Understanding Cable Materials and Tester Measure Categories

Various Gigabit Ethernet testers are available. Estimated world markets (total revenues) for network cable testers, according to Frost & Sullivan, were about $119 million in 1999, $137 million in 2000, a projected $161 million in 2001, an estimated $188 million in 2002, and a projected $220 million in 2003. These amounts include testers for all categories of cable, not just Gigabit Ethernet ones. Nevertheless, an often-stated recommendation for networks is to install the highest-performance cable available at installation time. Testers often adopt the same strategy, testing all speeds up to and including the current standard speed, which is 1000 BASE-T or Gigabit Ethernet. Individual testers may vary in price from $100 to $5,000, depending upon the number, sophistication, and precision of the tests performed, as well as the ease-of-use features included. These testers are classified according to the type of cable they test: • unshielded twisted pair (UTP), copper only, • fiber (glass) only, or • hybrid (both UTP and fiber). Each of these categories includes multiple measurements. For convenience, most are packaged as easily carried, hand-held devices. Packed with functions, these cable testers deliver a wide range of sophisticated analyses as well as simple practical checks, such as the wire map. The wire map indicates end-to-end continuity, shorts, crosses, and reversed pairs. Analysis includes the measurement categories discussed in last month’s column, including loss, pair coupling, delay, and signal-to-noise ratio. Although most testers will perform any individual measurement upon demand, pushing a single button will usually initiate a sequence of tests, beginning with wiring faults and proceeding through a battery of electrical tests for loss, length, etc. In fact, a summary of all tests is then displayed or made available for uploading to a computer for storage or printing. Alternatively, a single pass/fail result is more efficient if the cable is being certified. Some major tester manufacturers include Agilent Technologies (the former test and measurement division of HP), DataCom, Fluke, Microtest, and Wavetek, Wandel & Golterman. UTP testers Among the features usually supported is the ability to measure the cable length. This should be measured and recorded after initial installation. Not only does this ensure that the cable is shorter than the specified maximum length, but it can also simplify and speed subsequent troubleshooting. Any change in length can help determine where the cable is broken. Length is measured by the time domain reflectometer (TDR) principle, which consists of creating a pulse at the tester and timing its return after it reflects from the distant cable end, like radar. Such measurements depend upon accurate specification of the type of cable, so that well-known electrical properties can be used in the calculation. One disadvantage of such devices is that they must be applied to isolated cables. This is because the testers will corrupt data on “live” cables, possibly falsifying the tests. Usually the cable end opposite the tester must be attached to a specified loopback plug or attenuator. Another potential disadvantage is that the standards they test against are an ever-moving target. For instance, the original five EIA/TIA 568 UTP categories have been extended with Category 5e, but Categories 6 and 7 have never been officially specified. Yet cable consumers demand and several vendors supply cables that support greater than 200 to 250MHz performance. Fortunately, the testers can be upgraded with new software to make the measurements required in the future, as long as the hardware can provide the appropriate connections. Fiber cable testers The fiber testers are actually simpler than UTP ones, because optical environments don’t have the crosstalk and noise corruption of copper UTP environments. Generally, fiber optic cables must be tested only for loss and continuity. Light from a calibrated source connected at one end of the cable is simply measured by an optical power meter connected to the other end. If no light reaches the other end, the failure point can sometimes be found by looking along the cable for light leaks where the sheath is broken. Otherwise, a considerably more expensive tester, called an optical time domain reflectometer (OTDR), must be used to measure where the fault has occurred. SHINN is principal of Compututor. He can be reached via e-mail at cshinn@compututor.com. THE FOLLOWING WEB SITES OFFER FURTHER INFORMATION: Microtest, www.microtest.com Gigabit Ethernet Alliance, www.gigabit-ethernet.org/ Testsmart, www.testmart.com/news/newstmp.cfm/v/001~~/news/fs_20000911_1.cfm.html

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