Is ethernet an electrical specification

Power over Ethernet (PoE)

IEEE / IEEE 802.3at / IEEE 802.3bt

Power-over-Ethernet are IEEE standards and procedures for supplying end devices in a network with power via the network cable.
With Power-over-Ethernet, a terminal device is supplied with power via the existing twisted pair cable and the RJ45 plug connections through a network-side Ethernet interface. In other words, data and the power for the energy supply are transmitted together via a cable to the end device. The feeding device is, for example, a switch.


The power supply of end devices in network technology is typically within the sphere of influence of the end device manufacturer. They solve the power supply via internal power supply units, or for devices with low power via plug-in power supply units. This means that there must be at least one 230V socket next to each network connection socket. With Power-over-Ethernet (PoE) there is no power connection or plug-in power supply on the end device.

Typical PoE end devices or consumers:

  • Wireless Access Points (WAP)
  • simple IP cameras
  • IP telephones
  • Time recording and access control devices

Possible future PoE consumers:

  • swiveling IP cameras (with motors)
  • Thin clients
  • Monitors
  • Lamps (with integrated WAP)

Advantages of Power-over-Ethernet

  • Less space required: External power packs are usually bulky and, together with the socket, take up unnecessary space.
  • Falling installation costs: Unnecessary cable ducts, strands or bundles can be avoided. Consumers no longer have to be placed near the socket.
  • Power supply: In the event of a power failure, the power supply to the devices can be controlled and ensured centrally. For example with a UPS on the switch.

Overview: Standards for Power-over-Ethernet (PoE)

  • IEEE 802.3af (PoE) provides 15 W output power or up to 12.95 W at the end device.
  • IEEE 802.3at (PoE +) provides 30 W output power or up to 25.5 W at the end device.
  • IEEE 802.3bt (4PPoE) provides 90 W output power or up to 71.3 W on the end device.
  • IEEE 802.3bu (PoDL) for single-pair Ethernet

The IEEE standards for Power-over-Ethernet define the energy source (Power Sourcing Equipment, PSE) and the maximum amount of power it can feed into the network. In addition, the end device (powered device, PD) is defined and how much power can or must arrive there.

Performance classes

With the functions of the end devices, their energy requirements also increase, which is why there are several standards that provide different usable power. There are standardized performance classes for this.

classdefaultOutput voltageMax.
Output current
maximum power
Supply (PSE)
Minimum performance
Withdrawal (PD)
0IEEE 802.3af36 - 57 V.350 mA15.4 W0.44 - 12.96 W.
14.0 W0.44-3.84 W.
27.0 W3.84 - 6.49 W.
315.4 W6.49-12.95 W.
4IEEE 802.3at42.5-57 V.600 mA25.5 W12.95-25.50 W.
5IEEE 802.3bt42.5-57 V.2 × 960 mA45 W40 W
660 W51 W
775 W62 W
890 W71 W

When the device is switched on, the voltage is> 42 volts. In normal operation, the voltage is in the range from 36 to 57 volts.

Variants of the energy supply in the cable

Depending on the Ethernet standard, there are differences in the way energy is fed into the cable.

  • Phantom power or remote power: The power is transmitted on the data-carrying wire pairs.
  • Spare pairs supply: The power is transmitted on the unused wire pairs.

Note: Because the polarity is not fixed and can be swapped again with a crossover cable, an input circuit in the end device recognizes the polarity.

Assignment of the RJ45 plug connection depending on the supply

Pin codeSpare pair supplyPhantom power
1RX +Rx + / DC-Rx + / DC +
2RX-Rx- / DC-Rx- / DC +
3TX +Tx + / DC +Tx + / DC-
4DC +--
5DC +--
6TX-Tx- / DC +Tx- / DC-

Variants of the energy supply or energy source

The IEEE 802.3af standard describes a consumer, the Powered Device (PD), and the power source, the Power Source Equipment (PSE).

  • Endspan process: The energy is supplied directly from a central power source. For example with a PoE-enabled switch.
  • Midspan process: The energy is supplied via an interconnected or looped-in power supply unit. For example through a PoE injector or PoE splitter.

Endspan process: Power supply with a PoE-capable switch

In the Endspan process, the power source (Power Source Equipment, PSE) is usually a PoE-capable switch that feeds itself and the connected PoE end devices from its power supply unit.

Note: There is usually a limit to the total PoE power in the power source. This means that full power consumption is not possible at all ports at the same time. Therefore you always have to consider the power consumption and the power consumption of the individual PoE end devices and measure them if necessary. In addition, the actual power consumption must be compared with the possible nominal power of the power source.

Midspan process: power supply with a PoE injector

If you want to supply power to individual PoE end devices, you don't need a PoE-capable switch. A simple power injector is sufficient. The is switched into the network cable from the switch and the outgoing network cable to the end device.

If the end device to be supplied does not support Power-over-Ethernet, the power can be separated from the network cable with a PoE splitter in order to supply the end device with power in the conventional way.

The energy supply with injector and splitter works like a tinkering solution, which is why this only makes sense in small networks with individual devices. For example, to test Power-over-Ethernet. As soon as you have several Power-over-Ethernet end devices, but do not yet have a PoE-capable switch, a power hub is useful, which should be installed at the switch. In addition, you should prefer a PoE-enabled switch. There are already PoE switches with 8 ports at an affordable price. The advantage is less wiring and fewer sources of error.

Spare pairs procedure

With the spare pairs method, the transmission of power and data takes place via separate wire pairs. With the Ethernet standards 10Base-T and 100Base-TX, only wire pairs 1/2 and 3/6 are used for data transmission. The two unused wire pairs 4/5 and 7/8 can be used for the power supply as part of the spare pairs procedure.

Phantom power

With phantom power, the data-carrying wires in the cable are also used for the power supply. The data signal is superimposed with the current for the power supply. The power device must take over the decoupling, which is why it is important that only PoE end devices receive a PoE supply.

Note: If all wire pairs of a network cable are used for data transmission, then one is inevitably dependent on the phantom power supply.

IEEE 802.3af / Power-over-Ethernet (PoE)

The IEEE 802.3af standard only applies to 10Base-T (Ethernet) and 100Base-TX (Fast Ethernet). The special feature here is that in the twisted pair cable used, only wire pairs 1/2 and 3/6 are used for data transmission and the other two wire pairs are unused. That is why there are two variants of feeding into the network cable.

  • Spare pairs supply: Only the two free wire pairs are used for the energy supply. The data-carrying wire pairs are excluded from this.
  • Phantom power supply: Alternatively, the wire pairs occupied by the data transmission can be overlaid with the power supply.

Because RJ45 plugs and twisted pair cables are not designed for currents in the ampere range, a voltage between 44 V and 57 volts, on average 48 volts, is used, which corresponds to the requirements for a safety extra-low voltage. A maximum current of 175 mA is provided for each wire pair. With two pairs of wires this is a total of 350 mA. When switching on, 400 mA are allowed for a short time. The maximum power is 15.4 watts per line.
Due to the relatively high voltage, the development of heat in the cables and at the transitions of the plug connection remains low. The twisted pair cable and the RJ45 plug connections still lead to a loss of performance. At the end of a 100 meter long line, of the 15.4 watts fed in, around 12.95 watts of usable power remain.
With a maximum extraction capacity of 12.95 watts, this technology is ideal for supplying webcams, print servers, IP telephones and wireless access points (WAP for WLAN) with power.

IEEE / Power-over-Ethernet Plus (PoE + / PoE Plus)

The IEEE standard is also called Power-over-Ethernet Plus, or PoE + or PoE Plus for short. It was developed so that Power-over-Ethernet is also possible for Gigabit Ethernet with 1 GBit / s via twisted pair cable (1000GBase-T). At the same time, the performance is almost twice as high.
Because Gigabit Ethernet uses all 4 pairs of wires in the twisted pair cable for data transmission, the power can only be supplied via the phantom power supply.
IEEE enables a power of up to 25.5 watts per port. The minimum voltage is increased from 44 to 50 volts. The maximum current has been increased from 350 mA to 600 mA. With these high outputs, at least a Cat 5e or Cat 6 cable is recommended. That has less resistance.
However, the 25.5 watt output is not directly available to the end device. You still have to subtract the losses between the feeding power source and the end device. One speaks of an efficiency of around 85%, which corresponds to a power at the end of the cable of around 21.90 watts.

IEEE 802.3bt / Four-Pair-Power-over-Ethernet (4PPoE / PoE ++)

The IEEE 802.3bt standard is known as four-pair Power-over-Ethernet. The short notation is 4PPoE or PoE ++.

IEEE 802.3bt enables end devices to be supplied with up to 60 or 90 W per port via four pairs of wires parallel to the data traffic. The power of the power source then only reaches the end device with 51.0 or 71.3 W. That is enough to supply monitors, swiveling cameras and thin clients. You could completely supply an entire workplace with electricity. From the desktop PC, monitor, telephone and lighting. Here, only one network cable would be necessary for complete supply. Other applications in the field of lighting technology that are fed and also controlled from the network are conceivable.
Like the USB in computer technology, the RJ45 connector is becoming a globally standardized connector for the electrical energy supply for end devices in the field of network technology.

The IEEE 802.3bt standard conforms to the SELV (Safety Extra Low Voltage) criteria of EN 60950 for protective extra-low voltages and the Limited Power Source of EN 60950-1.
The EN 50174 already takes into account IEEE 802.3bt in the specifications for the technical specification as well as for the planning and evaluation of the cabling for remote feeding.

IEEE 802.3bu / PoDL - Power over Data Line

IEEE 802.3bu is a standard for supplying energy to devices via a single-pair Ethernet connection with 100Base-T1 and 1000Base-T1. This standard is not compatible with the PoE standards IEEE 802.3af, 802.3at or 802.3bt. Single-pair Ethernet with IEEE 802.3bu is used, for example, in the automotive or fieldbus area.

PoE detection

Before the power source is fed in, it must first classify the end device. This avoids damage to non-PoE-capable end devices. For this purpose, the power source uses a classification current and a low voltage to determine whether the end device is PoE-capable at all and to which class it belongs. Depending on the end device, an exchange of information (handshake procedure) between the power source and the end device is possible in which the end device communicates the PD class.

In order to first differentiate PoE-compatible end devices from unsuitable end devices, a process called Resistive Power Directory is used in the PoE power source. For this purpose, a PoE-capable terminal device has an input circuit with passive components. The power source uses a measuring circuit to check the internal resistance of the consumer. If it is between 19 and 26.5 kOhm and has a maximum capacity of 10 µF, the energy supply is activated. The performance class is determined in a second recognition phase.
Power-over-Ethernet end devices must be able to use both methods of power consumption in any case. The energy source is free to choose which method it supports. However, the simultaneous use of both methods is prohibited.

Note: Be careful when using Power-over-Ethernet in a network. With PoE detection, all connected end devices should be checked for PoE support before switching on the PoE power supply. However, in order to avoid damage to non-PoE-capable end devices, you should only connect the lines with PoE supply to which PoE-capable end devices are connected.

Why does the additionally transmitted direct current not interfere with the data transmission of Ethernet?

With Ethernet, the data is transmitted as a differential signal with opposite polarity on a pair of wires in each direction. The transmitter feeds a positive voltage into one wire on each side of the cable and a negative voltage into the second wire. At the other end of the line, a differential receiver subtracts the two voltages from each other. This means that it calculates the difference between the two signals in order to determine the desired signal voltage from which the data signal can be derived. It is therefore possible that differential signals, which are fed into the two conductors of a wire pair with opposite polarity, are received error-free at the other end.

If you now want to use the data-carrying signal for power supply, then, to put it simply, you have to raise the voltage of the signal to the potential that is required for power supply. You just have to ensure that the signal receiver can also handle the higher voltage. If not, it will not be able to read any data from the signals or, in the worst case, it will break. It is therefore important that the PoE power source detects a non-PoE-capable end device.

For the power supply, the power source applies a common-mode voltage to all wire pairs. It is important here that the DC resistance of the wires in a pair is largely the same or symmetrical. However, the line resistance increases with the length and the cross-section of the conductor.
The resistance differences in the cores lead to a direct current resistance imbalance. End devices can usually tolerate a certain asymmetry. However, the transformer in the power source can saturate due to the asymmetry. This then disrupts the transmission of the data signals and leads to bit errors.

These asymmetries are mostly the result of poor installation quality when handling the cables and when laying them on the cable ends. Furthermore, the minimum bending radii were often not adhered to.
In order to be able to assess the PoE capability of a certain twisted pair cable, one can measure the loop resistance and the resistance asymmetry with many cable testers.

Requirements for the power supply with Power-over-Ethernet

After Ethernet-based networks have established themselves in the industrial sector and in building technology, parts of the standards DIN and EN 50174, as well as VDE 0800-174, have been revised and future types of installation, such as for Power-over-Ethernet, have been taken into account.

The power supply causes a higher current to flow through the cable. And where more current flows, the line resistance generates more heat. However, warmer cables also dampen data transmission more. In the worst case, this can mean that not enough of the signal arrives at the receiver and the data transmission no longer works. This must be taken into account when planning PoE-compatible LAN cabling with twisted pair cables.

  • Suitable network planning must be drawn up for the power supply via twisted pair cables.
  • A safety extra-low voltage must be maintained to prevent personal injury.

Data distributors must be viewed as electrical distributors and be adequately ventilated. The data wiring, like the electrical wiring, must be laid appropriately.

  • Temperature increase: Cables can heat up due to the higher current flow.
  • Resistance asymmetry: Minimum bending radii or the correct twisting of wire pairs play an increasingly important role.
  • Attenuation: The transmission performance of the cable deteriorates with the length and with each additional cable. Therefore the choice of the patch cable and the limitation of the cables are important (maximum of 24 cables).
  • Wear of the plug connection: Under load, removing a patch cable in the contact area can lead to sparks and burn spots. The plug connection (plug and socket) can wear out faster and also be damaged. It must be taken into account that higher inrush currents are to be expected and that these also increase with power consumption.

To avoid problems caused by cables that are too long, it is recommended that you only use twisted pair cables of category 7, 7A or better.

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