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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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PoE on Data or Spare Pairs?
PoE was originally intended for 10Base-T and 100Base-TX (10/100 Mbps) Ethernet applications, in which, as we learned in Chap. 1, (only) two pairs of the cable are used for data. These are called the data pairs; one pair is for transmitting, one for receiving. So two pairs of the Ethernet cable were unused, these are called its “unused pairs” or “spare pairs.” That is strictly from the viewpoint of data. When the first IEEE PoE standard (the AF standard) was under debate, the only thing clear from the very start was that the scope of the PoE standard was restricted to power over only two pairs—one pair for forward-current flow (PSE to PD), one for return. But the question was: Which two pairs of the four should be ...

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