For the past three newsletters, we’ve been highlighting portions of interviews with some industry thought-leaders concerning the use of innovative technologies. We’re wrapping up this series today with an extremely hands-on and practical conversation that Steve recently had with Jason Tarn from Fluke Networks concerning “Encircled Flux” technology. Steve: Jason, as I understand the issue, especially as you go to higher speeds (such as 10 Gbps), some of the readings for testing the quality of a link can get kind of inconsistent. For instance, if we were to use a different source, the power meter at the remote end may show a value of 3.0 dB as compared to 2.0 dB.
For the past three newsletters, we’ve been highlighting portions of interviews with some industry thought-leaders concerning the use of innovative technologies. We’re wrapping up this series today with an extremely hands-on and practical conversation that Steve recently had with Jason Tarn from Fluke Networks concerning “Encircled Flux” technology.
Steve: Jason, as I understand the issue, especially as you go to higher speeds (such as 10Gbps), some of the readings for testing the quality of a link can get kind of inconsistent. For instance, if we were to use a different source, the power meter at the remote end may show a value of 3.0 dB as compared to 2.0 dB.
Jason: To clarify, as the expectations of higher bandwidth supportability increase, particularly at 10Gbps and through 40/100Gbps, it is not so much that the readings become inconsistent, but more that because the loss budgets are even tighter at higher bandwidths, any measurement variability has a larger impact on whether a link will pass the loss limits.
In other words, as networks move from 10Gpbs to 40Gbps, the maximum channel loss drops more than 25%, from 2.6 dB to 1.9 dB! Because 1 dB (the difference between a loss reading of 3.0 dB vs. 2.0 dB, in this case) of measurement uncertainty accounts for HALF of the loss budget for 40Gpbs, it is critically important that all steps are taken to ensure that one’s readings are as precise and repeatable as possible.
Steve: So why do we see these variations? I assume that the variation pretty much must be in the source since same power meter is reading the results.
Jason: Great question Steve. Because multiple sources are often used when making measurements, the effect of obtaining different readings is not unusual – even when the same power source is used. Differing readings are possible even when one uses the same type of laser transmitter (such as a VCSEL) because of the variability of launch conditions into the test reference cord and the link-under-test. Conversely, LED transmitters overfill the fiber leading to overly pessimistic results (and potentially false failures).
Steve: OK. So now that we know what the problem is, what do we do? In as simple terms as possible, what is “Encircled Flux” (EF)?
Jason: Encircled Flux is a new standard that dictates multimode launch conditions. Because of the possibility of variability in link-loss measurements in using equipment from different manufacturers, Encircled Flux was formulated to reduce uncertainty in launch conditions of Gigabit Ethernet fiber transceivers – especially relative to previous standards – by setting universal upper and lower limits in order to obtain repeatable and consistent loss measurements.
Steve: And when you mention that EF “tightly controls the number of mode groups,” can you give us a feeling for what that means?
Jason: Modal dispersion is a phenomena where an optical signal spreads over time and distance due to the nature of varying wave propagation velocities in multimode fiber. This signal dispersion results in multiple wave paths, or “modes”, that travel throughout the fiber which contribute to variability in optical power and loss measurements. By controlling the number of mode groups, one increases measurement repeatability – reducing uncertainty. Under the Encircled Flux standard, variability in loss measurement readings will be limited to 10%, which is a major improvement over the previous standard of Mode Power Distribution (MPD), which allowed for up to 40% variability in loss measurements.
Steve: So, not surprisingly, Fluke Networks has something to bring to the table here to help eliminate this uncertainty. Can you tell us a bit about the Encircled Flux “Launch Controller” and how it works? Is there an integral linkage so that I can only use this with other Fluke Networks’ gear?
Jason: Fluke Networks’ EF Launch Controller is a specially tuned device that controls the number of mode groups (or paths) launched from the test cord in accordance with Encircled Flux requirements. Its application is extremely straightforward: to test with the EF Launch Controller, one only needs to substitute it for the test reference cord connecting the source to the link-under-test (also recognized as the test lead with a mandrel on it) – and then follow normal testing procedures. And one of the great benefits of Fluke Networks’ EF Launch Controller is that because it is not internal nor specific to a source type, its compatibility is not limited to Fluke Networks equipment. Provided that a manufacturer’s source is Encircled Flux compliant, the EF Launch Controller will ensure your measurements are in within Encircled Flux standards.
Many thanks to Jason for this quick tutorial! And, again, there are diagrams to complement the discussion at Webtorials, where you may also ask additional questions.
