A Practical Explanation of the 3 Test Lead Methods Suggested by the TIA Standards
Comment: We often are asked questions about testing installed fiber optic cables that indicate the industry standards are confusing, have little information on measurement accuracy and no guidelines for troubleshooting. This presentation is an attempt to clear up some of this confusion. But remember, as Bob Metcalfe, co‐inventor of Ethernet, says, "Standards are wonderful, because we have so many to choose from!“ And –they are suggested ways to do something.
OK now ‐‐‐ How many ways are there to test fiber? : Answer – a bunch!
But this presentation is only going to deal with 3 of the most basic methods used in practical applications.
Update from years ago! Don't you remember, the old TIA OFSTP‐14 was replaced by a new ISO standard. The TIA adopted IEC 61280‐4‐1 as the replacement of OFSTP‐14. Most of the two documents is the same, with some important exceptions.
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For insertion loss, three reference methods are still approved, but the nomenclature is different ‐ no more "Method A, B, or C" designations‐ it's now 1, 2, or 3 reference cables.
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OTDR testing is now an approved second tier test method as long as you use both launch and receive cables
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Reference test cables with "reference grade connectors" are recommended.
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Methods are given for testing and verifying the loss of reference test cables.
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For multimode modal control, CPR with a mandrel wrap is gone, at least for 50/125 fiber at 850nm, replaced by "Encircled Flux," a complex ‐ and not completely debugged ‐method of measuring the source output.
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Note: Most of the changes are nomenclature. In the meantime, continue testing as usual until advised by the FOA.
Internet Explanation as to why the three methods exist.
Instructions: Read once. Have a drink. Read again. Go to sleep. Wake up in the morning. Drink some coffee or stiff tea. Read again. Wait 30 minutes. Write down what you know without copying the words below. Now ‐‐‐go out and do it. How did that work out for you???
Special Note: Most fiber optic LANS and WANS use the same connector style throughout except, perhaps, where it plugs into an active device via a patch cable at either end.
GET READY HERE WE GO!
Two Reference Test Lead Method
Instructions, Applications, and Benefits
Instructions: Zero out two test leads remembering to use a good adapter during the process. Disconnect the zeroed out test leads. Lay the adapter off to the side. Plug in the Launch and Receive Cable to the network under test using customer adapters mounted within the patch panels.
Applications: This procedure is commonly used and recommended by the TIA and others to determine insertion loss across a customer’s network as it includes the loss created within the mating adapters in the customer’s patch panels due to misalignment, the loss of the connector itself AND the loss in the fiber running between the patch panels.
Benefits: Quick determination of the insertion loss between patch panels. But, this is a “partial” system test as it DOES NOT include the loss of the patch cables that are attached or will be attached at either end of the network.
Three Reference Test Lead Method
Instructions, Applications, and Benefits
Instructions: Zero out three test leads remembering to use good adapters during the process. Disconnect the “Golden” test lead. While keeping your adapters attached, plug in one end of the cable to be tested to one test lead adapter and go to the other end of the cable tobetested and plug in the other end of the cable to be tested to the other test lead adapter.
Applications: You will be able to determine the actual attenuation of the fiber under test without the adapters in the customer’s patch panels affecting the final test.
Benefits: This is a great procedure to use when troubleshooting bad adapters in the patch panels. For example: Using this method, If you test the installed cable plant and it tests at .5dB and then 3.5dB when it is plugged back into the adapters in the patch panels, you know you have an adapter issue and NOT a cable issue. Another BIG benefit is on the next slide!
____“Golden” Test Lead
____Your “Good” adapters from your test kit. (Both Sides)
____Connectors + Mating Adapter (Both Sides)
Insert cable to be tested between YOUR adapters!
Three Reference Test Leads (Old Method “C”)
Instructions, Applications, and Benefits
One of The Main Benefits
“Home Run” cable between
patch panels.
Patch Cables connecting active devices at both ends
Point B: “Fire” out.
Point A: “Fire” in.
Adapters (customer’s) typical both
patch panels
Customer Optoelectronic Device:
Computer, Server, switch, Router, etc.
(TxRx)
Customer Optoelectronic Device:
Computer, Server, switch, Router, etc.
(TxRx)
Customer Patch Panel
Customer Patch Panel
Considering the above network (most networks look like this), if a customer is knowledgeable about LANS and WANS he/she will ask you to perform a “Channel” test. A “Channel test includes all patch cables and everything in between. Channel testing looks at the whole combined cabling circuit. An example of a combined channel cabling circuit is a patch cord, wall plate, horizontal cable, patch panel, and patch cord. The maximum length for this combined cabling circuit is for MM networks is 100m or 500m depending on 10/100, 1Gig, or 10Gig requirements.
This test is designed to be run using cable test equipment connected to both ends of the cabling circuit after it has been installed. Because there are no unknown components the testing limits are more relaxed. Circuits that pass this test are suitable for network deployment.
“Home Run” cable between
patch panels.
Patch Cables connecting active devices at both ends
Point B: “Fire” out.
Point A: “Fire” in.
Adapters (customer’s) typical both
patch panels
Customer Optoelectronic Device:
Computer, Server, switch, Router, etc.
(TxRx)
Customer Optoelectronic Device:
Computer, Server, switch, Router, etc.
(TxRx)
Customer Patch Panel
Customer Patch Panel
Instruction: To perform a “Channel” test on a piece of fiber (patch cables and everything in between), one must connect a test lead at Point A and the other at Point B.
Thus, you MUST use the 3-cable reference method. As in the previous slide, zero out 3 test leads. Remove the “Golden” test lead and plug Point A into one of your existing adapters and Point B into the other. Presto! You have just performed a Channel Test. This is a great test as it provides the loss (dB) the signal will experience
through the media through which it is traveling – from entry to exit.
One Reference Test Lead (Old Method “B” Setup)
Instructions, Applications, and Benefits
Instructions: Zero out one test lead. Disconnect one end from the power meter only and never disconnect from the power source at anytime during the procedure. If it is removed for any reason, then the zero process must be repeated. One cannot hold a fiber optics connector in the sky and determine its insertion loss unless another connector (one which has been zeroed out) is connected to it. Test one connector and write down the result and then test the other and write down the result. The one with the higher loss is the one to replace first! If both have high loss --- well you know what to do --- either replace the connectors or replace the patch cable (if you have one).
Applications: There are several. But, the most useful is troubleshooting connectors to see which one is worthless and which one is not so worthless. This procedure will work on short patch cables and on some installed networks.
Zero out one test lead.
FOTP – 171: Provides for documentation on how to check individual connectors of a patch cable.
Benefit: Lets say the technician is on a job site and he has several patch cables and none seem to be working properly and he needs to replace the connectors on one or two of them to get the network up and running. Rather than just cutting off both ends, the tech can use this procedure to determine which one to cut off first.
It is interesting to note that, in some circumstances, this same single ended test can be used to troubleshoot network connectors as well. But be sure to use adapters out of the test kit and NOT those in the customer’s patch panels.
If you would like to learn more about how to PROPERLY test your fiber optics network (inside and outside plant), join with us in our Certified Fiber Optics Specialist in Testing & Maintenance (CFOS/T) Course. You will be glad you did!
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