Overview Of PoE – Power Over Ethernet Concepts and Devices List

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.

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.

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.

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:

• D-Link Power over Ethernet (PoE) Adapter – DWL-P50
• D-Link Power Over Ethernet Adapter – DWL-P200
• Linksys Power Over Ethernet Adapter Kit

PoE Wireless Access Point:

• D-Link PoE Wireless Access Point – DWL-2200AP
• D-Link PoE Outdoor Wireless Access Point – DWL-7700AP

PoE Switch:

• Netgear 24 Port Switch with 2 Gigabit Ports and 12 Port POE – FS726TP ProSafe

PoE Network Surveillance Camera:

• Airlink Power over Ethernet Network Camera – SkyIPCam310
• Panasonic 21x Optical Zoom Pan/Tilt Network Camera – BB-HCM580A
• Mobotix Outdoor, Wide Angle Lens, Motion Detection Camera D22M-IT

PoE VoIP Phone:

• Polycom IP Phone with Integrated PoE – IP550

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

Additional PoE References

• PoE White Paper from PowerOverEthernet.com
• PoE on Wikipedia

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