Power over Ethernet (PoE) is a networking feature defined by the IEEE 802.3af and 802.3at standards. PoE lets Ethernet cables supply power to network devices over the existing data connection.
PoE-capable devices can be power sourcing equipment (PSE), powered devices (PDs), or sometimes both. The device that transmits power is a PSE, while the device that is powered is a PD. Most PSEs are either network switches or PoE injectors intended for use with non-PoE switches. Common examples of PDs include VoIP phones, wireless access points, and IP cameras.
What are the benefits of PoE?
- Because PoE allows you to use one cable for both power and data transmission, PoE saves you money on purchasing and running cable for networking equipment and VoIP phones.
- PoE makes installing or expanding a network much simpler and cheaper in buildings where it is too expensive or inconvenient to install new power lines.
- Using PoE lets you mount devices in places where it would be impractical to install power, such as drop ceilings.
- Using PoE can reduce the number of cables and electrical outlets needed in a crowded equipment room or wiring closet.
What is PoE+?
The latest update to PoE is the IEEE 802.3at standard, known as PoE+. The major difference between 802.3af (PoE) and 802.3at (PoE+) is that PoE+ PSEs can provide almost twice as much power over a single Ethernet cable.
PoE+ PSEs can supply power to both PoE and PoE+ PDs, but PoE PSEs can only supply power to PoE PDs. PoE+ PDs require more power than PoE PSEs can provide.
How much power can PoE devices supply?
PoE+ devices can supply a maximum of 30 watts per port, while PoE devices can supply a maximum of 15.4 watts per port. However, some power is always lost over the length of the cable, and more power is lost over longer cable runs. The minimum guaranteed power available at the PD is 12.95 watts per port for PoE and 25.5 watts per port for PoE+.
PSEs also have a maximum power budget, which is the total amount of power they can supply to PDs at one time, measured in watts. Most PSEs do not have a high enough power budget to supply the maximum possible power to all PoE-capable ports, because most users do not require that much power. When you are shopping for a PoE-capable PSE, make sure that you calculate your required power budget carefully for all of the PDs you plan to connect.
What do the PoE Classes mean?
PoE and PoE+ powered devices are assigned a class from 0-4 based on how much power they require. When a PD is connected to a PSE, it provides its class to the PSE so that the PSE can supply the correct amount of power to it. Class 1, Class 2, and Class 3 devices require very low power, low power, and medium power, respectively. Class 4 (PoE+) devices require a high amount of power and are only compatible with PoE+ PSEs. For more information about PoE classifications, see https://en.wikipedia.org/wiki/Power_over_Ethernet#Standard_implementation.
Class 0 devices have not received a proper class designation from the manufacturer, so most PSEs must allocate the same amount of power for Class 0 devices as they do for Class 3 devices, even if the Class 0 device actually uses very little power. However, newer models of NETGEAR PoE+ switches allow you to define the upper power limit for a PD regardless of its class. These models also allocate power to other PDs based on the power limits that you define, not the theoretical power allocation according to device class, reducing power budget waste.
Can I mix PoE and non-PoE devices in my network?
PoE devices can be mixed in a network with non-PoE devices, but non-PoE devices cannot provide power for PDs or be powered by PSEs. The non-PoE devices must have a separate source of power.
Where can I learn more about the PoE standard?
For a more detailed look at PoE standards and implementations, see https://en.wikipedia.org/wiki/Power_over_Ethernet.
To download a copy of the IEEE Ethernet standards, which include PoE and PoE+, visit http://standards.ieee.org/about/get/802/802.3.html.