Long single mode fiber runs naturally have attenuation (loss of light power) over the run. Tx power values are higher than Rx values because Rx represents sensitivity to light pulses. This allows for link loss . Optical loss is measured in “dB” which is a relative measurement, while absolute optical power is measured in “dBm,” which is dB relative to 1mw optical power Loss is a negative number (like –3. 2 dB) while power measurements can be either positive (greater than the reference) or negative (less than. This Applications Engineering Note (AEN 135) explains and recommends standard measurement methods for characterizing optical fiber system performance. Receive power is the power at which the receiver of an optical transceiver module receives optical signals, in dBm. In real-world deployments, fiber optic loss directly constrains transmission distance, split ratio, network. At TREND Networks, we are frequently asked how much loss is allowed when conducting testing on fibre optic cabling.