What is Power Over Ethernet (POE)?

Power over Ethernet (PoE) technology can be used to power devices using ethernet cables that are usually used to transfer data. When using this technology, the devices do not require a separate power connection or cord other than an Ethernet cable. PoE does not require any special wiring, it uses the same ethernet cables – Cat 5e, Cat 6, etc - and the "RJ45" connectors that are used for regular local area networks. 

Standard Ethernet cables - Cat 5e (or better) consist of four twisted pairs of cable, and PoE sends power over these pairs to devices. In one method, two wire pairs are used to transmit data, and the remaining two pairs are used for power. In the other method, power and data are sent over the same pair. When the same pair is used for both power and data, the power and data transmissions don't interfere with each other. Data transmission is not affected since twisted-pair Ethernet uses differential signaling. Ethernet cables can carry 15.4 – 60 W power using two wire pairs and 60 – 100 W using all 4 wire pairs of the cable.

PoE Compatibility and PoE Network Components

For purely PoE networks to be built, all the devices connected to the network should be PoE compatible i.e. they should be able to use the power they receive via the ethernet cables. Additionally, These devices should be connected to a PoE-compatible network switch which is different from regular network switches. Only PoE-compatible network switches can transmit power through ethernet cables.

If a Non-PoE network switch is to be used, an additional component called injector is attached at the network switch end (as shown in the image) to make it a PoE network. A data only ethernet cable from the network switch and a power supply cable is input into the injector and it output is an ethernet cable carrying power and data.

If the device to be connected in the network is non-PoE, then a component called a splitter is used at the non-PoE device’s end (as shown in the image) to make it compatible with PoE. The PoE power and data carrying ethernet cable is connected to the splitter’s Input. The splitter separates the power from the data and it is then transmitted through a power ‘OUT’ cable connected to the device’s power inlet. The data is sent separately through a data only carrying ethernet cable.

If both the device and the network switch are Non-PoE compatible, both the splitter and injector need to be used to make it a PoE network (shown in the image above).

Initially many temporary and proprietary schemes were employed to get power over network cables, but with the standardization of PoE, it gained universal adoption and popularity which also assures compatibility with all modern PoE equipment.

Standards and power parameters of PoE

There are several common techniques for transmitting power over Ethernet cabling. The Institute of Electrical and Electronics Engineers (IEEE) has standardized three of these techniques in standard IEEE 802.3 since 2003. These standards are known as alternative A, alternative B, and 4PPoE:

• Alternative A transports power on the same 2 twisted pair wires (out of a total of 4) as data in a typical Cat5 Ethernet cable. Because twisted-pair Ethernet uses differential signalling, the voltage applied does not interfere with data transmission. 
• 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 -- 2 for data transmission and 2 for power transmission. 
• 4PPoE provides power using all four pairs of a twisted-pair Cat5 cable. This enables higher power for applications like Pan–Tilt–Zoom (PTZ) cameras, high-performance WAPs, or even charging laptop batteries.

The original IEEE 802.3af-2003 PoE standard provides up to 15.4 W of DC power for type 1 devices. But only 12.95 W is assured to be available at the powered device as some power is dissipated in the cable. The updated 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 of a Cat5 cable for power. Both of these standards have since been incorporated into the IEEE 802.3-2012 publication. To increase the amount of power transmitted, IEEE defined IEEE 802.3bt 4PPoE in September 2018 which uses all 4 pairs of an Ethernet cable. This standard introduces two additional power types: up to 51 W delivered power (Type 3) and up to 71.3 W delivered power (Type 4).

Advantages and Applications of PoE 

Power over Ethernet reduces the time and expense of having electrical power cabling installed as it minimizes the number of wires that are required to install a network. Since devices using PoE don't have to depend on proximity to electrical outlets, they can be located wherever they are needed most and repositioned easily if required. POE delivery is intelligently designed to protect network equipment from overload, underpowering, or incorrect installation. As POE power comes from a central and universally compatible source, rather than a collection of distributed wall adapters, it can be backed up by an uninterruptible power supply, or controlled to easily disable or reset devices. PoE makes the installation and distribution of network connections simple and effective.

Applications of Power Over Ethernet (POE): 

• RF Test Equipment – Many programmable test devices that were remote-controlled via ethernet, can now also be powered by ethernet and do not need a separate power cable.
• Outdoor point to point microwave and millimeter-wave radios and some Free Space Optics (FSO) units usually featuring proprietary PoE
• Outdoor roof-mounted radios with integrated antennas, 4G/LTE, 802.11 or 802.16 based wireless CPEs (customer premises equipment) used by wireless ISPs
• Wireless - Wifi and Bluetooth APs and RFID readers are commonly PoE-compatible, to allow remote location away from AC outlets, and relocation following site surveys.
• Wireless Access Points (WAPs) - A networking device that allows other wireless to connect to a wired network.
• Industrial control system components including sensors, controllers, meters etc.
• VoIP phones - The original PoE application. Using POE means phones have a single connection to a wall socket and can be remotely powered, just like with the older analog systems.
• IP cameras including PTZs - PoE is now ubiquitous on networked surveillance cameras, where it enables fast deployment and easy repositioning.
• Mini network switches installed in distant rooms support a small cluster of Ethernet ports from one uplink cable. PoE power is fed into the device (or PoE in) port and they in turn power remote PoE devices using PoE pass through.
• PoE Splitters can even output the power at a different voltage (e.g. 5V), to power a remote device or charge a mobile phone.