GE DS-82 - Manual (Page 216)

Chapter 8: Power Over Ethernet Overview
212 GE-DS-82 and 82-PoE Ethernet Managed Switch User Manual
PoE System Architecture
The specification of PoE typically requires two devices: the Powered Source
Equipment (PSE) and the Powered Device (PD). The PSE is either an End-Span or a Mid-
Span, while the PD is a PoE-enabled terminal, such as IP Phones, Wireless LAN, etc.
Power can be delivered over data pairs or spare pairs of standard CAT-5 cabling.
How Power is Transferred Through the Cable
A standard CAT5 Ethernet cable has four twisted pairs, but only two of these are used
for 10BASE-T and 100BASE-T. The specification allows two options for using these
cables for power, shown in Figure 99 and Figure 100:
The spare pairs are used. Figure 99 shows the pair on pins 4 and 5 connected
together and forming the positive supply, and the pair on pins 7 and 8 connected and
forming the negative supply. (In fact, a late change to the spec allows either polarity
to be used).
Figure 99: Power Supplied over the Spare Pins