In fiber optics, the glass fibers are not perfectly clear. The amount of light coming out of the distant end of a cable is always somewhat less than the amount transmitted into the fiber. This loss of light is called attenuation. Attenuation is measured in decibels (dB) and is the ratio between the output power of the fiber and the input power. Attenuation is usually a linear quantity, increasing in direct proportion to the length of a fiber optic cable run. Excessive attenuation results in a recieved signal which is too weak to be useful. Likewise, no metal is a perfect conductor of electricity, and even the best cable designs will have a certain amount of capacitance and inductance which will cause any signal propagated down the cable to lose strength over distance. Electrical attenuation is also generally directly proportional to the length of the cable.