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All about PoE (Power over Ethernet)

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.

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.

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.

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?

  1. 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.
  2. The PD will be classified to determine its rated power consumption, based on its class signature.
  3. During startup, the PSE will limit the inrush current with the appropriate value based on the PD class.
  4. The PSE will periodically perform a keep alive function with the PD to maintain operation.
  5. 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.

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