Intro to Networking - Power Over Ethernet (PoE)


This article will cover the basics of power over ethernet (PoE).

Table of Contents

  1. Introduction
  2. Active vs. Passive Power-over-Ethernet
  3. IEEE Standards 802.3af/at/bt
  4. Mode A vs Mode B
  5. Related Articles


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Power-over-Ethernet (PoE) streamlines both of the processes of powering and providing data to the device through one ethernet cable. PoE is a relatively straightforward technology and PoE standards have been established to ensure greater compatibility of devices. This article goes over the different types of PoE that are commonly used and provides the information users will need to understand the information included in each device's Data Sheets.

Active vs. Passive Power-over-Ethernet

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In simple terms, active PoE refers to any type of PoE that negotiates the correct voltage between the switch and the PoE-powered device. Passive PoE does no such negotiation, and as such is always sending electric current out over the ethernet cable at a certain voltage regardless of the device it’s going to.

IEEE Standards 802.3af/at/bt

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In 2003, to combat issues with the incompatibility of various PoE devices/PoE suppliers, the standard IEEE 802.3af was established. This standard established a baseline standard PoE current to support low power devices. 802.3af compliant PoE provides a maximum of 12.95 W to the powered device.

As this low wattage ultimately became insufficient for many PoE devices, a new standard was established in 2009- IEEE 802.3at. 802.3at permits for greater power (up to 25.5W) to be delivered to devices via PoE, and also included an advanced method of negotiating power transmission between the PoE supply and device. PoE+ is another name for 802.3at compliant PoE.

To even further the ability for more power to be supplied 802.3bt was ratified in 2018. With this specification, the max power draw can be up to 60W (type 3) or 100W (type 4).

User Tip: This can be seen as PoE++ on UniFi devices. See the Related Article below for more.  

Mode A vs Mode B

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The 10/100 and gigabit Ethernet cables have four pairs of twisted cables that correspond to 8 pins. established that for 10/100 Ethernet cables, only two of the four pairs of wires are permitted to transmit data, thus the other two could be used for power. Of these pairs, 10/100 Ethernet only allows for the transmission of data over two wire pairs (four wires). Two methods of doing this were preferred based on the power source, Mode A and Mode B. Mode A leaves two twisted pair unused and transmits both data and power on the outside pairs- 1 and 2, and 3 and 6. This method is best when both PoE and data originate from the same power supplying device, hence it is also known as Endspan.

Mode B works by having two dedicated pairs of wire (Pins 4,5+ and 7,8-) dedicated to carrying power, and the other two pair for data. As PoE injectors take standard data ethernet and "inject" PoE into the cable, they more often use Type B to power devices. Thus Mode B is sometimes referred to as Midspan.

Related Articles

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UniFi - Supported PoE Protocols

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