What is Power Over Ethernet (POE) ?

Power over Ethernet

Power over Ethernet or PoE technology describes a system to transmit electrical power, along with data, to remote devices over standard twisted-pair cable in an Ethernet network. This technology is useful for powering IP telephones, wireless LAN access points, webcams, Ethernet hubs, computers, and other appliances where it would be inconvenient or infeasible to supply power separately. The technology is somewhat comparable to POTS telephones, which also receive power and data (although analog) through the same cable. It works with an unmodified Ethernet cabling infrastructure.

There are several general terms used to describe this feature. The terms Power over Ethernet (PoE), Power over LAN (PoL), Power on LAN (PoL), and Inline Power are synonymous terms used to describe the powering of attached devices via Ethernet ports.

There are several PoE implementations, including ad-hoc techniques, but supplying power over ethernet according to the IEEE standard is strongly recommended.

IEEE 802.3—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.^[1]

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 onfigure a new Ethernet switch, and only PoE needs to be added to the network.