Premises optical fiber links should be tested:

Testing in only one direction at one wavelength misses directional loss differences and wavelength-dependent attenuation, providing a fundamentally incomplete characterization of the link.

Testing in one direction only, even at both wavelengths, fails to detect loss anomalies that only manifest when the optical signal travels in the reverse direction.

Bidirectional testing at only one wavelength is insufficient because attenuation characteristics differ between the two supported operating wavelengths, leaving one wavelength completely unverified.

Fiber optic links must be tested bidirectionally because reflections, connector endface polish, and splice geometry can produce different loss readings depending on which direction light travels. Both corresponding wavelengths must be used (e.g., 850 nm and 1300 nm for multimode, or 1310 nm and 1550 nm for singlemode) because attenuation is wavelength-dependent and a link may pass at one wavelength while failing at another. This bidirectional, dual-wavelength method is required by TIA-568.3-D and ISO/IEC 14763-3 to ensure a complete and accurate characterization of the installed link.