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At the Fiber Optic Association (FOA), we get many technical questions from contractors, installers and end-users (www.foaguide.org). This month’s quiz includes real questions we’ve been asked recently. Can you answer them? Answers and explanations are in red.
1. We’re installing a fiber optic backbone with patch panels on each end. To connect fiber optic transceivers correctly, each fiber pair needs to be crossed. How is that most easily accomplished in the least confusing way?
A. Crossing the fiber pairs in the backbone between the patch panels
B. Using a duplex patchcord that is crossed on one end
C. Using a duplex patchcord that is crossed on both ends
D. Using simplex patchcords at both ends
Actually A, B and D all work, but B and D are harder to manage in a network environment. So A is the preferred method.
2. How can I measure the attenuation of the fibers in a cable on a reel without terminating them with connectors?
A. Use a bare fiber adapter.
B. Use a pigtail and a mechanical splice.
C. Use a pigtail and a fusion splice.
D. Any of the above
Any of these three methods work, depending on what tools are available.
3. It is not “connector loss” but really “connection loss” of a mated pair of connectors.
A single connector has no loss. Loss is defined as the light lost when it is mated to another connector.
4. Why do we see a “gain” in some OTDR splice measurements?
A. It is caused by different backscatter from two fibers spliced together.
B. The OTDR parameters are not set correctly.
C. The splice is too reflective.
D. You are testing a fiber amplifier.
“Gainers” are common in OTDR tests. They are caused by the difference in backscatter in the two fibers joined, with gainers shown when the lower loss fiber is closer to the OTDR. Reverse the direction of test and the splice loss will be higher than actual loss, so average the two measurements to get a trustworthy reading.
5. What is the minimum bend radius for fiber optic cable being pulled into conduit?
A. 2 × the cable diameter
B. 10 × the cable diameter
C. 20 × the cable diameter
D. 50 × the cable diameter
Manufacturers generally specify minimum cable bending radius as 20X the diameter of the cable under tension and 10X the cable diameter after installation.
6. Should we include bending losses when calculating a link-loss budget for a fiber optic cable pulled in conduit?
A. Yes, it is a legitimate loss factor.
B. No, it is impossible to quantify.
C. No, unless you know how many bends are in the conduit
D. No, because if properly installed, there should be no bending losses.
A loss budget should include estimates of component loss if they are properly installed. Bending losses are excess losses caused by poor installation and should not be included.
7. What determines the accuracy of the length measurement of fiber optic cable with an OTDR?
A. The measurement accuracy of the instrument itself
B. How well you know the speed of light in the fiber
C. How much excess fiber is in the cable
D. All of the above
All three of these factors contribute to the error. The biggest error is usually the excess fiber included in the cable, around 1 to 2 percent more fiber than cable length.
8. When we fusion-splice, the fiber splicing machine shows splice losses of 0.00 to 0.04 dB, but tests with an OTDR indicate splice losses of 0.2 to 0.3 dB. Which is correct?
A. The splice machine
B. The OTDR
The fusion splicer estimates loss based on its alignment system, while the OTDR uses backscatter trace analysis which may be different due to backscatter differences in the fibers spliced. If you need to know the real splice loss, it’s necessary to test with an OTDR in both directions and average.
9. What is the difference between “certification” and “verification” of Category 6 cable?
A. Certification tests to UL requirements, and verification tests for network transmission.
B. Certification tests to network standards, and verification calls for UL tests.
C. Certification tests to TIA cabling standards, and verification tests for network transmission.
D. They are the same thing.
Certification refers to testing to cabling standards, testing all the parameters specified for cabling without regards to its use. Verification refers to testing with a network simulation tester using actual network signals.
10. My Ethernet system will not work on UTP cabling longer than 100 meters.
The 100 meter limit is what is in the standards but many Ethernet systems will work over longer cables depending on the bit rate and cable rating (e.g. Cat 6).
HAYES is a VDV writer and educator and the president of The Fiber Optic Association. Find him at www.jimhayes.com.