Performance parameters like crosstalk and delay skew still matter. And now, so does much more.
When structured cabling systems were installed in commercial office spaces to support voice lines and data transmission such as email, testing those systems for compliance with specific standards provided the network owner with assurance that the infrastructure would capably handle the job. The suite of tests administered to ensure compliance with a standard, like Category 5e for example, examined specific electrical-performance characteristics. If the cabling system met or exceeded all the requirements in the Category 5e test suite, the user could confidently deploy certain applications—most notably 1000Base-T—knowing that the installed cabling system had, via testing, proven its ability to withstand the stresses that 1000Base-T would place on it.
In one sense, not much has changed with twisted-pair copper cabling, testing, and the support of Ethernet applications. A system that passes the full suite of Category 6A tests provides assurance of 10GBase-T support.
But in another sense, much has changed. Chances are, twisted-pair cabling installed in buildings today will have to support more than just voice (which is really data now anyway) or data applications like email and file sharing. A newly installed link very well may carry direct-current power as well as data. And the data packets it does carry may be far less tolerant of latency or retransmission than Ethernet is. This is today’s reality for cabling systems, and it affects both the techniques and the tools used for testing.
AV, MPTL, PoE
In a recently published technical paper titled “AV Cabling Consideration,” Fluke Networks says, “To support everything from high-definition video streaming and video conferencing, to digital signage and content sharing, many of these applications use the HDBase-T standard, while others may use Internet Protocol (IP)-based video. When it comes to deploying and testing structured cabling to support these AV systems, there’s more than meets the eye.”
The paper describes some old-school electrical characteristics—propagation delay and delay skew—that have some new implications when AV is the application of choice. It explains there are advantages to deploying a cabling system with margin beyond the standard’s minimum delay-skew requirements. “While equipment can typically resolve the time differentials between pairs, when the delay skew is too high, it can result in increased bit error rates and jitter. For high-resolution RGB video signals where each color is sent down a separate pair, too much delay skew can cause a jittery picture on a video display. While industry standards require less than 50 nanoseconds (ns) for delay skew, cables that exhibit less than 25ns are better for video applications. And given the ever-increasing use of AV systems in commercial enterprise environments, many cabling vendors are now offering ‘low-skew’ cables with delay skew values closer to 2 or 3 ns.”
With regard to conducting tests of these systems, Fluke Networks says, “While there are specific AV tools for testing resolution, frame rate and other video specifications once systems are up and running, the cabling infrastructure for both HDBase-T and IP-based AV systems need to be tested just as they would when deploying an IP-based LAN for voice and data transmission. In fact, the HDBase-T Alliance specifically states that whichever cable type is deployed, it must be tested for compliance with the appropriate TIA standard.”
While the applications running over twisted-pair cabling have grown and changed, so too have the methods by which cabling circuits are installed and used. The modular plug terminated link (MPTL) is a relevant current example. Formally recognized in the TIA’s 568.2-D standard, the MPTL “is nothing more than an Ethernet cable that is terminated with a female socket (jack) on one side and a male plug on the other,” explains Ideal Networks’ Dan Payerle Barrera in the document “The Rise of Modular Plug Terminated Links.” But, Barrera warns, “Installing is one thing, testing is another.
“When certifying an MPTL, one would expect all they need to do is change the adapters on the certifier so one end has the test cord with a plug and the other end has the adapter with the socket. Unfortunately,” he points out, “it’s not quite that simple. Measuring the performance of the plug at the end of the MPTL takes special care. A non-centered socket cannot be used to measure the performance of a plug. So simply changing the adapter on the certifier doesn’t necessarily mean the MPTL can be tested. Depending on the manufacturer of the certifier, the standard channel adapter may not test the plug, resulting in a bad plug or termination that does not meet required performance going unnoticed. The only way an MPTL can be properly and accurately tested is with a channel adapter that uses the specific centered socket required for certification of plugs.” In some cases, he notes, that means using a patch-cord test adapter or a test cord with a centered socket.
Parameters that matter
IEEE 802.11-based wireless communication technologies, and IP video cameras, are the two most commonly referenced end devices that take advantage of MPTLs. In many cases, these devices also receive power via Power over Ethernet. In cases like this, as well as in applications deploying the technology Power over HDBase-T, Fluke Networks emphasizes the importance of the electrical-performance parameter DC resistance unbalance. In its AV-focused paper, the company states that because both HDBase-T and IP-based systems deliver DC power via POH and PoE, “DC resistance unbalance is a key parameter to test for, as too much DC resistance unbalance can distort a picture.
“While too much DC resistance unbalance can be avoided by choosing quality cable from reputable manufacturers and by ensuring quality workmanship with consistent terminations, it’s a good idea to test for it as it could have a significant impact on the performance of AV systems. If the difference in DC resistance between two conductors or between any two pairs is greater than the maximum allowed, there’s a pretty good chance that your HDBase-T and IP-based video application won’t be picture perfect.”
The coexistence of data and power on twisted pairs has prompted not only new emphasis on long-known electrical characteristics, and new considerations about test equipment’s componentry and its fitness for the tasks. Data/power coexistence also has prompted the development of new test devices, one of which is AEM’s TestPro. The company recently announced that TestPro “received Level 2G accuracy certification, the highest level specified in TIA-1152-A and ISO 61935-1 standards for cable certification testers, from the ETL/Intertek agency.” AEM boasts of TestPro’s “six-second Category 6A certification,” but also emphasizes the tester’s versatility.
“TestPro can qualify multi-gig link speeds up to 10GigE and verify loaded Power over Ethernet up to 90 watts,” the company said. “With testing for all three of these essential core functions [cabling performance, speed capability, Power over Ethernet] in one platform, cable contractors and those responsible for deploying IoT-connected devices can quickly understand the problem’s source.”
Harshang Pandya, general manager of test and measurement at AEM International, commented, “Digital transformation is having a huge impact on today’s networks, and the demand for more bandwidth is driving increased use of multi-gig Ethernet over copper cabling. In addition, sophisticated PoE devices are turning the network switch into a power distribution center. With the TestPro, contractors and network owners can quickly and accurately certify the cable, qualify link speed, and validate PoE++.”
In fall 2017, Netscout Systems Inc. introduced LinkRunner G2, an Android-based network tester. The company explains the tester “accelerates deployments and eliminates finger-pointing by combining high-wattage PoE and copper and fiber network connectivity testing with vendor apps on the Android OS. This creates a more-effective workflow.
“With LinkRunner G2 technicians can review installation documentation, test loaded PoE and network services, document the results, and use other vendor-specific apps to configure and triage connected devices,” according to Netscout.u
Patrick McLaughlin is our chief editor.
