POE+ (IEEE 802.3at) powers network devices that require higher power

By Rajesh K

We all know about the POE Standard (IEEE 802.3af). It is used to provide electrical power along with data using the ethernet (cat x) cable that connects to the network device so that a separate power cable is not required to power the device. But, there are certain network devices that may require higher power to be provided through the ethernet ports, than what is offered by IEEE 802.3af/POE. A standard called POE+ or IEEE 802.3at is used to provide that additional power to compliant devices.

The convenience and perhaps cost/power saving of POE (Power Over Ethernet) technology has enabled many companies to power up their access points/ IP Phones, IP Surveillance Cameras and many more devices using the ethernet cable itself. So, one cable carries both data and power, simplifying installation / maintenance.

But, there is a limit to the maximum power that can be supported by POE (IEEE 802.3af) technology. POE can provide a maximum of 15.4 W of power with a minimum assurance of 12.95W. This is sufficient for powering many commonly used network devices.

But there are some devices which require higher power than what is supported by the POE technology. For example – PTZ Cameras, 802.11n Access Points, IP Video Phones, etc.

An IEEE Standard called POE+ or IEEE 802.3at can provide higher power (than POE) which may be able to power up some of these devices, if they are compliant to this standard. To be precise, POE+ standard can provide 25.5W of assured power and a maximum power of 34.2W. POE+, like POE requires two pairs of cables to be used for data transmission and two pairs to be used for power transmission in an ethernet (Cat 5+) cable.

However, there are certain proprietary standards (UPOE - for example) that might provide a higher power than POE+ by using all the four pairs of cables in an ethernet cable to transmit data as well as power.

POE+ Standard is backward compatible with POE enabled devices. In fact, there are five classes of power ratings specified by the POE/POE+ Standards out of which four (Class 0 to Class 3) can be utilized by both POE and POE+ and one (Class 4) is exclusive to POE+.

Class    –   Power

Class 0 – 0.44 to 12.94 W

Class 1 – 0.44 to 3.84 W

Class 2 – 3.84 to 6.49 W

Class 3 – 6.49 to 12.95 W

Class 4 – 12.95 to 25.5 W

These power class levels are required to be negotiated initially (during the initial connection setup between the network device and POE injector/ switch port). The network device generally specifies its class, but if the class is not specified, POE standards connect the devices using the default class 0. Obviously, Class 4 power is specified by network devices that support POE+ / IEEE 802.3at.

If both Network devices (that need to be powered up) and the POE+ Compliant Switches/ Midspan Devices support a technology called DLL – Data Link Layer Classification, they can even participate in a dynamic power allocation which enables the network devices to be powered with granular increments/ decrements of 1.1W. Of course, the network device needs to periodically communicate its power requirement to the POE enabled network switch/ midspan POE injectors.

This is mainly done to reduce the total power consumption (per port) as the power supplied to the network device can be adjusted based on its requirement at any point of time. Otherwise, full power is always supplied which results in inefficient power consumption.

Source: Wikipedia article on POE, White paper by Microsemi/PowerDsine : Understanding 802.3at

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