Latency is a time delay between a stimulation and its response. It is caused by velocity limitations in a physical system. In simplest terms, latency is the time it takes for a signal to travel (or propagate) from point A to point B.
In telecommunications, latency describes the time delay of a packet traveling through a network or the delay imposed on a signal traveling in a transmission medium such as a copper cable, optical fiber waveguide, or even free space.
In radio transmissions it is the time it takes a radio signal to propagate through free space from the transmitter to the receiver.
For electrical transmissions, it is the time the electrical signal takes to propagate through a metallic or conductive medium.
In optical transmission, latency describes the time required for the optical signal to propagate through free space or in the core of an optical fiber.
The speed of light in a vacuum is a universal physical constant denoted by the symbol ‘c’ and is used to determine velocity limitations in all transmission media.
It is based on two physical constants: the permittivity in a vacuum (electric constant), denoted as ɛ0, and the permeability in a vacuum (magnetic constant), denoted as μ0. Using these two values, the speed of light in a vacuum can be determined using the formula:
Latency is a function of distance and the speed of light. In a vacuum (or free space), light travels at 299,792,458 meters per second (m/s). To make the math easier, the value is rounded up to 300,000,000 m/s (or 3 X 108 m/s).
Therefore, light traveling over a distance of 1 kilometer in a vacuum will result in a time delay, or latency, of 3.33 microseconds (μsec). While the speed of light in air is slightly slower than in a vacuum, the difference (0.3 percent) is so slight it is usually ignored.
To the human eye, electrical and optical phenomena are perceived as being instantaneous. When we turn on a flashlight, the object at which it is pointing appears to illuminate instantly.
However, we know it takes a finite amount of time for the flashlight’s beam to travel to the object—and a finite amount of time for the reflected light to return to our eye.
If the object to be illuminated is 100 meters from the flashlight and the latency from flashlight to object is 0.33 μsec, the time required for the light to travel from the observer’s flashlight to the object and back to the observer (round-trip delay), is 0.66 μsec.
In the same way, we can calculate the time it takes light to travel from the sun to the Earth (approximately 8.3 minutes) or for a radio signal to travel from Earth to Mars (about 12.5 minutes).
Content sourced from paper by Joseph Coffey, senior principal engineer, CommScope