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Neither the plugs (RJ45) nor the cables (CAT3/5/7) are designed to transmit current above one ampere. The standard takes account of that by setting a cap of a relatively low current of 350 mA at a relatively high voltage, 48 V. The low current ensures that losses in the line due to heat generation within the UTP cables are kept to a tolerable level.

Higher current could cause cables to overheat, especially when they are tightly packed together; it could also damage plugs due to the resistance in the contacts. The 48 V meets the requirements of "extra-low voltage" regulations and will not cause injuries if a person touches an open RJ-45 contact. As a result, the maximum power is 15.4 W, though only around 13 W is actually available to devices with the maximum allowable cable length of 100 m.


Twisted pair Ethernet cables consist of four pairs of conductors and are used in various networks such as 10BaseT, 100BaseTX, Gigabit Ethernet and ISDN. But not all systems used the same pairs of conductors (see box [anchorlink pinbelegung]Pin assignment[/anchorlink]).

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It is easy to install WLAN access points that get their power supply directly from the LAN, even in hard-to-reach places.

Power over Ethernet was initially only designed for Ethernet over 10/100BaseT. It specifies two varieties of power supply, with the source deciding which one to use. The consumer needs to adapt.

Phantom power sends electricity through the data conductors (1/2 and 3/6). The device developer has to keep in mind that polarity is not specified, but has to be assigned. Decoupling is not a problem because Ethernet devices use signal transformers for the data lines so that devices are galvanically isolated from the LAN.

PoE partially does away with this isolation. If any problems occur, developers have to remedy the matter themselves. Ground loops will rarely occur because a number of devices -- such as wireless base stations and IP telephones -- will only need a single cable thanks to PoE.

If phantom power is not used, the power source can also use the two spare pairs (4/5 and 7/8). For instance, Zyxel's Access Point B-3000 gets all of its power this way, in violation of the standard. In contrast, Lancom Systems' AP L-54g complies with the standard correctly.

The simultaneous use of phantom power and spare pairs is explicitly prohibited. Only phantom power works with Gigabit Ethernet because 1000BaseT uses all four pairs to transmit data.

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