Power over Ethernet (PoE) - Explained

14/11/2009 by David Slade.

Power over Ethernet (PoE).

Powering remote Ethernet devices can be a problem when you need to find an AC power outlet to plug a bulky transformer into. But with Power over Ethernet (PoE) you don’t need a power outlet because your Ethernet device draws power from the same Ethernet UTP cable that connects it to the network. You simplify installation by getting power where you need it with PoE.

What is PoE?
The seemingly universal network connection, CAT5 cable, has another role to play: to provide electrical power as well as data. Power over Ethernet (PoE) was ratified by the Institute of Electrical and Electronic Engineers (IEEE) in June 2000 as the 802.3af-2003 standard. It defines the specifications for the transmission of electrical power over CAT5 data cable.

This standard promotes the delivery of low levels of power—just 13 watts or less—over data lines to PoE devices such as IP telephones, wireless access points, Web cameras, and audio speakers. PoE is also ideal for applications such as video surveillance, building management, retail video kiosks, smart signs, vending machines, and retail point of information systems.

By eliminating the need to install separate outlets for data and power, users can save up to 50% in installation costs.

How does PoE work?
Very simply, CAT5 Ethernet cable consists of four twisted pairs of cable. Only two pairs are used for 10BASE-T and 100BASE-TX Ethernet data transmission; the remaining pairs are available for power. The PoE standard offers two options for using the twisted pairs for power. The first option sends the electrical power through the spare pair. The second option uses the data pairs for both power and data.

The data and power transmissions don’t interfere with each other either. Electricity has a low frequency of 50 - 60 Hz or less, and data transmissions have frequencies that can range from 10 million to 100 million Hz. Because data and electricity are at opposite ends of the frequency spectrum, they can travel over the same cable.

PoE devices
To install Power over Ethernet, you need to add a PoE injector (also known as Power Sourcing Equipment or PSE), to insert DC voltage onto the CAT5 cable. The injector is usually installed near the Ethernet switch and may be a single-port model that inserts power onto only one cable or may be a PoE hub, which inserts power onto multiple cables.

At the other end of the powered CAT5 Ethernet cable, you need a way to get the power from the Ethernet cable and back into a device. Many network devices are now made as PoE-compatible devices that can take power directly from the CAT5 cable. These devices are sometimes also described as active Ethernet compatible.

Additionally, you can power some network devices that aren’t PoE compatible by using a device called a picker or a tap. It “picks” the DC voltage from the CAT5 cable and routes it to the device.

PoE applications and benefits:

• Use one set of twisted-pair wires for both data and low-wattage appliances.
• Save money by eliminating the need to run electrical wiring.
• Easily move an appliance with minimal disruption.
• If your LAN is protected from power failure by a UPS, the PoE devices connected to your LAN are also protected from power failure.
• Supports the addition of end-span, standalone, and mid-span devices.
• As PoE becomes more common, the 8-pin modular connector will become the industry-standard power jack.

Posted in Cat5E, 10BASE-T, 802.3af-2003, Power over Ethernet (PoE), 100BASE-TX | No Comments »