Knowledge base · Measuring
Reading a PON trace: the splitter, three pulse widths, and many fiber ends
In a PON, one fiber path is split among several subscribers. For an OTDR that changes everything: a step of ten decibels or more sits in the middle of the trace, and beyond it there is no single end of fiber but several at once.
This page explains what is on the curve and why a single path usually comes as several files. There are no requirements or thresholds here — those arrive with the design.
Three differences from an ordinary span
Everything else is the same fiber and the same instrument.
The splitter takes the budget at once
Power is divided evenly between the outputs, and every doubling of the output count costs about three decibels: 1x2 is roughly 3 dB, 1x4 is 6, 1x8 is 9, 1x32 is 15. Add the device's own excess loss and its connectors, usually about another decibel. This is the arithmetic of the splitter, not an acceptance threshold: a step of that size is normal on a PON and a failure on an ordinary span. Instruments keep a recognition table of their own beside that arithmetic, and it can sit below the calculation: Anritsu will call a step of 8 dB a 1x8 splitter and one of 11 dB a 1x16, where the arithmetic gives 9 and 12. That is a threshold for naming the part, not a budget: the instrument agrees to name a ratio without waiting for the full figure.
There is more than one fiber end
Past the splitter the light goes into every branch at once, and the instrument sees their sum. Reflections off the subscribers' end faces arrive each at its own distance, so the end of fiber on such a trace is a group of events rather than a single point. Branches shorter than the one being measured hide inside the common curve entirely.
Resolution against reach
A short pulse separates events that stand close together; a long one reaches the end of the link — but drags a long dead zone behind it, inside which neighbouring events merge. On a PON these two demands conflict more than usual: the connectors near the head stand back to back, while past the splitter another ten decibels of reach are needed.
Hence several files for one and the same path
No single setting resolves that conflict, so the link is shot in several passes. The order EXFO gives in its own PON guide runs like this: first a short pulse, a few nanoseconds, to qualify the first connector and everything up to the splitter; then a medium one, to measure the splitter itself; then a long one, to reach the end of the link.
That leaves three or four traces of the same path, none of which answers every question. They are then compared with one another to decide which pass measures which event more credibly. This is manual work, and it is where the time goes — not into the acquisition.
A practical consequence for whoever receives such files: the file name is usually the only note on which pulse was used. The pulse width itself is recorded in the instrument's header, but no format carries a label saying this pass goes up to the splitter and that one to the end. When opening a batch, start with the longest trace — it shows the link as a whole, and the short ones refine its beginning.
A bend only shows up at a second wavelength
A splice attenuates the same at any wavelength; a bend in the fiber attenuates more the longer the wavelength, because light forced around a corner leaves the core more readily. On one trace a bend and a splice look like the same step down, and no processing of a single curve will tell them apart.
This is why vendor procedures repeat the whole acquisition at a second wavelength and compare the loss of each event between the two traces. That comparison of two measurements is the only way to separate a bend from a joint — the shape of one curve is not.
What we do, and what we do not
We read and display files; we do not analyse networks. To us a splitter is an event with a large loss, and it sits in the table exactly as the instrument recorded it. Several traces of one path can be laid over one graph, each in its own colour, so that passes at different pulse widths can be compared; comparing event losses between two wavelengths is available in the experimental mode and flags a suspected bend — with the caveat that this is our inference and not the instrument's word.
What we do not have: threshold values, a verdict on whether a network passes, any knowledge of PON topology, or any way to say which subscriber a branch belongs to. The loss budget is set by the class of the system in the design, comes from the site documentation, and is not supplied by us in any form.
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A PON can also be measured while it carries service: instruments have a separate port at a wavelength the subscriber's filter blocks, and the working channels are left untouched. That is usually 1625 or 1650 nm, and vendor PON acceptance procedures are written around them. In the file such a trace differs from an ordinary one only by the wavelength in its header — which is the only mark saying the network was live when it was measured.