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By David Green, P.Eng.Convergedcommunications demandnew troubleshooting skillsFigure 1Figure 2Photo © Fluke NetworksWith the escalating move toward voice, data and video(VDV) convergence, today’s cabling installations arebecoming increasingly sophisticated. These newer,high-performance Cat 5e and Cat 6 cabling systems must supportfaster data communication rates and more precise timingof the delivery of VDV information. As a result, professionalsmust know how to test installations to the more complex performancespecifications that have been developed to addresshigher bandwidth requirements.Testing and certification for newer cabling systems entails newtest parameter and link definitions, more data points, higherbandwidth, a higher level of performance and new connectortypes—as well as patch cord requirements. After all, propercertification is the guarantee that the installer left the job ingood working order, and that the installation will perform at thenecessary levels to support a customer-approved SLA (servicelevel agreement).What follows is a brief overview of what you need to knowabout certifying cabling for today’s VDV triple-play installations.There are, in fact, three key areas that you need to considerin the certification process:• The link model: choosing the proper test setupand eliminating a common source of measurement error.• The measurements: what is tested and reportedduring certification.• The diagnostics: when errors are found, how they canbe fixed as rapidly as possible.Link modelsThe term ‘link model’ refers to the configuration of the cabling linkunder test and the manner in which the test equipment is connectedto it. It is important to ensure you are using the correct link modelstandard with the appropriate tester interface hardware.The majority of all certification tests in the field should usethe Permanent Link model, which consists of the cable and thepermanent connectors in the wall outlet or patch panel. Theconnection from the test instrument to the Permanent Link ismade via a Permanent Link Adapter (Figure 1).Keep in mind that the channel link model defines the performanceof the completed end-to-end link as it is used by theoperating network. Therefore, the patch cords that are used toconnect the PC or network device must remain in the link andplugged into the channel adapter of the test instrument.(This is why channel measurements are typically done whenrestoring service, or for verifying cabling for application support. Itis uncommon to perform channel tests during initial installation,since the patch cords are rarely available at that time. See Figure 2.)From a practical standpoint, installers need more sophisticatedfield testers, because they must to be able to measure theperformance of the link without any of the effects of the testcords with which they are connected to the link-under-test.(The reference point for the measurements is not at the testinstrument but at the far end of the test cord.)If the effect of the adapter is not totally transparent, a goodlink can produce a failing test result. The probability of this happeningincreases with the performance requirements. In otherwords, Cat 6 is more likely to produce these false Fail results.So, when performing Permanent Link tests for Cat 6/Class Elinks, it is important to take special care to ensure you are usingadapters that are appropriate for the cabling under test.Certification measurementsLink certification requires a number of complex measurements,which change in degree of precision depending on the categoryof the installed link. For Cat 5e and Cat 6 links, a certificationtool needs to measure or calculate:• Wire map• Propagation delay (delay skew and link length)• Insertion loss• NEXT (Near-End Crosstalk)• Power sum NEXT• ELFEXT (Equal Level Far-End Crosstalk)• Power sum ELFEXT• Return lossThe accuracy level of the certification tool determines its abilityto make these measurements at an appropriate level for the typeof link. For Cat 5e installations, Level IIe defines the minimumrequired accuracy. For Cat 6, a certification tool needs to meetLevel III accuracy. Level IV defines a higher level of accuracy overa wider frequency range to certify the ISO Class F (Cat 7) links.The certification tool must perform the measurements of allrequired test parameters over the prescribed frequency rangefor the category of cabling and with the number of frequencypoints defined in the standards. The test results can be inspectedon the tester or they can be stored in full detail to be inspected▼22 • NOVEMBER/DECEMBER 2007 • www. mag.com