An estimate made before testing by adding fiber length times its attenuation coefficient, the number of mated connector pairs times an allowance such as 0.75 dB, and the number of splices times about 0.3 dB, plus any splitters. Measured insertion loss is compared with it to pass or fail a link. Designers also compare it with the equipment's power budget to confirm the link will work with margin left over.
Loss budget for that run is 3.1 dB with two connectors and two splices, you measured 2.4, so we're good.
A measured loss lower than the fiber alone should lose, or a negative reading, means a bad reference, not a great link.
Also heard as
- link loss budget
- cable plant loss budget
Do not mix it up with
- Margin: Margin is what is left over after subtracting the actual or budgeted loss from what the equipment can tolerate.
Related terms
Insertion loss
The optical power lost when a component or a whole link is placed in the light path, measured in dB.
Margin
The spare optical power left after link loss is subtracted from what the transmitter and receiver can tolerate.
Attenuation coefficient
Fiber loss per unit length, in dB/km, which sets how much a given length of glass will lose.
Splitter
A passive optical device that divides the light from one fiber among several output fibers.
OLTS
Optical Loss Test Set: a light source and power meter used together to measure end-to-end link loss.