Otdr Trace Analysis Essay

Introduction

The Telecommunications Industry Association, in TIA 568.3-D has defined a two-tier certification process that not only certifies compliance with cabling standards but also optimizes the installation quality by identifying marginal components. ANSI/TIA 568.3-D uses the terms “Tier 1” and “Tier 2” while IEC 14763-3 uses the terms “Basic” and “Extended” Test Groups:

  • Tier 1 / Basic Tests measure performance of the overall channel/permanent link and can be performed using a light source and power meter (LSPM) or an automated Optical Loss Test Set (OLTS).
  • Tier 2 / Extended Tests add measurements that evaluate components within the channel and require an Optical Time Domain Reflectometer (OTDR).

Tier 2 complements Tier 1 for a very simple reason: Tier 2 is more detailed but is more uncertain than Tier 1. At first this might sound like an oxymoron but it results from fundamental technical principles, true for OTDRs whether past or future models. In fact, OTDR's can deliver accurate measurements, but in order to do so, proper techniques must be employed – techniques that are all too often bypassed for being too complex. This paper describes new methods and procedures that deliver more correct and repeatable results and, at the same time, significantly reduce the overall testing time.

OTDR’s Have A Point View

We will now take a step back and describe various OTDR test scenarios as they typically occur today. Measuring the loss of individual events, such as connectors and splices, as well as the overall link loss unfortunately depends on the direction from which the measurement is made. Even though you won't find the term on Wikipedia, we will call this “Directivity”.

“Directivity” results from differences in diameter, backscatter, numerical aperture and index of refraction of the link under test as well as the launch and tail fiber. In singlemode fiber, directivity is influenced by differences in backscatter coefficients between different fibers. In multimode fibers, core diameter and numerical aperture play a bigger role.

One of these directivity effects, differences in backscatter coefficient, can be removed through bi-directional testing. If the fibers on either side of a connector have different backscatter coefficients, the connector will appear to have greater loss when tested in one direction than it does when tested in the opposite direction.

OTDR - Optical Time Domain Reflectometer

 

OTDR - Optical Time Domain Reflectometer

An Optical Time Domain Reflectometer (OTDR) is an important instrument used by organizations to certify the performance of new fiber optics links and detect problems with existing fiber links.

Certifying New Links

The health of your network depends on the quality of your network infrastructure. This quality begins with complete certification by contractors or systems integrators that the fiber cabling infrastructure was properly installed. Maintaining a reliable fiber plant is also essential in protecting your business-critical applications. As a network administrator, it is important to understand how to get the best performance from your cabling investment and how to solve problems quickly when they occur.

Most customers are familiar with Basic Certification - sometimes known as Tier 1 fiber certification – which measures attenuation (insertion loss), length and polarity. This test ensures that the fiber link exhibits less loss than the maximum allowable loss budget for the immediate application. Simple Light Source/ Power Meters or more automated Optical Loss Test Sets can perform this function.

Extended or Tier 2 fiber certification supplements Tier 1 testing with the addition of an Optical Time Domain Reflectometer (OTDR) from end to end. An OTDR trace is a graphical signature of a fiber's attenuation along its length which provides insight into the performance of the link components (cable, connectors and splices) and the quality of the installation by examining non-uniformities in the OTDR trace. More advanced units can provide easy to understand Event Maps and loss values for individual components as well as the link.

An OTDR trace helps characterize individual events that can often be invisible when conducting only loss/length (tier 1) testing. Only with a complete fiber certification can installers have a complete picture of the fiber installation and network owners have proof of a quality installation.. This fiber test certifies that the workmanship and quality of the installation meets the design and warrantee specifications for current and future applications.

Viewing trace results is simplified with advanced features such as pinch and zoom

Bi-Directional Testing

Bi-directional testing of fiber links for Tier 2 (OTDR) testing is not only required by industry standards and most manufacturers for warranty, it's also the only way to know the actual overall loss for a link. That's because measuring the loss of fiber connectors and splices, as well as overall link loss, depends on the test direction. Testing a fiber link in one direction can give you different results than testing the same fiber link in the opposite direction.

Because of the significant time and cost involved in testing from both ends, technicians often try to save as much time as possible by testing all links from one end before moving to the other end. Unfortunately this method does not work. To accurately test a fiber link in both directions, the launch and tail cords must remain in their initial measurement positions (even the standards say so) during both tests. But that is simply not possible if you test all the links from one end before moving to the other.

To solve this dilemma, you can test two fibers at the same time and use a loop to connect the two fibers together. This allows the two fibers of a duplex link to be tested in one shot without moving the OTDR to the far end. OTDRs like Fluke Networks’ OptiFiber® Pro feature “SmartLoop” Technology that checks for the presence of the launch, loop and tail fiber when testing a duplex fiber link.

The OptiFiber Pro Event Map displays Pass/Fail results and the loss for the overall fiber as well as for each event.

With SmartLoop, technicians can deploy multiple loops at the far end and perform a set of bidirectional tests without ever having to leave the near end--cutting test time by at least 50%.

Detecting Problems

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OTDRs are also used for maintaining fiber plant performance. An OTDR maps the cabling and can illustrate termination quality and location of faults that may hinder network performance. An OTDR allows discovery of issues along the length of a channel that may affect long term reliability. OTDRs characterize features such as attenuation uniformity and attenuation rate, segment length, location and insertion loss of connectors and splices, and other events such as sharp bends that may have been incurred during cable installation or afterwards.

When selecting the right OTDR, network engineers should make sure the tool has certain functionality, such as loss-length certification, channel/event map view, power meter capabilities, an easy-to-use interface, and smart-remote options. In addition, the OTDR needs to provide a reliable means to document the results. Features that make the OTDR easy to operate such as automated setup and Event Map are essential for users who aren’t OTDR experts but need to locate problems fast.

Tools such as the award winning OptiFiber® Pro OTDR provide the ultimate testing and troubleshooting solution to ensure the health of your most critical network cabling. With the OptiFiber Pro OTDR, network engineers have the in-house capability to perform inspection, verification, certification, troubleshooting, and documentation of fiber cabling in a single, easy-to-use OTDR tool.

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