IEEE Compliance in a PoE WorldRichard Mei
Those of us who work in the structured cabling industry know that once a network is active, administrators are unable to continuously measure DC resistance unbalance without impairing the Ethernet transmission. But what if you could measure and monitor in real-time, DC resistance unbalance on an active PoE link, without impairing transmission? NFLEXON is working on a solution.
What if you could measure and monitor in real-time, DC resistance unbalance on an active PoE link, without impairing transmission?
DC Resistance Unbalance
PoE is designed to use two or four pairs in a four pair network cable. The center taps of the transformers for the Power Sourcing Equipment (PSE) and Powered Device (PD) split the common-mode current evenly between both conductors in a twisted pair before transmitting and receiving the current. Once the current is split, however, the PSE and PD are unable to detect DC resistance unbalance and the extent to which the current ratio for each conductor changes.
Depending on the Ethernet application, power can be transmitted over unused cable pairs or the same pairs used for Ethernet transmission. When DC resistance unbalance is present, power and Ethernet transmission can appear to function normally for some time while masking latent network issues.
In a 10BASE-T/100BASE-TX application where Ethernet and power are transmitted over different pairs — or in a 10BASE-T/100BASE-TX/1000BASE-T application where Ethernet and power are transmitted over the same pairs — Ethernet and power transmission can appear to function normally when there is DC resistance unbalance. For the pairs that transmit power and have DC resistance unbalance, an uneven amount of common-mode current will be transmitted over the conductors in those pairs and go undetected by the PSE and PD.
Structured Cabling Temperature
The uneven flow of common-mode current can cause an excessive temperature rise in the cable. As cable temperature rises, the insertion loss of the cabling channel will increase. As the insertion loss increases, the cabling channel electrical length will increase even though the physical cabling channel length is unchanged. In addition, the attenuation to crosstalk ratio, an important indicator of usable bandwidth for a cabling channel, will decrease. As this occurs, the Ethernet signal may become distorted or the application may stop running on the network links where the cabling channel physical length is close to the ANSI/TIA Standard limit of 100-meters.
While Ethernet and power transmission issues are usually apparent to IT network administrators, the long- term degradation of the network cabling physical and mechanical properties will be less evident. More importantly, they are irreversible. Over time, as higher powered PoE devices are integrated in the network, this degradation could lead to latent Ethernet and power transmission issues. The only remedy, replacing the cable, is a costly solution.
DC Resistance Unbalance Testing
DC resistance unbalance can be tested during field acceptance testing using an independently verified handheld cable tester to ensure compliance with IEEE 802.3-2012 and IEEE802.3bt-2018. However, network cables are bumped and moved quite frequently after field acceptance testing during moves, adds and changes. This can cause the conductors to shift within or be dislodged from the IDC slot due to marginal conductor insertion depth. Depth tolerances in the IDC slot are typically several mils and with visual inspection, it is difficult to determine if the conductor is seated far enough and consistently in the IDC slot when punched down.
As an increasing number of power-hungry PoE devices are added to structured cabling networks, ensuring compliance will become a bigger challenge. The Dell’Oro Group estimates that PoE port shipments will exceed 624 million over the next five years. It is becoming more critical to not only test DC resistance unbalance during field acceptance testing, but to continuously measure and monitor it once PDs are turned up.
We believe the most optimal and cost-effective way to achieve this measuring capability for an active PoE link is through structured cabling connecting hardware. NFLEXON is developing a solution to do this. To learn more, call 855-NFLEXON (855-635-3966) or email us at email@example.com.