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Refinement Tool - Use of automated test limits for cost and error reduction

July 15, 2010

Based on a white paper by Fluke Networks
The quality of an enterprise fibre optic network depends on the test equipment used during its installation. Greater precision and depth of test coverage make it possible to build and certify high quality networks.

Technologies and standards for fibre optic cable testing
There are two primary technologies for testing fibre optic cabling: optical loss test sets (OLTS) and optical time domain reflectometers (OTDR). OLTS measures the absolute end-to-end loss of light with a power meter and source for each link.OTDR analyses the loss and reflectance of light along each link. Together, these technologies provide complete test coverage for installing enterprise fibre optic networks.

The standards for the use of these two technologies are addressed in published guidelines from the Telecommunications Industry Association. The guidelines specify two levels of testing, Tier 1 and Tier 2, which govern the use of OLTS and OTDR technologies.

Tier 1 testing is required in all fibre optic cabling links. In Tier 1, an OLTS is used to measure the total attenuation on each link; fibre length and polarity are also determined, either by measurement or calculation. Tier 1 testing certifies the overall performance of each link, but it does not provide any direct information about the quality of its components and construction.

Tier 2 tests each link with an OTDR and analyses the connectors, splices and the fibre itself by capturing and measuring the light that is reflected back from them.

Tier 1 and Tier 2 testing provide full test coverage for enterprise fibre optic networks. However, traditional test instruments require the time and attention of a skilled technician. This makes the process expensive, time consuming and vulnerable to human error.

To address this problem, OLTS equipment manufacturers began to build the functionality for automatically performing the evaluation of test results.This was done through the use of test limits entered by the operator. The OLTS would apply the limits to each test result, automatically compare it against performance standards, give a pass/fail result and record all the test data.

For Tier 1 testing, this has led to increased productivity, lower costs and the elimination of human error. However, Tier 2 test results using an OTDR need to be evaluated by a skilled technician and continue to be expensive and difficult.

To solve this problem, new-generation OTDR test equipment with automated test limit analysis capability has been developed. The use of this equipment has resulted in a significant reduction in Tier 2 certification cost and time.

Automated OTDR test limits
Analysing and evaluating Tier 1 and Tier 2 fibre test results require time and expertise.Historically, high test coverage also meant high costs. The solution to this problem was the use of automated test limits. Test limits have long been available with OLTS equipment, and the industry has been relying on them for Tier 1 testing.

Some of today's new-generation OTDR testers include automated test limit capability. They perform many of the functions that would otherwise require a skilled technician, and the benefits have been significant.

Benefits of using automated OTDR test limits
Rapid and accurate testing: With automated test limits, an OTDR can analyse a trace much faster. Manual testing requires moving the cursor to each of the many peaks and drops in the OTDR trace, measuring them on-screen and saving the results. The technician also has to read and evaluate every connector and each splice on each trace. With automated test limits, the entire process happens in seconds and the technician only has to look at anything that fails.

Custom limits; per project specifications: Test limits can be set directly to the project's contract specifications. Network designers write specifications that reference various standards and dictate what testing must be performed. Once the limits are entered into the OTDR, every test result is automatically evaluated against these limits, thereby allowing the technician to do rapid, accurate testing without necessarily knowing the project specifications.

Increased profitability: Automated test limits increase profitability by making OTDR testing faster and error free, and by reducing the need for specialised skills or training.

Verifying fitness for future usage demands: Completely certifying a network requires testing the performance of all the components, not just the end-to-end optical loss. OTDR testing can certify that the network is ready for future demand.

Certification: The OTDR stores the results of each test. These results show that the installation is certified, implying that it has been tested against the design parameters for the project.

Tier 1 testing: Methodology using an OLTS
An OLTS determines the total light loss along the fibre link using a known light source at one end of the fibre and a light meter at the other end. The total loss is compared to the test limits, and if the loss is within the specified limits, the link passes.

This method gives a pass/fail result for each fibre link, but treats each link as a black box and does not provide information on anything internal to the black box. This is acceptable when a link fails but can be a liability when a link passes.The reason is that if a link fails Tier 1 testing, the technician knows there is a problem and can repair it. But when a link passes Tier 1, the technician does not know whether there are potential problems. A passing link might have marginal connectors, bad splices, or other problems that are not revealed. Therefore, relying on Tier 1 alone may result in a false sense of security.

A link might have a dirty connector with a relatively high loss. As long as the total loss is within limits, the link would pass Tier 1 testing.

Tier 2 testing: Methodology using an OTDR
An OTDR sends a known light pulse to the fibre link, and measures the strength and timing of the reflected light. The OTDR then creates a trace of the strength of the reflected light as a function of distance along the fibre. This trace provides detailed information about the link and its fibre, connectors, splices, bends and breaks.

Tier 2 testing with an OTDR is valuable both for troubleshooting and for preventing future problems. For links that fail Tier 1 testing, the OTDR can troubleshoot and isolate the cause. For links that pass Tier 1, an OTDR may reveal latent problems, including sub-par splices, dirt or marginal connectors.

Latent problems that Tier 2 testing can prevent
A drop in performance when utilisation increases: Marginal components can raise the transmission control protocol/internet protocol bit error rate and the packet retransmission rate, thereby affecting performance. This may not be noticeable on a lightly loaded network but will matter as network utilisation goes up.

Low power: Dirty, contaminated and damaged connectors may lead to major problems; they cause the majority of all network failures. A sensitivity analysis of optical communication links shows that the three major contributors to a link's robustness are the amount of power the laser couples into the fibre, the receiver's optical sensitivity and the amount of connector loss within the link. These three factors can determine the magnitude of optical power reaching the receiver. If a marginal connector reduces the power reaching the receiver, the robustness of the link is diminished.

Mode selective loss: Marginal components can result in mode selective loss, which discriminates the modes propagating within the fibre. As mode selective losses increase, the bit error rate falls.

Reflections: Contamination on a connector's surface may cause a small air gap which causes high reflection. The reflected light travels back into the laser cavity, causing its output power to fluctuate.

Multi-path interference: This occurs when light is reflected back from an air gap in a connector and interferometrically combines with the rest of the incoming light to cause constructive or destructive interferences. Consequently, the transmitted power can vary by 0.7 dB.

Failures during moves/additions /changes: Poor splices or connectors on two different links can cause failures -­ even when each link passes Tier 1 testing -­ if they are later brought together on to the same link through moves, adds or changes.

Failures can also occur during changes if connectors are dirty. Each time a dirty connector is moved, the dirt can move towards the core and block the light. Moreover, dirt can be ground into the glass when the connector is mated or re-mated, and damage the glass with nicks, scratches and chips. This can progressively degrade the fibre and result in link failure.

Failures from component degradation: A dirty connector can degrade over time. Even without the effects of mating and re-mating, a fibre that passes optical test at the time of installation can fail in the future.

Conclusion
With the spread of enterprise fibre optic networks and rise in network utilisation, in-depth test coverage becomes essential to solve latent problems. Complete certification testing of fibre optic cabling is more important now than ever before.Performing both Tier 1 and Tier 2 testing using OLTS and OTDR provides the necessary depth of coverage. The use of automated test limits keeps costs down and reduces human errors.


 
 

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