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Sources and sinks

As a closer look at the various device types reveals, this distribution was cleverly selected: endspan suppliers integrate the power source in the Ethernet switch. They are especially useful in new installations.

Midspan systems lie between the switch and the consumer and only provide power; they merely pass on data unchanged. Power supply via reserve lines is much less complicated with such injectors, which are available for less than about £20 as single-port versions. Nonetheless, they also have to detect PoE-capable consumers and protect old devices.

A number of manufacturers now offer injectors and endspan power supplies for both small and large installations. Those who only wish to power a single PoE consumer can simply use single-port injectors, such as 3Com's 3CNJPSE.

For less than about £ 20, power supplies that largely comply with 802.3af can be had.

Although it only costs less than £15, it completely implements the recognition of consumers. This device provides a stable supply of power, though it also exceeds the maximum admissible constant current of 350 mA. For most applications, that should not be a serious problem, however. Zyxel's PoE-12 costs a little more. While it complies with the standard, it also requires an external power supply, which 3Com's injector does not. You should be careful before choosing other power sources that may be less expensive. Not all of them take the time to check each consumer's signature before they put 48 volts on the lines. In other words, older components may get damaged.

Sometimes, the 4/5 and 7/8 pairs are already assigned. If you have such a network, the only option is phantom power. In the midterm, that will be the only solution anyway because gigabit Ethernet uses all of the pairs.

Some manufacturers offer splitters or active taps to provide power to older devices via PoE. These devices take care of communication with the power supply, decouple power from the mains, and adapt voltage to consumption. The use of passive splitters, which only decouple the two reserve lines, is not recommended. Because they did not provide any feedback to the source of power, the power source may refuse to supply power.

Zyxel's PoE-12 is a typical midspan injector. It works between the switch and PoE consumers and only provides power to the latter.

Investigative work

To ensure the safety of older network components, the standard requires that sources must identify PoE consumers before they activate power supply. To do so, a method called Resistive Power Discovery is used; on the consumer side, only passive components are required during detection. In contrast, the power supply requires a bit more electronics to inspect a consumer's signature. That not only saves costs for consumers; it also works before consumers have to start drawing power.

For detection, the power source has measurement switch that outputs a maximum of 30V and 5mA. If the specified impedance is within a certain range (19 to 26.5 kΩ, maximal 10 µF), the PSE can start providing power. In a second, optional detection phase, the power supply can detect the consuming device's [anchorlink poeklasse]device type[/anchorlink]. To do so, it measures power consumption and a given voltage between 15.5 and 20.5 V.

The five categories reflect the energy required at the source so that current can be limited. For instance, if manufacturers want to properly design power supplies for PoE-capable switches, a switch with eight ports will have to have a power pack of at least 130 W to supply the full 15 W at all outputs. Unfortunately, such potential would not only be expensive, but also wasted if the switch only has to power VoIP telephones. On the other hand, if you want to power large wireless installations by means of PoE, you should make sure that the switches you purchase have sufficient maximum power output.

Consumers sometimes briefly require greater current when they are switched on. But the source should refuse to provide energy to a consumer if it cannot provide the maximum current for the detected category.

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