Power over Ethernet

What is Power over Ethernet (PoE)?

Power over Ethernet (PoE) is technology that passes electric power over twisted-pair Ethernet cable to powered devices (PD), such as wireless access points, IP cameras, and VoIP phones in addition to the data that cable usually carries. It enables one RJ45 cable to provide both data connection and electric power to PDs instead of having a separate cable for each.

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Terminology

Here are some of the most common terms related to PoE technology.

PoE, PoE+ and Ultra PoE

As Power over Ethernet technology has developed, the amount of power that can be sent over Ethernet cable has increased. IEEE-compliant PoE switches and injectors can output anywhere from 12 watts to over 70 watts of power per port. Here are the names and power outputs that PoE offers.

PoE StandardPoE Common Name  Power OutputYearCommentComment
IEEE 802.3afPoE15.40 W 200312.95 W power available for connected device (PD)
IEEE 802.3atPoE+ 30 W200925.50 W power available for connected device (PD) 
IEEE 802.3bt Type 34PPoE, Ultra PoE, UPoE60 W201851 W power available for connected device (PD) 
IEEE 802.3bt Type 4 Ultra PoE, UPoE100 W201871 W power available for connected device (PD) 


Powered Device (PD)

Any network device that is powered by Power over Ethernet is referred to as a powered device, or PD. Wireless access points, IP security cameras and IP phones are common examples. The rise of the more powerful IEEE 802.3bt standard has paved the way for more power-hungry applications such as PoE LED lighting and High-Speed HD Outdoor PoE network cameras with climate control.


Power Sourcing Equipment (PSE)

PSE devices send power and data over the Ethernet cable to a connected PD. PSE devices are classified as either “midspan” or “endspan.”


ENDSPAN

Also called endpoint, the typical endspan is a PoE Network Switch or a Power over Ethernet Injector. Since the switch itself can power connected devices, there is no need for an additional power source between the PoE switch (PSE) and the connected PoE edge device (PD).


MIDSPAN

If a non-PoE-enabled network switch is to be used with a Power over Ethernet device, an power source that adds power to the connection is required. This device is placed between ("in the middle") the non-PoE network switch and the PoE device. A very common type of a PoE midspan is a PoE injector.

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What are the advantages?

PoE technology offers several benefits. First, the delivery of data and power over a standard Ethernet cable eliminates the need for AC/DC power supplies and power outlets to give power to PD equipment. That lowers the costs of adding or installing compatible PD equipment since you don’t need an electrician to install a power outlet if there isn’t one where you want to place your new PDs. Additionally, regular Ethernet cable is rather inexpensive and is often already installed in the location. Second, with PoE technology in general, there are fewer points of failure. Connect your managed or unmanaged PoE switch or Power over Ethernet injector to an uninterruptible power supply (UPS) and your PoE-powered devices are guaranteed a constant power delivery, which is important for mission-critical PoE edge devices. Third, Power over Ethernet installations, which utilize managed PoE switches, offer the ability to remotely restart connected PoE devices. That can be done either manually, automatically or based on a schedule. Modern PoE switches are equipped with watchdog functionality (i.e. Powered Device Manager [PDM]) that cuts power to offline devices and then resupplies it so they reboot. This function helps to greatly reduce the downtime of your connected devices.

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Is Power over Ethernet technology safe to use? Can it damage my equipment?

IEEE 802.3af/at/bt compliant PoE technology is safe. PoE injectors and switches will not damage any equipment, even if the equipment is not designed for PoE applications. Before the PSE sends any power to a connected PD, the PSE initiates a handshake procedure that establishes how much power the connected device requires. This procedure uses low voltage and is harmless to any connected device, PoE or non-PoE. If the handshake is completed, the PoE injector or switch begins sending power, which triggers the PD to start up. If that handshake is not completed for any reason, the PSE never sends any power. It is this built-in feature of all IEEE 802.3af/at/bt-compliant devices that makes PoE technology inherently safe.

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What kind of network cable can I use?

Power over Ethernet switches and PoE injectors are based on the rules defined by the IEEE 802.3 working group. As such, no special cabling is required. Keep in mind, though, that Ultra-PoE connections require 8-pin network cabling. Almost all networking cabling is 8-pin, but there is inexpensive cable out there that may only come with 4-pins. As those can only be used for Fast-Ethernet connections, they have become increasingly rare.

PoE StandardMinimum Cable CategoryPins requiredSupported Modes
IEEE 802.3afCategory 54-pins / 2-pairsMode A, Mode B
IEEE 802.3atCategory 54-pins / 2-pairsMode A, Mode B
IEEE 802.3bt Type 3 Category 58-pins / 4 pairs4-pair
IEEE 802.3bt Type 4 Category 58-pins / 4 pairs4-pair


Can I use CCA (Cu/Al) Cable?

There’s a lot of (mis)information out there about the drawbacks of this cable type. Copper-Clad Aluminum (CCA) cable uses an aluminum (aluminium) core and is coated with copper. It contrasts with regular network cable that has a 100% copper core.

Simply put, aluminum isn't as good an electrical conductor. It has a higher DC-resistance value than copper, so more power is lost and dissipated as heat (and it only gets worse the longer the cable connection is). Higher heat and the larger power loss on the cable are both major concerns in PoE applications, so we do not recommend using them with PoE injectors or switches.

For standard networking applications, CCA cables usually get the job done just fine and cost less than 100% copper cable.

Conclusion: Demand 100% copper network cable from your supplier for PoE applications.

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What are the electrical specifications of the PoE standards?

For most users, the "Minimum power for PD" value is the most significant, as that value dictates which PoE standard provides sufficient power for the required application.

PoE StandardVoltage @ PDVoltage @ PSE Minimum power for PD*Minimum output @ PSE Supported ModesMaximum cable length
IEEE 802.3af37-57 V44-57 V12.95 W15.40 W Mode A + B100 m
IEEE 802.3at42.5-57 V50-57 V25.5 W30 WMode A + B100 m
IEEE 802.3bt Type 3 42.5-57 V50-57 V51 W60 WMode A + B, 4-pair mode 100 m
IEEE 802.3bt Type 4 41.1-57 V52-57 V71 W100 WMode A + B, 4-pair mode 100 m

* Short distances via high-quality cable result in power values that are closer to the power output at the PSE.

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How do I connect PoE edge devices that are further than 100 meters away?

The connection distance can be extended by using the aptly called PoE extender or PoE repeater. A Power over Ethernet extender is installed in-line between the PSE and the PD. Each side the connection distance can be 100 meters (328 ft.). Some PoE extenders allow you to daisy-chain multiple units, pushing the total connection distance past 500 meters (1640 ft.).

How does it work?

A power over ethernet extender receives power and data from the PSE it is connected to. It uses some of that power for itself to operate, and it passes the remaining power to the next PoE device. A Gigabit PoE extender will typically use 4 to 5 watts. If you connect it to an IEEE 802.3at PSE that can deliver roughly 25 watts of power to the extender, you will have approximately 20 – 21 watts available for your connected PoE device. If you cascade PoE extenders, you will lose 4 to 5 watts for each extender, so you would have about 15 watts for the next PD, 10 watts for the PD after that, and so on. The table below shows the relationship between power availability and the number of PoE extenders used.

Number of extenders  Distance Maximum power available from PSE (input)Maximum power for PD (output)
1200 m25 W20 W
2300 m20 W15 W
3400 m15 W10 W
4500 m10 W5 W

The figures above assume that the PoE extender draws 5 watts for itself. While that is on the high side to be sure - you may only lose 4 watts per extender - it is good to be conservative about power availability in scenarios like these.

Anything else to keep in mind about Power over Ethernet extenders?

  • Some PoE extenders can be used outdoors, but not all. 
  • Some PoE extenders can be daisy-chained, but not all.
  • Not all PoE extenders support Gigabit speeds; some are only Fast-Ethernet.
  • Some PoE extenders have two outputs and allow you to connect two PDs to the PSE at a distance of 200 meters.

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