POE INJECTOR

Power Over Ethernet

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 for low power levels.^[1] 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^[2] PoE standard provides up to 15.4 W of DC power (minimum 44 V DC and 350 mA^[3][4]) to each device.^[5] Only 12.95 W is assured to be available at the powered device as some power is dissipated in the cable.^[6]

The updated IEEE 802.3at-2009^[7] PoE standard also known as PoE+ or PoE plus, provides up to 25.5 W of power.^[8] The 2009 standard prohibits a powered device from using all four pairs for power.^[9] 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 Cat.5 cable.^[10]

Numerous non-standard schemes had been used prior to PoE standardization to provide power over Ethernet cabling. Some are still in active use.

Advantages over other integrated data and power standards

This technology is especially useful for powering IP telephones, wireless LAN access points, cameras with pan tilt and zoom (PTZ), remote Ethernet switches, embedded computers, thin clients and LCDs.

All these require more power than USB offers and very often must be powered over longer runs of cable than USB permits. In addition, PoE uses only one type of connector, an 8P8C modular connector (often called RJ45), whereas there are numerous types of USB connectors.

PoE is presently deployed in applications where USB is unsuitable and where AC power would be inconvenient, expensive or infeasible to supply. However, even where USB or AC power could be used, PoE has several advantages over either, including the following:

• Cheaper cabling — even category 5 cable is cheaper than USB repeaters, and the task of meeting building code requirements to run AC power cable is eliminated.
• A Gigabit connection to every device is possible, which exceeds 2009 USB and the AC powerline networking capabilities.
• Global organizations can deploy PoE everywhere without concern for any local variance in AC power standards, outlets, plugs, or reliability.
• Direct injection from standard 48 V DC battery power arrays; this enables critical infrastructure to run more easily in outages, and make power rationing decisions centrally for all the PoE devices. The priority for power-supply via PoE can be configured via the switches.
• Symmetric distribution is possible. Unlike USB and AC outlets, power can be supplied at either end of the cable or outlet. This means the location of the power source can be determined after cables and outlets are installed.

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