OTDR testing can only be performed on one side of the splitter.

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When an OTDR tests a network that includes a passive splitter, the pulse sent into the fiber is divided among the splitter’s outputs. The backscattered signal observed at the input side is the combination of reflections and losses from all branches after the splitter. That combined trace makes it impossible to resolve individual branch lengths, losses, or events from a single measurement taken on the side with the splitter. To get meaningful, branch-specific details, you’d need access to the ends of each branch or perform separate measurements on each branch, which isn’t practical from the opposite side of the splitter. Therefore, in typical practice, OTDR testing is done from one side of the splitter, and the statement that it can be performed only on one side reflects that practical limitation.