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Latency is a time interval between the stimulation and response, or, from a more general point of view, as a time delay between the cause and the effect of some physical change in the system being observed.

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. 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. 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 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, 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.