Power over Ethernet (PoE) describes any of severalstandards orad hoc systems that passelectric power along with data ontwisted-pair Ethernet cabling. This allows a single cable to provide both a data connection and enough electricity to power networked devices such aswireless access points (WAPs),IP cameras andVoIP phones.
| Pin | Pair | Color |
|---|---|---|
| 1 | 3 |  white/green |
| 2 | 3 |  green |
| 3 | 2 |  white/orange |
| 4 | 1 |  blue |
| 5 | 1 |  white/blue |
| 6 | 2 |  orange |
| 7 | 4 |  white/brown |
| 8 | 4 |  brown |
There are several common techniques for transmitting power over Ethernet cabling, defined within the broaderInstitute of Electrical and Electronics Engineers (IEEE)802.3 standard since 2003.
The three techniques are:
Alternative A transmits power on the same wires as data for common 10 and100 Mbit/s Ethernet variants. This is similar to thephantom power technique commonly used for powering condenser microphones. Power is transmitted on the data conductors by applying a common voltage to each pair. Because twisted-pair Ethernet usesdifferential signaling, this does not interfere withdata transmission. The common-mode voltage is easily extracted using thecenter tap of the standard Ethernetpulse transformer. Forgigabit Ethernet and faster, both alternativesA andB transmit power on wire pairs also used for data since all four pairs are used for data transmission at these speeds.
4PPoE provides power using all four pairs of the connectors used for twisted-pair Ethernet. This enables higher power for applications likepan–tilt–zoom cameras (PTZ), high-performancewireless access points (WAPs), or even charginglaptop batteries.
In addition to standardizing existing practice for common-mode data pair (Alternative A), spare-pair (Alternative B), and four-pair (4PPoE) transmission, the IEEE PoE standards provide for signaling between thepower sourcing equipment (PSE) andpowered device (PD). This signaling allows the presence of a conformant device to be detected by the power source and allows the device and source to negotiate the amount of power required or available while avoiding damage to non-compatible devices.
The original PoE standard,IEEE 802.3af-2003,[1] now known asType 1, provides up to 15.4 W ofDC power (minimum44 V DC and 350 mA)[2][3] on each port.[4] Only 12.95 W is guaranteed to be available at the powered device as some power dissipates in the cable.[5]
The first update to PoE,IEEE 802.3at-2009,[6] introducedType 2, also known asPoE+ orPoE plus. It provides up to 25.5 W and prohibits the use of four pairs simultaneously for power.[7][8]
Both of these standards, 802.3af and 802.3at, were later incorporated into theIEEE 802.3-2012 publication.[9]
LaterType 3 andType 4 were introduced inIEEE 802.3bt-2018, respectively allowing up to 51 W and up to 71.3 W delivered power, optionally by using all four pairs for power.[10] Each pair needs to handle a current of up to 600 mA (Type 3) or 960 mA (Type 4).[11] Additionally, power capabilities are defined for2.5GBASE-T, 5GBASE-T and10GBASE-T.[12] This development opens the door to new applications and expands the use of applications such as high-performancewireless access points and surveillance cameras.
IEEE 802.3bt was incorporated into 802.3 in the 2022 revision.[13]
TheIEEE 802.3bu-2016[14] amendment introducedsingle-pairPower over Data Lines (PoDL) for the single-pair Ethernet standards100BASE-T1 and1000BASE-T1 intended for automotive and industrial applications.[15] On the two-pair and four-pair standards, the power voltage is applied between one conductor of each of two pairs, so that within each pair there is no differential voltage other than that representing the transmitted data. With single-pair Ethernet, power is transmitted in parallel to the data. PoDL initially defined ten power classes, ranging from 0.5 to 50 W (at PD).
Subsequently, PoDL was added to the single-pair variants10BASE-T1,[16]2.5GBASE-T1, 5GBASE-T1, and10GBASE-T1,[17] and as of 2021[update] it includes a total of 15 power classes with additional intermediate voltage and power levels.[16]
Examples of devices powered by PoE include:[18]
802.3 refers toPower Sourcing Equipment (PSE), which provides power on the Ethernet cable. This device may be anetwork switch, in the standardEndpoint PSE (commonly called anendspan device) or aPoE injector,Midspan PSE in the standard, an intermediary device between a switch that does not provide PoE (or one that cannot provide sufficient power) and a PoE-powered device.[21]
802.3 refers to any PoE-powered piece of equipment as aPowered Device (PD). Examples includewireless access points,VoIP phones, andIP cameras.
Many powered devices have an auxiliary power connector for an optional external power supply. Depending on the design, some, none, or all of the device's power can be supplied from the auxiliary port,[22][23] with the auxiliary port also sometimes providing backup power in case PoE-supplied power fails.
Advocates of PoE expect PoE to become a global long-term DC power cabling standard and replace a multiplicity of individualAC adapters, which cannot be easily centrally managed.[24] Critics of this approach argue that PoE is inherently less efficient than AC power due to the lower voltage, and this is made worse by the thin conductors of Ethernet. Advocates of PoE, like theEthernet Alliance, point out that quoted losses are for worst-case scenarios in terms of cable quality, length and power consumption by powered devices.[25] In any case, where the central PoE supply replaces several dedicated AC circuits, transformers and inverters, the power loss in cabling can be justifiable.
The integration of PoE with the IEEE 802.3azEnergy-Efficient Ethernet (EEE) standard potentially produces additional energy savings. Pre-standard integrations of EEE and PoE (such asMarvell'sEEPoE outlined in a May 2011 white paper) claim to achieve a savings upwards of 3 W per link. This saving is especially significant as higher-power devices come online.[26]
Standards-based Power over Ethernet is implemented following the specifications in IEEE 802.3af-2003 (which was later incorporated as Clause 33 intoIEEE 802.3-2005) or the 2009 update, IEEE 802.3at. The standards requireCategory 5 cable or better for high power levels but allow usingCategory 3 cable if less power is required.[27]
In multi-pair cases, PoE supplies power as acommon-mode signal over two or more of thedifferential pairs inEthernet cables. This power comes from a PoE-providing device like anEthernet switch or aPoE injector
Thisphantom power technique works with10BASE-T,100BASE-TX,1000BASE-T,2.5GBASE-T, 5GBASE-T, and10GBASE-T because all twisted pair standards usedifferential signaling withtransformer coupling. The DC supply and load connections can be made to the transformer center-taps at each end. Since each pair operates incommon mode as one side of the DC supply, two pairs are needed to complete the circuit.
The powered device must operate with either pair: the spare pairs on pins 4 and 5, and 7 and 8, or the data pairs on pins 1 and 2, and 3 and 6. Polarity is defined by the standards on spare pairs (4+5+, 7-8-). The polarity of the DC supply on data pairs may be inverted bycrossover cables and hence the polarity is ambiguously implemented for data pairs, with the use of adiode bridge. (It is customary to use 1+2+, 3-6- on T568A.)
| Official name in IEEE 802.3 | Type 1 | Type 2 | Type 3 | Type 4 |
|---|---|---|---|---|
| Common name(s) | PoE | PoE+ | PoE++ / 4PPoE[28] | |
| Defining IEEE document | 802.3af | 802.3at | 802.3bt | |
| Power available at PD[note 1] | 12.95 W | 25.50 W | 51 W | 71.3 W |
| Maximum power delivered by PSE | 15.40 W | 30.0 W | 60 W | 90 W[29] |
| Voltage range (at PSE) | 44.0–57.0 V[30] | 50.0–57.0 V[30] | 52.0–57.0 V | |
| Voltage range (at PD) | 37.0–57.0 V[31] | 42.5–57.0 V[31][32] | 41.1–57.0 V | |
| Maximum currentImax | 350 mA[33] | 600 mA per pair[33][32] | 960 mA per pair[32] | |
| Maximum cable resistance per pairset | 20 Ω[34] (Category 3) | 12.5 Ω[34][32] (Category 5) | ||
| Power management | Three power classes(1–3) negotiated by signature | Four power classes(1–4) negotiated by signature or0.1 W steps negotiated by LLDP | Six power classes(1–6) negotiated by signature or0.1 W steps negotiated by LLDP[35] | Eight power classes(1–8) negotiated by signature or0.1 W steps negotiated by LLDP |
| Derating of cable maximum ambient operating temperature | None | 5 °C (9.0 °F) with only two pairs active, atImax | 10 °C (18 °F) with all of the bundled cables pairs active, atImax[36] | 10 °C (18 °F) with temperature planning required |
| Supported cabling | Category 3 and Category 5[27] | Category 5[27][note 2] | ||
| Supported modes | Mode A (from Endpoint PSE), Mode B (from Midspan PSE) | Mode A, Mode B | Mode A, Mode B, 4-pair mode | 4-pair mode mandatory |
Notes:
Three modes,Mode A,Mode B, and4-pair mode, are available. (In the standard these are discussed as two Modes, with the term4-pair mode for both simultaneously.) Mode A delivers power onT568A andT568B pairs 2 and 3 – the data pairs of100BASE-TX or 10BASE-T. Mode B delivers power on pairs 1 and 4 – the pairs not used by 100BASE-TX or 10BASE-T. 4-pair mode delivers power using all four pairs. PoE can also be used with 1000BASE-T, 2.5GBASE-T, 5GBASE-T and 10GBASE-T Ethernet, in which case there are no spare pairs and all power is delivered using thephantom technique.
Mode A has two alternative configurations (MDI and MDI-X), using the same pairs but with different polarities. In Mode A, pins 1 and 2 (pair 3 in T568A wiring, pair 2 in T568B) form one side of the 48 V DC, and pins 3 and 6 (pair 2 in T568A, pair 3 in T568B) form the other side. These are the same two pairs used for data transmission in 10BASE-T and 100BASE-TX, allowing the provision of both power and data over only two pairs in such networks. The free polarity allows PoE to accommodate crossover cables, patch cables andauto MDI-X.
In Mode B, pins 4–5 (pair 1 in both T568A and T568B) form one side of the DC supply and pins 7–8 (pair 4 in both T568A and T568B) provide the return; these are the pairs 10BASE-T and 100BASE-TX do not use. Mode B, therefore, requires that all four pairs of the connectors be wired.
ThePower Sourcing Equipment (PSE), not thePowered Device (PD), decides whether Mode A or Mode B shall be used. PDs that implement only Mode A or Mode B are disallowed by the standard.[37] The PSE can implement Mode A, Mode B, or both (4-pair mode). A PD indicates that it is standards-compliant by placing a 25 kΩ resistor between the powered pairs. If the PSE detects a resistance that is too high or too low (including a short circuit), no power is applied. This protects devices that do not support PoE. An optionalpower class feature allows the PD to indicate its power requirements by changing thesense resistance at higher voltages.
To retain power, the PD must use at least 5–10 mA for at least 60 ms at a time. If the PD goes more than 400 ms without meeting this requirement, the PSE will consider the device disconnected and, for safety reasons, remove power.[38]
There are two types of PSE:Endpoint andMidspan. Endpoint devices (commonly PoE switches) are Ethernet networking equipment that includes the power-over-Ethernet transmission circuitry. Midspan devices arepower injectors that stand between a non-PoE Ethernet switch (or one that cannot provide sufficient power) and the powered device,injecting power without affecting the data. Endpoint devices are normally used in new installations or where the switch has to be replaced for other reasons (such as moving from10/100 Mbit/s to1 Gbit/s), which makes it convenient to add the PoE capability. Midspan PSE can be used e.g. to power a single piece of equipment added to a network that does not provide PoE.
| Stage | Action | Volts specified (V) | |
|---|---|---|---|
| 802.3af | 802.3at | ||
| Detection | PSE detects if the PD has the correct signature resistance of19–26.5 kΩ. | 2.7–10.1 | |
| Classification | PSE detects resistor indicating power range (see below). | 14.5–20.5 | |
| Mark 1 | PD signals it is 802.3at-capable. PD presents a0.25–4 mA load. | — | 7–10 |
| Class 2 | PSE outputs classification voltage again to indicate 802.3at capability. | — | 14.5–20.5 |
| Mark 2 | PD signals it is 802.3at-capable. PD presents a0.25–4 mA load. | — | 7–10 |
| Startup | PSE supplies startup voltage.[39][40] | > 42 | > 42 |
| Normal operation | PSE supplies power to device.[39][40] | 37–57 | 42.5–57 |
IEEE 802.3at-capable devices are also referred to asType 2. 802.3at PSE may also useLLDP communication to signal 802.3at capability.[41]
| Class | Usage | Classification current (mA) | Power range at PD (W) | Max power from PSE (W) | Class description |
|---|---|---|---|---|---|
| 0 | Default | 0–5 | 0.44–12.94 | 15.4 | Classification unimplemented |
| 1 | Optional | 8–13 | 0.44–3.84 | 4.00 | Very Low power |
| 2 | Optional | 16–21 | 3.84–6.49 | 7.00 | Low power |
| 3 | Optional | 25–31 | 6.49–12.95 | 15.4 | Mid power |
| 4 | Valid for Type 2 (802.3at) devices, not allowed for 802.3af devices | 35–45 | 12.95–25.50 | 30 | High power |
| 5 | Valid for Type 3 (802.3bt) devices | 36–44 & 1–4 | 40 (4-pair) | 45 | |
| 6 | 36–44 & 9–12 | 51 (4-pair) | 60 | ||
| 7 | Valid for Type 4 (802.3bt) devices | 36–44 & 17–20 | 62 (4-pair) | 75 | |
| 8 | 36–44 & 26–30 | 71.3 (4-pair) | 90 |
Class 4 can only be used by IEEE 802.3at (Type 2) devices, requiring valid Class 2 and Mark 2 currents for the power-up stages. An 802.3af device presenting a Class 4 current is non-compliant and, instead, will be treated as a Class 0 device.[44]: 13
Link Layer Discovery Protocol (LLDP) is a layer-2 Ethernet protocol for managing devices. LLDP allows an exchange of information between PSE and a PD. This information is formatted intype–length–value (TLV) format. PoE standards define TLV structures used by PSE and PDs to signal and negotiate available power.
| TLV Header | TLV information string | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| Type (7 bits) | Length (9 bits) | IEEE 802.3OUI (3 octets) | IEEE 802.3 subtype (1 octet) | MDI power support[46] (1 octet) | PSE power pair[46] (1 octet) | Power class (1 octet) | Type/source priority (1 octet) | PD-requested power value (2 octets) | PSE-allocated power value (2 octets) |
| 127 | 12 | 00-12-0F | 2 | Bit 0: port class (1: PSE; 0: PD) Bit 1: PSE MDI power support Bit 2: PSE MDI power state Bit 3: PSE pairs control ability Bits 4–7: reserved | 1: signal pair 2: spare pair | 1: Class 0 2: Class 1 3: Class 2 4: Class 3 5: Class 4 | Bit 7: power type (1: Type 1; 0: Type 2) Bit 6: power type (1: PD; 0: PSE) Bits 5–4: power source Bits 3–2: reserved Bits 0–1 power priority (11: low; 10: high; 01: critical; 00: unknown) | 0–25.5 W in0.1 W steps | 0–25.5 W in0.1 W steps |
| TLV Header | TLV information string | |||||
|---|---|---|---|---|---|---|
| Type (7 bits) | Length (9 bits) | IEEE 802.3OUI (3 octets) | IEEE 802.3 subtype (1 octet) | MDI power support[46] (1 octet) | PSE power pair[46] (1 octet) | Power class (1 octet) |
| 127 | 7 | 00-12-0F | 2 | Bit 0: port class (1: PSE; 0: PD) Bit 1: PSE MDI power support Bit 2: PSE MDI power state Bit 3: PSE pairs control ability Bits 7–4: reserved | 1: signal pair 2: spare pair | 1: class 0 2: class 1 3: class 2 4: class 3 5: class 4 |
| TLV Header | MED Header | Extended power viaMDI | |||||
|---|---|---|---|---|---|---|---|
| Type (7 bits) | Length (9 bits) | TIAOUI (3 octets) | Extended power via MDI subtype (1 octet) | Power type (2 bits) | Power source (2 bits) | Power priority (4 bits) | Power value (2 octets) |
| 127 | 7 | 00-12-BB | 4 | PSE orPD | Normal orBackup conservation | Critical, High, Low | 0–102.3 W in0.1 W steps |
The setup phases are as follows:
The rules for this power negotiation are:
There are more than ten proprietary implementations.[49] The more common ones are discussed below.
Some Cisco WLAN access points andVoIP phones supported a proprietary form of PoE[50] many years before there was an IEEE standard for delivering PoE. Cisco's original PoE implementation is not software upgradeable to the IEEE 802.3af standard. Cisco's original PoE equipment is capable of delivering up to10 W per port. The amount of power to be delivered is negotiated between the endpoint and the Cisco switch based on a power value that was added to the Cisco proprietaryCisco Discovery Protocol (CDP). CDP is also responsible for dynamically communicating the Voice VLAN value from the Cisco switch to the Cisco VoIP Phone.
Under Cisco's pre-standard scheme, the PSE (switch) will send afast link pulse (FLP) on the transmit pair. The PD (device) connects the transmit line to the receive line via alow-pass filter. The PSE gets the FLP in return. The PSE will provide a common mode current between pairs 1 and 2, resulting in48 V DC[51] and6.3 W[52] default of allocated power. The PD must then provide Ethernet link within5 seconds to the auto-negotiation mode switch port. A later CDP message with a TLV tells the PSE its final power requirement. A discontinuation of link pulses shuts down power.[53]
In 2014, Cisco created another non-standard PoE implementation calledUniversal Power over Ethernet (UPOE). UPOE can use all four pairs, after negotiation, to supply up to 60 W. It is backward compatible with PoE+ (802.3at). It uses a new LLDP TLV to negotiate.[54]
In 2017, Cisco released Catalyst 9300, a switch with support for UPOE+, backwards compatible with both UPOE and PoE++ (802.3bt).[55]
A proprietary high-power development called LTPoE++, using a single Cat 5e Ethernet cable, is capable of supplying varying levels at 38.7, 52.7, 70, and 90 W. LTPoE++ is backward compatible with PoE+ (802.3at).[56] LTPoE++ works by extending the current-sensing mechanism.[57]
Powered devices compatible with both 802.3bt (PoE++) and non-standard LTPoE++ are available.[58]
PowerDsine, acquired byMicrosemi in 2007, which was then acquired by Microchip in 2018, has been selling power injectors since 1999. Using Microchip's multi-PoE PSE ICs, PoE injectors and switches can support the IEEE 802.3 PoE standards and also pre-standard configurations. Several companies such asPolycom,3Com,Lucent andNortel used PowerDsine's olderPower over LAN PoE implementation.[59]
In a passive PoE system, the injector does not communicate with the powered device to negotiate its voltage or wattage requirements but merely supplies power at all times. Most passive systems use the pinout of 802.3af mode B (see§ Pinouts) – with DC positive on pins 4 and 5 and negative on 7 and 8. This was to simplify the construction of devices as it does not interfere with100 Mbit/s signaling on pins 1, 2, and 3 and 6, but with Gigabit ethernet a transformer is required anyways. As a result, Gigabit vary between using the mode B-style (4+5+, 7-8-) and mode A-style (1+2+, 3-6-).
There is no voltage negotiation in a passive system, so the PSE and the PD must agree on a voltage beforehand. Common choices include:
12 V, 18 V.[60] and 54 V are also used.[61] The maximum current is generally 1 A or 2 A.[60]
Passive PoE power sources include switches and injectors, just like standard (active) PoE have. The injectors can be powered from AC or DC.[60]
Passive PoE powered devices include not only natively PoE-aware devices, but also devices that are powered through splitters. For example, one splitter converts a wide range of passive PoE voltage to 5 5 volts.[62][unreliable source?]
TheISO/IEC TR 29125 andCenelec EN 50174-99-1 draft standards outline the cable bundle temperature rise that can be expected from the use of 4PPoE. A distinction is made between two scenarios:
The second scenario largely depends on the environment and installation, whereas the first is solely influenced by the cable construction. In a standard unshielded cable, the PoE-related temperature rise increases by a factor of 5. In a shielded cable, this value drops to between 2.5 and 3, depending on the design.
| Pins at switch | T568A color | T568B color | 10/100 mode B, DC on spares | 10/100 mode A, mixed DC & data | 1000 (1 Gbit/s) mode B, DC & bi-data | 1000 (1 Gbit/s) mode A, DC & bi-data | 1000 (1 Gbit/s) mode A+B (4PPoE), DC & bi-data[note 1] | |||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Pin 1 | White/green stripe | White/orange stripe | Rx + | Rx + | DC + | TxRx A + | TxRx A + | DC + | TxRx A + | DC + | ||
| Pin 2 | Green solid | Orange solid | Rx − | Rx − | DC + | TxRx A − | TxRx A − | DC + | TxRx A − | DC + | ||
| Pin 3 | White/orange stripe | White/green stripe | Tx + | Tx + | DC − | TxRx B + | TxRx B + | DC − | TxRx B + | DC − | ||
| Pin 4 | Blue solid | DC + | Unused | TxRx C + | DC + | TxRx C + | TxRx C + | DC + | ||||
| Pin 5 | White/blue stripe | DC + | TxRx C − | DC + | TxRx C − | TxRx C − | DC + | |||||
| Pin 6 | Orange solid | Green solid | Tx − | Tx − | DC − | TxRx B − | TxRx B − | DC − | TxRx B − | DC − | ||
| Pin 7 | White/brown stripe | DC − | Unused | TxRx D + | DC − | TxRx D + | TxRx D + | DC − | ||||
| Pin 8 | Brown solid | DC − | TxRx D − | DC − | TxRx D − | TxRx D − | DC − | |||||
| Notes: | ||||||||||||
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