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UNDERSTANDING POE
Power over Ethernet (or PoE) is a system which passes electrical power along with data over Ethernet cabling. This
allows a single cable to provide both data connection and electrical power to devices such as wireless access points or
IP cameras. This feature can simplify network installation and maintenance by using the switch as a central power source
for other network devices.
There are several common techniques for transmitting power over Ethernet cabling, and two have been standardized by
IEEE 802.3. Power may be transmitted on the unused conductors of an Ethernet cable, or by applying a common-mode
voltage to each pair. The latter is similar to phantom power commonly used for powering balanced microphones.
In addition to standardizing existing practice for spare-pair and common-mode data pair power transmission, the IEEE
PoE standards provide for signalling between the power source equipment (PSE) and powered device (PD). This signaling
allows the power source to sense and negotiate the amount of power required for a connected device.
Power Budget
One challenge during system design and installation is to consider and calculate the total device power consumption to
ensure it is less than the total power budget of the switch. The power supply in the switch - not the number of ports -
determines your total power budget.
For instance, the AMS-1208P AV Series Gigabit Managed Switch features a total PoE power budget of 130 Watts. If you
were planning an 8-camera serveillance system, and each camera consumed 10W, your total power consumption would
be 80W, which is well under the 130W total power budget. However, if each camera consumed 20W, total consumption
would exceed the 130W total power budget.
Depending on the devices you intend to connect, you may need a larger switch, not because of the number of ports,
but because of the total power budget.
Cable Length
Power over ethernet (POE) will only power a connected device if there is enough voltage after the cable run. The maxi-
mum cable run for PoE devices is approximately 300ft (100m). Beyond this point, voltage will drop and become unstable.
If you need to connect PoE devices more than 300ft from a switch, PoE extenders are available for that application.
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