Fiber links are subject to the optical equivalent of UTP impedance discontinuities. When light encounters an optical discontinuity, some of the light signal is reflected back in the opposite direction with only a fraction of the original light signal continuing down the fiber towards the receiver. This results in a reduced amount of light energy arriving at the receiver, making signal recognition difficult. Just as with UTP cable, improperly installed connectors are the main cause of light reflection and signal strength loss in optical fiber.

Because noise is not an issue when transmitting on optical fiber, the main concern with a fiber link is the strength of the light signal that arrives at the receiver. If attenuation weakens the light signal at the receiver, then data errors will result. Testing fiber optic cable primarily involves shining a light down the fiber and measuring whether a sufficient amount of light reaches the receiver.

On a fiber optic link, the acceptable amount of signal power loss that can occur without dropping below the requirements of the receiver must be calculated. This calculation is referred to as the optical link loss budget. A fiber test instrument checks whether the optical link loss budget has been exceeded. If the fiber fails the test, the cable test instrument should indicate where the optical discontinuities occur along the length of the cable link. Usually, the problem is one or more improperly attached connectors. The cable test instrument will indicate the location of the faulty connections that must be replaced. When the faults are corrected, the cable must be retested.

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