73
9.1.3 Power over Ethernet
Power over Ethernet (PoE) provides the option of supplying devices connected to an Ethernet
network with power using the same cable as for data communication. Power over Ethernet is
widely used to power IP phones, wireless access points and network cameras in a LAN.
The main benefit of PoE is the inherent cost savings. Hiring a certified electrician and installing
a separate power line are not needed. This is advantageous, particularly in difficult-to-reach
areas. The fact that no power cable has to be installed can save, depending on the camera loca-
tion, up to a few hundred dollars per camera. Having PoE also makes it easier to move a camera
to a new location, or add cameras to a video surveillance system.
Additionally, PoE can make a video system more secure. A video surveillance system with PoE
can be powered from the server room, which is often backed up with a UPS (Uninterruptible
Power Supply). This means that the video surveillance system can be operational even during a
power outage.
Due to the benefits of PoE, it is recommended for use with as many devices as possible. The
power available from the PoE-enabled switch or midspan should be sufficient for the connected
devices and the devices should support power classification. These are explained in more detail
in the sections below.
802.3af standard and High PoE
Most PoE devices today conform to the IEEE 802.3af standard, which was published in 2003. The
IEEE 802.3af standard uses standard Cat-5 or higher cables, and ensures that data transfer is not
affected. In the standard, the device that supplies the power is referred to as the power sourcing
equipment (PSE). This can be a PoE-enabled switch or midspan. The device that receives the
power is referred to as a powered device (PD). The functionality is normally built into a network
device like a network camera, or provided in a standalone splitter (
see section below
).
Backward compatibility to non PoE-compatible network devices is guaranteed. The standard
includes a method for automatically identifying if a device supports PoE, and only when that is
confirmed will power be supplied to the device. This also means that the Ethernet cable that is
connected to a PoE switch will not supply any power if it is not connected to a PoE-enabled device.
This eliminates the risk of getting an electrical shock when installing or rewiring a network.
In a twisted pair cable, there are four pairs of twisted wires. PoE can use either the two ‘spare’
wire pairs, or overlay the current on the wire pairs used for data transmission. Switches with
built-in PoE often supply electricity through the two pairs of wires used for transferring data,
while midspans normally use the two spare pairs. A PD supports both options.
NETWORk TECHNOLOGIES - CHAPTER 9
