Interlogix GE-DS-242-PoE - Manual (Page 206)

Chapter 8: Power Over Ethernet Overview
202 GE-DS-242-PoE Managed Ethernet 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 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