Power over Ethernet (PoE) is a widely used term that refers to any technology enabling an Ethernet device to receive and send power over the same cable as data. Below I’ve highlighted key information about PoE technology to help you determine which PoE solutions are most compatible with your network application.
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 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 PoE 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 power demands
As mentioned earlier, IEEE 802.3af specifies a maximum output of 15.4W per port. However, in instances of 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 Industrial Internet of Things (IIoT) applications.
To remedy this situation, the IEEE 802.3 Working Group approved IEEE 802.3at in 2009. This PoE standard 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 topped out at a safe 57V port voltage. IEEE 802.3at allowed for high power on-hungry devices, such as pan/tilt/zoom cameras, alarm systems and wireless access points, to be powered by a nearby PoE-enabled industrial Ethernet switch or other power sourcing equipment (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 (operations technology) 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 vs. passive
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 PDs and 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 discussed here. 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 networks
The simplest method of adding 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.
Henry Martel is field application engineer at Antaira Technologies.
