Power over Ethernet or PoE technology describes a system to pass electrical power safely, along with data, on Ethernet cabling. The IEEE standard for PoE requires category 5 cable or higher for high power levels, but can operate with category 3 cable if less power is required. Power is supplied in common mode over two or more of the differential pairs of wires found in the Ethernet cables and comes from a power supply within a PoE-enabled networking device such as an Ethernet switch or can be injected into a cable run with a midspan power supply.
The original IEEE 802.3af-2003 PoE standard provides up to 15.4 W of DC power to each device. 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. The 2009 standard prohibits a powered device from using all four pairs for power. Some vendors have announced products that claim to be compatible with the 802.3at standard and offer up to 51 W of power over a single cable by utilizing all four pairs in the Category 5 cable.
Numerous non-standard schemes had been used prior to PoE standardization to provide power over Ethernet cabling. Some are still in active use.
Comparison with other integrated data and power standards
PoE provides both data and power connections in one cable, so equipment doesn't require a separate cable for each need. For equipment that does not already have a power or data connection, PoE can be attractive when the power demand is modest. For example, PoE is useful for IP telephones, wireless LAN access points, cameras with pan tilt and zoom (PTZ), and remote Ethernet switches. PoE can provide long cable runs e.g., 100 m (330 ft) and deliver 12 W of galvanically isolated power. PoE-plus provides even more power.
There are competing data and power technologies. The Universal Serial Bus (USB) provides both data and power,but it is designed for short cables with a maximum length of 5 m (16 ft) and provides less than 2.5 W of non-isolated power. It is less expensive than PoE, and works well for low power peripherals such as a computermouse, a headset/microphone, or a serial port. Some peripherals, such as speakers, scanners, and printers, need more power than USB can provide. Firewire (IEEE 1394) is similar to USB, but can provide substantially more power (45 W) at a distance of 4.5 m. On the other hand, USB peripherals can operate using very little power;while maintaining an Ethernet connection uses a significant amount of power.
If a device already has power available but no data link, then PoE may not be attractive. A wireless data connection such as IEEE 802.11 may be more economical than running a data cable for the device. Alternatively,there are power line communication technologies that can use power cables for transmitting data. Using some power line modems may be more economical than running a cable.
When data rate and power requirements are both low, other approaches may be viable. Mobile phones, for example,use batteries for power and antennas for communication. Remote weather sensors use very low data rates, so batteries (sometimes supplemented with solar power) and custom wireless data links are used. Depending on the application, some of the advantages with PoE over other technologies may be:
Fast data rate
No batteries required
Peer-to-peer network access. Once a device is connected to the network, it is accessible to many users.
Some types of devices with PoE include:
● IP Security Cameras
● Network routers
● A mini network switch installed in distant rooms, to support a small cluster of ports from one uplink cable. (These ports on the mini-switch do not themselves provide PoE.) (In most modern VoIP phones a two-ports witch is embedded to which a local workstation can be installed using a different VLAN from the voice-VLAN used by the phone itself)
● Network webcams
● Network Intercom / Paging / Public address systems and hallway speaker amplifiers
● VoIP phones
● Wall clocks in rooms and hallways, with time set using Network Time Protocol
● Wireless access points
● Outdoor roof mounted radios with integrated antennas, 802.11 or 802.16 based wireless CPEs (customerpremises equipment) used by wireless ISPs.
● Industrial devices (sensors, controllers, meters etc.)
● Access control and Help-points (intercoms, entry cards, keyless entry, etc.)
● Lighting controllers
● Remote Point of Sale (POS) kiosks
● Physical Security devices and controllers