Power Over Ethernet Overview - Planet Switch User Manual

5. 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.
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 IEEE 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 taps 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 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-5e cabling.

How power is transferred through the cable
A standard Cat5e Ethernet cable has four twisted pairs, but only two of these are used for 10BASE-T,
100BASE-TX and 1000BASE-T. The specification allows two options for using these cables for power as
shown in Figure 5-1-1 and
The spare pairs are used.
positive supply, and the pair on pins 7 and 8 is connected and forms the negative supply. (In fact, a late
change to the specification allows either polarity to be used.).
Figure 5-1-2.
Figure 5-1-1 shows the pair on pins 4 and 5 are connected together and form the
Industrial Renewable Energy 4-Port 10/100/1000T
802.3at PoE+ Managed Ethernet Switch/Router
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BSP-360