Typical SMF cable attenuation at 1310nm wavelength is 0.3 to 0.5dB/km. The actual value should be obtained from the cable manufacturer. I use 0.4dB/km for calculating expected loss over the spans we build.
For a hypothetical 10km run, total attenuation expected would be:
cable attenuation of 4.0dB (10km * 0.4dB/km); plus
connector pair loss of 3.0dB (1.0dB per pair, or 0.5dB per connector); plus
splice loss of 0.4dB (0.1dB per fusion splice);
This is within the 1000BASE-LX/LH GBIC's power budget of 9.5dB.
This fiber span consists of a 2-meter patch cable at each end (with each patch cable having a pair of connectors); and 9.996km of SMF with four fusion splices along its length (and a connector at each end, located in a fiber patch panel).
Cisco has a page which is helpful in illustrating the concept:
Calculating the Maximum Attenuation for Optical Fiber Links
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