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The data pairs are used. Since Ethernet pairs are transformer coupled at each end, it is possible to apply DC power to the
center tap of the isolation transformer without upsetting the data transfer. In this mode of operation the pair on pins 3 and 6
and the pair on pins 1 and 2 can be of either polarity.
Figure 2 - Power Supplied over the Data Pins
When to install PoE?
Consider the following scenarios:
• You're planning to install the latest VoIP Phone system to minimize cabling building costs when your company moves into
new offices next month.
• The company staff has been clamoring for a wireless access point in the picnic area behind the building so they can work
on their laptops through lunch, but the cost of electrical power to the outside is not affordable.
• Management asks for IP Surveillance Cameras and business access systems throughout the facility, but they would
rather avoid another electrician's payment.
References:
IEEE Std 802.3af-2003 (Amendment to IEEE Std 802.3-2002, including IEEE Std 802.3ae-2002), 2003 Page(s):0_1-121
White Paper on Power over Ethernet (IEEE 802.3af)
http://www.poweroverethernet.com/articles.php?article_id=52
Microsemi /PowerDsine
http://www.microsemi.com/PowerDsine/
Linear Tech
http://www.linear.com/