Testing fiber-to-the-desk installations

A new class of measurement equipment emerges to verify horizontal fiber runs.

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Eric Anderson / Microtest

A new class of measurement equipment emerges to verify horizontal fiber runs.

The cabling industry is beginning to see more optical-fiber cable in the horizontal. The growth of fiber-to-the-desk applications is attributable to fiber cable's advantages over unshielded twisted-pair (UTP) cable, improvements in connectivity technology, the falling prices of optoelectronics, and economical cabling infrastructures.

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CertiFiber's main unit (left) includes a liquid-crystal display screen and user-interface features that are not necessary on the remote unit (right).
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Fiber-optic cable's advantages over UTP products are well-documented and include superior bandwidth and lower attenuation, so more information can be sent further distances. For example, a multimode fiber-optic network has the capacity to transmit information at 50 times the speed and 20 times the distance of a traditional Category 5 network. Additionally, glass optical fiber's electrical immunity eliminates concerns about electromagnetic interference (EMI), radio-frequency interference (RFI), ground loops, and electrical emissions. Fiber cable is approximately 40% smaller in diameter, 65% lighter in weight, and four times stronger in tensile strength than Category 5 cable. Also, new multimode-fiber designs can transmit high data rates over long distances with laser-based light sources and without any need for mode-conditioning patch cords.

Advances in fiber-optic-based connectivity hardware have also made fiber in the horizontal more of a reality. Small-form-factor (SFF) connectors make terminating fiber-optic cable easier and more affordable than it has been historically. Installers in the field can now terminate two fibers in about two minutes with some SFF designs. These connectors also permit densely populated crossconnects, thereby minimizing the real estate required in telecommunications closets (TCs).

Centrally managed cabling infrastructures are generally more economical than distributed networks. Fiber's low attenuation allows users to bring cabling from multiple floors into a central closet, minimizing the number of closets and hubs, and consequently lowering costs and saving floor space. Reducing the number of hubs in a network also reduces the number of other active electronic components, so it increases network reliability. With a centrally managed cabling infrastructure, troubleshooting and upgrades are easier because all electronics are in one location.

Standards acceptance

Several standards that support fiber-to-the-desk applications are fully ratified. These standards provide guidelines that ensure the installed cabling plant will successfully support network communications. Standards generally fall into one of two categories: application standards and building standards. An application standard defines the performance requirements for a particular transmission protocol. Ethernet, put out by the Institute of Electrical and Electronics Engineers (IEEE-New York City), as well as Fiber Distributed Data Interface and Fibre Channel, put out by the American National Standards Institute (ANSI-New York City), are examples of application standards.

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A building standard provides generic performance criteria for a premises communications cabling system. The TIA/EIA-568A and ISO/IEC-11801, put out by the Telecommunications Industry Association/Electronic Industries Alliance (TIA/EIA-Arlington, VA) and the International Organization for Standardization/International Electrotechnical Commission (Geneva, Switzerland), respectively, are examples of commercial building standards. Within these building standards are requirements specific to horizontal applications. The TIA/EIA-568A requirements for horizontal fiber-optic cabling include a conservative 90-meter maximum length that does not recognize fiber's ability to span longer distances. As such, networks using this limit do not take full advantage of centralized cabling architectures.

Most fiber-to-the-desk networks possess some common characteristics. Multimode fiber dominates, with 62.5/125- and 50/125-micron fibers the most common fiber types. Most of these networks contain many closet-to-outlet links, with each link including a transmit-and-receive fiber pair. Links are short-most are less than 90 meters. Furthermore, users expect the installed network to comply with an industry standard. Often the installer must provide a report documenting compliance with the standard as proof that the cabling system was installed properly.

New measurement capabilities

Within these building standards are specifications for test methodology. With this information in mind, it should be no surprise to learn that developers have produced a new class of test equipment specifically designed to test standards-based fiber-to-the-desk local area networks (LANs). These new tools are more productive, easier to use, and more reliable than the test equipment traditionally used for fiber-cable measurement. CertiFiber, manufactured by Microtest and introduced in late 1997, was the first in this new class of fiber-optic measurement tools.

Engineered to address the special characteristics of these networks, the new fiber-optic cable testers measure length and link attenuation as specified in the appropriate standards. The testers use the end-to-end power-through measurement method to test attenuation, which is the most reliable and accurate loss-mea surement method. Time-of-flight techniques, which yield immediate accurate length results without interpretation or manipulation of cursors, measure optical length.

These new cable testers feature two units-main and remote-that work in unison to make loss and length measurements. The main unit incorporates a dual-wavelength 850/1,300-nm light source, power meter, liquid-crystal display screen, pushbutton keypad, and serial PC port. The remote unit is equivalent, except for the user-interface features.

This main-and-remote-unit setup has several advantages. It excludes the jumpers' loss and length from the true fiber link's loss and length mea surements. It also allows for the testing of the transmit fiber and receive fiber simultaneously, thus speeding the overall testing process. Such speed is critical in a fiber-optic network with many links (which, as previously stated, most of them have).

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The main and remote-unit setup provides several advantages over previously used testing methods, such as the ability to test two fibers simultaneously and the exclusion of the jumpers' loss and length from the true fiber link's loss and length measurements.
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Additionally, because the user tests each fiber at both 850 and 1,300 nm through a common transmit port, that user does not have to physically move connectors. Users accomplish bidirectional testing of each fiber by swapping the transmit/receive connectors at the crossconnect, avoiding the walk that is necessary to exchange source and meter units when using dedicated sources and meters.

Not only do these new tools provide reliable loss and length measurements quickly, but they also compare the actual measurements against the specifications of whatever standard the user selects. A built-in list of LAN standards lets the user select the parameters against which the installed fiber-optic cabling system will be compared. The tester immediately displays a pass or fail indication based upon the comparison. Finally, this new series of test tools and the supporting PC software enable the user to print summary or detailed certification reports.

With the CertiFiber fiber-optic verification tool, job-site setup tasks typically take less than 5 min. Only those items that differ from the previous day's setup must be adjusted. In what is frequently called an autotest, the tester measures the optical loss of two fibers at two wavelengths in one direction, measures optical length, calculates the optical-loss budget if the user selected a formula-based standard as the measurement baseline, internally compares results against the selected standard, calculates the performance margin, and displays a pass or fail indication for each fiber. This autotest takes 7 sec. The tester user can save results for both fibers in less than 30 sec. So, just as a fiber infrastructure allows for high-speed information transmission, testing that fiber system can also be accomplished in a speedy manner.

Productivity and usability

In fact, the speed with which you can accurately and reliably test horizontal fiber-cabling runs makes these new testers stand out from traditional fiber-test equipment. Not only is the method of measuring loss with an optical time-domain reflectometer (OTDR) noncompliant with industry standards, but the time required to acquire and analyze OTDR waveforms, change wavelengths, and save data is slow when compared to the new testers. Fiber LAN testers yield time savings of approximately 85% over OTDR testing. Test times are cut by about 75% compared to traditional power-meter techniques. The new fiber-testing tools' productivity advantages are significant.

Networking professionals require that test results be presented in easy-to-read, easy-to-interpret formats. New fiber-optic cable testers facilitate the reporting process by including PC software written specifically for the LAN environment. For example, Microtest's ScanLink is included with every CertiFiber. Each test report includes the actual measured loss and length, the loss and length limits, and the margin between the two. The report also uses a "fail" notation to highlight any individual test in which the performance does not comply with the selected standard's specifications.

Additionally, the report includes the applicable reference method, project-specific information, fiber-cable information, and test-equipment information. The reporting software prints summary or detailed certification reports, in hardcopy or electronic format, as evidence that the network is indeed certified for operation. Software lets the user export this report data in spreadsheet and database-compatible formats.

The new handheld, portable fiber-optic LAN testers are designed for field use. Their compact size and light weight are well-suited for testing in TCs and confined spaces. Impact-resistant cases stand up to rough handling, and temperature-compensated components permit stable, accurate operation over a wide temperature range. Long battery life ensures hours of uninterrupted testing.

Measuring and comparing

The use of fiber-optic cable in horizontal runs will continue to grow as associated network components continue to decline in price, improve in performance, and become easier to use. New tools specifically designed to test multimode fiber-to-the-desk networks are available to make certifying these systems simple, accurate, and fast. Measuring optical characteristics and comparing the results to the user-selected requirements can ensure the installed cabling plant will effectively support network communications.

Eric Anderson is senior product manager with Microtest (Phoenix).

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