Things We Should Know

July 1, 1995
Twisted-pair cabling?it is not just for dial tone anymore. We as an industry are building a balanced transmission line for future applications.

Twisted-pair cabling?it is not just for dial tone anymore. We as an industry are building a balanced transmission line for future applications.

EIA/TIA-568, TSB-36 and TSB-40A specify components. For years, the standards have told us that if we buy Category 5 components and install them in a workmanlike manner, we have a Category 5 cabling system: But there have been skeptics. OWhat constitutes a workmanlike manner?O and OHow can I tell if I am getting the performance I am paying for?O were questions that could be heard in the user community. The answer was to test it, but with what? Enter the handheld cable test instrument manufacturers with the solution. But now who is going to police the test instrument manufacturers?

Another technical system bulletin will address that issue. TSB-67 says that if we choose to test the cabling system to ensure we have installed the Category 5 components in a workmanlike manner, then the cabling system must meet certain electrical parameters, all of which the document carefully details.

This being the case, all failures must implicate the installation. After all, we are still using the same Category 5 components that we were before there were test instruments and TSB-67.

This is not always the case, however. At the BICSI meeting held in January 1995, the topic of short links was discussed. The matter was then studied in a series of tests by the TIA?s link performance task group, which determined that a performance issue did exist. For the purpose of the test, a short link was defined as less than 20 meters. A phrase was added to draft 11 of TSB-67 to alert the reader that the connecting hardware task group is aware of and studying the performance issue.

Every short link that fails the near-end crosstalk test is one more line on the punch-list that is preventing the contractor from getting paid. While most cabling contractors are not engineers, they are clever and resourceful. Many have discovered that when a link fails the near-end crosstalk test and is less than 20 meters long, they can place a longer piece of the same cable with it and when the link is retested, it will pass. Why does this work? Because the cable has so many decibels of headroom above the TSB-36 specification that the additional cable can be used to mask an otherwise marginally balanced connector. I do not recommend this remedy. While you will have a OpassO reading on the cable tester, you will still have an unbalanced link that is susceptible to noise at the receiver.

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