Power over Ethernet

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Power over Ethernet or PoE technology describes any 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.

Power over Ethernet is standardized in IEEE 802.3af. There are several earlier, ad-hoc techniques, but the IEEE standard will probably become dominant.Image:PoE adapter.jpg

IEEE 802.3af provides 48 volts DC over two pairs of a four-pair cable at a maximum current of 350 mA for a maximum load power of 15.4 watts, although, after counting losses, only about 13 watts are available. 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 with the use of a bridge rectifier.

As of May 2005 there is discussion about increasing the amount of power available on the cable. This may be done by sending power through all four pairs of wire which would double the amount of power. Other discussions include increasing the amount of current. The new standard got the name IEEE 802.3at.

Before applying power, an IEEE 802.3af power source first "probes" the remote device to determine if it can accept power, and if so, which pairs should be used to supply it. Two modes, A and B, are available. In mode A, pins 1&2 (pair #2 in TIA-568B wiring) form one side of the 48VDC supply, and pins 3&6 (pair #3 in TIA-568B) provide the 48VDC 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.

A popular misconception is that a load (technically a Powered Device (PD)) may choose either mode A or B. This is not so. It must be capable of using both modes (although only one can be active at any one time) to be compliant with the IEEE 802.3af standard.

The supply (technically the Power Sourcing Equipment(PSE)) can implement Alternative A or Alternative B or both (but must not supply power on both at the same time). If the supply detects either an open or a short circuit, no power is applied, thus protecting devices that do not support IEEE 802.3af.


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