Issue link: https://www.e-digitaleditions.com/i/1386104
22 I ICT TODAY OTDR CHARACTERIZATION IS VITAL While most data center project specifications have traditionally called out Tier 1 testing for insertion loss using an OLTS, digital transformation is driving the need for a more complete testing strategy with OTDR testing. Unlike an OLTS that measures the amount of light coming out of the far end, an OTDR measures the amount of light reflected back to the source from events, such as connections, breaks, cracks, splices, sharp bends, or the end of the fiber. This allows the OTDR to infer not only the amount of loss in the link, but also to measure the loss and reflectance of specific events. Unless Tier 2 OTDR testing was required by the specification, the ability to measure and locate specific events meant that an OTDR was usually regarded as a troubleshooting tool. However, characterizing fiber links via an OTDR during initial testing offers several benefits, and it can help reduce the risk of using only an OLTS. Although an OLTS accurately calculates the total loss of the entire link as required by industry standards and application compliance, specific event losses are completely invisible to Tier 1 testers. During installation, air gaps, poor fiber core alignment, dirty fiber endfaces, macrobends, and microbends can occur as a result of poor workmanship and challenges, such as higher density pathways and more complex routing both between and within racks and cabinets. Characterizing fiber links with an OTDR makes it possible to pinpoint the location of any fault and verify that the quality of the installation meets the design specifications for current and future applications and ensures that there are no unplanned loss events due to poor cable management or errors in installation. It allows technicians to see the performance of specific connection points and their locations within the link, making it easy to identify any questionable connection points that may need to be addressed. While simple point-to-point permanent links that have pre-terminated connectors on both ends with no splices likely do not warrant Tier 2 testing (unless it is specified), fiber links today are becoming more complex with several connectors and/or splices. Further- more, with more links than ever, terminations and splices are often performed by different technicians, some of whom may be more skilled than others. With today's stringent insertion loss requirements, characterizing specific events in a fiber link is more valuable than ever, especially considering that total loss can increase over time due to poor cable management, splice degradation, dirty fiber endfaces, and even loss of power due to transmitter age. In fact, due to ever-tighter loss budgets and less room for error, many network owners and designers are setting not only overall loss budgets, but also loss budgets for individual splices and connectors, which can only be verified with an OTDR. It is also possible that a link can pass a Tier 1 insertion loss test yet still fail to carry network traffic due to reflectance issues, and only an OTDR can locate the specific reflectance problem. The ability to measure reflectance of specific connections with an OTDR is even more critical for the aforementioned short-reach single- mode applications where the maximum channel insertion loss now depends on the number and reflectance of connectors. Characterizing the fiber link with an OTDR also confirms exactly how many connections exist within a link, which is not information that can be acquired with only Tier 1 insertion loss testing. This is valuable for identifying when a link contains too many connection points due to a cross-connect or links being patched together, which can cause the end-to-end link to exceed loss limits for a given application. If Tier 2 OTDR testing is used, some may question if Tier 1 insertion loss testing with an OLTS is still neces- sary. The answer is yes—Tier 1 testing with an OLTS is always required. Tier 2 testing with an OTDR does not replace Tier 1 testing but rather is considered "extended" testing. The use of an OTDR cannot replace Tier 1 testing because the total insertion loss measurement achieved with an OTDR is an inferred calculation that does not necessarily depict the total loss that will occur on a link once it is live. Furthermore, when characterizing a fiber link via OTDR testing, it is recommended to do so before Tier 1 insertion loss testing. The ability to measure the number, location, and performance of each splice and connector with an OTDR allows problems to be corrected during the installation process and prior to final insertion loss testing rather than afterward when the network is live.