On a previous post, we have already discussed about PoE, which stands for power over Ethernet. It describes various standards and ad-hoc systems allowing passing electric power along with the bandwidth data through twisted pair cables. Two of these techniques are standardized by IEEE 802.3 starting from the year 2003. These standards (802.3 af vs 802.3 at) are known as below:
- 3af Generally referred to as PoE Type 1
- 3at Generally referred to as PoE Type 2
As the main title implies, this post is dedicated for comparing major technical and performance difference between these two “Power Over Ethernet (PoE) Standards.” These standards define specifications of the devices being powered using PoE technique. They define specifications of power sourcing equipment or PSE as well. PSE devices are simply used for delivering power / electric energy on Ethernet cable, while PSE is the PD (powered device) is the device consuming the energy delivered by PSE. (i.e., IP compliant mobile phones, IP compliant cameras, wireless APs, etc.)
Also Read: Difference between POE Switch and Injector
Related: List of Best POE Switches
Now let’s move to the primary topic of this post. Before jumping deep into details, following is a simple comparison of features offered by these PoE and PoE+ standards. Have a look below:
Feature Comparison: IEEE 802.3 af vs 802.3 at:
Before jumping deep in technical details, here are general features of IEEE 802.3af and 802.3at, which also roughly describe their differences:
IEEE 802.3af (PoE Type 1) :
- A voltage range of 37 – 57 for PD
- A voltage range of 44 – 57 Volt for PSE
- Provides DC power up to 15.4 W, actual power will be around 12.95 due to Ethernet cable power dissipation
- Deliver current up to 350mA
- 20 Ohm of cable resistance for Cat 3 cable
IEEE 802.3at (PoE Type 2 / PoE Plus / PoE+) :
- A voltage range of 42.7 – 57 for PD
- A voltage range of 50 – 57 Volt for PSE
- Provides DC power up to 25.5 W
- Delivers 51 W of power by utilizing four pairs in Cat 5 cables
- Deliver current up to 600mA
- 5 Ohm of cable resistance for Cat 5 cable
Related: Best Powerline Ethernet Adapter
Following table describes the differences with further details, you may want to have a look before we discuss other technical stuff:
Comparison of PoE Parameters:
| Properties | 802.3af | 802.3at |
| Power for PD (Watt) | 12.95 | 25.50 |
| Power by PSE (Watt) | 15.40 | 30.00 |
| PSE Voltage Range (Voltage) | 44 – 57 | 50 – 57 |
| PD Voltage Range (Voltage) | 37 – 57 | 42.5 – 57.0 |
| Max. Current (mA) | 350 | 600 |
| Max. Cable Resistance (Ohm) | 20 Per Pair, Cat 3 Cable | 12.5 Per Pair, Cat 5 Cable |
| Power management Features | Triple power class levels Signature negotiation | Quadruple power class levels Signature negotiation 0.1 W step negotiation by LLDP |
| Operating Temperature Derating | None | 5 C with one mode active (i.e., dual pair) |
| Cable Support | Category 3 Cable (Cat 3) Category 5 Cable (Cat 5) | Category 5 Cable only (Cat 5) |
| Mode Support | Mode A (Endspan) Mode B (Midspan) | Mode A Mode B |
As the tables above show, IEEE 802.3af PoE standard delivers up to 15.4 W DC power on each port while delivering minimum 44 V DC and 350ma. With this standard, some power gets dissipated in the cable, and thus only 12.95W is available for the powered device. 802.3at PoE standard is an updated version of the former standard. It is also referred to as PoE+ or PoE plus. It promises to deliver 25.5 W of power to devices with Type 2 support.
Powered devices are prohibited from utilizing all four pairs for power in this standard. Note that both of these PoE and PoE+ standards are incorporated under IEEE 802.3 2012. Generally, power devices supporting IEEE 802.3at PoE+ standard are referred to as PoE Type 2 devices. On the other hand, devices compatible with only IEEE 802.3af PoE standard are known as PoE Type 1 devices.
Stages of powering up PoE Link: 802.3 af vs 802.3 at Voltage:
| Stages | Action | Volts Specified (Voltage / V) | |
| 802.3af | 802.3at | ||
| Detection | Verify if PD has compatible signature resistance | 2.7 – 10.1 | |
| Classification | Detection of resistor indicating power range | 14.5 – 20.5 | |
| Mark 1 | Signal that PSE is 802.at capable, PD presents a 0.25-4 mA load | – | 7 – 10 |
| Class 2 | Classification voltage output indicating 802.3at capability | – | 14.5 – 20.5 |
| Mark 2 | Same procedure as Mark 1 | – | 7 – 10 |
| Startup | Startup Voltage | > 42 | > 42 |
| Normal Operation | Supply Power to Device | 37 – 57 | 42.5 – 57 |
To improve the performance of 802.3 at, layer two communication signals are used by PSE. IEEE 802.3at devices are capable of utilizing Class 4 current as they possess valid class 2 and mark two currents for the power-up stages. If an 802.3af compliant device presents class 4 current, it is treated as a Class 0 devices since its non-compliant.
Advantages of PoE Protocols:
Having all these differences, here are some common advantages of these protocols or standards offer in networking:
- Reduces wiring & installation costs by using a single cable for transmission of both internet bandwidth and electricity
- By using proper hardware, you can build a wired network as wide as 100 meters
- Highly secure and reliable networking solution. As the voltage is below 60, the risk is an electric shock is a novice; moreover, the technology utilizes auto test procedures
- The devices that do not go well with the standard remain safe
- The installation gets comparatively easy
- Most of the PSEs and Powered Devices are backward compatible with older protocols.
Conclusion:
That’s it for the difference between 802.3af PoE and 802.3at PoE+ protocols. We hope this post helps you to understand the major differences between these two protocols. There are even new updates protocols available, we will discuss about them in another post. If there is any query, suggestion or doubt on your mind, feel free to contact us directly or comment down below. We are always looking forward to hearing from our readers. Make sure to subscribe for regular tech updates, See you until next post!
