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PoE Technology Simply Put

PoE Technology Simply Put

PoE (Power-over-Ethernet) technology was created for IP telephony, access points, IP cameras and other devices where it is undesirable to lay a separate power cable. PoE technology has no influence on the quality of data transmission, it uses the Ethernet-level potential, that is, network cables.

poe devices

- PoE enabled cameras

- PoE switches

- PoE injectors and splitters

It is important to understand that a power supply device (such as a PoE router) supplies power to the cable only if the connected device (such as an IP camera) supports PoE technology. How does it work?

1) First, a test is carried out: whether the connected device is powered (PD). Voltage of 2.8 to 10 V is supplied to it, the input resistance of the connected device is determined. If the settings correspond to the required ones, the power supply device proceeds to the next step.

2) The power supply device determines the power consumption of the connected device, for subsequent control of this power. Depending on the power, devices are assigned a class: from 0 to 4.

Class Watt on PoE port Watt per device
0 15,4 from 0,44 to 12,95
1 4,5 from 0,44 to 3,84
2 7 from 3,84 to 6,49
3 15,4 from 6,49 to 12,95
4 30 from 12,95 to 25,5

Once the device is classified, voltage of 48V with the rising edge of no more than 400 ms is supplied to it, and the power supply device starts to control its operation:

1) If the device consumes less than 5 mA current within 400ms, the power supply is stopped;
2) If the resistance of the connected device is greater than 1980 kOhm within 400ms, the power supply is stopped.
3) If the current consumption exceeds 400mA within 75ms, the power supply is stopped.

There are 3 PoE standards, what is the difference between them?

1. PoE - IEEE 802.3af

The first generation of PoE (IEEE 802.3 af) provides DC power up to 15.4W to each connected device.

2. PoE+ - IEEE 802.3at

The next standard, IEEE 802.3at, provides power up to 30W to each device. Thus, PoE+ is able to supply power to devices with greater capacity, such as Pan-Tilt-Zoom (PTZ) surveillance cameras and high-performance wireless access points 11n.

Differences between PoE and PoE+ standards

Method of power supply



DC voltage range on the powered device

36 to 57 V (nominal 48V)

42.5 to 57 V

Source voltage range

44 to 57 V

50 to 57 V

Maximum capacity of PoE source

15.4 W

30 W

Maximum capacity received by a PoE consumer

12.95 W

25.50 W

Maximum current

350 mA

600 mA

Maximum cable resistance

20 Ohm (for cat.3)

12.5 Ohm (for cat.5)

Power supply classes




3. IEEE 802.3bt

Currently, a new standard IEEE 802.3bt has been developed, this technology provides for powering devices up to 51 W with one cable, in this case, all four pairs of category 5 cable are used. Using previously inactive pairs of wires to supply power increases efficiency and capacity without any additional cable costs.


PoE IEEE 802.3af

PoE+ IEEE 802.3at

Cable requirements

Category 3 (UTP CAT3) or higher

Type 1: Category 3 (UTP CAT3) or higher

Type 2: Category 5 (UTP CAT5) or higher


0.35 A

Type 1: 0.35 A

Type 2: 0.6 A

Injector output voltage

44 - 57 V

Type 1: 44 - 57 V

Type 2: 50 - 57 V

Input voltage of the powered device

37 - 57 V

Type 1: 37 - 57 V

Type 2: 42.5 - 57 V

Maximum power consumption of the powered device

PoE Class 0, 3: 12.95 W

Type 1: PoE Class 0, 3: 12.95 W

PoE class 1: 3.84 W

PoE class 1: 3.84 W

PoE Class 2: 6.49 W

PoE Class 2: 6.49 W

PoE Class 4: not used

Type 2: PoE Class 4:

25.5 W

Supported powered devices

IP cameras, IP phones, access points

All PoE devices, PTZ cameras for outdoor installation,

WiMAX access points, LED boards, some computers


Power requirements for PoE devices:




Resistance, kilo ohm



Start-up time (> 10 mA), ms



Power consumption, W.



Input voltage range, V



Voltage on, V



Voltage off, V

30 V


Input current (@ 36VDC), mA



Input current, Peak, mA



POE standard IEEE 802.3af pinout:


Passive PoE

What if you need to connect devices that do not support PoE to your infrastructure? In such cases, Passive PoE technology is used. Its peculiarity is that the power source does not inquire the connected device and does not coordinate its power capacity. The power is merely supplied over the vacant conductors of the twisted pair cable using a PoE splitter.

The PoE splitter splits the signal incoming over a twisted pair cable into data and power (12V-24V). This makes it possible to power and integrate the device without PoE support into the existing infrastructure.

POE Splitter Scheme

With this method of connection, it is necessary to carefully select the capacity of the power source and the powered device.


PoE injector


There are two kinds of devices - PoE splitters and PoE injectors. We have made it clear about a splitter, but how does a PoE injector work?Let's take an example. Imagine that your infrastructure uses a switch that does not support PoE, and the network cable transmits only data.
How is it possible to connect and supply power over a twisted pair cable to devices that support this technology in such a system? It is in those cases that a PoE injector is used to supply voltage in the network cable.
A PoE injector is connected both to RJ45 and to the power supply source. As a result, the PoE injector receives data at the input and at the output provides both data and voltage that can be used to connect devices that support this technology.

PoE adapter


It is an AC-DC converter with a built-in splitter and a stabilizer at the output. A PoE adapter does not use phantom power but uses free pairs (which means the inability to use Gigabit ports, the inability to use two-pair cables, the inability to share the cable). The only difference from the splitter is that the adapter supports the lines with a length of up to 100m by increasing the voltage at rated capacity and activates the power supply scheme via PoE. There is no guarantee that the equipment that is powered by PoE-B standard will work. That is, the equipment that uses the same contacts 1, 2, 3, 6 for power and data transfer

poe adapter

Cable requirements:

• A four-pair twisted pair cable is required with a category not lower than cat.5e;
• The twisted pair should be copper, not copper-plated;
• The thickness of the conductors is not less than 0.51 mm (24 AWG);
• The conductors resistance should not be higher than 9.38 Ohm/100 m (if more, there will be a large loss of power).
Standards 802.3af and 802.3at characterise the length of the twisted pair cable for PoE as equal to 100m. However, in practice the recommended maximum cable length should not exceed 75m. When using Passive PoE, the cable length should not exceed 60m.
Thus, PoE technology has extensive communication capabilities that allow you to create networks with devices of different types and purposes. Installation costs for PoE systems are generally much lower than those for traditional power distribution systems.

Add comment
Polarity for power pins looks reversed
I found this article very clear and explanatory.


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