How PoE Works

Using the Datacom Cable Plant

Powering in PoE networks can originate from two sources – the Ethernet switch (hub) unit, or a mid-span injection point. The latter can be used with an Ethernet switch that does not incorporate the power electronics for PoE connections. The dc power can be supplied over two or four pairs, depending on which type of PoE technology is used – that is, which standard. All versions operate over distances up to 100 meters using standard category cabling, such as Cat-3, Cat-5e, Cat-6, or Cat-6a.


In PoE systems, the powered devices do not need to meet the requirements of the local power utility line voltages, such as 110 V, 230 V, etc. The power source equipment converts line voltages to the appropriate levels required by the devices, and all dc power (and data) connections are made through the category cables’ 8P8C terminations.


Hub vs. Mid-span Power Sources

The use of mid-span power sources also can provide greater flexibility in using existing Ethernet switches, upgrading the Ethernet equipment to faster bit rates, and in scaling the number of powered devices connected as needed. Mid-span sources are availability with different port counts, from one to 96, although lower counts, such as 4, 8, 12 and 24, are widely used. The use of mid-span sources also can optimize power efficiency by tailoring the number of ports with power sources to the number needed, and powering up and down as needed. High-port-count Ethernet switches, with say 48 or 96 ports, if fully enabled as PoE power supplies, will waste more power.


Benefits of Centralized Control

The power management and power efficiency is a big advantage of PoE systems compared with the use of traditional building wire for power delivery. In a September 2017 webinar organized by Leviton, Kirk Krahn, the company’s senior product manager, copper cable and assemblies, said that PoE can save energy costs for some buildings by up to 40%. The use of central control systems can allow some diagnostics and maintenance from a remote location and can reduce the time to effect repairs. The centralized control and the use of backup power or uninterruptible power supplies also can result in more”up time” or fewer outages.


As for operating efficiency, Mr. Krahn discussed office lighting and other building functions, such as temperature and humidity controls. Lighting and other networked systems become part of the IT network, so the building operator can collect data and develop stronger management plans. The use of air conditioning, for example, can be carefully tailored to parts of the building that heat up more due to sunlight or other factors.


PoE May Drive Cable Demand

These intrinsic advantages suggest strong potential for PoE systems and category cabling. In addition, some of the applications are experiencing rapid growth. In the case of enterprise wireless devices, the CAGR for wireless traffic is progressing at growth rates of 50% or more. This means not only more data on the networks, but also more wireless access devices.  Mr. Krahn said that POE lighting is expected to grow 42% annually, and use of sensors for controlling LED lighting in commercial buildings is expected to increase with a CAGR of 79% through 2020. Thus, LED lighting could prove to be a relatively new and high-growth market for category cables.