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 PoE 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 Standard | PoE Common Name | Power Output | Year | Comment |
|---|---|---|---|---|
| IEEE 802.3af | PoE | 15.40 W | 2003 | 12.95 W power available for connected device (PD) |
| IEEE 802.3at | PoE+ | 30 W | 2009 | 25.50 W power available for connected device (PD) |
| IEEE 802.3bt Type 3 | 4PPoE, PoE++ | 60 W | 2018 | 51 W power available for connected device (PD) |
| IEEE 802.3bt Type 4 | 4PPoE, PoE++ | 90 W | 2018 | 71 W power available for connected device (PD) |
Powered Device (PD)
Any network device that is powered by PoE is referred to as a powered device, or PD. Wireless access points, IP security cameras and VoIP 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. 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 PoE 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 of PoE?
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 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 power if there isn’t any 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 PoE switch or PoE 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, PoE 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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What is a PoE budget? What is maximum power per port? What is average power per port?
The final helpful concept is average power per port, which is the total power budget divided by the number of ports. Average power per port gives you a rough idea about whether your devices can function on the power available at each port, especially if you plan on connecting many of the same PoE standard.
Example
Consider a 24-port Gigabit PoE+ Switch with a PoE budget of 240 W. The maximum power per port is 30 W. The average power per port is 10 W. While that is enough for many PoE-enabled devices — e.g., IP phones (5 W) and wireless access points (up to 10 W) — it may not be enough if you want to connect many that require up to 15 W. In that case, you'd be more suited to a switch with a higher PoE budget, like this one with 370 W.
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Is PoE 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?
PoE switches and 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 Standard | Minimum Cable Category | Pins required |
|---|---|---|
| IEEE 802.3af | Category 5 | 4-pins / 2-pairs |
| IEEE 802.3at | Category 5 | 4-pins / 2-pairs |
| IEEE 802.3bt Type 3 | Category 5 | 8-pins / 4 pairs |
| IEEE 802.3bt Type 4 | Category 5 | 8-pins / 4 pairs |
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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PoE Pinout - Mode A or Mode B?
Power over Ethernet (PoE) Mode A and Mode B are two different methods of delivering power through Ethernet cables. Both modes are defined by the IEEE 802.3af/at standard.
Mode A uses the same pairs for data and power, potentially impacting data transmission if not properly managed. Mode B uses separate pairs, minimizing interference with data transmission. It is typically implemented in PoE switches (endspan), while Mode B is commonly used with midspan injectors.
PoE Mode A (Endspan):
Power Delivery: In Mode A, power is delivered over the data pairs in the Ethernet cable (pairs 1-2 and 3-6). This means the same pairs that carry data are also used to deliver power.
Wiring: Mode A uses the following pins in the Ethernet cable:
- Pair 1-2 (Pins 1 and 2)
- Pair 3-6 (Pins 3 and 6)
Typical Use: This mode is commonly used in devices where the power sourcing equipment (PSE), such as a PoE switch, directly injects power into the Ethernet cable.
Voltage: The power is delivered as a differential voltage, meaning the voltage difference between the two wires of each pair.
PoE Mode B (Midspan)
Power Delivery: In Mode B, power is delivered over the spare pairs in the Ethernet cable (pairs 4-5 and 7-8). These pairs are typically not used for data transmission in 10/100 Mbps networks but can be used in gigabit networks.
Wiring: Mode B uses the spare pairs of the Ethernet cable: Pair 4-5 (Pins 4 and 5) Pair 7-8 (Pins 7 and 8)
Typical Use: This mode is often used in situations where a midspan injector is placed between a non-PoE switch and the powered device (PD).
Voltage: The power is supplied as a common-mode voltage, meaning the same voltage is applied to both wires of each pair.
Do I have to check the PoE pinout to make sure my PoE device is compatible with the PoE injector?
No. If your PoE device is IEEE802.3af/at/bt compliant, it will work with any PoE mode. Here is why:
The IEEE dictates that any PoE (powered) device, such as a PoE phone, PoE network camera or PoE WiFi access point, must support both PoE mode A and PoE mode B. The IEEE standard also mandates that any PSE, whether it is a PoE switch, midspan or injector, must support at least one of the two modes.
In short: The device that sends out power must support either PoE mode A or PoE B. The device that receives power must support both mode A and mode B. That ensures compatibility.
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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 Standard | Voltage @ PD | Voltage @ PSE | Minimum power for PD* | Minimum output @ PSE | Supported Modes | Maximum cable length |
|---|---|---|---|---|---|---|
| IEEE 802.3af | 37-57 V | 44-57 V | 12.95 W | 15.40 W | Mode A + B | 100 m |
| IEEE 802.3at | 42.5-57 V | 50-57 V | 25.5 W | 30 W | Mode A + B | 100 m |
| IEEE 802.3bt Type 3 | 42.5-57 V | 50-57 V | 51 W | 60 W | Mode A + B, 4-pair mode | 100 m |
| IEEE 802.3bt Type 4 | 41.1-57 V | 52-57 V | 71 W | 100 W | Mode 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 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 PoE 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 PoE 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) |
|---|---|---|
| 1 | 200 m | 25 W |
| 2 | 300 m | 20 W |
| 3 | 400 m | 15 W |
| 4 | 500 m | 10 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 PoE 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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