From the November, 2014 Issue of Cabling Installation & Maintenance Magazine
Testing for the presence of power sourcing equipment can be accomplished easily, but determining a cabling system's ability to carry DC power and successfully transmit data is more challenging.
The Institute of Electrical and Electronics Engineers (IEEE; www.ieee.org) published its 802.3af standard-commonly referred to as Power over Ethernet (PoE)-in 2003. In 2009 the IEEE published 802.3at, which is commonly referred to as PoE Plus. While industry groups drum up support for the establishment of an official program to recognize standard-compliant and standard-friendly PoE devices (see "Why the industry needs a PoE Logo Program," October 2014, p. 6), there is little debate about the proliferation of PoE. Claims that tens of millions of PoE powered devices (PDs) have shipped since IEEE 802.3af was ratified are widely made and accepted. Advocates of the technology make the claim that the 8-position, 8-contact (8P8C) RJ-45 jack is the global power outlet, enabling certain PDs around the world to receive direct current (DC) via power sourcing equipment (PSE) through the familiar interface-unlike the interface for alternating current (AC) power, which as world travelers are painfully aware, requires the use of country-specific adapters.
The delivery of PoE can come in the form of endspan or midspan. In either scenario, the existence of DC power on the line is a reality. And in the early days of PoE, that was a potentially perilous situation when a cabling installer or technician plugged a test device into a port. Plugging a tester into a PoE-enabled port prompted the PSE to engage the device on the other end to detect whether or not it would accept PoE; that engagement was enough to damage some testers' circuitry.
Detecting PoE
Generations of test equipment that have been manufactured in the decade-plus since those first painful discoveries were made have done more than just protect against a PoE-enabled port damaging the tester. The testers have become tools used proactively to discover PoE-enabled lines in a network. Tester manufacturers have been able to build this detection capability into a full range of copper-cabling test equipment, not just their higher-end units.
For example, in June of this year Ideal Industries (www.idealindustries.com) introduced its VDV II tester series, which the company characterizes as "affordable, easy-to-use cable verifiers." Jeff Meader, business unit manager with Ideal Industries, commented when the product line was announced, "Installers and technicians spend too much time troubleshooting with testers that are inaccurate or do not provide enough detail about the installation to really diagnose and solve the problem. With our VDV II series, problems can quickly and easily be located, saving time and money. They have a range of features to match all levels of network and coax installation." The product line's VDV Pro incorporates what the company describes as "intelligent technology" that "lets the unit know if it has been plugged into an outlet where potentially damaging voltages are present ... By detecting the presence of voltage and checking polarity, the VDV II Pro quickly determines which type of media service is running over the cable, such as ISDN, PBX and PoE, resulting in faster fault diagnosis."
The WowTester, the test-hardware portion of the WowClowd (www.wowclowd.com) platform that was introduced in 2013 and began shipping in October 2014, also includes PoE and PoE Plus-detection capability. The unit performs other test functions including mapping, shield continuity, length, ping, distance-to-fault, trace route, and return-loss analysis. Like Ideal Industries's VDV II, the WowTester is not a "certifier" tool in that it does not carry out all the tests specified in the ANSI/TIA-568-C.2 twisted-pair cabling standard. As such, both are considered "verification" testers with appropriately accompanying price tags; and both give users the ability to check copper lines for the presence of PoE power.
Fluke Networks' LinkSprinter (www.linksprinter.com), introduced in March 2014, also fits that general category of test products. It conducts five tests of an installed link-Internet connection, gateway/router connections, DHCP (Dynamic Host Configuration Protocol), link to switch, and Power over Ethernet. After a single button-push by the user, the LinkSprinter runs through those five tests in under nine seconds, Fluke Networks says, with LED indicators for each of the five tests lighting green, yellow or red upon completion.
These three testers are by no means the only ones to offer PoE-detection capability; they simply are three of the more-recently introduced tools to do so-underpinning the reality that the ability to detect PoE on a copper cabling circuit is readily available in the market and does not require an investment in the industry's top-of-the-line equipment.
A more-resistant issue
The industry has progressed to a point at which detecting the presence of PoE on a copper cabling line is well understood and fairly easily accomplished. The industry, however, likely is just beginning to face the challenges associated with determining the ability of an installed twisted-pair cabling circuit to capably handle next-generation powering technologies-and the extent to which handheld cabling testers can help in that determination.
Within standards-making realms, and as we have reported in the past, the IEEE has established a P802.3bt 4-Pair Power over Ethernet Task Force. According to a recent "Standards Advisor" document published by CommScope (www.commscope.com), the task force adopted a timeline with expected publication date in early 2017. Also from that recent CommScope document, "an ad-hoc cabling group within IEEE is studying cabling issues such as the effects of high elevation, cable bundle sizes, bundle sizes for smaller patch cords, and installation bundles in trays and conduits." Additionally, "another ad-hoc group continues to study the DC resistance unbalance of the complete channel, including the electronics on both ends."
DC resistance unbalance is an electrical-performance characteristic of twisted-pair copper cabling that is specified in some TIA cabling standards but not others. Specifically, certain DC resistance unbalance performance levels are required in component standards-meaning the performance levels must be met by manufacturers of twisted-pair cable. But no such performance levels are included in the specifications for testing installed twisted-pair cabling systems-meaning they are not required when systems are tested in the field.
Fluke Networks has been voicing concern over this issue for several months. The company recently published a white paper titled "DC Resistance Unbalance Testing: Easy, Low-Cost Insurance for Your PoE Systems." That document explains that DC resistance unbalance becomes a factor when PoE is deployed via Alternative A, in which power and data are delivered over a cable's pairs 2 and 3. In this scenario, "the power is transmitted over the data pairs by applying a common-mode voltage," Fluke Networks says. "Power is received and returned using the center tap of a PD's transformer, which splits the current between each conductor of the pair. When the resistance of each wire in the pair is equal, the DC resistance unbalance (the difference between two conductors) is at zero, current is split evenly, and common-mode current is achieved.
"While devices can tolerate some DC resistance unbalance, too much unbalance causes the potential for saturation of the transformer. This can ultimately distort the waveform of Ethernet data signals, causing bit errors, retransmits and even non-functioning data links."
The white paper further says there can be several causes of DC resistance unbalance, but the most frequent causes are "poor workmanship, inconsistent terminations and subpar cable quality." It cites consistency in individual conductor terminations and precision manufacturing processes as being necessary to protect against DC resistance unbalance.
Fluke Networks' DSX-5000 CableAnalyzer can test DC resistance unbalance, even though the characteristic currently is not required by field-test specifications. "DC resistance unbalance testing verifies that both conductors in a pair have equal resistance and will therefore enable the common-mode current needed to effectively support PoE and avoid distortion of the data signals transmitted on the same pair," the company says. It also cautions against confusion between DC resistance unbalance and DC loop resistance, explaining: "The ability to deliver a certain amount of power is dependent on the total DC loop resistance of a specific length of cable. DC loop resistance is calculated as the sum of the DC resistance of two conductors in a pair. According to IEEE standards, the channel loop resistance of a pair shall be 25 Ohms or less while permanent DC loop resistance shall be 21 Ohms or less." The IEEE also addresses DC resistance unbalance: "IEEE 802.3-2012 specifies a maximum DC resistance unbalance of 3 percent between conductors, meaning that the difference in DC resistance between two conductors is no more than 3 percent of the total DC loop resistance of a pair."
Will DC resistance unbalance make its way into an upcoming TIA or ISO/IEC cabling-test specification? Fluke Networks optimistically states in its white paper, "With TIA field testing standards now open for review, there might be just cause for defining DC resistance unbalance testing as a field test requirement ..."
Regardless of the pace at which certain test parameters get worked into industry standards, an in-the-field reality for users of PoE is that their cabling systems either do or do not have what it takes to support current and future generations of device-powering technologies. As the wattages delivered via twisted-pair cabling increase, network owners hope to avoid the fate that befell some field-test instruments in the early 2000s-finding out about the full extent of PoE's capabilities the hard way.
Patrick McLaughlin is our chief editor.
