Knowledge base · Measuring
Why there are two losses and only one span
You place cursors A and B and get two numbers: 2P and LSA. That is neither an error nor a matter of taste — the two methods answer slightly different questions, and the gap between them is itself useful.
Both are computed from the same samples between the cursors. What differs is what each takes as the level.
Three figures in the readout
The status bar prints them while the cursors sit on the curve.
2P — two-point
The difference of the levels directly under the cursors. It reads instantly and honestly shows what the eye sees, but it depends entirely on where those two points landed: noise under a cursor goes straight into the result.
LSA — across the whole span
A straight line is fitted by least squares through every sample between the cursors, and the loss is taken from it. Noise averages out, so on a long, even fiber LSA is steadier than 2P.
Deviation — the judge
How well the samples hold to that fitted line. A small deviation means the span really is uniform and LSA can be trusted on it. A large one means something is inside the span: an event, a step, the edge of a pulse.
Which is closer to the truth, and when
On an even stretch of fiber between two events, take LSA: it uses every sample and does not depend on which noise spike a cursor happened to land on. On an event itself — a splice, a connector, a bend — no straight line goes through a step, and the steeper the step the less LSA resembles the truth; there you read 2P and what the event itself reports. If the two numbers have drifted apart and the deviation is large, there is almost certainly an event inside the span, and the span should be cut in two with the cursors.
What we do not say
Neither 2P nor LSA answers "did this span pass". The threshold is set by your project and your customer's requirement, and there is not one threshold value in the service — not for a splice, not for a connector, not per kilometre. A plausible number sitting in a default field would read as a norm, and that is the only reason none is there.
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The figures are computed by the same code everywhere: in the browser and on the server it is literally the same arithmetic.