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4. Power Over Ethernet Overview
What is PoE?
Based on the global standard IEEE 802.3af, PoE is a technology for wired Ethernet,
the most widely installed local area network technology adopted today. PoE allows
the electrical power necessary for the operation of each end-device to be carried
by data cables rather than by separate power cords. New network applications,
such as IP Cameras, VoIP Phones, and Wireless Networking, can help enterprises
improve productivity. It minimizes wires that must be used to install the network
for offering lower cost, and less power failures.
IEEE 802.3af also called Data Terminal equipment (DTE) power via Media
dependent interface (MDI) is an international standard to dene the transmission
for power over Ethernet. The 802.3af is delivering 48V power over RJ-45 wiring.
Besides 802.3af also dene two types of source equipment: Mid-Span and End-
Span.
Mid-Span
Mid-Span device is placed between legacy switch and the powered device. Mid-
Span is tap the unused wire pairs 4/5 and 7/8 to carry power, the other four is for
data transmit.
End-Span
End-Span device is direct connecting with power device. End-Span could also tap
the wire 1/2 and 3/6.
PoE System Architecture
The specication 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 specication allows two options for using
these cables for power, shown in Figure 2 and Figure 3:
The spare pairs are used. Figure 2 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).
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