GE -DSH-73 User Manual page 170

Chapter 8: Power Over Ethernet Overview
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 8-1 and Figure 8-2:
The spare pairs are used. Figure 8-1 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 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.
166 GE-DSH-73/DSH-82 and DSH-82-PoE User Manual