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What is the IEEE 802.3 Standard for Power over Ethernet?

Currently recommended (IEEE 802.3-2005)

Clause 33 of IEEE 802.3-2005 (commonly referred to as IEEE 802.3af) provides 48 volts DC over two out of four available pairs on a Cat. 3/Cat. 5 cable with a maximum current of 400 milliamperes for a maximum load power of 15.4 watts. Only about 12.95 watts are available after counting losses, and most switched power supplies will lose another 10-20% of the available energy. A "phantom" technique is used so that the powered pairs may also carry data. This permits its use not only with 10BASE-T and 100BASE-TX, which use only two of the four pairs in the cable, but also with 1000BASE-T (Gigabit Ethernet), which uses all four pairs for data transmission. This is possible because all versions of Ethernet over twisted pair cable specify differential data transmission over each pair with transformer coupling; the DC supply and load connections can be made to the transformer center-taps at each end. Each pair thus operates in "common mode" as one side of the DC supply, so two pairs are required to complete the circuit. The polarity of the DC supply is unspecified; the powered device must operate with either polarity or pair 45+78 or 12+36 with the use of a bridge rectifier.

The standard describes two types of devices: Power Sourcing Equipment (PSE) and Powered Devices (PD). Power Sourcing Equipment provides power to the Powered Devices.

Powering devices

Two modes, A and B, are available. In mode A, pins 1-2 (pair #2 in TIA-568B wiring) form one side of the 48 volt DC supply, and pins 3-6 (pair #3 in TIA-568B) provide the 48 V return. These are the same two pairs used for data transmission in 10Base-T and 100BASE-TX, allowing the provision of both power and data over only two pairs in such networks.
In mode B, pins 4-5 (pair #1 in both TIA-568A and TIA-568B) form one side of the DC supply and pins 7-8 (pair 4 in TIA-568A and TIA-568B) provide the return; these are the "spare" pairs in 10BASE-T and 100BASE-TX. Mode B, therefore, requires a 4-pair cable.
The Power sourcing equipment (PSE) decides whether power mode A or B is to be used, not the powered device (PD).

The PSE can implement mode A or B or both (but must not supply power in both modes at the same time). If the PSE detects either an open or a short circuit, no power is applied, thus protecting devices that do not support IEEE 802.3af. To stay powered, the PD must continuously use 5-10 mA for at least 60 ms with no less than 400 ms since last use or else it will be unpowered by the PSE.

There are two types of PSE's specified by IEEE802.3-2005: Endspans and Midspans. Endspans are Ethernet Switches that include the Power over Ethernet transmission circuitry. Endspans are commonly called PoE Switches. Midspans are power injectors that stand between a regular ethernet switch and the powered device, injecting power without affecting the data. Endspans are normally used when the switch has to be replaced for other reasons (such as moving from 10/100 to 1 Gigabit or adding security protocols), and then it is convenient to add also the PoE capability. Midspans are used when there is no desire to replace and configure a new Ethernet switch, and only PoE needs to be added to the network.

Under development extension (IEEE 802.3at)

A future standard, commonly referred to as PoE+, is being developed by the IEEE 802.3at task force, which officially began work in September 2005. The draft standard describes extending the IEEE Power over Ethernet by using all four pairs of standard Ethernet Category 5 cable to provide up to 56 watts of power. The higher power available with this future standard should make self-powered equipment with higher power requirements such as WiMAX transmitters, pan-tilt-zoom cameras, videophones and thin clients possible. The 802.3at Task Force objectives are along the following lines:

802.3at should operate on CAT5 and higher infrastructure, unlike 802.3af, that had take into account the CAT3 limitations
802.3at should follow the power safety rules and limitations pertinent to 802.3af
A 802.3at PSE must be backwards compatible with 802.3af, being able to power both 802.3af and 802.3at PD's
802.3at should provide the maximum power to PD's as allowed within practical limits, at least 30W
802.3at PDs, when connected to a legacy 802.3af PSE, will provide the user an indication that a 802.3at PSE is required
Research the operation of midspans for 1000BASE-T
Research the operation of midspans and endspans for 10GBASE-T