Power over Ethernet, PoE, IEEE 802.3af / 802.3at

- a summary or tutorial about Power over Ethernet, PoE, also called Power over LAN, PoL and its specification under IEEE 802.3af and IEEE 802.3at.

This Ethernet, IEEE 802.3 tutorial is split into several pages each of which addresses different aspects of Ethernet, IEEE 802.3 operation and technology:

Powering network devices can sometime present problems, especially if they are located remotely. One convenient solution is to supply the power over an Ethernet LAN cable. Power over Ethernet, is defined under two IEE standards, namely IEEE 802.3af and later IEE 802.3at which defined a number of enhancements.

This Power over Ethernet, PoE is now being used for a wide variety of applications including powering IP telephones, wireless LAN access points, webcams, Ethernet hubs and switches and many more devices. It is convenient to use and as a result, Power over Ethernet is widely used and many products are available.

PoE Development

With Ethernet now an established standard, one of the limitations of Ethernet related equipment was that it required power and this was not always easily available. As a result some manufacturers started to offer solutions whereby power could be supplied over the Ethernet cables themselves. To prevent a variety of incompatible Power over Ethernet, PoE, solutions appearing on the market, and the resulting confusion, the IEEE began their standardisation process in 1999.

A variety of companies were involved in the development of the IEEE standard. The result was the IEEE802.3af standard that was approved for release on 12 June 2003. Although some products were released before this date and may not fully conform to the standard, most products available today will conform to it, especially if they quote compliance with 802.3af.

A further standard, designated IEEE 802.3at was released in 2009 and this provided for several enhancements to the original IEEE 802.3af specification.

PoE overview

The standard allows for a supply of 48 volts with a maximum current of 400 milliamps to be provided over two of the available four pairs used on Cat 3 or Cat 5 cable. While this sounds very useful with a maximum available power of 19.2 watts, the losses in the system normally reduce this to just under 13 watts.

The standard Cat 5 cable has sets of twisted pair cable, and the IEEE standard allows for either to be used for 10Base-T and 100Base-T systems. The standard allows for two options for Power over Ethernet: one uses the spare twisted pairs, while the second option uses the wires carrying the data. Only one option may be used and not both.

When using the spare twisted pairs for the supply, the pair on pins 4 and 5 connected together and normally used for the positive supply. The pair connected to pins 7 and 8 of the connector are connected for the negative supply. While this is the standard polarity, the specification actually allows for either polarity to be used.

When the pairs used for carrying the data are employed it is it is possible to apply DC power to the centre tap of the isolation transformer that are used to terminate the data wires without disrupting the data transfer. In this mode of operation the pair on pins 3 and 6 and the pair on pins 1 and 2 can be of either polarity.

As the supply reaching the powered device can be of either polarity a full wave rectifier (bridge rectifier) is used to ensure that the device consuming the power receives the correct polarity power.

Within the 802.3af standard two types of device are described:

• Power Sourcing Equipment, PSE     This is the equipment that supplies power to the Ethernet cable.
• Powered Devices, PD     This is equipment that interfaces to the Ethernet cable and is powered by supply on the cable. These equipments may range from switches and hubs to other items including webcams, etc.

Power Sourcing Equipment, PSE

This needs to provide a number of functions apart from simply supplying the power over the Ethernet system. The PSE obviously needs to ensure that no damage is possible to any equipment that may be present on the Ethernet system. The PSE first looks for devices that comply with the IEEE 802.3af specification. This is achieved by applying a small current-limited voltage to the cable. The PSE then checks for the presence of a 25k ohm resistor in the remote device. If this load or resistor is detected, then the 48V is applied to the cable, but it is still current-limited to prevent damage to cables and equipment under fault conditions.

The PSE will continue to supply power until the Powered Device (PD) is removed, or the PD stops drawing its minimum current.

Powered Device, PD

The powered device must be able to operate within the confines of the Power over Ethernet specification. It receives a nominal 48 volts from the cable, and must be able to accept power from either option, i.e. either over the spare or data cables. Additionally the 48 volts supplied is too high for operating the electronics to be powered, and accordingly an isolated DC-DC converter is used to transform the 48V to a lower voltage. This also enables 1500V isolation to be provided for safety reasons.

PoE Summary

Power over Ethernet, PoE, defined as IEEE 802.3af or the enhancements under IEEE 802.3at provide a particularly valuable means of remotely supplying and controlling equipment that may be connected to an Ethernet network or system. PoE enables units to be powered in situations where it may not be convenient to run in a new power supply for the unit. While there are limitations to the power that can be supplied, the intention is that only small units are likely to need powering in this way. Larger units can be powered using more conventional means.

Further pages from this tutorial
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