Intro to Networking - Power Over Ethernet (PoE)


This article will cover the basics of Power over Ethernet (PoE). PoE streamlines the process of powering and providing data to a connected device by sending electric power over the Ethernet cable. There are different PoE types, standards modes available. This article will focus on the differences between some of the PoE types and standards that are relevant to Ubiquiti equipment.

Table of Contents

  1. Active and Passive PoE Types
  2. Mode A and Mode B
  3. Related Articles

Active and Passive PoE Types

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Some of the PoE terminology used is:

  • Power Sourcing Equipment (PSE) A network device (switch, router, etc) that provides power.
  • Powered Device (PD) A network device (IP phone, Access Point, camera, etc) that requires power.

Ubiquiti devices can use two types of PoE output modes:

  • Active PoE The voltage that the Powered Device (PD) needs is negotiated. Example output modes are IEEE 802.3af, 802.3at and 802.3bt.
  • Passive PoE A fixed output voltage is provided and there is no negotiation performed. Example output modes are 24V, 48V and 54V. 

Be aware that different devices can support different types of PoE which may not be compatible with each other. Please see the Datasheets and Quick Start Guides (QSG) in the Ubiquiti Downloads section for more information on the type(s) supported by each PoE-capable device.

The PoE types commonly found on Ubiquiti devices are:

  • 802.3af Active PoE IEEE standard that supports low power devices and provides a maximum of 15.4W at the PSE.
  • 802.3at Active PoE IEEE standard that provides a maximum of 30W at the PSE. 
  • 802.3bt Active PoE IEEE standard that provides a maximum of 60W (type 3) or 100W (type 4) at the PSE.
  • 24V Passive PoE Provides 24V power.
  • 48V Passive PoE Provides 48V power.
  • 54V Passive PoE Provides 54V power.

On Ubiquiti equipment, 802.3af PoE is often reffered to as 'PoE', whereas 802.3at and 802.3bt are referred to as 'PoE+' and 'PoE++'.

Mode A and Mode B

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The 10/100 and gigabit Ethernet cables have four pairs of twisted cables that correspond to 8 pins. The 802.3af standard established that for 10/100 Ethernet cables, only two of the four pairs of wires are permitted to transmit data, while 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 1,2 and 3,6. This method is best when both power and data originate from the same PSE, 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 adapters (injectors) take standard data Ethernet and 'injects' power 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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