The IEEE 802.3at-2009 PoE standard, also known as PoE+ or PoE plus, provides up to 25.5 W of power. Some vendors have announced products that claim to comply 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 Numerous non-standard schemes had been used prior to PoE standardization to provide power over Ethernet cabling. Some are still in active use.

Standards-based power over Ethernet is implemented following the specifications in IEEE 802.3af-2003 (which was later incorporated as clause 33 into IEEE 802.3-2005) or the 2009 update, IEEE 802.3at. A phantom power technique is used to allow the powered pairs to 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 may be inverted by crossover cables; the powered device must operate with either pair: spare pairs 4-5 and 7-8 or data pairs 1-2 and 3-6. Polarity is required on data pairs, and ambiguously implemented for spare pairs, with the use of a diode bridge.

Powering devices

Two modes, A and B, are available.

Mode A has two alternate configurations (MDI and MDI-X), using the same pairs but with different polarities. In mode A, pins 1 and 2 (pair #2 in T568B wiring) form one side of the 48 V DC, and pins 3 and 6 (pair #3 in T568B) form the other side. 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. The free polarity allows PoE to accommodate for crossover cables, patch cables and auto-MDIX.

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 PSE, not the powered device (PD), decides whether power mode A or B shall be used. PDs that implement only Mode A or Mode B are disallowed by the standard.

The PSE can implement mode A or B or both. A PD indicates that it is standards-compliant by placing a 25 kO resistor between the powered pairs. A major difference between IEEE 802.3af and IEEE 802.3at is that while IEEE 802.3af clearly precluded collocating two PD interfaces on a single RJ45 connector, IEEE 802.3at changes the definition of a PD, and therefore allows two PDs collocation, one mode A and the other mode B. 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 PoE. 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.

There are two types of PSEs: 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 Mbit/s to 1 Gbit/s or adding security protocols), which makes it convenient to add 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.

IEEE 802.3at capable devices are also referred to as "type 2". An 802.3at PSE may also use layer2 communication to signal 802.3at capability.

Non-standard Implementations

Cisco

Under Cisco's pre-standard scheme, the PSE (switch) will send a Fast Link Pulse (FLP) on the transmit pair. The PD (device) connects the transmit line to the receive line via a low pass filter. And thus the PSE gets the FLP in return. And a common mode current between pair 1 and 2 will be provided resulting in 48 V DC and 6.3 W default of allocated power. The PD has then to provide Ethernet link within 5 seconds to the auto-negotiation mode switch port. A later CDP message with a type-length-value tells the PSE its final power requirement. A discontinued link pulses shuts down power.

PowerDsine

Active Splitter

Network devices which can accept the injected DC power directly from the Category 5 cable through their network connection are considered to be “PoE Compatible” or “Active Ethernet Compatible”.

Devices which are not PoE Compatible can be converted to Power-over-Ethernet by way of an “Active Ethernet Splitter (also known as a DC “Picker” or “Tap”). These devices take the DC voltage that has been injected into the Category 5 cable, by the injector, and make it available to the connected device via a conventional DC power socket.

Two basic types of “Active Ethernet Splitters” are available; passive and regulated.

A passive Active Ethernet Splitter simply takes the voltage from the Category 5 cable and directs it to the device for direct connection.

A regulated Active Ethernet Splitter takes the voltage from the Category 5 cable and converts it to a different voltage. Several standard regulated voltages are available; 5VDC, 12VDC, 18VDC and 48VDC. This allows a wide range of non-PoE devices to be powered through a Category 5 cable.