This article will explain in detail how Power-over-Ethernet (PoE) works and how UniFi devices are powered with PoE.
Table of Contents
- Active vs. Passive Power-over-Ethernet
- IEEE Standards 802.3af/at
- Mode A vs Mode B
- PoE and UniFi Switches
- What PoE Method is Supported by UniFi Devices?
- Related Articles
Most UniFi products run off of Power-over-Ethernet (PoE) as it 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 provide information on what types of PoE can be used by UniFi access points. We will also define some common terms you may hear when discussing PoE.
Active vs. Passive Power-over-Ethernet
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
In 2003, to combat issues with 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.
An additional PoE standard, IEEE 802.3bt has been approved and will likely be implemented in coming years.
Mode A vs Mode B
The 10/100 and gigabit Ethernet cables have four pairs of twisted cables that correspond to 8 pins. 802.af 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 7,8+4,5-) 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.
PoE and UniFi Switches
One of the challenges with large PoE deployments is figuring out how to provide power to your UniFi access points. When you have many access points it becomes less viable to power devices using AC PoE Injectors. With non-PoE capable switches, you can add a Midspan device which acts a collection of individual PoE injectors by receiving ethernet from the switch with only data being transmitted and adding power out over ethernet through the connection. Such a piece of equipment takes up additional space on your rack, while also costing you a lot of money.
To help with such deployments, UniFi Switches come in a few different models with varying numbers of ports from 8, 16, 24 and 48. These switches are endspan devices as they act as both the switch and provide PoE to devices. UniFi switches give you greater functionality when used with APs/Secure Gateway and cost well under the amount of the midspan device alone.
See the following section for information on which UniFi AP models can be powered by each model of the UniFi Switch.
UniFi AP/UniFi Switch Compatibility Matrix
*Note: US-8 only has one PoE passthrough port, so can power one of the devices shown. US-8-60W can power four devices.
What POE method is supported by UniFi Devices?
UniFi Access Points - PoE Input Methods Supported
|UAP-AC-M||X||X (Mode A only)||X|
*UAP-AC-IW only supports PoE Pass-Through when powered by a UniFi Switch.
Other UniFi Devices - PoE Input Methods Supported
Legacy Devices - PoE Input Methods Supported
*UAP-IW only supports PoE Pass-Through when powered by a UniFi Switch.
UniFi PoE Switches - PoE Output Methods Supported
|US-8||X (1 port)|
|US-8-60W||X (4 ports)|
For more info, see other guide here showing maximum PoE output by each UniFi Switch Model.
UniFi Non-PoE Switches
US-16-XG, US-24, US-48 were designed to be used in applications where PoE output is not required, hence offer no PoE out capabilities.