Power Over Ethernet (PoE)


The need to install network devices in places without electrical outlets nearby gave rise to the creative PoE solution (Power over Ethernet).

Let’s start with answering the question “What is PoE?”.

What is Power over Ethernet?

Power over Ethernet (PoE) is a standard that provides direct current electrical power over Ethernet twisted-pair cabling. This makes it possible for an Ethernet switch port, for example, to provide both Ethernet data and the power needed to operate a low-power Ethernet device connected to the other end of the cable, such as a wireless access point. The system is carefully designed to provide both power and Ethernet data over the same cable, without causing any interference with the data.

Power over Ethernet (PoE)

PoE supports devices with relatively low power requirements, including wireless access points, Voice over IP (VoIP) telephones, video cameras, and monitoring devices, making it possible to reduce costs by avoiding the need to provide a separate electrical circuit for the connected device. The power being provided is classed as Safety Extra Low Voltage (SELV), which is defined as a voltage that is limited to a peak of 60 volts DC, provided by a power supply that has no direct connection to primary power (AC grid), and which derives its power via a transformer or equivalent isolation device.

In other words, the power being provided over the Ethernet cable is carefully engineered so as not to present a shock hazard. The low voltages, electrical isolation from the AC grid, and limited current levels mean that the power being delivered is safe to work with and that you do not need an electrician to install or manage these power circuits.

Power Over Ethernet Standards

Power over Ethernet was first developed in 2003, in the 802.3af supplement, which became Clause 33 of the 802.3 standard. The original 802.3af version of PoE is the most widely deployed version of the standard, and provides up to 15.4 watts of DC power for transmission over the Ethernet cable. Power over Ethernet is defined to work over 10BASE-T, 100BASE-T, and 1000BASE-T links.

Goals of the PoE Standard

The goals listed in the IEEE PoE standard include:

Power

Provide both power and data through the twisted-pair cable.

Safety

Ensure that only safe (SELV) power is allowed onto the cable, isolating the cable from any other power source.

Compatibility

Work without modification over existing twisted-pair Ethernet systems.

Simplicity

Add no complexity for the end user beyond what is needed to connect a twisted-pair Ethernet link.

Devices That May Be Powered Over Ethernet

Many access points, telephones, and video cameras can be powered over the original 802.3af PoE system that provides roughly 15 watts (the powered device may draw a maximum of 12.95 watts to allow for power losses over the cable). However, access points supporting the newer 802.11 standards and containing multiple radios, or video cameras that have motors for zoom, pan, and tilt functions, can draw more than 12.95 watts. The 802.3at revision of the PoE standard provides up to 34.20 watts, resulting in a minimum of 25.50 watts at the Powered Device.

Access Point working with Power over Ethernet
Access Point working with PoE

Meanwhile, the widespread success of PoE has resulted in demands to support devices that have even higher power requirements, such as display monitors, medical monitoring devices, and building automation systems (door locks, card-key systems, HVAC monitoring). Adopting light-emitting diodes for display monitors and general lighting has reduced power requirements for those devices, increasing the number of devices that could be powered with PoE if more wattage could be provided.

As mentioned previously, several vendors are already providing vendor-specific versions of four-pair PoE that provide high power levels (up to 60 watts, or even higher) to meet these increased demands. These systems may not be interoperable. When the new IEEE standard is completed, it will provide interoperable and vendor-neutral technology for achieving higher power over four cable pairs.

Benefits of PoE

PoE capability saves costs by avoiding the need for the installation of power circuits for things like wireless access points. Providing dedicated power circuits to hundreds or even thousands of APs located in the ceilings of buildings across a campus network system would be very expensive.

Power over Ethernet has been instrumental in the adoption of 802.11 wireless LANs, by making it possible to provide power and data over a single cable. This vastly simplifies installation and reduces the cost of an access point installation. With PoE, you can provide power anywhere there is an Ethernet cable, providing far more flexibility than would hard-wired electrical outlets.

PoE also improves remote management, monitoring, and troubleshooting. PoE switches with management interfaces make it possible to provide manual power management of the powered devices. Using the switch management features, you can find out from the switch whether or not a Powered Device is drawing power, and how much power it is using. You can also control power to a device such as an access point by sending management commands to the switch, making it possible to toggle power on and off as part of troubleshooting efforts.

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References

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