Measure DC Resistance Unbalance in Structured CablingRichard Mei
Because Power over Ethernet (PoE) is the most economical way to transmit an Ethernet signal and power over twisted-pair cables, savvy IT managers know it is their best solution. The Dell’Oro Group estimates PoE port shipments will exceed 624 million over the next five years. Along with this growth, however, comes a new cabling concern: How do you know if DC resistance unbalance will remain in compliance with the IEEE 802.3-2012 and IEEE 802.3bt-2018 standards after field acceptance testing is completed and the network is active?
The fact is, it is becoming more critical to not only test DC resistance unbalance but also to monitor it once Powered Devices (PDs) are turned up.
How do you know if DC resistance unbalance will remain in compliance with the IEEE 802.3-2012 and IEEE 802.3bt-2018 standards after field acceptance testing is completed and the network is active?
Measuring DC Resistance Unbalance
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 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.
Once the network is active, IT network administrators are not able to continuously measure DC resistance unbalance without impairing the Ethernet transmission. The most optimal and cost-effective way to achieve this measuring capability may be through the structured cabling connecting hardware.
Uneven Current Flow
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 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, 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 caused by the mechanical degradation of the network cables.
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.
- The insertion loss of the cabling channel will increase. As this occurs, the cabling channel electrical length will increase even though the physical cabling channel length remains unchanged.
- 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.
Continuous Measurement and Monitoring of DC Current
For IT network administrators, Ethernet and power transmission issues are a major concern. Unfortunately, few may be aware of this long- term degradation of their network cabling because physical and mechanical properties are less evident. Critically, this degradation is irreversible. Over time as higher powered PoE devices are integrated in the network, the degradation could lead to latent Ethernet and power transmission issues. Replacing the cable is a costly remedy.
With the proliferation of IoT devices requiring PoE and the advent of high-power PoE devices, more onus is on IT network administrators to be aware of latent network issues caused by DC resistance unbalance. More importantly, they will need the capability to continuously measure and monitor DC resistance unbalance. We believe the most optimal, cost-effective way to achieve this for an active PoE link is through the structured cabling connecting hardware. NFLEXON is developing a solution to do just this. To learn more, call 855-NFLEXON (855-635-3966) or email us at firstname.lastname@example.org.