The standard also includes a mechanism to protect devices that do not support PoE: A 25-kW resistor is added between the power pairs on the powered device, and the power source provides power only if something close to that resistive value is detected. The first of these standards, IEEE 802.3af, was ratified in 2003 and specified that the power must be able to be carried by either spare pairs (pins 4 and 5 or pins 7 and 8) or data pairs (pins 1 and 2 or pins 3 and 6). In 1999, both the IEEE and the Ethernet Alliance started working to standardize PoE to ensure interoperability across a broader range of connected powered devices and power sourcing equipment. Cisco developed and first delivered a proprietary version of PoE in 2000, to enable scalable and manageable power delivery to Cisco IP-telephony handsets.Įmulating how traditional PSTN (public switched telephone network) landline phones operate-delivering 48V DC power over the same copper cabling used for communications-PoE initially took advantage of unused pairs of the four twisted pairs of copper wires found in typical Category 5 (Cat 5) cable.
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