Power over Ethernet

“PoE” redirects here. For other uses, see Poe (disambiguation).

“PoE++” and “4PPoE” redirect here. For the point-to-point protocol, see PPPoE.

Not to be confused with Ethernet over power (HomePlug) particularly IEEE standard IEEE 1901.

Power over Ethernet (PoE) encompasses a range of standards and ad hoc systems that enable the transmission of electric power alongside data through twisted-pair Ethernet cabling. This innovative technology allows a single cable to deliver both a data connection and sufficient electricity to support networked devices like wireless access points (WAPs), IP cameras, and VoIP phones.

Techniques

Since 2003, the Institute of Electrical and Electronics Engineers (IEEE) has standardized three common techniques for transmitting power over Ethernet cabling.

The three techniques are:

Alternative A, which uses the same two of the four signal pairs that 10BASE-T and 100BASE-TX use for data in typical Cat 5 cabling.
Alternative B, which separates the data and the power conductors for 10BASE-T/100BASE-TX, making troubleshooting easier.
4PPoE, which uses all four twisted pairs in parallel, increasing the achievable power.

Both alternatives A and B transmit power on wire pairs used for data transmission at speeds of Gigabit Ethernet and faster. This method is similar to phantom power used for condenser microphones, where power is transmitted on the data conductors by applying a common voltage to each pair. This does not interfere with data transmission, as twisted-pair Ethernet uses differential signaling. The common-mode voltage can be easily extracted using the center tap of the standard Ethernet pulse transformer.

4PPoE utilizes all four pairs of a twisted-pair cable to deliver power, allowing for higher power output for devices such as pan-tilt-zoom cameras, high-performance wireless access points, and even laptop battery charging. The IEEE PoE standards also include signaling capabilities between the power sourcing equipment (PSE) and powered device (PD) to detect compatible devices and negotiate power requirements, ensuring safe and efficient power delivery.

Standards development

Two- and four-pair Ethernet

The original IEEE 802.3af-2003 PoE standard provides up to 15.4 W of DC power (minimum 44 V DC and 350 mA) on each port.Only 12.95 W is assured to be available at the powered device as some power dissipates in the cable.

The IEEE 802.3at-2009 PoE standard, also known as PoE+ or PoE plus, provides up to 25.5 W of power for Type 2 devices.The 2009 standard prohibits a powered device from using all four pairs for power.Both of these standards have since been incorporated into the IEEE 802.3-2012 publication.

The IEEE 802.3bt-2018 standard further expands the power capabilities of 802.3at. It is also known as PoE++ or 4PPoE. The standard introduces two additional power types: up to 51 W delivered power (Type 3) and up to 71.3 W delivered power (Type 4), optionally by using all four pairs for power.Each pair of twisted pairs needs to handle a current of up to 600 mA (Type 3) or 960 mA (Type 4).Additionally, support for 2.5GBASE-T, 5GBASE-T and 10GBASE-T is included.This development opens the door to new applications and expands the use of applications such as high-performance wireless access points and surveillance cameras.

Single-pair Ethernet

The IEEE 802.3bu-2016 amendment introduced single-pair Power over Data Lines (PoDL) for the single-pair Ethernet standards 100BASE-T1 and 1000BASE-T1 used in automotive and industrial settings. In the two-pair or four-pair standards, each conductor of the pair receives the same power voltage to ensure no differential voltage aside from the transmitted data. However, with single-pair Ethernet, power is transmitted simultaneously with the data. PoDL originally defined ten power classes ranging from 0.5 to 50 W (at PD). Later, PoDL was incorporated into the single-pair variants 10BASE-T1, 2.5GBASE-T1, 5GBASE-T1, and 10GBASE-T1, and as of 2021, includes a total of 15 power classes with additional intermediate voltage and power levels.