high, it can reduce the PoE power available for a device. DC resistance unbalance evaluates the difference in resistance between conductors and pairs. Too much DC resistance unbalance can cause Ethernet signals to become distorted, resulting in bit errors, retransmits, and even nonfunctioning data links. Measuring DC resistance unbalance between pairs is especially critical in IEEE 802.3bt four-pair PoE (Type 3 and Type 4), which transmits power and data over all four pairs of a twisted-pair cable. IEEE 802.3 PoE standards specify 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 the pair. The standards specify a maximum DC resistance unbalance of 7 percent between two pairs, meaning that the difference in parallel resistance is no more than 7 percent of the total parallel resistance of the two pairs. DC resistance unbalance between pairs requires six measurements to compare each pair to every other pair. Figure 1 shows the DC resistance unbalance between conductors for Pair 1,2 and Pair 4,5, and the DC resistance unbalance between both pairs. In this example, DC resistance unbalance passes for Pair 4,5 but fails for Pair 1,2. The cabling also fails pair-to-pair DC resistance unbalance.
Category 6A/Class FA cabling that supports 10 Gb/s Ethernet (i.e., 10GBASE-T) often also requires alien crosstalk testing, including power sum alien near-end crosstalk testing (PSANEXT) and power sum alien attenuation to crosstalk ratio far-end (PSAACR-F). This is due to higher-frequency signals causing interference from adjacent cables. Industry cabling standards recommend two Category 6A/Class FA connections to Wi-Fi 6/6E APs and four Category 6A/Class FA connections to Wi-Fi 7 APs. Depending on the project specification, Wi-Fi APs connected via fiber also necessitate Tier 1 certification testing with an optical loss test set (OLTS) that measures insertion loss on a link, or Tier 2 certification testing using an optical time domain reflectometer (OTDR) that characterizes the loss of the link for individual splices and connectors by transmitting light pulses into a fiber and measuring the amount of light reflected from each pulse. ENSURING SUPPORT FOR PoE While not typically required for certification of installed cabling, extended field testing of direct current (DC) loop resistance and DC resistance unbalance can help ensure adequate support for PoE delivery to wireless APs and other PoE-enabled devices. In twisted pair cabling, DC loop resistance measures the total resistance of a pair. If DC resistance is too
FIGURE 1: DC resistance unbalance for Pair 1,2 and Pair 4,5 and between Pair 1,2 and Pair 4,5 (Source: Fluke Corporation)
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