What is Power over Ethernet (PoE)? / by DataStrait Networks


Ethernet is a tried and true method of computer network communication. In addition to providing a convenient way for different “connected” devices to interact with each other, it also offers an excellent way to deliver power to subsidiary connected devices. Colloquially known as “Power Over Ethernet” or “ PoE," this capability is a genuinely significant secondary benefit of this technology. Here is a quick rundown on the basics of POE...

Power Over Ethernet (PoE) Basics

The Ethernet cabling system was initially designed to merely provide a data connection between two connected devices. Subsequent development allowed the same – single, twisted pair – cable to also provide electric power along the same line. An Ethernet cable can provide both electrical power and a data connection to devices as diverse as IP cameras, VoIP phones, and other wireless devices. The user needs only connect the devices to each other to gain this functionality. The "plug and play" capability of POE is indeed a wonder of modern technology.

What is the Goal of PoE?

The use of Power over Ethernet aims to attain two goals:

1.     Transmission of data

2.     Providing power to the device

The system accomplishes these goals through the technology of differential signaling. More technically, this is a method for electrically transmitting information using two complementary signals via separate but physically paired conductive wires instead of just a single wire.



Essential elements of Power over Ethernet

There are some standard techniques for pushing power across an Ethernet-cabled network with the two most common being named 10 BASE-T and 100 BASE-TX. In the industry, they are also known – not surprisingly - as Alternative A or "Alt-A" and Alternative B or "Alt B."


Using Cat 5 cabling, Alternative A transports the data as well as the power on the same wires. It is a power transfer technique that mimics a much older one used on conventionally wired microphones.


Quite differently, Alternative B completely separates the data and the power conduction. Power transmits to the data conductors by applying a same-source voltage to each pair. Differential signaling is then used to not interfere with data transmission. In short, the common-mode voltage is easily extracted using the center tap of the standard Ethernet pulse transformer.

For Gigabit Ethernet and faster, all four pairs are used for data transmission, so both alternative A and B transport power on wire pairs also used for data. All in all, a quite intelligent way to use the same set of wires for different purposes.

How are PoE standards governed?

Before PoE, two connections (a network connection and a separate electrical one) were required to join any subsidiary devices like IP cameras to the network. PoE efficiently combines these two tasks through a single network cable. While no actual governing body oversees the PoE standards, some essential protocols control PoE development and implementation. Currently, there are four types of Ethernet standards:

  • 2-pair PoE (IEEE 802.3af)
  • PoE+ (IEEE 802.3at)
  • 4P PoE ( PoE++ or UPOE)
  • PoE Type 4

What devices use POE?

While most people recognize that Power over Ethernet cabling can support and recharge such items as smartphones and small tablets, they do not realize that the latest generation of Type 4 cabling can also be used to recharge such items as IP cameras, laptops, and even television monitors.