Power over Ethernet (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.

What is Ethernet?

Ethernet CablesEthernet” 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.

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.

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 PD’s. 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 gray 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.

How power sourcing equipment manages PoE

PSE’s 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 PD’s 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 25 kΩ resistor. This ensures that PSE’s 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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