Clause 33 of IEEE 802.3-2005 (commonly referred to as IEEE 802.3af) is how Power over Ethernet is usually implemented. It provides a device selectable 36–57 V DC, though usually 48 V, over two of the four available pairs on a Cat. 3/Cat. 5e cable with a selectable current of 10–400 mA subject to a maximum load power of 15.40 W. Only about 12.95 W are available after counting cable losses, and most switched power supplies will lose another 10–20% of the available energy. A "phantom power" 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 T568B wiring) form one side of the 48 V (+) DC supply, and pins 3-6 (pair #3 in T568B) provide the 48V (-) 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 T568A and T568B) form one side of the DC supply and pins 7-8 (pair #4 in both T568A and T568B) 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 shall 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). A PD indicates that it is standards-compliant by placing a 25 kΩ resistor between the powered pairs. If the PSE detects a resistance that is too high or too low (including a short circuit), no power is applied. This protects devices that do not support IEEE 802.3af. An optional "power class" feature allows the PD to indicate its power requirements by changing the sense resistance at higher voltages. 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. [1]

There are two types of PSEs specified by IEEE 802.3-2005: 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 on new installations or 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.

Stage Action Volts specified per 802.3af Volts managed by chipset (LM5071) 

Detection Measure whether powered device has the correct signature resistance of 15–33 kΩ 2.7-10.0 1.8–10.0 

Classification Measure which power level class the resistor indicates (see below) 14.5-20.5 12.5–25.0 

Startup Where the powered device will startup >42 >38 (LM5072) 

Normal operation Supply power to device 36-57 25.0–60.0

This higher power standard, commonly referred to as PoE+, was developed by the IEEE 802.3at task force with work beginning in September 2005. The standard describes the extension of the 803af PoE standard using all four pairs of standard Ethernet Category 5 cable to provide up to 56 W of power. The higher power available with 802.3at standard allows for the power consumption requirements of some multi-radio access points and high transmit power 802.11ac and 802.11ax devices. It also allows for the inclusion of 802.3 Ethernet pass-through ports that are PoE (802.3af) source ports, built into access points for the attachment of surveillance cameras or other equipment without the need for separate power cabling.