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5 December 2020

An Introduction to Power Over Ethernet (PoE)

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What is Power Over Ethernet (PoE)?

Power Over Ethernet (PoE) is exactly what it says on the tin, it’s the combined delivery of Ethernet and electrical power over a single cable. This simplifies devices by removing the need for an electrical mains plug, reduces wiring complexity and means an electrician isn’t required to install any mains based cabling. It has been designed for use with 10/100/1000 Mbps systems and one of the main advantages is it can use legacy cabling (CAT5 or CAT3 for low power applications).

As the DC power is injected and received on the isolated side of the Ethernet transformers PoE equipment can coexist with non-PoE equipment on the same network, the non-PoE equipment simply won’t see the power. This is achieved in one of 3 ways; mode (A) uses the same wiring pairs as the data, mode (B) uses the unused wiring pairs and the 4-pair configuration uses all 4 wires pairs.

PoE equipment falls into two categories, Powered Devices (PD) or Power Sourcing Equipment (PSE). PD equipment is the device being powered, IP phone, security camera etc. As the name implies PSE provides the power and is categorised as either end-span or mid-span. End-span equipment will typically be a PoE network switch and supply the PD equipment directly. Mid-span equipment is used when non-PoE equipment is still required but the PD still needs powering. In this scenario a PoE injector can be used, it takes the data cable from the non-PoE device on one port, adds the power and a second port goes to the PD. PoE injectors can also be daisy-chained together to increase the distance beyond 100m.

PoE Overview
PoE Overview

Power Over Ethernet Benefits and Applications

Power over Ethernet offers many benefits to end-users, IT teams, system integrators and product designers. Removing the need for mains power not only simplifies the cabling but has the added safety factor of operating at a SELV ( Safety Extra Low Voltage) level. This can be particularly important for retrofit industrial applications where installing mains wiring can present many hazards. PoE is also plug and play meaning the network can be left online when adding or removing devices. Another advantage is simplified power redundancy and backup power. A single UPS can now backup multiple devices through a PoE switch for example.

Typical applications for PoE include security controls, kiosks, security cameras, signage, telephones, wireless access points, lighting systems and increasingly PoE is finding many industrial applications.

What Can PoE do?

PoE uses standard Ethernet cabling (CAT5 etc) and normal distance rules apply for the data, typically 100m. For the power delivery, the standards specify the minimum power at the powered device with 100m of cabling. Higher power levels can be achieved by deducing the cable length.

There are techniques to extend the maximum 100m distance of PoE. Injectors and extenders are Midspan pieces of equipment that can be daisy-chained thus increasing the overall reach. An injector (see diagram 1) has one data Ethernet port connected to non-PoE equipment and a second PoE port that goes to the powered device, it does require an external power supply.

 A PoE extender (see diagram 1) is similar to an injector but doesn’t require an external power supply. Obviously, each extender requires power which it gets from the upstream power sourcing equipment. Therefore depending on the power requirements of the PD, there are limits on the ultimate length of the daisy chain.

One other piece of PoE terminology is a PoE splitter. A splitter supplies power to non-PoE equipment by separating the power from the data, then providing separate outputs for data and power. Like a PoE injector but in reverse.

PoE Standards and History

The original standard was the 2003 IEEE 802.3af and allowed 12.95W delivered to the powered device, also known as PoE or type 1. In 2009 it was updated to IEEE 802.3at (PoE+ or type 2) which allowed for 25.5W at the powered device and the minimum cabling requirements were changed to CAT5. In 2018 IEEE 802.3bt was introduced, this covered both type 3 and type 4 applications (PD power of 51W and 71W respectively). All 4 conductor pairs are used in 802.3bt.

How can Spark Product Innovation Help?

There are a great deal of companies who make off the shelf ready to go PoE equipment. This isn’t Spark Product Innovations market segment as we are predominantly a design consultancy. Where we fit in is if you need bespoke PoE hardware designing then with our high speed digital and power electronics skills we are ideally suited to help. For example, if you are developing an industrial product and an off the shelf solutions simply won’t fit into your packaging requirements then Spark Product Innovation can help.

POE Overview