Knowledge base · Measurement
Three lengths of one OTDR trace: acquisition, span and fiber
A single trace has three different lengths, and the file keeps them in three different places. Until they are told apart everything looks consistent: the figures sit next to each other and resemble one another, so the discrepancy is easy to miss.
Loss per kilometre is loss divided by a length, and which of the three goes into the denominator changes the answer several times over.
By Ilya (engineer, 6 years in fibre optics) ·
What the three are
Each answers its own question, and they coincide only on a short link shot carefully.
Acquisition length
How far the recording went: the number of samples times the sampling step. A property of the instrument's settings, not of the cable: the operator set a range and the instrument wrote up to it. The fibre may well have ended much earlier, and then the whole tail of the recording is noise past the end of the link.
Span length
The distance between the two markers the operator placed: from here to there is what we hand over. The near marker is not always at zero, since a launch cable or a splice-in point may sit before it, and this is usually the figure that gets copied into the record.
Fibre length
Where the fibre physically ended: the last event, past which the curve drops into noise. It is the same figure as the span only when the section handed over is the entire fibre; on a live network the fibre often carries on beyond the span being accepted.
What it looks like in a signed record
The example comes from a real acceptance package for eight fibres. The engineer copied two figures from the "Measurement result" block of the vendor report into the record: total loss and attenuation. Attenuation there is computed over the fibre length, and the report says so plainly.
Only the fibres turned out to be of different lengths. Five of the eight terminate at a closure, and for them fibre length and span length agree at 1.024 km, while the rest run on into the existing network: 1.839 and 3.954 km. For those the instrument divided the loss by its own length, and 0.405 and 0.282 dB/km went into the record. Recomputed over the span actually handed over, they are 0.727 and 1.088 dB/km.
So the paper carries a figure four times too low, and it sits on the same line as a length it was not computed over: the neighbouring column of the record says 1.024 km for all eight fibres. Spotting this on paper is next to impossible, because the cable's specified figure lives in another volume and the rows look consistent. The arithmetic is right on both sides, the instrument's and the engineer's; what went wrong is that nobody stated which length the attenuation was computed over.
The fourth length: the cable
All three lengths above are measured by the instrument along the fibre. Digging follows the cable, and the fibre inside a cable is longer than the cable itself. It is laid into the tube with slack, so that pulling and frost stretch the sheath rather than the glass; the tubes are also stranded around a strength member, which adds a second allowance on top. How much depends on the design: a fraction of a percent in a central-tube cable, up to 4% with stranded tubes and in aerial cable.
A break that sits at one kilometre on the screen lies 5–40 m nearer along the cable, and that is before slack in closures and cable coils in manholes. The exact percentage is usually not in the cable datasheet. Manufacturers say it depends on the design and advise measuring it yourself: take a section with known sheath markings and divide the sheath length by the instrument's length to the same point. That factor converts the instrument's reading into metres of cable.
This is not the same allowance as the one in the loss budget guide. There the cable in the trench is longer than the distance between manholes: route allowance, coils and detours. Here the fibre is longer than the cable inside the sheath. The causes and the figures differ, but both shift the same way, so they add up.
Where to dig: turn the OTDR reading into metres
Enter the distance the OTDR showed to the fault and pick the cable design. The calculator gives two windows: by the sheath markings and on the ground. A window rather than a point, because how much longer the fibre is than the cable is almost never in the cable datasheet.
By the sheath markings
On the ground from the start
How this shows up here
Two chips sit above the curve, Optical length and End of fibre, and they stand side by side when the figures differ. When there is only one figure there is only one chip: the same value under two names would read as a discrepancy.
Acquisition length is deliberately kept out of the summary; its place is in the acquisition conditions line above the chart and in the Trace length field in the parameters. It is the longest of the three, and it is exactly the one that can end up copied into a record.
Where the instrument recorded no length, we stay silent too: an empty field is not filled in with our estimate, because an acceptance package is signed against what is on the screen, and our end-of-fibre estimate is not a measurement. It can be seen by turning on the experimental mode, and it is labelled as ours.
Next
A simple check for somebody else's test record: take the total loss and divide it by the length the record itself names. If that does not match the printed attenuation, it was computed over a different length, and it is worth finding out which.