Understanding fiber network requirements and the role of the tools available to test these networks will make testing quicker and more accurate
You've just installed and terminated a premises fiber network in a commercial building, so you're ready to pack up the tools and move on to the next project. The general contractor and/or building owner, however, want assurance that you've properly installed the cabling infrastructure.
So, how do you tell a good fiber link from a bad one? Furthermore, what's the definition of a good fiber? And what type of tester should you use for which applications? These questions are common in our industry. Let's try to answer them.
Look to the standards for help. Part 1 (General Requirements) of TIA/EIA-568-B.1 “Commercial Building Telecommunications Cabling Standard” rates the quality of a link based on several performance characteristics. This classification or definition of quality, based on the performance characteristics for horizontal, backbone, and centralized applications that are listed in Table 1 is widely accepted throughout the industry. BICSI has adopted this definition and references it in the organization's Telecommunication Distribution Methods Manual (TDMM).
The fiber link test procedure consists of three steps:
Measure the optical link loss at the appropriate wavelength(s) as dictated by the application and fiber type.
Measure the link length.
Compare the results to the link loss budget, which represents the uppermost limit above which a fiber is considered no longer usable.
Premises LAN networks differ from their long-haul WAN cousins in several ways. First, multimode fiber dominates in premises applications. Second, premises link lengths are relatively short with few or no splices. Third, optical loss events are closely spaced and the number of terminated links to test is large. For example, it's not unusual for the number of links to exceed 1,000, especially when the installation includes fiber in the horizontal to desktops. For these reasons, the tools you use to test LAN networks will be different from those you use to test WAN networks. But knowing which one to use and when can be a challenge if you don't know what you're looking for.
While you have a choice of troubleshooting and testing tools, certain types of fiber test equipment are better suited for premises testing.
The following four fiber testers have their own capabilities that are necessary for different applications. Table 2 at right gives a overview of their strengths and weaknesses.
For visual verification and troubleshooting, you'll need a visual fiber tracer and a VFL. The latter uses a bright red laser that's visible through some cable jackets to pinpoint the location of significant optical events like breaks in the fiber or loss caused by overly tight bends in the cable (Photo on page 33). However, the faults must occur where the cable is in the open and visual location is possible. You can also use VFLs to verify the integrity of mechanical splices and factory-polished/no-epoxy connectors during termination.
Optical loss test set (OLTS). This test tool provides a quantitative optical loss measurement using a light source and an optical power meter. The tool directly measures loss by computing the difference between the optical power entering a fiber and the optical power exiting it. This test method can accurately predict actual network loss because it duplicates the transmission path and the wavelength of the active network. A wide variety of sources and meters are available. Basic test sets for multimode fiber include LED sources emitting at 850nm and 1,300nm and an appropriately calibrated meter. More advanced OLTS sets include vertical cavity surface emitting laser diode optics (VCSEL) and laser sources for multimode and single-mode fibers and meters with results storage to speed testing and simplify reporting.
This advanced diagnostic tool for optical fibers allows you to take a snapshot of a fiber link. Like optical radar, the OTDR sends short pulses of light down one end of a fiber at a specified repetition rate. Light reflected back from fiber discontinuities and light continuously backscattered from the fiber itself travels back to the OTDR, where the tool records the optical power and arrival time. The arrival time of the pulse from a given point in the fiber is related to its distance from the OTDR. With this information, the OTDR graphically displays returned power versus distance. OTDRs are well-equipped for troubleshooting problems because they allow you to visually locate reflective events like connections and fiber breaks and nonreflective events like splices and tight bends by studying the graphical “trace.” The power difference between two points on the trace is an estimate of optical loss (Fig. 1 on page 36).
There is one caveat here: You must understand OTDR behavior when making measurements. Keep in mind that an OTDR is limited in its ability to resolve two closely spaced features by the width of the transmitted pulse. In other words, two closely spaced events may appear as one. OTDRs also have a dead-zone that limits their ability to locate events near the instrument, which can result in inaccurate length measurements, especially on short lengths found in the horizontal plant. Also, the reported end-to-end cable loss is an estimate based on the assumption that the fiber link's backscatter level is homogenous over its length. This is often incorrect due to manufacturing variations or different fibers on each side of a feature. For this reason, an OTDR end-to-end link loss measurement often differs from that obtained by an OLTS.
This test tool incorporates power meter technology for accuracy and standards-compliance with end-to-end length measurement capability and standards-based pass/fail analysis. A CTS consists of a main and remote unit, one for each end of the link. Each unit houses a power meter and a dual wavelength source. Most of the user interface is performed on the main unit, which also includes a nonvolatile memory and a PC communications port. Each hand-held unit is reasonably compact for convenient usage, transport, and storage. A CTS includes software for uploading the saved results to a PC for creation of reports.
The CTS generally tests two fibers at two wavelengths in one operation. The tool measures the end-to-end length of each fiber using the zero dead-zone time-of-flight technique. It then compares each measurement against the user-selected standard for immediate pass/fail analysis. You can make bi-directional measurements quickly without swapping main and remote units. As with a loss test kit, the CTS is equally effective testing a PON network with splitting devices.
The concept of testing two fibers at two wavelengths using main and remote test modules parallels the practice followed in certifying UTP and STP copper cable networks.
As a network installer, you're only as good as the tools on your belt and your ability to know when to use them. You have several testers at your disposal when completing an installation, each with its own strengths. Knowing those strengths and understanding when to apply each tester could be the difference between a satisfied customer and an angry one.
Anderson is a marketing manager for Fluke Networks, Everett, Wash.