Fiber Optics Cable Testing
Fiber is now beginning to appear in traditional 'UTP territory'. There have been continual improvements in optical fiber performance, fiber cable designs, connectivity technology, and test 16216t192q equipment. Not only are these fiber products more craft friendly than ever, they are also less expensive. New advancements in transceiver products will make fiber even more attractive in the LAN environment.
Fiber optic cable comes in two basic types: multimode or single mode. Multimode fiber has a relatively large core diameter (typically 50 or 62.5 microns). Light from LED sources can be efficiently coupled into multimode fiber. Multimode fiber is most often used in LED-based LAN systems. Single-mode fiber has a very small core diameter (8.3 microns). Single-mode fiber propagates only one optical mode, significantly increasing bandwidth. Single-mode fiber is primarily used in laser-based long haul and interoffice applications. Single-mode fiber is beginning to be used in LAN as backbone cabling and to "future proof" networks.
How is fiber tested? Historically, the quality of fiber cable was so good and bandwidth more than adequate that some network designers specified that only a simple continuity check was required for fiber cable certification. Today's higher speed networks demand more from the fiber and are making this simple approach obsolete. Industry standards bodies including the TIA/EIA, IEEE, ISO and ANSI have published standards that define maximum supportable distance and maximum channel attenuation for LAN. See Fiber Standards for pass/fail limits by application. Therefore when installing cable to support a standardized network application (i.e. Ethernet, FDDI, and ATM), it is appropriate to test cable and compare the results to the appropriate standard. In practice, network designers and architects are frequently unaware of the standards or chose to use their own user-defined pass/fail criteria. This can result in a cabling plant that is either not tested as thoroughly as necessary, jeopardizing network performance, or tested too severely which can needlessly add to the cost and time of the cable installation and testing.
What is really
required? It is best to comply with the appropriate fiber application standard,
all of which require direct attenuation measurement. These standards have been
painstakingly developed and approved by a large group of leading companies in
the industry. You can be confident of acceptable network performance when you
certify that the cabling plant meets the requirements of the standard. If you
are installing cable and the transmission standard is unknown or is a new
transmission protocol for which a standard has not yet been published, it is recommended
that you follow the guidelines as set forth by the networking equipment
manufacturer or a general building standard. That means TIA/EIA-568-A in North America or ISO/IEC 11801 in
There are a variety of tools available for testing fiber cable. Which tool(s) to use will depend upon the type of job to be performed, how frequently you test fiber networks, and your test equipment budget. Tools for the field include:
It is interesting to contrast measurements necessary to qualify a fiber optic cable installation with those of a copper cable installation. 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 fiber installation only attenuation, at one or two wavelengths, is usually measured. Length may also be measured or physically recorded. Fiber cable can be tested one fiber at a time using the end-to-end technique. This methodology uses an optical source and power meter for direct measurement of attenuation. Microtest's SimpliFiber optical loss kit is an example of the tools used for this form of testing. The fibers are tested, results recorded and later compared to an industry or user-defined standard to evaluate the success of the installation. For those who test fiber frequently, new fiber test technology is available for quicker, more productive network testing and certification. With tools such as the Microtest's CertiFiber and OMNIFiber, multiple measurements can be made over a Rx/Tx fiber pair with the push of one button and the pass/fail status of the fiber is instantly displayed. Optical Time Domain Reflectometers (OTDRs) are used for troubleshooting fiber optic cabling. An OTDR can measure optical length and display the distance to an optical event (fiber break, end of fiber, fiber splice or connector). OTDR are not suitable for making end-to-end power loss measurements.
We expect the use of LAN fiber networks to become more commonplace as technology and costs improve. Fiber standards do exist and networks should be tested to the appropriate application standard (Ethernet, FDDI, ATM) when possible. If unknown, a general commercial building standard such as TIA-568A or ISO 11801 should be followed. Network designers and equipment venders may have proprietary standards that must be followed. A range of test tools is available to ensure that you can accurately and reliably test fiber networks.
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