Published: July 2004
Testing is a critical component in installing data communications systems and requires equipment that can accurately test, diagnose and certify many different types and grades of copper and fiber cabling.
Continuity and wiremap verification tests tell installers whether a new network will or will not function when it is activated. Certification tests confirm if a system’s performance meets industry standards.
Because datacomm networks are configured in a wide range of specific installations, including workgroup-level LANs, horizontal links, permanent links and campus-level backbones, testing devices needed for jobs vary with design of the network and type of cabling it uses—twisted copper, fiber optic or coaxial.
The network application defines the cable type and interconnections required and the types of testers that will be needed for verification and certification.
Toners inject a frequency on conductors within a cable. Probes sense the frequency to locate cable behind walls, in bundles and in closets. Some models also can make basic continuity tests and perform polarity testing. Cable mappers examine conductor continuity and test for basic miswires that are pair-based, such as reversals and rolls.
Cable mapper capabilities are included on many higher-performance testers used for certification. Wire mappers provide comprehensive testing, including cable map functions, while also testing for split pairs, and they perform complete verification testing as called out in industry standards.
Before certification testing can be performed, tests must confirm that connections are properly made and that problems identified are corrected.
Tests to verify connectivity determine whether the installer has made good connections and that every wire pair is properly connected from one end to the other, said Hugo Draye, copper test marketing manager, Fluke Networks.
“These tests verify that the wire-pairs are properly connected from one end to the other—commonly referred to as the wiremap test,” continued Draye. “In addition, the test equipment shall measure the propagation delay of the link—the time required by an electrical signal to travel the length of the link. The difference between the propagation delay of the different wire-pairs in a link—called the delay skew—is to be evaluated as well. Test equipment finally calculates the length of the link using the measured propagation delay and a cable characteristic that defines the speed with which electrical signals travel over the link. The latter is called the Nominal Velocity of Propagation (NVP). Cabling manufacturers specify this parameter for each cable type.”
More than 80 percent of datacomm installation problems are caused by poor terminations, said Caroline C. Chen, product manager of datacomm test equipment, Ideal Industries Inc.
“This is especially true when you are dealing with Category 6 cables where precision is required,” said Chen. “To eliminate wiremap problems, you need a good tester to perform accurate wiremap tests. A tester with ‘pass or fail’ criteria is easy to use and requires minimal training. When you have a wiremap failure, it is helpful to be able to identify the cause of failure—open, shorts, split-pairs, miswire, etc. The best course of action may be to re-terminate. Length measurement is another important feature for this class of tester. The installer can use length measurement to verify the length of patch cords or installed links, making the installation job easier.”
Chen said that basic testers to perform essential wiremap, open/short, miswires test one remote at a time for voice (RJ11), data (RJ45), and video/security (coaxial F-Connector/BNC) are available for about $100. More advanced models that also measure length and are capable of testing multiple remotes, one at a time, are priced from $300 to $500.
In the United States, the Telecommunications Industry Association (TIA) is the standards body concerned with cabling performance and other related standards.
That basic verification tests are positive doesn’t necessarily mean the system will pass certification, and more sophisticated equipment is needed for certification testing.
Certification testers perform higher-level tests to evaluate performance based on parameters set by industry standards including NEXT (near-end cross talk), FEXT (far-end cross talk), and return loss, and they provide documentation of test results. Certification testers and standards will be covered by Cool Tools in October.
“High-end certification testers perform a full suite of tests to meet standards and are capable of supporting a full range of interfaces for voice, data and video/security applications,” Chen continues. “In addition, they support test record storage.”
Advanced models cost several thousand dollars.
Certification tests are based on a signal-to-noise ratio analysis, said Draye. Standards prescribe the test parameters to be selected to accomplish this analysis.
“The first and maybe most important transmission parameter measures the strength of the signal arriving at the end of the link,” he said. “This test parameter is identified as insertion loss (formerly called attenuation). Noise (disturbance) parameters are NEXT, FEXT, and Return Loss, which captures the reflected signal energy due to imperfections in the cabling, the connectors, or in the workmanship of cable terminations. Test equipment directly measures these parameters and calculates other parameters using from these measurements. Calculated parameters are Attenuation-to-Crosstalk Ratio (ACR); Equal Level Far-End Crosstalk (ELFEXT), and Power-Sum parameters such as PSNEXT, PSELFEXT, and PSACR.”
To certify a fiber optic link, test equipment must measure only the insertion loss (attenuation), which is called the loss measurement in the fiber optics field. The loss must be tested for two wavelengths except for very short links of about 300 feet or less.
Top-of-the-line certification models automate most testing and store results for transfer to an office computer.
“The user,” Draye said, “selects the appropriate test standard and cable type and executes ‘Autotest’ and with the press of one button the tester automatically makes all the necessary measurements and calculations. If each test parameter meets the minimum requirements established by the selected test standard, the tester issues a ‘Pass’ result.”
Documentation is an important part of the certification process; contractors for commercial installations typically require documentation of the completed installation be provided to the end-users.
Tester buyers should expect tester manufacturers to provide software to manage test results and prepare necessary copies in printed or electronic formats. Some models accommodate removable memory cards, which increase the tester’s memory capacity and allow contractors the option of storing data from different projects on individual memory cards.
Twisted copper and fiber
While some testers or limited to twisted copper or fiber cabling, most field testers today can test both using separate attachments for fiber and copper testing. However, Chen points out, in the basic to medium-range tester categories, it may be necessary to have one tester for twisted copper network and another for fiber to measure light loss (attenuation).
“In the high-end testers,” she added, “there are often fiber adapters available so you can use the same system to test voice cable, structured data cable, coax cable and fiber optic single-mode and multimode cable.”
Testers must achieve minimum accuracy standards to be used for certification.
Said Draye: “Accuracy specifications define a set of performance parameters that are uniquely applicable to the tester. The tester must meet or exceed all performance criteria over the applicable frequency range. As the measurement frequency increases and the number of link parameters increase, more and tougher requirements are specified for the testers. At this time, accuracy Level III is therefore the highest level of accuracy defined in TIA standards. It is furthermore very important to realize that different accuracy specifications apply to the tester by itself versus the specifications of the tester with different adapters.”
Whatever the datacomm project, installers want lightweight and versatile test instruments.
“Field testers should be highly portable, easy-to-use, and powerful enough to accommodate the higher frequencies used in today’s networks,” observed Chen. “At the same time, field testers also must offer the versatility and options to handle a variety of situations so they do not burden organizations making datacomm installations with buying, learning to use, and carrying multiple test devices.”
Tester quality continues to evolve, and today’s products provide more for the money than earlier models.
Chen said that troubleshooting features are important when testing to Category 5e or Category 6 and beyond.
“Whenever a specific test fails,” she explained, “the installer needs to be able to move directly into a comprehensive troubleshooting mode without having to change test instrumentation or reconfigure their set up. There are testers available now, for example, that integrate high-end TDR-like functions to enable users to simply switch into the instrument’s ‘Analyze’ mode and use the TDR’s graphic display to interactively pinpoint the exact location and nature of the fault conditions.”
Datacomm testing is a complex process and instruments must be used correctly to achieve accurate results. Many tester, cable and connector manufacturers offer training to help field personnel learn how to use their products and trade associations such as BICSI and NJATC offer certification courses.
The trend for electrical contractors to extend services to datacomm installations will continue.
“Datacomm work has become a significant opportunity for many electrical contractors,” noted Tim Kopp, DSV technical application specialist, Greenlee Textron.
“More and more electrical contractors,” he continued, “are being asked to bid a complete cabling infrastructure, including not only the electrical power cabling portion of a building, but also cabling for computer networks, telecommunications, and perhaps security video and alarm systems. Many large electrical contractors are developing their own datacomm divisions in an effort to gain the competitive edge on these larger contract bids.”
Technologies driving datacomm cabling, he concludes, are rapidly changing, and electrical contractors who pursue datacomm opportunities should profit from it.
Datacomm testing equipment is keeping pace with technological developments.
“Datacomm testers are becoming more integrated, more cost-effective and easier to use,” concluded Chen. “For example, we are seeing an increasing array of testers capable of support voice, data and video/security applications—both copper and fiber—for residential, commercial and industrial environments.” EC
GRIFFIN, a construction and tools writer from Oklahoma City, can be reached at 405.748.5256 or firstname.lastname@example.org.