Overview of PoE
Overview Of PoE – Power Over Ethernet Concepts and Devices
http://www.thegeekstuff.com/2009/01/overview-of-poe-power-over-ethernet-concepts-and-devices-list/
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 cable caries both data and current together to operate devices like wi-fi access points, security cameras, kiosks, etc. This is very helpful to install wi-fi access points in ceilings, where it is hard to locate power outlets. Only very low-power devices can be powered using the PoE, as it carries limited power over the ethernet cable.
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 every device, 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 easily move the end-devices wherever you can lay a LAN cable.
Case Study - Purdue University Details how Purdue was able to save $300 to $1000 per device by utilizing PoE and eliminating the labor costs incurred in contracting expensive electricians to install wiring for new AC outlets. Excerpt: "Through the use of Power-over-Ethernet, Mr McCoy discovered that his utilization of PoE save between $350 to $1000 per installation by eliminating the labor costs incurred from contracting an electrician to run wiring for new AC outlets."
Veracity - Power without the Struggle Describes the benefits of PoE technology, and how to take full advantage of them. Excerpt: "The benefits of PoE as a power delivery method include drastically lower installation and maintenance costs, and its technology offers a supply that is reliable, safe, efficient and robust, while being as simple as possible to install."
Global Action Plan - An Inefficient Truth A GAP analysis of the impact of ICT (Information and Communication Technology) on carbon emissions, and describes several technological solutions to reducing the impact including the utilization PoE technologies. Excerpt: "Power over Ethernet (PoE) combines data and operating power (15W, 48V) into a single Ethernet cable. PoE can deliver power to IP telephones, wireless routers, security devices and various other low power equipment using standard cabling without any modification to existing infrastructure(66). This reduces the need for AC cabling and provides power to remote Ethernet devices. It allows cheap, fast and more resilient installation(67). High deployment of PoE gives control and flexibility to facility managers. The power supply to lights, computers and peripherals could all be controlled through PoE allowing these to be turned on or off centrally."