The throughput is the measurement of how fast data can pass through an entity (such as a point or network). In other words if we consider this entity as a wall through which bits pass, throughput is the number of bits that can pass this wall in one second. Propagation speed measures the distance a signal or a bit can travel through the medium in one second. The propagation speed of electromagnetic signals depends on the medium and on the frequency of the signal. For example, in a vacuum, light is propagated with a speed of 3 * 10 8 m/s. it is lower in air. It is much lower in a cable. Propagation time measures the time that is required for a signal or a bit to travel from one point of the transmission medium to another. The propagation time can be calculated by dividing the distance by the propagation speed. Wavelength is another characteristic of a signal traveling through a transmission medium. Wavelength binds period or the frequency of a simple sine wave to the propagation speed of the medium. In other words, while the frequency of a signal is independent of the medium, the wavelength depends on both the frequency and the medium. Although wavelength can be associated with electrical signals, it is customary to use wavelengths when talking about transmission of light in an optical fiber.