Power over Ethernet (PoE) Guide

Introduction to Power over Ethernet (PoE)

The need to power Wireless Networking devices in areas where mains power supply is not available or is not practical is a very limiting factor in the deployment of a wireless network. This is because many devices such as Access Points, Bridges, Routers, VoIP phones, IP CCTV cameras etc. are normally limited to having a mains AC power supply near to the DC adaptor. If the device has a built in transformer this would normally mean you would have to run a mains extension cable all the way to the device, which is not safe.

After the need to power remote devices became apparent, the IEEE developed a standard called 802.3af which was a system that provided low voltage power to devices via the use of a CAT-5 Ethernet cable, more commonly known as Power over Ethernet or PoE. This gives the added benefits of increased mobility for the location of the end devices, better safety as no AC mains power supply is involved in a significant way, the reliability of managing the activity of devices more easily in power outages and the decrease in installation costs.

With regard to outdoor wireless networks, Power over Ethernet (PoE) can be very useful when the Wireless Networking device, such as an Access Point or Bridge, needs to be close to an antenna because of the required short Co-Axial cable lengths. The power supply can be run over Ethernet cable up to 100m, without significant loss, from the injector up to the device without any mains power involvement.

How Power over Ethernet (PoE) Works

Put simply, Power over Ethernet (PoE) works by converting mains power supply into a low voltage supply that is sent down the two (usually) spare pairs of wires inside the CAT-5 cabling to power the end device. In the Power over Ethernet (PoE) system, there are two at least two units involved:

• PSE (Power Sourcing Equipment): These take the mains power supply and provide the low voltage, DC electrical supply down the Ethernet cable. PSEs can supply up to, around, 12 Watts at 48 Volts to a Power over Ethernet (PoE) compatible device requiring power. In Wireless Networking there are two types of PSEs known as Endspans and Midspans. Endspans work in the same way as Ethernet switches that also include the power supply over the Ethernet data transmission circuit. Endspans can be referred to as Power over Ethernet (PoE) switches. Midspans are power injectors that are located in between a regular Ethernet switch and the powered device whilst injecting power without affecting the data. Midspans can be referred to as Power over Ethernet (PoE) Injectors.
• PD (Powered Devices): These are the Power over Ethernet (PoE) enabled devices at the end of the Ethernet cable that require low voltage, DC power. In Wireless Networking the PD may have an integrated Power over Ethernet (PoE) Splitter that will split the power supply and data supply and source them to the relevant areas, however, if a device doesn’t, a Power over Ethernet (PoE) Splitter must be used before the PD with a DC cable running from the Power over Ethernet (PoE) Splitter to the device along with another Ethernet cable.

There are two ways of providing the power over the cable and are known as Alternative A and Alternative B.

• Alternative A: This uses a method called simplex or phantom feeding which carries the power on the same cables as the data. The PSEs superimpose the power feed onto the data feed pins through the centre tap of the internal signal coupling transformers.
• Alternative B: This uses a method whereby the power supply is carried over the two spare pairs of wires in the Ethernet cable.

Power over Ethernet (PoE) Powered Devices are designed to be compatible with either Alternative A or B up to around 48V and the use of Ethernet cable allows 1Gbps data rates.

When a Power over Ethernet (PoE) device is turned on, the PSE is designed to detect how much power the PD requires and supply it with only the necessary amount. This is done by using a class system which denotes the amount of wattage to send to the device.