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Power for everyone?

In the midterm, it might even be possible to power PCs from data networks because the IEEE has been working on the successor to PoE since mid-2005; it it calls this 802.3at. Also called "PoE Plus", it doubles the available power from two pairs to 30 W and even specifies that up to 60 W can be transported through gigabit connections by means of phantom power on all four pairs. A number of thin clients and perhaps one or the other desktop PC that specialises in energy efficiency could make do with that. Furthermore, wireless base stations that support the 802.11n standard expected to be adopted in 2008 or that have multiple wireless modules will require more power than 802.3af can provide.

Such requirements have brought about temporary solutions even though the new standard is not even expected to be adopted until the spring of 2008. For instance, at the end of 2006 chip manufacturer Texas Instruments presented its TPS23841 source controller along with consumer chip TPS2376-H. The combination complies with 802.3af, but if need be voltage can be increased to 53 V and the current almost tripled to 615 mA so that up to 25 W can be made available to the consumer.

For the foreseeable future, normal PCs will not be able to get their power from a LAN cable because they require far more power than even 802.3at can deliver. On the other hand, PoE provides the world's first standardised power supply with enough output for a number of small devices.

If we then take into consideration the costs for the installation of additional 230 V sockets, we see that investments in PoE equipment pay for themselves quickly, especially in outdoor installations or hard-to-reach areas. Furthermore, central management and uninterruptible power supply constitute added value for certain applications.

At the moment, PoE support still costs a pretty penny in LAN devices, but that may be changing. If manufacturers of voltage converters begin integrating the PoE function in their chips soon, costs will drop.

If you want to set up your own PoE networks, it's a good idea to check with device manufacturers to make sure that your products actually comply with 802.3af. Although the standard specifies that consumers have to accept phantom power and power from spare pairs, in practice not every device does. And you cannot tell from the outside which method a power supply uses. In any case, you should steer away from inexpensive power sources that do not comply with the standard because they could permanently damage other network components. (rek)

[anchor poeklasse]PoE power classes[/anchor]
Class Application Classification power1 Maximum power injected (PSE) Maximum power consumption
default 0- 5 mA 15.4 W 0.44 W - 12.95 W
1 optional 8-13 mA 4.0 W 0.44 W - 3.84 W
2 optional 16-21 mA 7.0 W 3.84 W - 6.49 W
3 optional 25-31 mA 15.4 W 6.49 W - 12.95 W
4 reserved 35-45 mA 15.4 W reserved
1 If none of these rules apply, the class is 0.

[anchor pinbelegung]Pin assignment[/anchor]
RJ45 pin Color code 586B 100BaseTX Phantom power Spare pairs 1000BaseT
1 orange stripes TX+ TX+ (V+) RX+ (V-) TX+ RX+ D1+ (V+)
2 orange TX- TX- (V+) RX- (V-) TX- RX- D1- (V+)
3 green stripes RX+ RX+ (V-) TX+ (V+) RX+ TX+ D2+ (V-)
4 blue n. a. n.a. n.a. V+ V+ D3+
5 blue stripes n.a. n.a. n.a. V+ V+ D3-
6 green RX- RX- (V-) TX- (V+) RX- TX- D2- (V-)
7 brown stripes n.a. n.a. n.a. V- V- D4+
8 brown n.a. n.a. n.a. V- V- D4-
n.a. Not assigned
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