Insertion loss, or the loss of signal that happens along the length of a fiber optic link, is expressed in dBs and should always be a positive number. But it can be a negative number (which isn't a good thing). The estimate, called a "loss budget" is calculated using typical component losses for. Insertion loss is the signal power loss caused by inserting devices (such as fiber connectors, fiber jumpers, couplers, etc. Now we're getting to the fourth grade math. When implementing optical fiber communication, a key challenge is minimizing the loss of signals within the fiber.
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