Overview Of PoE – Power Over Ethernet Concepts and Devices List

by Ramesh Natarajan on January 23, 2009

RJ45 Ethernet Cable
Photo courtesy of Olivander

1. What is PoE – Power Over Ethernet?

Power over Ethernet (PoE) does exactly what it says — it carries power over ethernet cables. So, an ethernet cables caries both data and current together to operate devices like wi-fi access points. This is very helpful to install wi-fi access points in ceilings, where it is hard to find power outlets. Only smaller devices can be powered using the PoE, as it carries limited power over the ethernet.

[PoE Setup]
Fig 1: A Typical PoE Setup (Image From: PowerOverEthernet.com)

2. How does PoE works?

Following are the terminologies used in the PoE network:

  • PD – Powered Devices: PDs are end devices that can accept the power transmitted over Ethernet Cat-5 cable.
  • PSE – Power Source Equipment: PSEs add power to the ethernet cable.
  • Midspan device: These are power injectors that stands between the standard ethernet switch and the end device.
  • Endspan devices (PoE Switches): These are Ethernet Switches that includes the circuit to inject power to the ethernet cable.


Cat5 ethernet cable has 4 twisted pairs. Out of 4 pairs:

  • 2 pairs are used for data transfer,
  • 2 pairs are spare and not used.


There are two approaches to transfer power over ethernet.

Approach 1 uses data pairs:

The same 2 pairs used for data transfer, is used to carry power. The PSE injects power to the 2 data pairs (pin 1,2 and pin 3,6) via the center tap of the transformers without affecting the data transfer.

PoE Approach2
Fig 2: PoE Approach 1 uses data pairs to send power (Image From: PowerOverEthernet.com)

Approach 2 uses the spare pairs:

The 2 spare pairs that are not used, is used to carry power. It applies +ve voltage to pins 4 and 5 of ethernet cable. It applies -ve voltage to pins 7 and 8.

PoE Approach1
Fig 3: PoE Approach 2 uses spare pairs to send power (Image From: PowerOverEthernet.com)

3. What are the Benefits of PoE?

  • Portability: Install end-devices where it is hard to get power.  For example, you can install a wireless access point in the ceiling, where you cannot find power outlet.
  • Cost Savings: For a large scale deployment of wireless access point, you don’t need to install power outlet for the access points, which can be a huge cost saving.
  • Simplicity: You need to run only ethernet cable to the end devices, which eliminates the cable clutter.
  • Safer: No AC power is involved, as you don’t need to install A/C power outlet for the end-devices.
  • Easy Maintenance: You can restart the end-device remotely. No need to press the power button to reset the equipment.  You can easily move the end-devices wherever you can lay a LAN cable.

4. Sample List of Equipments that uses PoE.

PoE Adapters:

PoE Wireless Access Point:

PoE Switch:

PoE Network Surveillance Camera:

PoE VoIP Phone:


PowerOverEthernet.com site maintains a Huge List of all PoE products, including a PoE Analog Clocks and PoE Gas Detector.

Additional PoE References


Linux Sysadmin Course Linux provides several powerful administrative tools and utilities which will help you to manage your systems effectively. If you don’t know what these tools are and how to use them, you could be spending lot of time trying to perform even the basic administrative tasks. The focus of this course is to help you understand system administration tools, which will help you to become an effective Linux system administrator.
Get the Linux Sysadmin Course Now!

If you enjoyed this article, you might also like..

  1. 50 Linux Sysadmin Tutorials
  2. 50 Most Frequently Used Linux Commands (With Examples)
  3. Top 25 Best Linux Performance Monitoring and Debugging Tools
  4. Mommy, I found it! – 15 Practical Linux Find Command Examples
  5. Linux 101 Hacks 2nd Edition eBook Linux 101 Hacks Book

Bash 101 Hacks Book Sed and Awk 101 Hacks Book Nagios Core 3 Book Vim 101 Hacks Book

{ 2 comments… read them below or add one }

1 william June 30, 2012 at 5:12 am

i have a nortel POE switch in one building and a smaller network in another building i have a fiber linking the tw obut have a normal media converter to change the media at both ends. my question is will the power from the POE switch affect the media converter since it is plugged directly into one of those ports .

2 Mark November 1, 2013 at 10:34 am

Hi William, the answer is No.
Part of the specification of POE 802.3af and POE+ 802.3at as not to cause damage to equipment, requires a negotiation to take place between both devices before applying the PoE power of which there are three phases called detection, classification and operation.

Hope this helps

Leave a Comment

Previous post:

Next post: