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intermediary device between a non-PoE capable switch and a PoE device, it is
called a mid-span. An external PoE injector is a mid-span device.
Powered Device
A powered device (PD) is a device powered by a PSE and thus consumes energy.
Examples include wireless access points, IP phones, and IP cameras. Many
powered devices have an auxiliary power connector for an optional, external power
supply. Depending on the PD design, some, none, or all power can be supplied
from the auxiliary port, with the auxiliary port sometimes acting as backup power
in case of PoE supplied power failure.
How Power is Transferred through Cable
A standard Cat5 Ethernet cable has four twisted pairs, but only two of these are
used for 10BASE-T and 100BASE-TX. The specification allows two options for using
these cables for power, shown in Figure 1 and Figure 2:
The spare pairs are used. Figure 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).
POWER SOURCING
EQUIPMENT (PSE)
POWERED DEVICE
(PD)
SPARE PAIR
SPARE PAIR
SIGNAL PAIR
SIGNAL PAIR
Converter
DC / DC
RX
TX
TX
RX
48V
4
5
1
2
3
6
7
8
4
5
1
2
3
6
7
8
+
-
Converter
Figure 1:
Power Supplied over Spare Pins
