Figure 8-1 - Power Supplied over the Spare Pins
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 8-2 - 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.
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