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Tue 26 Feb 2008 04:00 AM

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Simplifying OTDRs

Christian Schillab, segment product manager at Fluke Networks, Europe sets out to simplify and make easy OTDRs for integrators and end-users.

Christian Schillab, segment product manager at Fluke Networks, Europe sets out to simplify and make easy OTDRs for integrators and end-users.

Identifying bottlenecks is the strength of an OTDR. The OTDR sends a pulse of light into fibre and measures the light reflected back at each component as the light lost at that component.

Not long ago the yardstick for fibre optic cabling was the IEEE 100BASE-FX standard, which supported a bitrate of 100Mbps over a channel with an attenuation of 11dB. Today, in order for IEEE 10GBASE-S to support a transmission rate 100 times higher, the transmission channel must attenuate the light by no more than 2.6dB.

It is this tightening of requirements for the physical media that represents a challenge for all components used to build and test a transmission path.

A standards compliant connector may contribute up to 0.75dB (0.5dB typical) to the total loss. This would mean that if the user patches two fibre segments together there would be a total of four connectors, which could - even though each individual segment is compliant - result, in worst case, in a total loss of (4 x 0.75dB) 3dB.

This exceeds the loss budget left for the entire link, and with a negative allowance left for the fibre.

This is where new test methods are required. Installers who work with optical fibre are no doubt familiar with optical loss test sets (OLTS). Performing a loss length test with an OLTS is an essential part of fibre installation. Every link needs to be tested to ensure the link is within the loss limits.

But an OLTS will only show if a link has passed or failed. If it fails, it will not show you why it failed, or where. For these answers, an optical time domain reflectometer (OTDR) is the best option. Using an OTDR need not be complicated or confusing. Understanding a few basic concepts will make OTDR use as straightforward as using a copper certification tool.

Testing fibre links as defined by national (TIA-568B) and international (ISO-11801) standards includes the use of an OLTS. Recently updated standards which focus on test methods for installed fibre links, such as ISO 14763-3 and TIA TSB-140, now recommend the complementary use of an OTDR.

These new standards add the use of an OTDR to verify not just that the link has passed, but to ensure the quality of each installed component on the link. Two levels of testing are defined in these updated standards: basic (Tier 1) testing uses an OLTS and extended (Tier2) testing involves the use of an OTDR in addition to OLTS.

The following example should help demonstrate how an extended test regime can help to ensure consistent quality during installation. In the following we assume that the first connector of a link performs extremely well while the second on is poorly installed or contaminated.

The measurement with the OLTS will show that the link passed with a slim margin of (0.95 - 0.93 = 0.02dB) but does identify the second connector as the bottleneck.

Identifying bottlenecks is the strength of an OTDR. The OTDR sends a pulse of light into fibre and measures the light reflected back at each component as the light lost at that component. The same is true for back-scattered light along the length of the fibre.

Setup required

An OTDR can produce accurate, highly detailed measurements, if the correct setup of the OTDR and necessary accessories are employed effectively.

Recent versions of standards like ISO 14763-3 make an attempt to specify all the necessary elements for a correct measurement with an OTDR, thus eliminating common sources of measurement error.

These include:

(1) specifications for launch and receive fibres

(2) correct use of launch and receive fibres

(3) positioning the cursor for the correct reading of link, component and segment attenuation

(4) specifications for conditions under which it is vital to measure fibre in both direction.
Users may view these setup requirements as overly complex, and explains why many view the OTDR as a tool for experts only. This is also why installers and contractors may choose not to bid on projects which require an OTDR, or subcontract this work to a company specialised in fibre.

Such thinking is in strong contrast to the certification of twisted pair copper cabling systems, where after setting the correct standard a single press of the auto-test button does everything else.

Instead of thinking complicated and expensive, installers should think easy and simple. Installing and testing fibre may be new, but the right equipment can make the job easier.

Fortunately, the actual use of an OTDR is not a challenging as it appears. Making sure that test leads, launch and receive fibres are in a crisp condition, clean and correctly connected will always be the responsibility of the user. But the remainder of the setup steps can be taken care by the instrument. Newer OTDRs will draw a picture of the proper setup configuration.

The user merely needs to make connections and have the instrument "learn" the launch and receive fibres.

After this very quick step the tester will be ready to certify links and all included components for their compliance. Often a project specific standard which is derived from the manufacturer's data sheet or a reference implementation will be used to set these limits.

Pass, Fail ... or just squeak by

When the tester is properly configured, the tests are as simple as a common copper certification test. The most common situation should be that the link passes, and the pass indication on the summary screen should be sufficient. The user knows that the tester had evaluated all elements of link. Results are stored for later reporting.

The instrument also automatically subtracts the contribution of the launch and receives fibre to the total link, showing the reader only the total overall loss.

While this example is sufficient information for a passing link, the user will need to dig deeper and get more detailed information if the link or parts of it failed the specified limits. The user could see that the overall loss is 1.07dB and within the limits, but there one "bottleneck" which contributes 0.92 to the overall loss.

A fully automatic OTDR automates the test to the same level as a copper field tester, using internal expert diagnostics to interpret all the information from the OTDR test, and present the results in a simple and easy to understand table format.

Anyone can be a fibre expert

Many installers have a bad reaction when they hear the term "OTDR." But instead of thinking "complicated and expensive," they should think "it's just like my copper tester, and a chance to grow my business." Installing and testing fibre may be new to contactors, but the right equipment can make the job easier.

The key to today's OTDRs is you don't have to be an expert reader. If you are a copper installer, an OTDR can offer you three important things:

1. Expert diagnostics that make the OTDR work much like your copper certification tool

2. A way to bid on more jobs, grow your business and increase profits

3. The ability to move your copper knowledge into a new area and become a fibre expert.

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Dave Wilson 12 years ago

This is a good article simplifying the differences between OTDRs and OLTS. However I do take exception with "Often a project specific standard which is derived from the manufacturer's data sheet..." None of a project designer, network owner, manufacturer or even a training/educational group (i.e. BICSI) can publish a "standard". These are only the realm of organizations such as IEEE, ISO, EIA/TIA etc. Other bodies can create specifications, guidelines, and proceedures etc. but these cannot be referred to as standards. Thanx Dave Wilson