If you have ever wondered about PoE, how to use, and how to design it, you came to the right place. We'll dive here into PoE theory of operation, standards, configurations, and the current state of art. Some of this information you can find in the appropriate Ethernet standards, but this site may save you hours of research.

WHAT IS IT?

In short, Power over Ethernet (POE) is a technique to power compliant network devices over Ethernet cables- the same cables that carry data. This technology reduces the overall system cost by eliminating the need of running electrical lines, installing additional AC outlets and using AC/DC adapters. Any PoE system includes two main blocks: Power Sourcing Equipment (PSE) - a device that injects power into a network cable, and a Powered device (PD). The connection between PSE and PD is normally done via a four twisted pairs cable with two RJ45 connectors at both ends. Some PoE-like systems also use coaxial cables.



The following video provides a quick explanation of PoE concepts. For a detailed explanation of PoE operation see How PoE Works.

HISTORY

PoE (originally called Power Over LAN™) was first proposed in late 1990’s by PowerDsine from Petah Tikva and 3Com from Palo Alto. The various implementations of the system were described in a number of patents (such as US 5,994,998, US 6,473,608 B1, US 7,046,983 B2, EP 1 764 947 A2, and US 6,329,906 B1). PowerDsine released first PoE integrated circuit and midspan in the market around 1998. In 2000 Cisco introduced its proprietary technology to power VoIP phones over Ethernet cables. The PoE originators, PowerDsine and 3Com, right now are parts of other companies. PowerDsine was acquired in 2007 by Microsemi, which in turn was bought by Microchip in 2018. 3Com was acquired by H3C in 2007, and then by HP in 2010. Microchip remains a leading maker of PoE controllers and midspans. HP along with Cisco are major manufacturers of PoE-enabled products.

Early PoE systems could provide up to 15 watt power over Ethernet cable. As the technology advanced, the power level was increased sixfold. Common PDs are wireless access points, IP cameras, IP phones, LED lighting, bar code readers, "digital ceilings", and security devices. In general, nowadays any network equipment or lighting system with power consumption up to 90W can be designed to get power over data.

The way PoE works is DC voltage around 54-56 V is applied between center taps of data transformers at the PSE. At the PD end, the voltage is likewise extracted from center taps of the network transformers. See detailed principals of operation.

There are various industry standards designed to make sure that superimposing power on a network cable is done in a safe way, does not reduce data rates, and that complaint PSEs and PDs will work with one another.