User:Electron9/PoE missing

Power over Ethernet

0) 802.3af/at Implementation knowhow

Power over Ethernet is usually implemented following the specifications in IEEE std. 802.3af-2003 which added clause 33 to the IEEE 802.3 standard. It allows the powering device to use a voltage between 44–57 V DC, though the nominal voltage is 48 V, over two of the four available pairs on a Cat. 3/Cat. 5e cable with a selectable current of 10 – 350 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 10–25% of the available power. A "phantom power" like 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 cross 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.

1) Dynamic power amendment, more power, how

The newly released IEEE std. IEEE 802.3at-2009 amendment enhanced Power over Ethernet Category 5 cable to dynamically provide between 0.1 – 25.5 W of power.

This is achieved by three means: increasing the current from 350 to 600 mA, increasing the minimum voltage output from the PSE from 44 to 50 V and reducing the worst case resistance assumed to exist on the channel from 20 Ω (Cat.3) to 12.5 Ω (Cat.5).

Integration with the up to 50% energy saving initiative IEEE 802.3az standard, the energy management capabilities of the combined standards are expected to be formidable. However, that integration has not yet occurred.

2) Polarity is specified

Polarity is specified by IEEE 802.3-2008, but all implementations so far use diode bridges on inputs without exception.