The The power loss for one way travel of a beam of light, usually measured in decibels per unit length. It is necessary to include both the value (and its unit) and the wavelength. The wavelength varies with the refractive index, while the frequency remains constant. The wavelength given to specify this type is the wavelength in a vacuum (refractive index = 1). The value of the one way loss per unit of length. The usual uom is decibels per kilometre (dB/km), although this might vary depending on the calibration method used. The frequency (and uom) for which the one-way loss is measured. The wavelength (and uom) for which the one-way loss is measured. The reported wavelength should be the wavelength of the light in a vacuum. Unique identifier for the loss.