There are a variety of factors to consider when testing Category 6 installations in the field.The issue of testing Category 6 installations in the field with hand held testers is not as straightforwar...
February 1, 2002
There are a variety of factors to consider when testing Category 6 installations in the field.
The issue of testing Category 6 installations in the field with hand held testers is not as straightforward as it would seem. There are many factors that can affect what is reported as a test result, especially when testing at frequencies up to 250 MHz. The higher the frequency the more visible are variations in the cabling and variations introduced by the test adapter. If we compare this to testing Category 5 / 5e installations at 100 MHz, it is like looking at an object with 2.5 times the magnification.
Although the Category 6 standard is not yet published, many customers are already installing Category 6 products. It is normal for a customer to want the assurance that the installation complies with either the “Channel” or the “Permanent Link” draft specifications for Category 6 (TIA/EIA 568-B.2-1) or Class E (ISO/IEC 11801, 2nd edition).
But why are there two sets of specifications, and which one is controlling? Before answering these questions, it is important to understand the difference between a Permanent Link and a Channel.
PERMANENT LINKS AND CHANNELS
A Permanent Link is the “portion” of cabling between the termination in the work area (8-pin modular connector) and the termination in the Telecommunication Room (patch panel or connector block such as BIX or 110). It is called “permanent” because the cabling that runs in the wall, ceiling or under the floor will not be replaced during the lifetime of the cabling, which is usually about ten years.
The Permanent Link test is typically performed following initial installation of the cabling plant and prior to installation of the user equipment. A Permanent Link does not include the equipment cords at each end or the cross- connect jumpers and patch cords.
A Channel includes all the cabling components from the Work Area equipment to the equipment in the Telecommunications Room. The channel test is the controlling test because it is what the equipment actually sees and what the user actually uses. Channel testing also includes all of the weak links in the cabling system, in addition to any adverse effects due to interaction between components.
PERMANENT LINK TESTING
Let’s focus for a moment on the Permanent Link test. There are two ways to perform this test. One way is to equip the hand-held tester with a manufacturer-specific adapter cord. Another way is to use a “generic” test adapter cord equipped with a limited choice of Category 6 modular plugs (usually one or two). The problem is that these generic cords or plugs are not necessarily universal. Variations in NEXT of up to 2 dB are not uncommon, depending on the connecting hardware from one generic plug to another plug.
What does this mean to the end user? It means that the results reported for a Permanent Link test might be different when the actual equipment cord uses a different style plug than the generic plug. This is one reason why it is preferable to perform a Channel test using actual user cords, or to perform a Permanent Link test using test adapters or test plugs recommended by the manufacturer of the connectivity hardware.
There is another variable that can affect the reported test results when testing Category 6 Permanent Links or Channels in the field. This variable has to do with the quality of a twisted pair termination on the connector. As Category 6 installations are tested up to 250 MHz, any variations in the installation quality can affect performance much more than for a Category 5 / 5e installation.
It is very important to follow the manufacturer’s recommended installation procedures. Improper terminations can cause an additional variance of 2 or even 3 dB at high frequencies above 100 MHz.
The Category 6 cabling that is being installed today, is designed to provide a minimum bandwidth of 200 MHz and meet test parameters that are specified to 250 MHz. When testing Category 6 Permanent Link installations in the field, the results obtained are more sensitive to test adapter variations than Category 5 / 5e installations. In addition, a higher degree of precision is required when performing cable terminations to obtain the highest performance for your cabling system.
When in doubt, a Channel test is the definitive indicator of network performance because it includes all of the components and the effect of component interaction.
Paul Kish is Director of IBDN Systems & Standards at NORDX/CDT in Pointe Claire, PQ. He is also Chair of the TR-42 engineering committee.
Disclaimer: The information presented is the author’s view and is not official TIA correspondence.