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Pushing the Rainbow

OTDR's have been around for about 20 years. A mature product, they continue to be the main test instrument used in fiber installation. We explore, what's hot, new, needed and tips to get the most out of your trusty tester.


May 1, 2003  


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Since the first long distance fiber deployment in the 1980s in Saskatchewan, the Optical Time Domain Reflectometer (OTDR) has changed as technology has improved, but mostly because the networks under test have changed as well.

The price of fiber has dropped almost as fast as that of computers. Fiber counts especially in the Metro space can reach 864/cable. Ribbon fiber is also helping push the density. Over building has begun on the most popular routes.

Testing at 1310nm was early standard transmission. Signals at 1550nm travel with lower attenuation and has been so widely adopted that over 95 per cent of all OTDR’s are sold with both wavelengths.

Pushing the Rainbow to 1625nm, this wavelength is becoming more popular and being used to test for bending and to check that the fiber is ready for communication at that optical window.

OTDR’s of today are unrecognizable from earlier versions. It’s arguable that their change is so dramatic it actually is responsible to some degree for the pervasive nature of fiber in the network.

It’s easier and cheaper to test and maintain, so it gets installed faster and more often. Can you imagine spending a week testing a cable? Neither can your finance department.

While today’s OTDRs have much better specifications and are easier on your back, are they really better at solving today’s problems? The answer is absolutely. Here is a real example: A fiber breaks on a trunk link. It’s Sunday evening and every minute is costly. You use your OTDR and locate the problem 25 kilometres away, which you know is near the highway.

But you have two connections at that location — a buried fiber vault on the near side of the highway and a simple surface mount pedestal on the far side. You risk spending $50,000 to call in a contractor and dig up the vault, since you can’t take the time to wait and see.

The best OTDR’s can pay for themselves twice over in this example. With centimeter sampling distance and the ability to start and stop the measurement (Start at 24km and Stop at 26km) you can easily see that the splices at the vault are intact and the problem is a bend at the pedestal.

In one minute, you’ve saved having to call in a contractor and can dispatch a crew to fix, rather than find the problem. You can even e-mail a picture or trace right to the repair crew.

INFORMATION TRANSFER

When OTDR’s were born, only a few fibers needed to be tested. Simply having a method of measurement (trace), storage or printout was acceptable. Indeed, some customers preferred paper printouts to anything else. Fiber documentation is now more critical, and more people have access to the information.

Add to this that now thousands of traces are able to be generated in a single day, and trace storage and transfer is an issue. Where in the past, a floppy disk drive was used, the newest units come equipped with flash memory.

The advantages of ruggedness, reliability and size are obvious, but the best news is the transfer rates. Where some installers would reserve an hour a day for floppy storage, labeling and transfer to a desktop PC, a compact flash memory hard can do this in about one minute.

The most professional companies have created reference trace databases of their entire fiber network located on a server. OTDR’s can now use built-in LAN ports to mount network drives or themselves become drives on the network for high-speed media free transfers.

SOFTWARE IS MUSTWARE

Today, operators can have millions of different fiber segments and it can take fiber documentation records two years to catch up with the reality of the network. That makes a well-documented OTDR record priceless.

To handle all the documentation and analysis, trace view and analysis software has been created and enhanced over the years. Every manufacturer offers a free viewer that allows viewing of their files on PCs. The most common file type is Bellcore/Telcordia GR-196.

Saving files in this format will ensure it can be viewed in all software packages and in most high-performance OTDR’s currently on the market.

Documentation and analysis software helps users process and handle large amounts of traces.

The functions and steps exist to easily process and display entire cable worth of data instead of individual fibers. Batch functions for two-way averaging (to get accurate loss measurements) and batch print modes handle entire directories instead of one file at a time. Once again, a critical task is reduced to minutes from hours.

A free copy of a viewer compatible with all Bellcore/Telcordia OTDR trace files — the Trace Viewer III — is available for download at www.otdr.com .

THROW AWAY THE MANUAL

Today, users don’t have the time or the space to carry and read a manual. All modern OTDR’s have automatic settings that enable the measuring of most fibers with a minimum of keystrokes. Some actually incorporate the entire operating manual as context sensitive on-line help at the push of a button.

This can avoid embarrassing questions or make the difference between a useful measurement or waiting until someone is available during those inevitable middle of the night emergency restorations.

MORE THAN JUST AN OTDR

Modern OTDR’s are as useful as today’s SUVs. The difference being they cost less and you don’t need an engineering degree or license to operate it.

More specifically, high performance OTDR’s can also act as stabilized laser sources to be used for end-to-end loss measurements and fiber identification.

A ruby red light source (think of this as a precision laser pointer focused on your fiber) can help locate breaks within the first five kilometres and help when you are trying to determine the fiber of interest in a bundle.

Accurate power meters can also be incorporated, again minimizing the amount of training required, batteries to charge and bags to carry. Even within the OTDR, specialized modes can exist that are optimized for different tasks such as easy fiber break location or Multifiber Test modes that allow measurement analysis and storage with one button per fiber.

Some tips on using an OTDR:

Clean the front connector. A dirty or scratched connector reduces the dynamic range, increasing time to measure and reducing available distance and resolution;

Use the smallest pulse width you can. A smaller pulse width will allow you to see things happening on the fiber closer together;

Measure only as long as you need. Unless you’re testing submarine cables, the days where you need to measure three minutes to test are gone. Set the averaging time to a value that reduces the noise to an acceptable value;

Use Compact flash cards over floppies when available in order to save time and rework. Have you ever had a floppy fail? Try explaining to your boss that you sat on the test results and need to drive 200 kilometres to test again.

In summary, what does the future hold for the most common of fiber network instruments? As network speeds increase beyond 10Gb/s and multiple wavelengths are added at high power to long networks, signal dispersion becomes an important parameter to measure and control.

Dispersion is simply the cumulative effect of the fiber, most commonly through wavelengths and different polarization states in the signal, whereby the pulses that make up the signal get spread out creating interference and limiting range and speed.

An exotic measurement a few years ago, these measurements will become increasing popular as soon as the data rates climb.

As history has shown with OTDR measurements, once they become easier to do and the equipment became less expensive, installers and contractors will be asked to make dispersion measurements as commonly as they are asked for an OTDR trace at 1550nm.

Test groups looking to invest may wish to consider a portable platform that provides the speed they need today with effortless expandability to measurements they will be asked to make in the years to come. The perfect fit is out there, and it can make your job easier now and in the future.

Peter Schwe
iger has been with Agilent Technologies/HP for 16 years and is currently a Business Development Manager focused on Optical Network Test in the Americas. He can be reached at Peter_Schweiger@agilent.com.


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