POWER OVER ETHERNET

Power over Ethernet,

 PoE describes any of several standards or ad hoc systems that pass electric power along with data on twisted pair Ethernet cabling. This allows a single cable to provide both data connection and electric power to devices such as Wireless Access Points (WAPs), Internet Protocol (IP) cameras, and Voice over Internet Protocol (VoIP) phones.

There are several common techniques for transmitting power over Ethernet cabling. Three of them have been standardized by Institute of Electrical and Electronics Engineers (IEEE) standard IEEE 802.3 since 2003. These standards are known as alternative A, alternative B, and 4PPoE. For 10BASE-T and 100BASE-TX, only two of the four signal pairs in typical Cat 5 cable are used. Alternative B separates the data and the power conductors, making troubleshooting easier. It also makes full use of all four twisted pairs in a typical Cat 5 cable. The positive voltage runs along pins 4 and 5, and the negative along pins 7 and 8.

Alternative A transports power on the same wires as data for 10 and 100 Mbit/s Ethernet variants. This is similar to the phantom power technique commonly used for powering condenser microphones. Power is transmitted on the data conductors by applying a common voltage to each pair. Because twisted-pair Ethernet uses differential signaling, this does not interfere with data transmission. The common-mode voltage is easily extracted using the center tap of the standard Ethernet pulse transformer. For Gigabit Ethernet and faster, both alternatives A and B transport power on wire pairs also used for data since all four pairs are used for data transmission at these speeds.

4PPoE provides power using all four pairs of a twisted-pair cable. This enables higher power for applications like Pan–Tilt–Zoom (PTZ) cameras, high-performance WAPs, or even charging laptop batteries.

In addition to standardizing existing practice for spare-pair (Alternative B), common-mode data pair power (Alternative A) and 4-pair transmission (4PPoE), the IEEE PoE standards provide for signaling between the power sourcing equipment (PSE) and powered device (PD). This signaling allows the presence of a conformant device to be detected by the power source, and allows the device and source to negotiate the amount of power required or available.
Standards development

The original IEEE 802.3af-2003[1] PoE standard provides up to 15.4 W of DC power (minimum 44 V DC and 350 mA[2][3]) on each port.[4] Only 12.95 W is assured to be available at the powered device as some power dissipates in the cable.[5] The updated IEEE 802.3at-2009[6] PoE standard also known as PoE+ or PoE plus, provides up to 25.5 W of power for Type 2 devices.[7] The 2009 standard prohibits a powered device from using all four pairs for power.[8] Both of these standards have since been incorporated into the IEEE 802.3-2012 publication.

The IEEE 802.3bu-2016[10] amendment introduced single-pair Power over Data Lines (PoDL) for the single-pair Ethernet standards 100BASE-T1 and 1000BASE-T1 intended for automotive and industrial applications. On the two-pair or four-pair standards power is transmitted only between pairs, so that within each pair there is no voltage present other than that representing the transmitted data. With single-pair Ethernet, power is transmitted in parallel to the data. PoDL defines 10 power classes, ranging from .5 to 50 W (at PD).
Looking at ways of increasing the amount of power transmitted, IEEE has defined IEEE 802.3bt 4PPoE in September 2018.[11] 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). Each pair of twisted pairs needs to handle a current of up to 600 mA (Type 3) or 960 mA (Type 4).[12] Additionally, support for 2.5GBASE-T, 5GBASE-T and 10GBASE-T is included.[13] This development opens the door to new applications and expands the use of applications such as high-performance wireless access points and surveillance cameras. PoE is also used in the automotive industry.