science and technology
* Transformer coupled plasma
* Tool Center Point, see Robot end effector
Computing
* Transmission Control Protocol, a fundamental Internet standard
* Telephony control protocol, a Bluetooth communication s ...
. Furthermore,
routers and
internet service provider
An Internet service provider (ISP) is an organization that provides services for accessing, using, or participating in the Internet. ISPs can be organized in various forms, such as commercial, community-owned, non-profit, or otherwise privatel ...
s might apply different
traffic shaping policies to different protocols.
For more accurate measurements it is better to use specific software, for example:
hping,
Netperf or
Iperf.
However, in a non-trivial network, a typical packet will be forwarded over multiple links and gateways, each of which will not begin to forward the packet until it has been completely received. In such a network, the minimal latency is the sum of the transmission delay of each link, plus the forwarding latency of each gateway. In practice, minimal latency also includes queuing and processing delays.
Queuing delay occurs when a gateway receives multiple packets from different sources heading towards the same destination. Since typically only one packet can be transmitted at a time, some of the packets must queue for transmission, incurring additional delay.
Processing delays are incurred while a gateway determines what to do with a newly received packet.
Bufferbloat can also cause increased latency that is an order of magnitude or more. The combination of propagation, serialization, queuing, and processing delays often produces a complex and variable network latency profile.
Latency limits total
throughput in reliable two-way communication systems as described by the
bandwidth-delay product.
Fiber optics
Latency in
optical fiber
An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparency and translucency, transparent fiber made by Drawing (manufacturing), drawing glass (silica) or plastic to a diameter slightly thicker than that of a Hair ...
is largely a function of the
speed of light
The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit fo ...
, which is 299,792,458 meters/second in vacuum. This would equate to a latency of 3.33
µs for every kilometer of path length. The
index of refraction of most fiber optic cables is about 1.5, meaning that light travels about 1.5 times as fast in a vacuum as it does in the cable. This works out to about 5.0 µs of latency for every kilometer. In shorter metro networks, higher latency can be experienced due to extra distance in building risers and cross-connects. To calculate the latency of a connection, one has to know the distance traveled by the fiber, which is rarely a straight line, since it has to traverse geographic contours and obstacles, such as roads and railway tracks, as well as other rights-of-way.
Due to imperfections in the fiber, light degrades as it is transmitted through it. For distances of greater than 100 kilometers,
amplifiers or
regenerators are deployed. Latency introduced by these components needs to be taken into account.
Satellite transmission
Satellites in
geostationary orbits are far enough away from Earth that communication latency becomes significant – about a quarter of a second for a trip from one ground-based transmitter to the satellite and back to another ground-based transmitter; close to half a second for two-way communication from one Earth station to another and then back to the first.
Low Earth orbit
A low Earth orbit (LEO) is an orbit around Earth with a period of 128 minutes or less (making at least 11.25 orbits per day) and an eccentricity less than 0.25. Most of the artificial objects in outer space are in LEO, with an altitude never m ...
is sometimes used to cut this delay, at the expense of more complicated satellite tracking on the ground and requiring more satellites in the
satellite constellation
A satellite constellation is a group of artificial satellites working together as a system. Unlike a single satellite, a constellation can provide permanent global or near-global coverage, such that at any time everywhere on Earth at least one ...
to ensure continuous coverage.
Audio
Audio latency is the delay between when an audio signal enters and when it emerges from a system. Potential contributors to latency in an audio system include
analog-to-digital conversion,
buffering,
digital signal processing
Digital signal processing (DSP) is the use of digital processing, such as by computers or more specialized digital signal processors, to perform a wide variety of signal processing operations. The digital signals processed in this manner ar ...
,
transmission time,
digital-to-analog conversion and the
speed of sound
The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elastic medium. At , the speed of sound in air is about , or one kilometre in or one mile in . It depends strongly on temperature as ...
in air.
Video
Video latency refers to the degree of delay between the time a transfer of a video stream is requested and the actual time that transfer begins. Networks that exhibit relatively small delays are known as low-latency networks, while their counterparts are known as high-latency networks.
Workflow
Any individual
workflow
A workflow consists of an orchestrated and repeatable pattern of activity, enabled by the systematic organization of resources into processes that transform materials, provide services, or process information. It can be depicted as a sequence ...
within a system of workflows can be subject to some type of operational latency. It may even be the case that an individual system may have more than one type of latency, depending on the type of participant or goal-seeking behavior. This is best illustrated by the following two examples involving
air travel.
From the point of view of a passenger, latency can be described as follows. Suppose John Doe flies from
London
London is the capital and List of urban areas in the United Kingdom, largest city of England and the United Kingdom, with a population of just under 9 million. It stands on the River Thames in south-east England at the head of a estuary dow ...
to
New York
New York most commonly refers to:
* New York City, the most populous city in the United States, located in the state of New York
* New York (state), a state in the northeastern United States
New York may also refer to:
Film and television
* '' ...
. The latency of his trip is the time it takes him to go from his house in England to the hotel he is staying at in New York. This is independent of the throughput of the London-New York air link – whether there were 100 passengers a day making the trip or 10000, the latency of the trip would remain the same.
From the point of view of flight operations personnel, latency can be entirely different. Consider the staff at the London and New York airports. Only a limited number of planes are able to make the transatlantic journey, so when one lands they must prepare it for the return trip as quickly as possible. It might take, for example:
*35 minutes to clean a plane
*15 minutes to refuel a plane
*10 minutes to load the passengers
*30 minutes to load the cargo
Assuming the above are done consecutively, minimum plane
turnaround time is:
:35 + 15 + 10 + 30 = 90
However, cleaning, refueling and loading the cargo can be done at the same time. Passengers can only be loaded after cleaning is complete. The reduced latency, then, is:
:35 + 10 = 45
:15
:30
:Minimum latency = 45
The people involved in the turnaround are interested only in the time it takes for their individual tasks. When all of the tasks are done at the same time, however, it is possible to reduce the latency to the length of the longest task. If some steps have prerequisites, it becomes more difficult to perform all steps in parallel. In the example above, the requirement to clean the plane before loading passengers results in a minimum latency longer than any single task.
Mechanics
Any mechanical process encounters limitations modeled by
Newtonian physics
Classical mechanics is a physical theory describing the motion of macroscopic objects, from projectiles to parts of machinery, and astronomical objects, such as spacecraft, planets, stars, and galaxies. For objects governed by class ...
. The behavior of
disk drives provides an example of mechanical latency. Here, it is the time
seek time Higher performance in hard disk drives comes from devices which have better performance characteristics. These performance characteristics can be grouped into two categories: access time and data transfer time (or rate).
Access time
The ''acce ...
for the actuator arm to be positioned above the appropriate track and then
rotational latency for the data encoded on a platter to rotate from its current position to a position under the
disk read-and-write head.
Computer hardware and operating systems
Computers run
instructions in the context of a
process. In the context of
computer multitasking
In computing, multitasking is the concurrent execution of multiple tasks (also known as processes) over a certain period of time. New tasks can interrupt already started ones before they finish, instead of waiting for them to end. As a result ...
, the execution of the process can be postponed if other processes are also executing. In addition, the operating system can schedule when to perform the action that the process is commanding. For example, suppose a process commands that a computer card's voltage output be set high-low-high-low and so on at a rate of 1000 Hz. The operating system
schedules
A schedule or a timetable, as a basic time-management tool, consists of a list of times at which possible tasks, events, or actions are intended to take place, or of a sequence of events in the chronological order in which such things are i ...
the process for each transition (high-low or low-high) based on a hardware clock such as the
High Precision Event Timer. The latency is the delay between the events generated by the hardware clock and the actual transitions of voltage from high to low or low to high.
Many
desktop operating systems have performance limitations which create additional latency. The problem may be mitigated with real-time extensions and patches such as
PREEMPT_RT.
On embedded systems, the real-time execution of instructions is often supported by a
real-time operating system
A real-time operating system (RTOS) is an operating system (OS) for real-time applications that processes data and events that have critically defined time constraints. An RTOS is distinct from a time-sharing operating system, such as Unix, which ...
.
Simulations
In simulation applications, latency refers to the time delay, often measured in
millisecond
A millisecond (from '' milli-'' and second; symbol: ms) is a unit of time in the International System of Units (SI) equal to one thousandth (0.001 or 10−3 or 1/1000) of a second and to 1000 microseconds.
A unit of 10 milliseconds may be ca ...
s, between initial input and output clearly discernible to the simulator trainee or simulator subject. Latency is sometimes also called ''transport delay''. Some authorities distinguish between latency and transport delay by using the term ''latency'' in the sense of the extra time delay of a system over and above the reaction time of the vehicle being simulated, but this requires detailed knowledge of the
vehicle dynamics and can be controversial.
In simulators with both visual and motion systems, it is particularly important that the latency of the motion system not be greater than of the visual system, or symptoms of
simulator sickness may result. This is because, in the real world, motion cues are those of acceleration and are quickly transmitted to the brain, typically in less than 50 milliseconds; this is followed some milliseconds later by a perception of change in the visual scene. The visual scene change is essentially one of change of perspective or displacement of objects such as the horizon, which takes some time to build up to discernible amounts after the initial acceleration which caused the displacement. A simulator should, therefore, reflect the real-world situation by ensuring that the motion latency is equal to or less than that of the visual system and not the other way round.
See also
*
Age of Information
*
Feedback
Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...
*
Interrupt latency
*
Jitter
In electronics and telecommunications, jitter is the deviation from true periodicity of a presumably periodic signal, often in relation to a reference clock signal. In clock recovery applications it is called timing jitter. Jitter is a significa ...
*
Lagometer
*
Lead time
*
Memory latency
''Memory latency'' is the time (the latency) between initiating a request for a byte or word in memory until it is retrieved by a processor. If the data are not in the processor's cache, it takes longer to obtain them, as the processor will ha ...
*
Performance engineering
*
Response time (technology)
In technology, response time is the time a system or functional unit takes to react to a given input.
Computing
Response time is the total amount of time it takes to respond to a request for service. That service can be anything from a memor ...
*
Responsiveness
References
Further reading
*
External links
Simulating network link latency under Linux
{{DEFAULTSORT:Latency (Engineering)
Engineering concepts