POE is a method of sending electric power over the unused copper conductors in a standard twisted pair cable. For most implementations of Ethernet networks, only four of the eight conductors in a twisted pair cable are in use to transmit network data, so the remaining four conductors can be used to transit modest levels of electrical power.
Some VoIP phones, security cameras, switches, wireless access points, and other devices natively support POE. This means only a single cable needs to be run to the device to provide it network connectivity as well as power. However, to provide POE to an end device, electrical power must be injected into the twisted pair cable. This can be accomplished through several means.
One option is to use a POE switch that injects power into any cable plugged into the switch’s ports. Another option is to use a power-injecting add-on component. This can be a special patch panel or rack-mounted device that is positioned in line between a non-POE switch and the end POE device.
In some cases, a POE switch is present, but the end device does not support POE in its RJ-45 jack. In that case, you can use a splitter to pull off the power from the twisted pair cable into a compatible power adapter cable for the device and provide a network data-only twisted pair cable to be plugged into the device’s RJ-45 jack.
Another important consideration is that there are various standards for POE, ranging from the 802.3af standard to the 802.3at standard. There are also numerous proprietary implementations, most notably those of Cisco equipment. It is important to check the parameters of the POE of both the injector and the end devices for compatibility. It is easy to damage equipment if plugged into a more powerful POE source than it is designed to accept.