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Ethernet: The Definitive Guide, 2nd Edition by Joann Zimmerman, Charles E. Spurgeon

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Chapter 6. Power Over Ethernet

Power over Ethernet (PoE) is an optional standard that provides direct current (DC) electrical power over Ethernet twisted-pair cabling. This makes it possible for an Ethernet switch port, for example, to provide both Ethernet data and the power needed to operate a low-power Ethernet device connected to the other end of the cable, such as a wireless access point. The system is carefully designed to provide both power and Ethernet data over the same cable, without causing any interference with the data.

PoE supports devices with relatively low power requirements, including wireless access points, Voice over IP (VoIP) telephones, video cameras, and monitoring devices, making it possible to reduce costs by avoiding the need to provide a separate electrical circuit for the connected device. The power being provided is classed as Safety Extra Low Voltage (SELV), which is defined as a voltage that is limited to a peak of 60 volts DC, provided by a power supply that has no direct connection to primary power (AC grid), and which derives its power via a transformer or equivalent isolation device.

In other words, the power being provided over the Ethernet cable is carefully engineered so as not to present a shock hazard. The low voltages, electrical isolation from the AC grid, and limited current levels mean that the power being delivered is safe to work with, and that you do not need an electrician to install or manage these power circuits.

Power Over Ethernet ...

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