Comparing Keystone Jacks: The Importance of Permanent Link Certification in Structured Cabling

Written by Dave Harris, trueCABLE Technical Specialist, BICSI INST1, INSTC Certified

When it comes to structured cabling, it’s all about the “permanent link.” This is the part of the physical network that is permanently installed as part of a building’s infrastructure. A permanent link can be visualized as a keystone jack connected to a copper twisted pair Ethernet cable which is in turn connected to another keystone jack. One of the jacks would typically be housed in a wall plate for access to users, while the other jack would be in a patch panel frame with other similar jacks for close access to network switching hardware.

Figure 1. One end of a permanent link is often a keystone jack installed into a wall plate.

Figure 2. The other end of a permanent link is usually a keystone jack mounted in a patch panel.

Apart from the permanent link, the rest of an Ethernet channel is more or less temporary. Patch cords are made to be plugged and replugged. Switching hardware and user devices are expected to come and go, but the structured cabling system, including the permanent links, is expected to remain in service for as long as the rest of the building’s utilities.

Figure 3. A permanent link is part of a physical Ethernet channel.

Therefore, in a commercial structured cabling installation, each permanent link is tested to very high standards in a process called “Certification.” The standards for permanent link testing are high because the rest of the components of the physical network are user-changeable variables. The quality of patch cords available to users can vary widely. Patch cords of excessive length might be chosen. User devices can have a detrimental effect on a network signal. Some users will even bring in their own switching hardware to plug into their desk port.

In order to ensure that their installations will pass strict testing for permanent links, installers opt for connection hardware that has been engineered and tested to standards prescribed for that individual piece of hardware. These components are said to be tested at the “component level,” and are therefore sometimes referred to as “component-rated.” Performance metrics for each of the components is found in the ANSI/TIA 568-2.D standard document, amongst others.

Key components that must have a Category rating and the testing to back it up are:

• Keystone jacks
• IDC (punch down terminal) patch panels
• Field termination plugs
• Patch cords (yes, the cable and two RJ45 connectors are considered a component!)
• Solid copper twisted-pair communications cable

Alternatively, some manufacturers only test their connection hardware by making sure that an Ethernet channel containing their component passes a channel test, which is not as demanding as a permanent link test. These components are tested at the “channel level,” and although there is a standard test to describe such a rating, it is only in the context of a fully constructed channel being tested in the field which includes up to six terminations and three cables! In effect, by using channel testing on individual components that otherwise require component testing, the manufacturer is literally cheating you, the customer. As you might expect, channel-rated hardware is often available at a lower price point.

In the following video, Don performs a permanent link Certification test using trueCABLE’s component-rated keystone jacks and compares the results to those from a test using a competitor’s channel-rated jacks.

As shown in the video, the permanent link using channel-rated jacks suffers a “marginal pass.” This is because the NEXT (near-end crosstalk) headroom is lower than the accuracy limits of the instrument, which essentially means that there is no NEXT headroom at all. The “marginal pass” permanent link will function, but only if the other components in the channel are meticulously chosen for maximum performance. Still, Certification results for each permanent link are recorded and reported to the customer. Therefore, any test result that is a “marginal pass” must either be repaired or at least explained. Many contracts that installers work to will specify what to do with “marginal passes”, but not always! In such a scenario, it is usually much easier to just replace the jacks.

If you would like to learn more about component rated termination hardware, check out our Cable Academy blog article, “Ethernet Quality: Channel Level vs Component Level Rating.”

Thank you for reading and watching! Happy Networking!

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