The Next Generation Of PoE

The long-term goal of Power over Ethernet (PoE) has always been to make laptop power supplies unnecessary. That may be achieved this year. In November 2004, the IEEE began work on a new version aimed at laptops and a new class of "power forwarding" devices. These are switches that receive power over Ethernet and then distribute it to clients.

The existing 802.3af standard supports up to 12.95W of power--enough for a phone or a Wi-Fi access point, but not a cash register or a long-range WiMAX transmitter. The next version, known informally as PoE Plus, aims for about 70W.

At present, 802.3af sends power through only two of the four twisted copper pairs within most cables, so the obvious upgrade is to use all four. However, this would mandate carrying power over the same wires as data, requiring specialized PoE switches. These do exist, but 802.3af also offers the option of simpler midspan power supplies, which sit alongside non-PoE switches and inject power into the two pairs not used for data.

A second possibility is to increase the power in the existing pairs by turning up either the voltage or the maximum current. The problem with boosting voltage is that it risks incompatibility. The original 802.3af standard specified the same 72V as the PSTN so that phones could be adapted for PoE with minimal effort. Changing that would mean redesigning both phones and PoE gear.

This leaves us with current, which has always been variable and thus risks no incompatibility. The problem here is that the amount of power wasted as heat increases exponentially with current, which risks melting the cable. One solution is to use a cable with low electrical resistance, but this would entail stricter wiring requirements than the Cat 3 demanded by 802.3af.