Bi-directional testing of fiber links for Tier 2 (OTDR) testing is not only required by industry standards and most manufacturers for warranty, it’s also the only way to know the actual overall loss for a link. That’s because measuring the loss of fiber connectors and splices, as well as overall link loss, depends on the test direction. Testing a fiber link in one direction can give you different results then testing the same fiber link in the opposite direction.

Read the source article at flukenetworks.com