EXFO FTB-7000 OTDR Series for FTB-500 - Manual (Page 205)

Analyzing the Results Manually
OTDR 191
Getting Event Loss (Four-Point and Least-Square Approximation)
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A-B LSA loss: the loss of the event bordered by the markers A and B is
obtained by fitting a straight line to the backscatter data between these
two markers.
The event is then obtained by the reduction in power (dB) over the
distance between the two markers, as calculated from the slope of the
fitted line.
Although this method works fairly well for splice loss, it is clearly not
appropriate for reflective events (definitely not a “straight-line” event).
A-B LSA Loss is mainly used to rapidly compute loss over a given length
of a fiber section.
Note: A-B LSA event loss measurements should be used on fiber sections only.
Measuring events will not yield meaningful results.
A-B LSA loss
a
b