Overview

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. The original meaning of “latency”, as used widely in psychology, medicine and most other disciplines, derives from “latent”, a word of Latin origin meaning “hidden”.

Its different and relatively recent meaning (this topic) of “lateness” or “delay” appears to derive from its superficial similarity to the word “late”, from the old English “laet”.

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. 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 to which it has been exposed.

The precise definition of latency depends on the system being observed or the nature of the simulation. In communications, 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 at which 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 and usability in the field of human–machine interaction.

Communications

Online games are sensitive to latency (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

Joel Hasbrouck and Gideon Saar (2011) measure latency to execute financial transactions 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 that 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 acknowledgment (from the vendor's computer). 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 (the time from the source sending a packet to the destination receiving it), or round-trip delay time (the one-way latency from the source to the 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. Many software platforms provide a service called ping that can be used to measure round-trip latency.

Ping uses the Internet Control Message Protocol (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. Furthermore, routers and internet service providers 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 toward 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 is largely a function of the speed of light. 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.