All About PoE (Power over Ethernet)

1. Overview

Connected devices are more proliferated than ever. Increasingly, these devices are not confined to a desktop environment. They can be found anywhere, from light poles to cabinets at traffic intersections. As a result, usage of Power over Ethernet (PoE) has increased dramatically over the past decade or so. This paper will cover the topic of PoE in depth, such as benefits, technical details, common usage scenarios, etc.

2. The Basics and Benefits of PoE

PoE is a technology that allows power to be provided over the same cabling that’s used for Ethernet data, without the need for a separate power supply. This allows for the elimination of power wiring to hard-to-reach places. Because it uses the same cabling that’s used for data, typically cat5 up to cat6a, there is no need for modification of existing infrastructure. Below shows how a PoE connection would work compared to a non-PoE connection that uses separate power and data wiring.

In a PoE application, a PSE (power sourcing equipment) provides power to a PD (powered device). A common example would be an IP camera (PD) connected to a PoE switch (PSE). Below is an illustration of a PSE to PD connection.

The main benefits of PoE are:

Save time and money – Ethernet cables do not require an electrician to install without much limitation on where they can be installed. Because the power is coming from the PSE, there is no concern about where the nearest power source is.
Flexibility for installation – Because the device (PD) is powered by the PSE, it does not have to be installed near a power source. It can be up to 100 meters away (the distance limit for Ethernet), providing much more flexibility for installations.
Scalable – Since the power is provided through network cabling, PoE devices can be added quickly, as needed.

3. PoE Markets and Applications

Various market trend data has shown that the PoE usage continues to grow year over year. Here is one such example:

The growth can be attributed multiple factors. The most obvious is the fact that there is a rising number of devices that are PoE-compatible. Types of devices now go beyond the traditional IP cameras, to access points, smart lighting, sensors, and more.

In addition to the growing types of devices, the amount of power that can be provided over PoE has gone from 4W up to 90W over the past two decades. This has opened many possibilities on what can be powered through PoE. Moreover, as higher data speeds become more common in devices, there will be more networked devices out in the field with PoE capability.

The rise in PoE usage has resulted in a convergence of IP devices that are centrally managed, rather than being controlled separately. This can include: Lighting, HVAC, access control, surveillance, sensors, digital signage, Wi-Fi, and VoIP. Here is an example that shows different PoE devices that can be used within a building.

4. Technical Details

Active PoE vs. Passive PoE

As mentioned previously, PoE provides power over the same cabling used for data. There are two types of PoE protocols – active and passive. Below is a comparison table of the differences between active and passive PoE.

Active PoE provides protection for non-PoE devices that get plugged into a PoE port. If a port is passive PoE, it will provide power to the device regardless of it being PoE-enabled or not.

PoE Process

So then, how does active PoE provide power to a device that is active PoE enabled?

When a device (PD) is plugged in, the PSE will detect if the PD has the correct resistance value to correctly categorize it as a PoE device.
The PD will be classified to determine its rated power consumption, based on its class signature.
During startup, the PSE will limit the inrush current with the appropriate value based on the PD class.
The PSE will periodically perform a keep alive function with the PD to maintain operation.
When a PD is disconnected from the PSE, the PSE must remove voltage within 15-20ms of the cable being disconnected.

PoE Standards

In the Active vs. Passive comparison table above, Active PoE is referring to IEEE 802.3af/at/bt. These are standards defined by the IEEE organization. These standards are defined by types with the amount of power that’s provided allocated per class. There are 4 types, made up by 8 classes.

802.3af = Type 1, Class 1-3

802.3at = Type 2, Class 4

802.3bt = Types 3&4, Class 5-8

The chart below shows the power that the PSE can provide along with the minimum power that the PD receives, per class.

In a cat5 or cat6 cable, there are 4 pairs of wires.

802.3af and 802.3at operate over 2 pairs of the wires. There are 2 possible modes (A and B) for the pinouts. In both modes, 4 wires are not utilized for the power portion. The diagram below shows the pinouts for both modes.

802.3bt operates over 4 pairs of wires. In this case, all 4 pairs are used to deliver power. There are 4 main improvements made in 802.3bt, compared to 802.3af/at:

Support for Type 3 (60W) and Type 4 (90W)
Support for dual-signature PDs. This means that a device can have two separate detection and classification circuits. This would come in useful for a device such as an IP camera with a heater.
Lower standby power – minimum standby power is 20mW, which is 10 times lower than the value for 802.3at, which is at 200mW. This can save a lot of energy, especially in PoE LED lighting.
Autoclass – a minimum of 71W is delivered to the PD, but the PSE can automatically detect the actual max power drawn by the PD and adjust accordingly.

Additional Notes

Below are important factors to know:

PD power – Make sure to refer to the power that is delivered to the PD, rather than the power that is output from the PSE. For example, if a camera needs 28W of power, it will not be sufficient to power the camera with 802.3at, which outputs 30W from the PSE, but only 25.5W is delivered to the PD.
Voltage – for PSE devices that require a DC input to power up, be aware of the input voltage range of the different standards. For example, Type 4 requires a voltage range of 52 – 57VDC. 48VDC power supplies are very common, which means that the power supply needs to be able to be cranked up to 52V at a minimum to be able to power up an 802.3bt PD. Below is a table of the voltage ranges needed for each Type:

	Type 1	Type 2	Type 3	Type 4
PSE Voltage	44-57VDC	50-57VDC	50-57VDC	52-57VDC

Proprietary protocols – before 802.3bt was ratified by IEEE, there were devices that required more power than what 802.3at (30W) could provide. There were several proprietary protocols that were developed at that time to handle this issue. This was implemented mostly in the form of an injector. The basic mechanism of the operation was that the protocol would be negotiated to be 802.3at, but instead of delivering power over 2 pairs, it would deliver power over 4 pairs, allowing the power to be higher than 30W out of the PSE.
Backward compatibility – all IEEE PoE standards/protocols are backward compatible. A device classified as an 802.3af PD will still work without any issue on an 802.3bt PSE.

5. Managed PoE Features

The most common PSE type is a PoE switch. Utilizing a managed switch allows for configuration of the network as well as the switch, compared to an unmanaged switch, which is pretty much a plug and play device. A managed PoE switch from EtherWAN Systems provides additional configuration options specific to PoE.

PoE scheduling – individual PoE ports can be set up to provide power on specific days of the week at specific time intervals, allowing for flexibility.
Proprietary mode – this allows a high-power PD to be powered by the PoE port at greater than 30W, while being negotiated to be 802.3at.
PoE watchdog and recovery – the individual ports can be configured to ping the PoE device connected to the port. If the device does not respond to the ping within a specified amount of time, the port will automatically reboot. This essentially power cycles the device, which can recover the device many times, thereby reducing any truck rolls or maintenance.

6. Wrap-Up

As PoE standards continue to expand, there will be new devices and applications to take advantage of the new technologies. PoE will steadily become more widespread than ever. EtherWAN’s range of hardened-grade PoE switches provide power to handle the latest 802.3bt standard, uninterrupted operation even in extreme temperatures (-40F to 167F ambient), and quality of life features such as proprietary mode and PoE watchdog. EtherWAN’s unmatched quality is backed by a limited lifetime warranty for hardened PoE switches.

Go here for more information: Hardened PoE Switches | EtherWAN USA