Knowledge base · Measurement
Embedded OTDR: how to read baseline and current traces
An embedded OTDR sits inside the transport system itself or in a fibre monitoring unit. It takes traces on its own while traffic runs on the fibre, and it exports an ordinary .sor. Open that file here in the browser, lay the current trace over the baseline and find the spot where they part.
You don't need to install anything. It runs on Windows, macOS and Linux.
By Ilya (engineer, 6 years in fibre optics) ·
What an embedded OTDR is
The reflectometer inside is the same as in a handheld unit. The difference is that it lives in the central office and starts by itself.
A card in the transport system
The module sits in a DWDM shelf and looks into the same fibre that carries traffic. Ciena 6500 with PinPoint works this way, and so does the OTDR16-1 card in Coriant hiT 7300, now Infinera. One hiT 7300 card serves up to 16 fibres and couples its light into the line through a passive filter between the amplifiers.
A monitoring unit
A rack unit with an OTDR and an optical switch steps through fibres one after another, hundreds per unit, and reports a fault by itself. VIAVI OTU-8000 in the ONMSi system and EXFO Nova Fiber RTU-2 work this way.
A wavelength out of the traffic's way
To keep clear of the working channels, the OTDR shoots at a service wavelength. On Ciena 6500 that is 1527 or 1625 nm depending on the shelf, on hiT 7300 it is 1625 nm, and on VIAVI and EXFO monitoring units it is 1650 nm. On dark fibre the monitoring units also measure at the usual 1310 and 1550 nm.
When the trace takes itself
The system takes the first trace when a span is turned up and keeps it as the baseline. After that it takes new ones: on a fibre break, after a repair, when the card powers up, and monitoring units also run on a schedule. The system compares every new trace with the baseline.
That is why these files come in pairs, and the file name often says more than the header. At Ciena the long baseline trace has LONGBSLN in its name and the long current one has long-current. A hiT 7300 file name carries the card port number, for example OTDR_Port_00-1-001-1. Look for the node, date and time of the shot in the name too.
Three traces per run
Ciena PinPoint takes three traces with different pulses on every run. A single pulse cannot show both the connector at the office and the end of an 80 km span well, so the near and far ends are read from different traces.
| Trace | What it shows | Range | Default pulse | Event dead zone | Distance accuracy |
|---|---|---|---|---|---|
| Office | Reflections on the front panel | 2 km | 100 ns | 50 m | 2 m |
| Short | Reflections near the transmitter, including the panel connector | 20 km | 1 µs | 100 m | 4 m |
| Long | Loss along the whole span | 80 km, up to 120 km to an open end | 40 µs | 4 km | 15 m |
From the Ciena PinPoint OTDR data sheet.
On the long trace a 40 µs pulse gives a 4 km event dead zone, and nothing closer to the office can be made out on it. The panel connector and the first kilometres are read on the short and office traces of the same run. Compare a current trace with a baseline of the same kind, long with long.
Why the header is empty
A machine took the trace, so the fields a technician fills in on a handheld unit are often blank: cable, fibre, operator, location. Sometimes they hold the field names themselves instead of values. The system usually does write the wavelength, pulse and acquisition date, and those are enough to match the trace with its baseline.
We show the header exactly as the system wrote it. An empty field stays empty and does not turn into a zero, and what is missing can often be found in the file name.
Baseline and current on one plot
To find where the trace changed, put both on one plot.
- Open both files: drag them into the window or press Open file. You can drop a whole folder or archive of traces.
- On the second file's card press the eye, Show on the plot. Both curves land on one scale, each in its own colour.
- Find where the curves part. A step that is not on the baseline means new loss at that point. If the current trace ends before the baseline does, look for the break where it ends.
- Put cursor A there: the readout line under the plot gives the distance along the fibre.
The distance along the fibre is longer than along the cable, and the cable is longer than the route on the ground: fibre lies in the cable with some excess length, and cable is left as slack in the closures. The calculator in the article on three lengths turns the instrument reading into a window where to look on the route.
If the fibre is watched from both ends (hiT 7300 does this as standard), open both traces and press Choose the reverse trace in the event table header. Events from the two directions line up against each other.
When there is no OTDR inside
Not every system keeps an OTDR inside. Nokia 1830 LX learns about a break when the optical supervisory channel at 1574 nm disappears. That signal only says whether there is light or not and gives no location for the break.
For an OTDR, 1830 LX units have a separate input, OTDR Mon IN, with up to 15 dB of loss on it. Through that input the instrument cannot make out the section just behind the unit, and its range is shorter. The trace in this case comes from your handheld OTDR and opens here like any other.
Common questions
How do I open a .sor from a monitoring system?
The same way as a trace from a handheld unit. Embedded OTDRs write an ordinary Bellcore .sor, and the Ciena PinPoint data sheet names Telcordia SR-4731 and GR-196 outright. The file opens here in the browser with nothing to install.
Why is the wavelength 1625 or 1650 nm and not 1550?
The trace was taken while traffic ran on the fibre. The OTDR works at a wavelength that filters separate from the working channels, so the instrument's light and the traffic stay out of each other's way. The trace itself reads the same as at any other wavelength.
Why can't I see the office connector on the long trace?
The dead zone of the long pulse hides it. The Ciena long trace is taken with a 40 µs pulse, and its event dead zone is 4 km. Read near events on the short or office trace from the same run.
Why does the header have no cable or operator?
The system took the trace without a person, so there was nobody to fill those fields in. The node, port, trace type and date are often in the file name.
If the file doesn't open
We have not seen embedded OTDR traces from every maker, and firmware varies. If your file doesn't open, send it to support@otdrmaster.com: files like that are how the reader learns.
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Equipment names belong to their owners. We are not affiliated with Ciena, Infinera, Nokia, VIAVI or EXFO.