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Chapter 5.
5.1. 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.
IEEE802.3af 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 RJ-45 wiring. Besides 802.3af also define 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 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 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).
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.
PoE (Power over Ethernet) Overview
Figure 5-1 Power Supplied over the Spare Pins
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User's Manual of IAP-200x Series
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