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PoE - introduction
PoE (Power over Ethernet) is the name of a range of methods that allow network equipment to be powered via a twisted pair cable while transmitting data. It is possible to power devices such as cameras, telephones, switches, access points, etc. In 2003, the IEEE established the 802.3af PoE power standard, which is constantly being developed, leading to new versions that provide higher power transmission capabilities.
Advantages of PoE technology:
- transmission of power and data via a single cable reduces wiring and installation costs
- at least 100 m range (with appropriate cabling)
- high security and and reliability - in typical conditions the risk of electric shock is not high (the voltage is below 60 V) and the technology involves auto-testing procedures
- protection of devices not compliant with the standard
- ease of installation
- backward compatibility with older versions
The table below shows a comparison of the most important parameters for the different versions of the standard.
| Feature/standard | 802.3af (802.3at Type 1), PoE | 802.3at Type 2, POE+ | 802.3bt Type 3, PoE++, 4PPoE | 802.3bt Type 4, PoE++, 4PPoE |
| Power supply output power (W) | 15.4 | 30 | 60 | 90 |
| Minimum power available to the terminal equipment (W) | 12.95 | 25.5 | 51 | 71.3 |
| Power supply output voltage (V) | 44-57 | 50-57 | 50-57 | 52-57 |
| Voltage available to the end device (V) | 37-57 | 42.5-57 | 42.5-57 | 41.1-57 |
| Maximum current (mA) | 350 | 600 | 600 per pair | 960 per pair |
| Compatible network standards | 10BASE-T, 100BASE-TX, 1000BASE-T, 2.5GBASE-T, 5GBASE-T, 10GBASE-T | |||
| Range (m) | 100 | |||
| Modes supported | A, B | A, B | A, B, 4-pair | A, B, 4-pair |
| Cabling | Twisted-pair min. cat. 3 | Twisted-pair min. cat. 5 | Twisted-pair min. cat. 5 | Twisted-pair min. cat. 5 |
To summarise, there are the types:
- 1 – 802.3af – rated at 15.4 W, supporting the following modes: A, B
- 2 – 802.3at – rated at 30 W, supporting the following modes: A, B
- 3 – 802.3bt – rated at 60 W, supporting the following modes: A, B and 4-pair
- 4 – 802.3bt – rated at 90 W, supporting the following modes: A, B and 4-pair
Topology
Power transfer modes:
- A – power is transmitted together with data over pairs 1/2 and 3/6
- B – power is transmitted over free pairs 4/5 and 7/8
- 4-pair – power is transmitted over all pairs
For PoE 802.3af and 802.3at, a powered device can provide both or only one of the power modes while remaining compliant with the standard. The powered device must therefore support both modes.
Power connection diagram for a switch type power device with PoE and an 802.3af (802.3at type 1) powered device. Purple – mode A, yellow – mode B
Power connection diagram for a switch type power supply device with PoE and a device powered by 802.3at, 802.3bt. Purple-mode A, yellow-mode B
Testing procedures and classes of powered devices
The PoE standard has been optimised for safe use. In addition to a safe voltage range, devices must communicate according to established procedures. The power supply will not supply an unsafe voltage to an incompatible device. Before it is applied, the power supply device tests what has been connected using a low current (up to 5 mA) to detect the specific resistance – around 25 kOhms. The next phase is device classification, which occurs when 15.5-20.5 V is applied. Information on power requirements is sent to the power management processor of the entire system to optimise resources. It is only after detection and classification that the correct 44-57 V supply voltage is applied to the line and the electronics in the end device begin to work.
The power device constantly monitors the connection to ensure that it is still connected and requires power to be supplied. The idea is not to apply high voltage to a non-standard device that could be accidentally connected.
The power device constantly monitors the connection to ensure that it is still connected and requires power to be supplied. The idea is not to apply high voltage to a non-standard device that could be accidentally connected.
Device classification provides useful information about a device's power requirements. Classification is done by measuring the current flowing when testing the connection. Powered devices can be in one of the 8 classes described below.
| Class | End device power (W) |
|---|---|
| 0 | 0.44 – 12.95 |
| 1 | 0.44 – 3.84 |
| 2 | 3.84 – 6.49 |
| 3 | 6.49 – 12.95 |
| 4 | 12.95 – 25.5 |
| 5 | 25.5 – 40 |
| 6 | 40 – 60 |
| 7 | 60 – 75 |
| 8 | 75 – 100 |
Devices can also communicate at the second layer of the OSI model (data links), using the LLDP-MED protocol (an extended version of the automatic device detection protocol). Thanks to such communication, it is possible to determine the actual power demand with an accuracy of 1.11 W. The power device sends information about the power demand every specified period, which improves energy efficiency.
PoE power supply for 250 m – "Extend" function.
Some PoE switches have an "Extend mode" function (activated in the menu or via a switch on the chassis), which allows PoE devices connected with up to 250 metres of twisted-pair cable to be powered. When this function is activated, the switch on the PoE ports disables auto-negotiation of the connection speed and rigidly sets the speed to 10 Mbps (the uplink ports continue to operate at 10/100 Mbps).
How to extend or branch the PoE power supply?
There are devices that are miniature PoE switches that act as extenders (repeaters) of the LAN and PoE power supply. They regenerate the network signal and transfer PoE power to selected outputs. They make it possible to 'amplify' and extend the network over distances longer than 100 metres. Furthermore, they are ideal as "active splitters" where you need to run several PoE receivers on a single cable (e.g. several IP cameras) or when you need to create an additional network branch.
When using these devices, it should be borne in mind that the total power consumed by the cameras (PoE receivers) connected to the extender cannot exceed the power budget offered by the switch powering the entire line.
When using these devices, it should be borne in mind that the total power consumed by the cameras (PoE receivers) connected to the extender cannot exceed the power budget offered by the switch powering the entire line.
Using xPoE-4-11A-HS to install an additional camera in an existing camera point
Example of using xPoE-4-11A-HS as a LAN+PoE network extender for several cameras
Example devices compatible with PoE
Non-standard solution - passive PoE
Wiring diagram of a passive PoE injector and powered device
Passive PoE transmits power through selected conductors of UTP/FTP cable. The supplying voltage can be connected directly to the powered device or converted by a special adapter. There is no communication between the power source and the powered device - the power is provided continuously. The wiring is usually made according to option B of IEEE 802.3af (use of free pairs 4/5 (+) and 7/8 (-) in 10/100 Mbps Ethernet networks).
There are power devices that can operate in Gigabit Ethernet networks. They use transformers that enable transmit power along with the data (like in 802.3af option A).
It should be noted that passive PoE solutions are not compatible with the 802.3at standard and are not recommended for use in professional networks.
There are power devices that can operate in Gigabit Ethernet networks. They use transformers that enable transmit power along with the data (like in 802.3af option A).
It should be noted that passive PoE solutions are not compatible with the 802.3at standard and are not recommended for use in professional networks.
Passive PoE equipment
An example of passive PoE solution in a CCTV network
The N9205 PoE adapter allows data and power to be transmitted over a single twisted pair cable (data on wires 1, 2, 3 and 6, while power using wires 4, 5, 7 and 8).
