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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 define the transmission
for power over Ethernet. The 802.3af is delivering 48V power over RJ45 wiring.
Besides, 802.3af also defines 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
are for data transmission.
End-Span
End-Span device is directly connected with powered device. End-Span could also
tap wires 1/2 and 3/6.
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.
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