Amer Networks SRPM24 - Manual (Page 6)

Introduction
Power-over-Ethernet (PoE) eliminates the need to run AC power to other devices on a wired LAN.
Using Power-over-Ethernet system installers needs to run only a single Category 5 Ethernet cable that
carries both power and data to each device. This allows greater flexibility in the locating of network
devices and significantly decreasing installation costs in many cases.
There are two system components in PoE -- the PSE (Power Sourcing Equipment) and the PD (Powered
Device). The IEEE 802.3af specification defines PSE which inserts power onto an Ethernet cable. The PSE
may be located at the device/switch (Endspan configuration), or it may be a separate device located
between the device/switch and the PD (Midspan configuration). The PD is the natural termination of this
link, receiving the power, and could be an IP phone, a WLAN access point, or any other IP device requires
power. The current is transmitted over two of the four twisted pairs of wires in a Category-5 cable.
Power over Ethernet follows the IEEE 802.3af and is completely compatible with existing Ethernet
devices/switches and networked devices. Because the Power Sourcing Equipment (PSE) tests whether a
networked device is PoE-capable, power is never transmitted unless a Powered Device is at other end of
the cable. It also continues to monitor the channel. If the Powered Device does not draw a minimum
current, because it has been unplugged or physically turned off, the PSE shuts down the power to that
port. Optionally, the standard permits Powered Devices to signal to the PSEs exactly how much power
they need.
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