Power over Ethernet (PoE): Technology and Applications

Infrastructure


PoEPhoto credit: Olli Niemitalo
 
No doubt you’ve come across an operating electronic device, and looked to find that there was no power cable connected to it. Well, how does it get power? The answer lies in a technology called Power over Ethernet (PoE), on which we will be shedding some light in this article.

What is Power over Ethernet (PoE)?

PoE is a system for conveying electrical power over Ethernet cables allowing the same cable to transmit data and power from equipment acting as a power source (power sourcing equipment, PSE) to other devices that are being powered (powered devices, PD). It is often found in devices like wireless access points and IP surveillance cameras. The technology is governed by IEEE standard 802.3. The IEEE (Institute for Electrical and Electronic Engineers) is a technical association that works to regulate, spread, advance, and provide education on electrical, electronic, computer, and telecommunications engineering. It creates specification standards for different technologies and assigns them codes, to ensure that all devices that use the technology are on the same page and using the same protocols.


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What is Ethernet?

“Ethernet” is a term used to describe wired computer network technology that can be found in Local Area Networks (LAN), Wide Area Networks (WAN), and Metropolitan Area Networks (MAN). It is one of the core technologies upon which rests the structure of the internet. The Ethernet network is connected using cables.

ethernetPhoto Credit: Someone's Moving Castle
 
Ethernet cables come in several varieties that are able to deliver different speeds, such as 10BASE-T, 100BASE-T, and 1000BASE-T (also called Gigabit Ethernet cables, which also include higher speeds like 20GBASE-T). The number represents the speed that the cable can transmit in Mbit/s, “BASE” means that it uses a telecommunications technology known as baseband, and the letter “T” signifies that the cable is a twisted pair cable. These are the network cables you’ve probably seen somewhere that look like a telephone cable on steroids. 

ethernet cableCredit: Raysonho @ Open Grid Scheduler / Grid Engine
 
These cables contain 8 wires, with each 2 twisted over each other forming a pair. Hence, they are 4-pair cables. Lower speed Ethernet uses only 2 pairs to transmit data: one pair sends while the other receives, whereas higher speed Gigabit Ethernet uses all 4 pairs.

Techniques for PoE

Power over Ethernet (PoE) can be transmitted over Ethernet cables in one of three ways: Alternative A (common-mode data pair standard) uses the same 2 pairs used for data transmission to transmit power. Alternative B (Spare-pair standard) uses the 2 pairs not used for data transmission to transmit power. 4PPoE (4-Pair Power over Ethernet) uses all 4 pairs to transmit power in addition to data 4PPoE transmits higher power (up to 100W) and can be used to power Pan Tilt-Zoom Cameras or even charge laptop batteries.

Endspans, midspans, and splitters

A PoE-capable switch can be used as the power source, automatically supplying power to all PoE-capable PDs. This is called an endspan PSE. If the switch is already installed and is not PoE-capable, a PoE injector can be placed after the switch to supply power to PoE-compatible end devices, and is called a midspan PSE. A regular non-PoE device can also be installed in a PoE network and make use of the power supplied by having a PoE splitter (the grey and black device in the photo) installed before it. Similar to a regular network splitter, this device separates data from electrical power coming through the inbound Ethernet cable, and provides the device with each component in a separate cable. 

endspansCredit: Pelzi (talk · contribs)
 
 

How power sourcing equipment manages PoE

PSEs ensure that devices are PoE-compatible before supplying them with power, decide between using different Power over Ethernet (PoE) techniques for each PD, and supply PDs with the power they need according to power negotiation protocols and standards. According to the standards, any PoE-capable device contains a signature in the form of a 25kΩ resistor. This ensures that PSEs can identify non-PoE-capable devices and desist from supplying them with power that may damage their circuits, so non-PoE devices can safely be installed in a network containing a PSE.


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Practical applications

Each PD has a power need and each PSE has a maximum power budget. The cables themselves and the method of PoE employed all provide limitations on the electrical power that can be conveyed over the system. When installing a PoE network, these specifications must be taken into consideration, calculating the total power needed by all devices, and making sure the PSE can cover that, and that the cables connecting the two are able to transmit the required power.

Benefits

There are many benefits for devices to use Power over Ethernet (PoE) rather than standard power sources. These include:

  • Cost considerations: which may not reflect on small scales, but would definitely add up in a larger scale. Since the devices will already require being connected to a network, the redundant power cable can be done without, relieving the network of the cost of power cables.
  • Practicality: PoE works in areas where installing power circuits is not practical. A backup power source can also be centrally connected to all devices that can receive auxiliary power.
  • Simplicity: In the case of PoE, a single power source and outbound cables to all powered devices can replace several dedicated AC power circuits, transformers, and inverters for each device.
  • Device Design: Devices need only make room for the Ethernet port, and can therefore save space by omitting the separate power port.
  • Cabling hazards: Having 2 cables means that if either of them is damaged, the whole system fails (there are two points of failure), whereas in PoE, there is only one point of failure for each device.
  • Convenience: In PoE, everything can be managed centrally from the PSE, including disabling any device in the network.

Current Uses


avayaCredit: Pelzi (talk · contribs)
 
Power over Ethernet (PoE) technology provides power in an intelligent and highly-controlled manner. This protects devices from power surges and weak current. It is used in:

  • IP and PTZ cameras
  • VoIP phones (like the Avaya 1140E)
  • Network routers and switches
  • Industrial control system
  • Intercoms and PA systems
  • Intelligent lighting controllers and LED lighting fixtures
  • Antennas and radio receivers (like for mobile network signals)

poe


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Conclusion

Power over Ethernet is a powerful and useful upgrade for a technological structure that is already powerful and widespread. As it develops further and is able to carry more power, more devices may abnegate the need for separate power cables, moving towards simpler networks with fewer cables, lower costs, and easier installations.




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