Encircled flux: A practical view

by Eric Pearson, CFOS

I have been following the encircled flux (EF) standard from the sidelines. From this position I do not have the entire picture. With that understanding, I offer the following information.

Any measurement method has three objectives. First, the measured loss should indicate that the link will or will not work. Second, a subtle objective, the measured loss should indicate proper installation of the link components. Third, the measured loss should be the same with test equipment from different manufacturers.

Let's look at the first objective: The measured loss should indicate that the link will or will not work. The EF method of defining launch power distribution is a compromise of the distribution for the two types of multimode light sources: LEDs and VCSELs. In this characteristic, compromise, the EF method is no different from the coupled power ratio (CPR) method required by TIA/EIA-568-B. However, the EF method is a compromise that is different from that of the CPR method of TIA/EIA-568-B.

There are two consequences of this compromise. The first consequence is that the measured loss will be less than the loss experienced with an LED transmitter (100Base-F or 100Base-SX). The risk with testing for loss from LED transmitters is that an acceptable measured loss may not result in proper operation. Thus, troubleshooting and installation costs can increase.

The second consequence is that the measured loss will be greater than the loss experienced with a VCSEL transmitter (1000Base-SX or 10GBase-SX). The risk with testing for loss from VCSEL transmitters is that an unacceptable measured loss may not result in improper operation. Thus, installation or rework costs can increase unnecessarily.

Of course, if the link components (cable, connectors and splices) are properly installed, the measured loss is likely to indicate proper operation for either type of light source. But it is this objective -- determination of proper installation -- that is the second objective of any test method.

Let's look at the second objective: The measured loss should indicate proper installation of the link components. Both the EF and CPR methods have the same difficulty with indicating proper installation. This difficulty results not from the test method, but from the TIA/EIA-568 standard. The TIA/EIA-568 standard recommends a method of calculation of the loss acceptance value. This method uses the maximum attenuation rate and connector loss values. This method conflicts with the reality of proper product installation. Properly installed cables, connectors and splices will have actual losses that are closer to the typical losses than they are to the maximum losses. Thus, neither EF nor CPR test methods can fulfill the second objective of a test method.

Such fulfillment requires the designer and installer to establish their own method. In our fiber-optic installation and design training programs and texts, including Successful Fiber Optic Installation - The Essentials, we at Pearson Technologies recommend a method to resolve this difficulty.

For now, let's look at the third objective: The measured loss should be the same with test sources from different manufacturers. While writing about encircled flux recently you stated, "In the meantime, there was poor correlation among labs when international round-robin testing was initially conducted. Another round-robin test is now underway." Obviously, the EF method does not yet meet this objective.

What are the network designer and installer to do? Because, apparently, losses measured with test equipment from different manufacturers do not yet agree, designers and installers should reference the TIA/EIA-568-B test method (CPR). When the EF test method is shown to create no problems for the installer and tester, the designer and installer can reference the TIA/EIA-568-C test method (EF). Of course, doing so may require purchase of new test sources and/or equipment.

We justify this recommendation with two facts. First, the CPR method of TIA/EIA-568-B does result in the same measured losses with test sources from different manufacturers. Our testing indicates that losses are with 0.05 dB of one another. this value is less than the repeatability value of 0.2 dB stated on many connector data sheets. Second, the CPR method results in the same measured loss with test sources that have a CPR ratio that is slightly outside of that defined in TIA/EIA-568-B. In 2003, testing conducted by Pearson Technologies demonstrated these two facts.

Eric R. Pearson, CFOS is president of Pearson Technologies Inc. You can reach him via email at fiberguru@ptnowire.com.