《VDV World》杂志：如何防止光缆故障
VDV World 是 Electrical Contracting Products 杂志的特别版面，本期介绍了如何使用 Fluke Networks 的光缆测试工具防止光缆故障。Fluke Networks 的光缆工具产品经理 Harley Lang说：“光缆的正确安装和维护对于网络的健康状况是至关重要的”。
《VDV World》 全文如下：
VDV World - Preventing Fiber Failures
Techniques and Tools for Installation and Maintenance of Fiber Optic Networks
Enterprise Networks rely on fiber optic backbones to carry an increasingly heavy load of data and voice traffic. With new applications and higher transmission speeds, the tolerance for attenuation dramatically decreases. Proper installation and maintenance of fiber optic cabling infrastructure is now paramount to the health of networks.
In the last ten years, applications have migrated from 10 Mbps to 100 Mbps, 1 Gbps and now 10 Gbps in fiber backbones. As network speeds increase, cabling infrastructure must exhibit much lower loss than in the past. The majority of fiber backbones running Fast Ethernet 100BASE-FX could tolerate 11dB of attenuation, but backbones running 10GBASE-S must not exceed 2.6dB.
Network uptime, application performance, and even efficient cabling certification are dependent on following best practices for installing and maintaining fiber infrastructure. In a study commissioned by Fluke Networks, both independent contractors and owners of fiber optic networks indicated that most problems stem from poor cable installation and management or poor connections between fiber links (Table 1).
Table 1. Incidents of fiber failure cited by contractors and network owners.
Insertion loss from dirty or poorly terminated connectors is the major contributor to attenuation. Poor cable management, often in the form of excessive bends, also contributes to attenuation. Poor labeling and polarity problems can also affect turn on rate and installation time. Fortunately the techniques and tools for good installation and maintenance are readily available.
Fiber Cabling Installation and Certification
Fiber verification tools are necessary to install and maintain fiber links. Connector end-face inspection, link loss, power levels, continuity and polarity checks can be conducted with easy-to-use and relatively inexpensive verification tools. These tools provide the ability to identify simple problems and fix them before certification and/or service activation.
End-face inspection should be conducted during and after termination to determine that connectors are clean and properly polished. This should be done with either a hand held optical microscope or video microscope. 200X magnification is sufficient for multimode connectors, but due to the small core in singlemode fibers, 400X is required to get a good view of the end face. A handheld microscope will enable an installer to see the end-face during the polishing process. A connector end face that is insufficiently polished can shatter when plugged in and damage another connector. Under-polishing certain fiber connectors can also cause fibers to break behind the connector body when they are stressed. Conversely, a connector polished below the core will exhibit higher loss and reflectivity.
Once fibers are terminated with connectors, a visual fault locator (VFL) will enable installers and network technicians to verify link continuity and polarity. A VFL uses a bright red visible laser that can be set to emit a continuous beam or flash on and off. A steady beam helps visually identify simple cable faults such as broken fibers and too tight bends. A pulsing light is easy to detect in a bundle of tightly packed cables. Polarity issues are particularly important when installing small form factor connectors that include two fibers with a single connector housing. In some cases a polarity reversal will require re-termination of the fiber.
A simple optical power meter and source should be used to test fiber links for total loss and power levels. The power meter can be used by itself to verify the output power levels of opto-electronic equipment. It can also be used with a source to determine the total loss of the fiber link. The power meter is referenced to the power level of the light source. When connected to the source through the fiber link under test, it simulates the loss of the link once activated. A loss that exceeds the link’s loss budget and exceeds the typical loss for similar links will indicate a problem that needs to fixed prior to certification and service activation.
Certifying a Fiber Installation
Network owners and independent cabling contractors that install and maintain a lot of fiber are wise to equip their organization with powerful fiber optic certification and troubleshooting tools such as a certifying Optical Time Domain Reflectometer (OTDR). These tools have diagnostic capabilities to provide unique vision into fiber cabling infrastructure and save precious time troubleshooting during the installation of fiber infrastructure. They also have the capability to certify the fiber cabling infrastructure.
Network certification is the process of testing certain network parameters and then comparing the actual results against an industry or user-defined standard. Certification recommendations such as TIA’s TSB140 bulletin titled “Additional Guidelines for Field-Testing Length, Loss and Polarity of Optical Fiber Cabling Systems” provide guidelines on how to test fiber optic cabling systems in the field — offering two tiers of fiber network certification.
Tier 1 testing is required in all fiber optic cabling links. The Tier 1 tests are attenuation (insertion loss), length and polarity. When conducting Tier 1 testing, each fiber link is measured for attenuation with an optical loss test set (OLTS). Fiber length is measured optically or calculated via the cable sheath markings. Polarity is verified either with the OLTS or a visible light source, such as a visual fault locator (VFL).
Tier 2 testing supplements Tier 1 testing with the addition of an OTDR trace of each fiber link. An OTDR trace is a graphical signature of a fiber’s attenuation along its length. You can gain insight into the performance of the link components (cable, connectors and splices) and the quality of the installation by examining non-uniformities in the trace. An OTDR trace does not replace the need for insertion loss measurement, but is used for complementary evaluation of the fiber link.
An OTDR trace helps characterize individual events that are invisible when conducting only loss/length (Tier 1) testing. Incorporating the proposed two-tier testing method, installers have the most complete picture of the fiber installation and network owners have proof of a quality installation.
Maintaining Fiber Infrastructure Performance
Technicians who troubleshoot fiber links, or who conduct moves, adds and changes can rely on similar tools as are used during installation and certification. Fiber microscopes, power meter and source and visual fault locators are helpful for diagnosing simple problems.
A handheld microscope should be used to spot contamination on the end-faces of fiber connectors. This contamination should be cleaned prior to mating fiber connectors. If inspection and cleaning are not performed, debris will migrate from connector to connector, contributing to network failures. Debris can also become embedded in the end-face of connectors, making it necessary to cut them off and re-terminate the cable.
If both ends of the fiber link can be located, a power meter and source can be used to verify that attenuation meets the loss budget. If only one end of the link is accessible, the power meter can be used to make sure that the power levels received meet the requirements of the optical receiver.
Visual fault locators will enable technicians to verify continuity and polarity, and locate simple faults that are accessible.
Optical Time Domain Reflectometers (OTDRs) are also excellent tools for maintaining fiber plant performance. An OTDR furnishes more details on cable installation, termination quality and provides advanced diagnostics to isolate a point of failure that may hinder network performance. An OTDR makes possible discovery of features along the length of a fiber that may affect fiber reliability. OTDRs characterize features such as attenuation uniformity and attenuation rate, segment length, location and insertion loss of connectors and splices, and other events such as sharp bends that may have been incurred during cable installation.
High bandwidth fiber backbones are increasingly used to carry the bulk of network traffic. Network uptime, application performance, and efficient cabling certification are important goals of network technicians and installers. With the proper tools and techniques for both installation and maintenance, these goals can be easily achieved, even in an environment where the stakes are being raised to new heights.
Harley Lang is a BICSI Registered Communications Distribution Designer (RCDD) and Product Manager, Fiber Optic Tools, for Fluke Networks, provider of innovative solutions for the testing, monitoring and analysis of enterprise and telecommunications networks.