Power Over Ethernet, or PoE, is a widely used term to refers to any technology that enables an Ethernet device to receive and send power over the same cable as data. With this article, we’ll provide an overview of PoE technology to help you determine which PoE solutions are compatible with your network application.

Power over Ethernet: A brief history

In the 1990s, prior to the IEEE stepping in, “power injection” was the label used to describe the concept of supplying electricity over an Ethernet cable. So-called “power injectors” had no intelligent protocol or safety considerations. They only worked by supplying the power over Ethernet, either AC or DC current, utilizing the spare pairs that 100Base-TX Ethernet did not use. In doing so, power injectors basically emulated how traditional PSTN (public switched telephone network) landline phones operate.

Standardizing Power over Ethernet was first proposed by the IEEE 802.3 Working Group in 1999. The group’s original standard, IEEE 802.3af, was ratified in 2003 thus ensuring interoperability across a broader range of connected devices.

IEEE 802.3af established a uniform, safer way to deploy PoE. It defined Powered Devices (PDs) as equipment that received power and Power Sourcing Equipment (PSE) as those that supplied power. PDs and PSEs would operate within a voltage range of 44-57V with a maximum power output of 15.4W per port. Power could be sent to a PD using two of the four twisted pairs that are available in typical Category 5 (Cat 5) cable by pins 4 and 5 or pins 7 and 8, or by data using four twisted pairs by pins 1 and 2 or pins 3 and 6.

Notably, the IEEE 802.3af standard included a mechanism to protect connected devices that do not support PoE. Specifically, a 25-kW resistor is placed in between the power pairs of the powered device, so that transmit power is only supplied by the power source if something close to that resistive value is detected.

Increased PoE power demands

As mentioned earlier, IEEE 802.3af specifies a maximum output of 15.4W per port. Yet due to cable power loss, the minimum guaranteed power available at the PD is only 12.95W per port. While that is plenty of power for simple devices, it falls short of the power and consumption needs of new higher wattage devices being deployed in Smart City and Industrial Internet of Things (IIoT) applications.

To remedy this situation, the IEEE 802.3 Working Group approved IEEE 802.3at, its second PoE standard, in 2009 that featured a new power class that could deliver up to 30W on the port and maximum guaranteed power of 25.5W to a PD. Although this altered the existing standard, IEEE 802.3at still supplied power over two pairs and it topped out at a safe 57V port voltage. IEEE 802.3at allowed for high power on-hungry devices, such as PTZ cameras, alarm systems and wireless access points, to be powered by a nearby PoE-enabled industrial Ethernet switch or other PSE. IEEE 802.3at is also backward compatible with the original standard.

The newest PoE standard, IEEE 802.3bt: Type 3- 45W or 60W (PSE) / 51W (PD); Type 4- 90W (PSE) and 71.3W (PD), was ratified in 2018. IEEE 802.3bt was designed to stay ahead of power demands since there simply aren’t many devices on a network that need more power than 90W in PoE delivery today.

Unlike previous standards, IEEE 802.3bt focuses on using all four twisted-pair cables within a single cable or standard Ethernet cable rather than two. The standard includes support for 2.5GBASE-T, 5GBASE-T, and 10GBASE-T, while previous PoE standards have a maximum speed of 1-Gbps.
Because of IEEE 802.3bt, the ongoing shift to Ethernet-based industrial control systems and the merging of IT and data lines in OT networks is certain to be accelerated. Having data and 90W PoE supplied by a single Ethernet connection is ushering in a new era for industrial networks.

Active PoE vs Passive PoE

Now that we have a better handle on PoE standards, let’s look at two other terms that are often misunderstood: active PoE and passive PoE.

Active PoE, also referred to as standard PoE, is any form of PoE in which Powered Devices (PD) and Power Sourcing Equipment (PSE) negotiate the proper voltage via a handshake procedure to ensure a valid PD is connected. During the handshake, the active PoE switch (PSE) applies voltage to verify the presence and value of a resistor within the PD. It then delivers the amount of power based on the response of the PD. Should the handshake not be completed successfully for any reason, the PD won’t receive the maximum power delivered from the PSE, protecting the PD from harm.

Passive PoE is the non-standard form. Passive PoE switches, adapters, injectors or other PSE have no negotiation or communication procedure and therefore do not conform to any of the IEEE standards we discussed. In general, a passive PoE PSE will supply electricity on the spare wire pairs of the twisted pair cable it is connected to at a specific fixed voltage, whether the connected end-devices supports PoE or not. However, since there is no standard, it is possible for passive PoE to be implemented with gigabit Ethernet despite all four pairs of wires being used for data transmission.

Passive PoE PSEs are mainly divided into 12V, 24V, and 48V fixed output voltages. If you are planning on using Passive PoE as a power source, you must match the exact voltage of the passive PoE PSE to the exact voltage of the data terminal equipment or powered device. If not, it may sustain electrical damage once connected. If you know the voltage and have no plans on expanding the network, Passive PoE can serve as a simple, cost-effective way of powering cameras, Wi-Fi access points, VoIP phones, and other IoT devices.

Adding PoE to industrial networks

The simplest method of adding these PoE powered devices to a network is via a PoE enabled industrial switch. In that case, all that is required is to run an Ethernet cable from the industrial switch port that delivers power to the PD. But what if there is no PoE enabled Ethernet switch on the network to supply power? Here are a few PSE options:

PoE injectors are deployed with a PD and the non PoE enabled networking switches. It typically features an RJ45 Ethernet in, an RJ45 Ethernet out and a Power in that goes to a wall outlet or power source.

PoE midspans are a type of injector but are used to power devices at distances within the traditional 100-meter limit of Ethernet cables.
PoE media converters transparently link two different media, such as fiber to copper, supplying both power and distance extension when the PD is far away from the nearest available network cable or industrial switch.

PoE splitters are commonly used for deploying remote non-PoE devices with no power loss and no nearby AC outlets. PoE splitters are used together with industrial PoE switches and PoE injectors. They supply power by separating the power from the data and feeding it to a different input that a non-PoE compliant device can utilize, as opposed to accepting both data input and power input and combining them into a single output.

Since its introduction, power over Ethernet has been a game-changer in the world of connectivity and has greatly simplified the installation and operation of a variety of devices, from surveillance cameras and sensors to access points and HVAC. PoE has evolved to support today’s high-density, widely distributed industrial networks that increasingly rely on edge computing. PoE++ systems can now deliver up to 90W per port, enough to operate the newest and most power- hungry network IoT sensors, devices and controllers, reducing the dependence on traditional power outlets.

Henry Martel, Field Application Engineer, Antaira Technologies