Power over Ethernet

PoE sample application: A single Ethernet - cable goes into the PoE splitter, which the signals divided into data line ( gray cable ) and power ( black cable ) for the wireless access point .

The power supply over Ethernet , English Power over Ethernet (PoE) , describes a procedure with which network- compatible devices can be supplied with power via the eight-wire Ethernet cable .

application areas

The main advantage of PoE is that you can save a power supply cable and thus install Ethernet-connected devices even in hard-to-reach places or in areas where many cables would interfere. The power supply to the device does not have to be supplied separately with a power cable and power supply unit or disconnected with a battery. Instead, the device draws its energy from the data network. In addition to the data signals, electricity must also be fed into the data line - usually at a central point in the network distributor. Thus, on the one hand, installation costs can be saved, on the other hand, the easy-to-implement use of a central uninterruptible power supply ( UPS ) can increase the reliability of the connected devices.

PoE is used by network devices that require little power. It is typically used in IP phones , small hubs , cameras, small servers or wireless transmission devices, such as WLAN - access points or Bluetooth used devices.

Challenges

The higher current strength poses new challenges for data cabling: where more current flows, more heat is generated by the resistance. Warmer cables dampen the data transmission more than before. This can mean that not enough signal can reach the receiver and that data transmission becomes impossible. This effect must be taken into account when planning new, PoE-compatible LAN cabling. The maximum transmission length must be adapted to the temperature conditions and shortened.

The relevant draft standards ISO / IEC TR 29125 and Cenelec EN 50174-99-1 describe the temperature increase in the cable bundle when using 4PPoE. A distinction is made between two parts:

Heating from the inside of a bundle to the outside
Heating of the entire cable bundle from the outside to ambient temperature.

The second part depends mainly on the installation conditions of the cable bundle. The temperature rise within the cable bundle, on the other hand, depends exclusively on the cable construction. In the case of shielded cables, the metal of the shield helps to transport the heat from the inside of the bundle to the outside. With a typical U / UTP cable, the PoE-related heating increases by a factor of 5, while a shielded cable has a factor of 2.5 to 3, depending on the construction. In a bundle with U / UTP cables, there is a temperature increase that is twice as high as in a comparable bundle with S / FTP cables.

Therefore, in the design of networks for PoE applications of the length is twisted pair -dependent voltage drop (also known as voltage drop referred) to be considered. Cables with a larger cross-section are advantageous because of the smaller resistances. The coding of the conductor cross-sections is done i. d. Usually by an American Wire Gauge designation on the network cables. Usual values are - depending on the category:

Cat 5 / 5e: AWG 24 (this corresponds to Ø 0.51 mm or a cross-sectional area of 0.21 mm²)
Cat 6 _A / 6A: AWG 23 (this corresponds to Ø 0.57 mm or a cross-sectional area of 0.26 mm²)
Cat 7/7 _A : AWG 22 (this corresponds to Ø 0.64 mm or a cross-sectional area of 0.33 mm²)

From this, the voltage drop can be calculated (taking the forward and return lines into account), if necessary using online tools. When considering the permissible level of the voltage drop, one can orientate oneself on the specifications for the voltage drop in extra-low voltage lighting systems. According to DIN VDE 0100‐715, the voltage drop between the transformer and the luminaire installed at the greatest distance must not exceed 5% of the nominal voltage of the extra-low voltage system. That is comparatively little (at 48 V only 2.4 V). However, the standard thought of extra-low voltage halogen bulbs (which at 95% of the nominal voltage only produce 85% of the brightness).

The line losses play a role in particular with regard to an efficient power supply of the connected small devices. In 2005, a Swiss study came to the conclusion that PoE is the energetically more favorable solution compared to highly efficient decentralized switched-mode power supplies with outputs of up to 8 to 9 watts. The shorter the transmission length, the more efficient the PoE solution, because cable losses dominate.

specification

In a narrower sense, PoE today mostly refers to the IEEE standard 802.3 Clause 33 “DTE Power over MDI ” , which was first adopted in June 2003 as IEEE 802.3af-2003. There is also a newer IEEE 802.3at-2009 standard - also known as PoE + or PoE plus before standardization - which increases the maximum power output from 15.4 W to 25.5 W. Before that, there were already some manufacturer-specific implementations that were also traded under the name Power over Ethernet . In addition, there are still proprietary variants.

The standard divides the devices involved into energy suppliers ( Power Sourcing Equipment , PSE) and consumers ( Powered Devices , PD). The supply voltage is 48 V, the maximum current consumption of the end devices is 350 mA (802.3af, type 1) or 600 mA (802.3at, type 2) in continuous operation (400 mA are allowed briefly when switched on). The maximum power output is 15.4 watts. The af standard assumes that after line losses 12.95 watts of usable power remain or may be consumed in order not to exceed the maximum power output. The wire pairs in the Ethernet cable that are free for 10BASE-T and 100BASE-TX are often used for power transmission . If this is not possible (e.g. because ISDN is routed over the line or with Gigabit Ethernet), the signal-carrying wires can also be used. The data lines, which are decoupled by means of transformers, are free of direct voltage without PoE, so that the direct voltage can be coupled in and out ("placed under the signal") without disrupting the data transmission. The respective mode is determined by the PSE; the consumers must support both operating modes; Consumers that only support one operating mode are not permitted.

The standards organization IEEE has further increased the transferable supply capacity and now also supports 10GBASE-T . The IEEE 802.3bt-2018 (also 4PPoE ) standard provides five new power levels from 40 W (Class 5) over two pairs of lines up to 100 W (Class 8+) over all four pairs of wires. Up to 960 milliamperes flow through each pair of wires . This enables new applications, for example the operation of powerful WLAN antennas and surveillance cameras.

The challenge for the manufacturers of proprietary PoE solutions used to be to avoid damage to non-PoE-capable end devices. Although wires 4, 5, 7 and 8 are not used with 10BASE-T and 100BASE-TX, that does not mean that there are not network cards or similar with the corresponding pins looped through to somewhere. If Power over Ethernet is accidentally applied there, this can lead to irreparable damage to the device. 802.3af solves this problem with a technique called Resistive Power Discovery . In this case, the energy supplier initially applies minimal voltage to the wires several times, which normally does not damage the device. It detects whether and where the energy consumer has a 25 kΩ terminating resistor and is therefore PoE-capable. The consumer is then supplied with a low level of power and must now signal which of the four power classes defined in the standard it belongs to. Only then does the device receive full power and can start operation.

Comparison of PoE standards
IEEE standard	PoE (802.3af-2003)	PoE Plus (802.3at-2009)	4-pair PoE (802.3bt-2018)
Output voltage in V (DC)	36-57	42.5-57	42.5-57
Output current operation in mA (DC)	350	600	2 × 960
Output current start mode in mA (DC)	400	400	?
Power of the (PSE) supply in W	Max. 15.4	Max. 30th	45; 60; 75; 90
Power at the end device (PD) in W.	Max. 12.95	Max. 25.5	40; 51; 62; 71
PSE class	1; 2; 3	4th	5; 6; 7; 8th
supported devices (PD-Type)	1	1 and 2	1; 2; 3; 4th
Used wire pairs	2	2	2 and 4

Power feed

The power for the devices to be supplied (PD) can be fed in through so-called endspan devices (e.g. switches ) or midspan devices (units between switch and terminal).

Hubs or so-called PoE injectors are mostly used as midspan devices, which supply power to the respective wires. Due to the additional space required and the additional patch cables required in distribution cabinets, patch panels (distribution fields, PoE patch panels) are also available that supply the power. These replace the conventional patch panels and therefore do not take up any additional space in the distribution cabinets. With the help of the appropriate management software, the individual ports of these distribution panels can be defined as either current-free or live.

Activation steps for PoE

step	action	Permissible voltage range according to 802.3af
Detection	Determination of whether the end device has a resistance in the range of 19–26.5 kΩ	02.7-10.1V
classification	Measurement of the exact resistance value to determine the performance class	14.5-20.5V
Startup	Activate the actual power supply	0.00> 42 , 0V
Normal business. Business as usual	Power supply in supply mode	36 , 0-57 , 0V

PoE PCI network card with active
4/5 port switch . The 48 V
with a boost converter from the 12 V from the PC power supply generated.

Available performance classes and classification signature

class	Available power on the supplied device	Classification signature
0	00.44-12.96 W.	00 to 04 mA
1	00.44-3.84 0W.	09 to 12 mA
2	03.84-6.49 0W	17 to 20 mA
3	06.49-12.95 W.	26 to 30 mA
4th	12.95-25.50 W (only 802.3at / type 2)	36 to 44 mA

general characteristics

Standards

802.3 af: twisted pair cable from Cat-3 (also UTP cable ), max. 20 Ω per line pair
802.3 at: twisted pair cable from Cat-5 (also UTP), max. 12.5 Ω per line pair
802.3 bt: twisted pair cable from Cat-5 , max. 12.5 Ω per line pair or 6.25 Ω for two pairs with 4PPoE

power

The output voltage is between 44 V and 54 V (usually 48 V), the power up to 15.4 W (divided into 4 classes, 802.3af) or 25.50 W (5 classes, 802.3at) or 71 W (divided into three classes, 802.3bt) with a cable length of up to 100 m.

Efficiency / efficiency

Due to the small conductor cross-sections, the long cable lengths and the low system voltage, there is a significant power loss in the cable, which v. a. with class 4 PD leads to poor system efficiency.

Example: With class 4, 25.5 W can be drawn from the PD, the line can have a loop resistance of up to 12 Ω at a length of 100 m, and a maximum current of 0.6 A is permissible. This results in a power loss of up to 4.32 W in the cable, which corresponds to an efficiency of approx. 86%. Added to this are the losses in the PSE and PD power supplies.

Overall, efficiencies of less than 70% are not unusual.

Variants of energy transfer

Mode A, also called phantom power : the current is transmitted via the data pairs used by 10BASE-T and 100BASE-TX. In the case of the transformers , the center point tap is required, as the direct voltage is fed in via this, comparable to the phantom circuit, whereby the differentially transmitted data is decoupled from the direct voltage and changing currents.
Mode B: the current is transmitted via the data pairs not used by 10BASE-T and 100BASE-TX, which is why this is also referred to as a spare pair supply. The center tap is required for existing transformers (this is always the case with 1000BASE-T and faster) or the lines are used directly (not with 1000BASE-T and faster).
4-pair mode: the current is transmitted simultaneously via the pairs used in mode A and mode B.

Variants of energy supply

Endspan (direct supply through PoE switch)
Midspan (supply via interconnected sources, example: PoE injector)

Pin assignment

Standard 802.3af / at A and B viewed from the supplying device ( MDI-X )
Pins on the hub or switch	Colors according to		10/100 Mbit				1000 Mbit (= 1 Gbit)
Pins on the hub or switch	T568A	T568B	Alternative B, DC on unused lines		Alternative A, DC and data combined		Alternative B, DC & Bidirectional data		Alternative A, DC & Bidirectional data
Pin 1	white / green	white / orange	Tx +		Tx +	DC -	TxRx B +		TxRx B +	DC -
Pin 2	green	orange	Tx -		Tx -	DC -	TxRx B -		TxRx B -	DC -
Pin 3	white / orange	white / green	Rx +		Rx +	DC +	TxRx A +		TxRx A +	DC +
Pin 4	blue	blue		DC +	unused		TxRx D +	DC +	TxRx D +
Pin 5	White blue	White blue		DC +	unused		TxRx D -	DC +	TxRx D -
Pin 6	orange	green	Rx -		Rx -	DC +	TxRx A -		TxRx A -	DC +
Pin 7	White Brown	White Brown		DC -	unused		TxRx C +	DC -	TxRx C +
Pin 8	brown	brown		DC -	unused		TxRx C -	DC -	TxRx C -

Abbreviations: DC = direct voltage, Tx = sender, Rx = receiver of data

Individual evidence

↑ The line calculation: Voltage drop online, accessed on October 29, 2018
↑ Alois Huser: Efficient power supply using Power over Ethernet (PoE) . March 2005 ( https://nanopdf.com/downloadFile/poe-bundesamt-fr-energie-bfe_pdf nanopdf.com [accessed on August 28, 2019] on behalf of the Federal Office for Energy (CH)).
↑ https://ieeexplore.ieee.org/xpl/mostRecentIssue.jsp?punumber=7428774 , IEEE 802.3at-2009 Clause 33, Information technology - Part 3: Carrier Sense Multiple Access with Collision Detection Access Method and Physical Layer Specifications - - Amendment 3: Data Terminal Power via the Media Dependent Interface Enhancement.
↑ With type 1 the cable must not offer more than 20 Ω resistance, with type 2 a maximum of 12 Ω.
↑ IEEE P802.3bt DTE Power via MDI over 4-Pair Task Force . March 29, 2016. Retrieved August 3, 2016.
↑ Roland Dold: Key technology Power over Ethernet (PoE) . In: electrical practitioners . tape 71 , no. 12 . Berlin December 2017, p. 996–999 ( elektropraktiker.de [PDF; 1.1 MB ; accessed on October 31, 2018] (partly freely available)).
↑ IEEE 802.3at, Table 33-18 PD power supply limits
↑ IEEE 802.3 Clause 33.1.4 Type1 and Type2 system parameters

Web links

Commons : Power over Ethernet - collection of pictures, videos and audio files

[1] The line calculation: Voltage drop online, accessed on October 29, 2018

[Huser-2005-2] Alois Huser: Efficient power supply using Power over Ethernet (PoE) . March 2005 ( https://nanopdf.com/downloadFile/poe-bundesamt-fr-energie-bfe_pdf nanopdf.com [accessed on August 28, 2019] on behalf of the Federal Office for Energy (CH)).

[3] ttps://ieeexplore.ieee.org/xpl/mostRecentIssue.jsp?punumber=7428774 , IEEE 802.3at-2009 Clause 33, Information technology - Part 3: Carrier Sense Multiple Access with Collision Detection Access Method and Physical Layer Specifications - - Amendment 3: Data Terminal Power via the Media Dependent Interface Enhancement.

[4] With type 1 the cable must not offer more than 20 Ω resistance, with type 2 a maximum of 12 Ω.

[5] IEEE P802.3bt DTE Power via MDI over 4-Pair Task Force . March 29, 2016. Retrieved August 3, 2016.

[Dold-2017-6] Roland Dold: Key technology Power over Ethernet (PoE) . In: electrical practitioners . tape 71 , no. 12 . Berlin December 2017, p. 996–999 ( elektropraktiker.de [PDF; 1.1 MB ; accessed on October 31, 2018] (partly freely available)).

[7] IEEE 802.3at, Table 33-18 PD power supply limits

[8] IEEE 802.3 Clause 33.1.4 Type1 and Type2 system parameters