Fiber Optics Cable Testing
It is interesting to contrast measurements necessary to qualify a fiber installation with those of a copper installation. As we have seen, to certify a copper installation it is important to consider wiremap, length, attenuation, NEXT and PSNEXT at both ends, ACR at both ends, ELFEXT at both ends, return loss at both ends, delay, and delay skew. In contrast, to certify a multimode fiber installation requires only attenuation at two wavelengths, and possibly length. Because UTP testing is so complex, much effort was undertaken early on to speed up and simplify the process. UTP installers can push one button and have a myriad of measurements performed quickly over multiple pairs and test certification performed in a few seconds. In contrast, most fibers have been tested in a methodical, manual fashion, one fiber at a time. The good news is that fiber test technology has recently caught up with UTP test technology. With tools now available such as the Microtest CertiFiber, that can certify multiple fibers with multiple measurements with one push of a button, productivity of fiber testing should increase significantly.
In addition to falling test costs, the cost of fiber itself is continuing to fall. New techniques are making splicing and terminating faster and easier. But with falling prices, new ease of testing, and fiber's inherent advantages, we will see more and more fiber appearing in traditional 'UTP territory'.
Fiber optic cable comes in two basic types: multimode or single mode. Horizontal cables usually contain a minimum of 2 fibers, and backbone cables often contain 6 or 12 fibers. But how is it tested?
In the absence of the existence (or awareness) of today's standards, much of today's installed fiber plant has been tested against somewhat arbitrary requirements. We have seen fiber installed where the guidelines were: "They're only using Ethernet, so keep the loss under 6 dB". While this simple approach may work for 10BASE-FL, it will fail 1000BASE-SX. As a network manager, you need to know what types of applications your fiber needs to support. As an installer, you need to know what losses the applications support so you can put appropriate boundaries on your qualification tests.
The philosophy on fiber testing runs the full range of:
- if you can see a flashlight through it let's call it good,
- run a power meter loss test at one wavelength in one direction and keep the loss under X dB,
- measure loss at two wavelengths
- measure loss at two wavelengths in opposing directions on different fibers in each pair
- measure loss at two wavelengths in both directions
- measure length and loss (version b, c, d, or e above), or
- do a full OTDR trace and provide a graphical printout
What is really required? Some experts say that fiber performs so well, if it's been put in by a competent installer, all you need is a simple light continuity test and you can be sure it's good. Other experts swear by the need for full certification with an OTDR. Most fall in the middle. You need to comply with the appropriate standards, all of which require power meter testing (including single mode test standards). In North America, that means the new TIA 568-B.3 (Optical Fiber Cabling Standard), TIA 526-14 (Test methods for multimode fiber), and TIA 526-7 (Test methods for single mode fiber).
It's clear that you need to ensure there is a low enough loss to support any intended application you may run in the future. You also need to ensure any single fiber segment does not exceed the maximum supportable distance for its application. Those are the musts. Given that fiber losses can be different in each direction, it's a good idea to test in the direction of transmission too. Fiber loss can be VERY different in each direction. Consider a 62.5 micron fiber spliced by accident to a 50 micron fiber. If you're testing from the 50 micron fiber to the 62.5, you'll see little loss. But if you're testing in the other direction, from the 62.5 micron fiber to the 50 micron, you'll see a significant loss. It really is critical to test in both directions. For each fiber tested, a summary of the applications it can support should be provided.
But how can you know what's required? Some field testers, such as the Microtest CertiFiber, have all the data programmed in memory, and do these tests and comparisons automatically. But if you are like most people, you may only have access to a simple power meter and light source.
Optical Time Domain Reflectometers (OTDRs) are used for troubleshooting fiber optic cabling. OTDRs can measure length, find the location of a break in the cable, measure the loss through connectors and splices, and do much more. However, they are not suitable for making end to end power loss measurements. Since power meter measurements operate like a transmission link, all fiber test standards specify using a power meter to measure loss. Even the outside plant singlemode test standard, TIA 526-7, says you should accept the power meter results. In spite of this, many people persist in asking for OTDR traces on installed premise multimode fiber. Such results are far more expensive to make than loss measurements, and provide less useful information.
The important thing to remember in fiber optic cable testing is to ensure that the intended applications will run with a reasonable margin for safety.