Modal Bandwidth, in the discipline of telecommunications, refers to the signalling rate per distance unit. The signalling rate can typically be measured in MHz, and the modal bandwidth is expressed as MHz·km (multiplied).

The Modal bandwidth of a multimode fiber is determined from the optical output power frequency

Frequency

Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 spectrum. The frequency at which the amplitude

Amplitude

Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...

 drops three decibels (dB) relative to the zero frequency component of the fiber is defined as the −3 dB bandwidth (or modal bandwidth) of the fiber.