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Latency (engineering)
Latency (engineering)
Published: 2014/08/30
Channel: Audiopedia
Large-Scale Low-Latency Storage for the Social Network - Data@Scale
Large-Scale Low-Latency Storage for the Social Network - Data@Scale
Published: 2013/12/11
Channel: Facebook Developers
Low-Latency Application Engineering Workshop, Low Latency Summit, February 2013
Low-Latency Application Engineering Workshop, Low Latency Summit, February 2013
Published: 2013/04/29
Channel: tsa1Monica
What is latency? What affects latency?
What is latency? What affects latency?
Published: 2016/09/20
Channel: internet-class
Reduce Choke, Lag, Lower Latency and Ping Internet Tweaks for Gaming
Reduce Choke, Lag, Lower Latency and Ping Internet Tweaks for Gaming
Published: 2017/01/05
Channel: jason weber
How to Improve Latency (Lag) and Packet Loss on Xbox One [Easy]
How to Improve Latency (Lag) and Packet Loss on Xbox One [Easy]
Published: 2016/06/15
Channel: Epiawesic
Latency and Cost Tradeoffs for Efficient Peer-to-Peer Assisted Content Distribution
Latency and Cost Tradeoffs for Efficient Peer-to-Peer Assisted Content Distribution
Published: 2009/11/23
Channel: GoogleTechTalks
perfSONAR Latency-Ping
perfSONAR Latency-Ping
Published: 2016/08/30
Channel: UOregon
Powerline Adapter vs 50FT Ethernet Cable Speed and Latency Comparison
Powerline Adapter vs 50FT Ethernet Cable Speed and Latency Comparison
Published: 2017/03/03
Channel: TechBlade
196 Network Latency in WAN Networks and performance optimization
196 Network Latency in WAN Networks and performance optimization
Published: 2017/03/02
Channel: The Linux Channel
Vintage King @ AES: Lavry Latency Killer
Vintage King @ AES: Lavry Latency Killer
Published: 2012/11/17
Channel: Vintage King
ANAN OPENHPSDR Latency comparison
ANAN OPENHPSDR Latency comparison
Published: 2016/11/26
Channel: TurboQueaf
Tuning Kafka for low latency guaranteed messaging -- Jiangjie (Becket) Qin (LinkedIn), 6/15/16
Tuning Kafka for low latency guaranteed messaging -- Jiangjie (Becket) Qin (LinkedIn), 6/15/16
Published: 2016/09/20
Channel: LinkedIn Engineering
Bandwidth and Latency Project
Bandwidth and Latency Project
Published: 2016/10/01
Channel: Alex Cohen
Cisco IP SLA   The High Definition Latency Probe you Already Own
Cisco IP SLA The High Definition Latency Probe you Already Own
Published: 2012/09/18
Channel: SevOneInc
Ethernet Switch Latency Experiment
Ethernet Switch Latency Experiment
Published: 2014/04/21
Channel: HamWAN
Measuring audio latency on iOS
Measuring audio latency on iOS
Published: 2013/09/05
Channel: Nick Clark
TP-Link AV600 Powerline network and gaming latency
TP-Link AV600 Powerline network and gaming latency
Published: 2014/08/30
Channel: 0Tweaky0
How To Check Network Latency In Windows Using Command Prompt
How To Check Network Latency In Windows Using Command Prompt
Published: 2014/11/22
Channel: EverleaGroup
How has fiber addressed the network latency challenge?
How has fiber addressed the network latency challenge?
Published: 2016/12/19
Channel: CommScope
BlueNET: Low Latency, High Performance IP & Transport Network
BlueNET: Low Latency, High Performance IP & Transport Network
Published: 2011/11/15
Channel: BatBlueNetworks
How to reduce lag/ping/network latency for mmo fps games 2015!!
How to reduce lag/ping/network latency for mmo fps games 2015!!
Published: 2013/12/10
Channel: KosmicCat
11.3.2.4 Lab - Testing Network Latency with Ping and Traceroute
11.3.2.4 Lab - Testing Network Latency with Ping and Traceroute
Published: 2017/01/11
Channel: Christian Augusto Romero Goyzueta
Comp Sci Talks for Data Engineers: Distributed, Low Latency Scheduling with Sparrow
Comp Sci Talks for Data Engineers: Distributed, Low Latency Scheduling with Sparrow
Published: 2014/05/14
Channel: Hakka Labs
Ping and Latency
Ping and Latency
Published: 2015/10/26
Channel: Bogdan Constantin Ionita
Time Warner latency...
Time Warner latency...
Published: 2013/01/02
Channel: Caleb
Performance optimization & tuning in Red Hat Enterprise Linux - tuned (network latency profile)
Performance optimization & tuning in Red Hat Enterprise Linux - tuned (network latency profile)
Published: 2014/06/10
Channel: Red Hat Enterprise Linux
Icom ID-1 Data Latency - DSTAR
Icom ID-1 Data Latency - DSTAR
Published: 2011/03/27
Channel: rllacey
A Quick Note About Tool Placement - Latency vs Retransmissions
A Quick Note About Tool Placement - Latency vs Retransmissions
Published: 2015/08/29
Channel: The Technology Firm
Xangati Monitoring Network Latency
Xangati Monitoring Network Latency
Published: 2014/08/19
Channel: Ben Vaux
Latency of a Raspberry PI HD Camera Streamed over the Network
Latency of a Raspberry PI HD Camera Streamed over the Network
Published: 2014/10/01
Channel: Patrick Duffy
RPI controlled car over mobile network, video latency demo
RPI controlled car over mobile network, video latency demo
Published: 2014/08/27
Channel: Alan Ibrus
Packet Drop and Network Latency Simulation
Packet Drop and Network Latency Simulation
Published: 2015/06/19
Channel: BeardedManStudiosInc
Cisco Nexus® 9508 Sets High Performance Low Latency Record Of Under 3.5 microseconds Spine Switch
Cisco Nexus® 9508 Sets High Performance Low Latency Record Of Under 3.5 microseconds Spine Switch
Published: 2013/11/06
Channel: Nick Lippis
Using the Network Latency map - Datapath.io Network Performance Optimization for Devops
Using the Network Latency map - Datapath.io Network Performance Optimization for Devops
Published: 2016/03/22
Channel: Datapath.io
EXFO enables CSPs to better monitor and manage network latency
EXFO enables CSPs to better monitor and manage network latency
Published: 2017/06/23
Channel: TelecomTV
Latency test Hobbyking Aomway DVR 5.8GHz 32ch Video Receiver with internal DVR
Latency test Hobbyking Aomway DVR 5.8GHz 32ch Video Receiver with internal DVR
Published: 2014/09/27
Channel: jamesb72
Ring Video Doorbell Pro - ring timing, lag/latency, and chime sequence demonstration
Ring Video Doorbell Pro - ring timing, lag/latency, and chime sequence demonstration
Published: 2016/05/03
Channel: Paul Braren
Peter Lawrey - Low Latency Code in Java 8
Peter Lawrey - Low Latency Code in Java 8
Published: 2015/04/02
Channel: MelbJVM JavaUsersGroup
UNET Part 4 - Latency Simulation!
UNET Part 4 - Latency Simulation!
Published: 2015/05/17
Channel: Gamer To Game Developer
Delivering Ultra-Low Latency, Speed 2 Ethernet Applications
Delivering Ultra-Low Latency, Speed 2 Ethernet Applications
Published: 2009/03/24
Channel: Accedian
Performance test for the Akamai home page (IE8, 15ms latency)
Performance test for the Akamai home page (IE8, 15ms latency)
Published: 2011/01/25
Channel: Strangeloopnet
What Happens When I Press Q | Riot Games Player Support
What Happens When I Press Q | Riot Games Player Support
Published: 2017/02/28
Channel: Riot Games Support
What is bandwidth? How is it different than latency?
What is bandwidth? How is it different than latency?
Published: 2016/09/20
Channel: internet-class
Command Prompt | Network Latency (PING Check)
Command Prompt | Network Latency (PING Check)
Published: 2015/03/30
Channel: The Tech Space
Framehawk performance at high latency and packet loss
Framehawk performance at high latency and packet loss
Published: 2015/09/03
Channel: Citrix
Download High Speed Networking A Systematic Approach to High Bandwidth Low Latency Communication Boo
Download High Speed Networking A Systematic Approach to High Bandwidth Low Latency Communication Boo
Published: 2016/07/29
Channel: Ardelis. R
Low-Latency Visual Odometry using Event-based Feature Tracks
Low-Latency Visual Odometry using Event-based Feature Tracks
Published: 2016/07/14
Channel: ailabRPG
The Right Tools to Diagnose Office 365 Network Latency
The Right Tools to Diagnose Office 365 Network Latency
Published: 2016/04/18
Channel: GSXSolutions
O3b
O3b's low latency connectivity is boosting tourism
Published: 2015/04/07
Channel: O3b Networks
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WIKIPEDIA ARTICLE

From Wikipedia, the free encyclopedia
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Latency is a time interval between the stimulation and response, or, from a more general point of view, a time delay between the cause and the effect of some physical change in the system being observed.[1] Latency is physically a consequence of the limited velocity with which any physical interaction can propagate. This velocity is always lower than or equal to the speed of light. Therefore, every physical system that has spatial dimensions different from zero will experience some sort of latency, regardless of the nature of stimulation that it has been exposed to.

The precise definition of latency depends on the system being observed and the nature of stimulation. In communications, the lower limit of latency is determined by the medium being used for communications. 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 one moment. In the field of human–machine interaction, perceptible latency has a strong effect on user satisfaction and usability.

Communication latency[edit]

Online games are sensitive to latency since fast response times to new events occurring during a game session are rewarded while slow response times may carry penalties (for instance in a first person shooter a slow response time may leave a player in the line of fire for longer periods of time). Lag is the term used to describe latency in gaming. A player with a high latency internet connection may show slow responses in spite of superior tactics or the appropriate reaction time due to a delay in transmission of game events between said player and other participants in the game session. This gives players with low latency connections a technical advantage and biases game outcomes, so game servers favor players with lower latency connections, sometimes referred to as low "ping" times (typically measured in milliseconds).

Minimizing latency is of interest in the capital markets,[2] particularly where algorithmic trading is used to process market updates and turn around orders within milliseconds. Low latency trading refers to the network connections used by financial institutions to connect to stock exchanges and Electronic communication networks (ECNs) to execute financial transactions.[3] Joel Hasbrouck and Gideon Saar (2011) measure latency based on three components: the time it takes for 1)information to reach the trader, 2) the trader’s algorithms to analyze the information, and 3) 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).[4] With the spread of computerized trading, electronic trading now makes up 60% to 70% of the daily volume on the NYSE and algorithmic trading close to 35%.[5] Trading using computers has developed to the point where millisecond improvements in network speeds offer a competitive advantage for financial institutions.

Packet-switched networks[edit]

Network latency in a packet-switched network is measured either one-way (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 performs no packet processing; it merely sends a response back when it receives a packet (i.e. performs a no-op), thus it is a first rough way of measuring latency. Ping cannot perform accurate measurements,[6] principally because it uses the ICMP protocol that is used only for diagnostic or control purposes, and differs from real communication protocols such as TCP. Furthermore, routers and ISP's might apply different traffic shaping policies to different protocols.[7][8]

For more accurate measurements it is better to use specific software (for example: lft, paketto, hping, superping.d, NetPerf, IPerf)

However, in a non-trivial network, a typical packet will be forwarded over many links via many 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 minimum latency of each link, plus the transmission delay of each link except the final one, plus the forwarding latency of each gateway. In practice, this minimal latency is further augmented by 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. A new and emergent behavior called 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[edit]

Latency is largely a function of the speed of light, 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 fibre 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, the latency performance rises a bit more due to building risers and cross-connects and can push the latency over 5 µs per kilometer.

It follows that to calculate latency of a connection, one has to know the distance traveled by the fibre, 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 fibre, light degrades as it is transmitted through it. For distances of greater than 100 kilometers, either amplifiers or regenerators need to be deployed. Passive amplifiers typically add less latency than regenerators, at the cost of compounding attenuation, though in both cases it can be highly variable, and so needs to be taken into account. In particular, legacy spans are more likely to make use of higher latency regenerators.

Satellite transmission[edit]

This is illustrated when a news presenter in a studio talks with a reporter in a distant place. The signal travels from the newsreader via communication satellite situated in geosynchronous orbit to the reporter and then goes all the way back to geosynchronous orbit and then to the studio, resulting in a journey of over one hundred thousand kilometers . This full hop time lag is easily noticeable. Even though the signal travels at the speed of light, it still requires about half a second to travel that distance (not including the much smaller latencies inside the communications equipment).

Low-Earth orbit 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 to ensure continuous coverage.

Audio latency[edit]

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, transmission time, digital-to-analog conversion and the speed of sound in air.

Operational latency[edit]

Any individual workflow 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.

Consumer view[edit]

From the point of view of a passenger, latency can be described as follows. Suppose John Doe flies from London to New York. 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.

Producer view[edit]

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 one after another, 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 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.

Mechanical latency[edit]

Any mechanical process encounters limitations modeled by Newtonian physics. The behavior of disk drives provides an example of mechanical latency. Here, it is the time needed for the data encoded on a platter to rotate from its current position to a position adjacent to the read-write head as well as the seek time required for the actuator arm for the read-write head to be positioned above the appropriate track. This is also known as rotational latency and seek time since the basic term latency is also applied to the time required by a computer's electronics and software to perform polling, interrupts, and direct memory access.

Computer hardware and operating system latency[edit]

Computers run sets of instructions called a process. In operating systems, 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 may choose to adjust the scheduling of each transition (high-low or low-high) based on an internal clock. The latency is the delay between the process instruction commanding the transition and the hardware actually transitioning the voltage from high to low or low to high.

On Microsoft Windows, it appears[original research?] that the timing of commands to hardware is not exact. Empirical data suggest that Windows (using the Windows sleep timer which accepts millisecond sleep times) will schedule on a 1024 Hz clock and will delay 24 of 1024 transitions per second to make an average of 1000 Hz for the update rate.[citation needed] This can have serious ramifications for discrete-time algorithms that rely on fairly consistent timing between updates such as those found in control theory. The sleep function or similar windows API were at no point designed for accurate timing purposes. Certain multimedia-oriented API routines like timeGetTime() and its siblings provide better timing consistency. However, consumer- and server-grade Windows (as of 2011 those based on NT kernel) were not to be real-time operating systems. Drastically more accurate timings could be achieved by using dedicated hardware extensions and control-loop cards.

Linux may have the same problems with scheduling of hardware I/O.[citation needed] The problem in Linux is mitigated by support for posix real-time extensions, and the possibility of using a kernel with the PREEMPT_RT patch applied.

On embedded systems, the real-time execution of instructions is often supported by the low-level embedded operating system.

In simulators and simulation[edit]

In simulation applications, 'latency' refers to the time delay, normally measured in milliseconds (1/1,000 sec), between initial input and an 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 a detailed knowledge of the vehicle dynamics and can be controversial.
  • Importance of Motion and Visual Latencies. 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 and/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[edit]

References[edit]

  1. ^ "What is Latency?" / "Latency" Retrieved 2015-02-22.
  2. ^ TABB (2009). High Frequency Trading Technology: a TABB Anthology. 
  3. ^ Mackenzie, Michael; Grant, Jeremy (2009). "The dash to flash" (PDF). Financial Times. Retrieved 18 July 2011. extracting tiny slices of profit from trading small numbers of shares in companies, often between different trading platforms, with success relying on minimal variations in speed - or "latency", in the trading vernacular. 
  4. ^ Hasbrouck, Joel; Saar, Gideon. "Low-Latency Trading" (PDF). p. 1. Retrieved 18 July 2011. 
  5. ^ Heires, Katherine (July 2009). "Code Green: Goldman Sachs & UBS Cases Heighten Need to Keep Valuable Digital Assets From Walking Out The Door. Millions in Trading Profits May Depend On It" (PDF). Securities Industry News. Retrieved 18 July 2011. 
  6. ^ "Don't misuse ping!". Retrieved 29 April 2015. 
  7. ^ Shane Chen (2005). "Network Protocols Discussion / Traffic Shaping Strategies". knowplace.org. Archived from the original on 2007-01-09. 
  8. ^ "Basic QoS part 1 – Traffic Policing and Shaping on Cisco IOS Router". The CCIE R&S. Retrieved 29 April 2015. 

External links[edit]

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