The answer is through the use of phantom power—power sent over the same wire pairs used for data. When the same pair is used for both power and data, the power and data transmissions don’t interfere with each other. Because electricity and data function at opposite ends of the frequency spectrum, they can travel over the same cable. Electricity has a low frequency of 60 Hz or less, and data transmissions have frequencies that can range from 10 million to 100 million Hz.
10- and 100-Mbps PoE may also use phantom power. The 802.3af PoE standard for use with 10BASE-T and 100BASE-TX defines two methods of power transmission. In one method, called Alternative A, power and data are sent over the same pair. In the other method, called Alternative B, two wire pairs are used to transmit data, and the remaining two pairs are used for power. That there are two different PoE power-transmission schemes isn’t obvious to the casual user because PoE Powered Devices (PDs) are made to accept power in either format.
Gigabit PoE devices—such as high-resolution cameras and Gigabit switches—are appearing more frequently on the market. These devices offer lightning fast data throughout, plus the convenience of receiving their power from PoE, but they’re unfortunately still restrained by the 1000BASE-T 100-meter (328-ft.) distance limitation.
Gigabit PoE Media Converters from Black Box enable you to break through that distance limitation to link to faraway PoE devices over fiber. These converters provide a 10-/100-/1000-Mbps copper connection at the end of a 1000-Mbps fiber link (one- and two-port SFP units are also available), plus they act as power source equipment (PSE) on the copper side to power PoE devices.