What is POE (Power Over Ethernet) and How Does it Work?

[fa icon="calendar"] Aug 30, 2017 10:03:00 AM / by John Crawford

John Crawford

Power Over Ethernet is a technology that enables electrical current to be transmitted over standard Ethernet network cables rather than traditional power cords. POE is a convenient way to supply power to devices like VoIP phones, security cameras and RFID readers, without having to install dedicated electrical wiring.


If a device is POE-enabled then only the network connection needs to be made. The device receives its electrical power from the same cable. This greatly reduces the time and cost involved with deploying new equipment, as well as being more flexible and scalable. Devices can be repositioned more easily, and reliability is also improved because critical network equipment is usually backed up by an uninterruptible power supply.


IEEE standards 802.3af and 802.3at ensure the reliable operation of all compatible POE equipment. If your network doesn’t already support POE, then upgrading is relatively simple. There are two common approaches – POE Switch and POE Injection (or POE Midspan).


1. POE Switch


A POE switch is a normal rack-mounted network switch with integrated Power over Ethernet functionality. It automatically recognizes any POE-enabled devices connected to the

 switch, and supplies power to the device accordingly. Power will not be sent if the device is not POE-compatible.


2. POE Injector


A POE Injector (also known as a POE Midspan) injects power into non-POE network links. To upgrade a specific port, the cable simply needs to be patched through the Midspan. This is a flexible and cost effective solution when only a few POE ports are needed.


POE Splitters


If your network supports POE, but your device doesn’t, then it’s sometimes possible to use a POE splitter to power the device. The splitter connects to the network RJ45 port, and splits the simultaneous transmission of data and power into two separate ports. One port is a standard RJ45 network connection, and the other is a DC power jack that outputs a low voltage suitable for the IP phone, camera or other device.


How POE works


Ethernet cables are constructed of eight wires arranged in four twisted pairs. Two pairs are used for transmitting data under 10 and 100BASE-T Ethernet, and all four pairs are used for transmitting data under Gigabit Ethernet. POE treats each pair as a single conductor, and can use any two pairs to carry electrical current.


Power is injected onto the cable at level that is safe for humans – typically 48 volts DC. However, sensitive equipment that has not been designed to receive POE may suffer damage even at that level, so the POE switch performs a signature detection process and only sends power if a compatible device is detected.


Once power has been enabled, the switch monitors the electrical stream and will cut power to the cable if too much or too little power is drawn. This ensures that the supply of power is terminated if the device is unplugged.

More about VoIP Quality of Service here. 


Topics: Quality of Service, Glossary

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