Demystifying Power over Ethernet

Power over Ethernet or POE is an upcoming technology and most information available on the subject is either outdated or creates a lot of conflict. That puts off people wanting to use this new technology. Some misconceptions common to POE is discussed here.

POE leads to compatibility problems. In the early days of POE, several proprietary and home-brewed schemes were used to transfer power over networking cables. However, the standard IEEE 802.3af is now universally adopted for POE. That means compatibility between all modern equipment using POE is assured.

To use POE, one needs electrical knowhow. That was true of early implementation involving careful design. However, with IEEE802.3af, the design of POE ensures reliable operation in all configurations that is possible with the regular implementation of Ethernet. When the user has set up the network to work as normal, the equipment takes automatic care of delivery of power.

Special wiring is required for POE. This is not true. You can use the same cabling as used for regular Ethernet networks – RJ45 style of connectors with Cat6 and Cat 5e cabling. Both regular as well as POE-enabled local area networks can use the same connectors and wiring.

POE forces power into the device. This is a common misconception. Usually, manufacturers quote the maximum power ratings and the device draws only the power it needs. For example, if you plug in a 5W device into a socket capable of delivering 15W maximum, the power consumption is only 5W and there is no loss of 10W somewhere. Any electrical load will draw only the power it uses and no more.

The POE standard 802.3af is meant for network devices that draw around 13W of electrical power. However, there are devices in the market with POE that require a little more than 13W. Usually, these devices are proprietary and such high-power POE systems are not always compatible with devices complying with 802.3af POE.

The latest POE standard deals with such devices. This new standard IEEE 802.3at is called POE Plus and it handles double the electrical power – devices that can handle up to 25.5W.

Injectors and switches compatible to POE Plus can recognize any normal POE devices plugged into them and enable them as normal. Conversely, any POE Plus powered device plugged into POE injectors or switches are designed to restrict the amount of power they use.

The new standard 802.3at has scope for power budgeting. POE Plus devices can communicate with each other and simultaneously negotiate the allowance of electrical power. That means you can POE-power a more complete range of network equipment. This can include heaters and blowers, along with multichannel wireless access points.

However, both standards 802.3af and 802.3at coexist with each other and POE Plus does not replace POE. There are still many users of 802.3af for powering Power over Ethernet devices.

Most network cables utilizing Cat 6 or Cat 5e are made of eight wires arranged as four twisted pairs. Ethernet cables such as 10 and 100BASE-T use two of these pairs as data pairs for sending information. The balance two pairs, known as the spare pairs, remain unused. POE uses either the two data pairs or the two unused pairs at 48V to transfer power.