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force-draft

Power over Ethernet

Power over Ethernet or PoE technology describes a system to transfer electrical power, along with data, to remote devices over standard twisted-pair cable in an Ethernet network. This technology is useful for powering IP telephones, wireless LAN access points, network cameras, remote network switches, embedded computers, and other appliances where it would be inconvenient, expensive (mains wiring must often be done by qualified and/or licensed electricians for legal or insurance reasons) or infeasible to supply power separately. The technology is somewhat comparable to POTS telephones, which also receive power and data (although analog) through the same cable. It doesn't require modification of existing Ethernet cabling infrastructure.

There are several general terms used to describe this feature. The terms Power over Ethernet (PoE), Power over LAN (PoL), and Inline Power are synonymous terms used to describe the powering of attached devices via Ethernet ports.

There are several PoE implementations, including ad-hoc techniques, but using the IEEE standard for supplying power over Ethernet is strongly recommended.

IEEE 802.3—Power over Ethernet

Currently recommended (IEEE 802.3-2005)

Clause 33 of IEEE 802.3-2005 (commonly referred to as IEEE 802.3af) is how Power over Ethernet is usually implemented. The specification allows the powering device to use a voltage between 36–57 V DC, though the nominal voltage is 48 V, over two of the four available pairs on a Cat. 3/Cat. 5e cable with a selectable current of 10–400 mA subject to a maximum load power of 15.40 W. Only about 12.95 W are available after counting cable losses, and most switched power supplies will lose another 10–25% of the available power. A "phantom power" technique is used so that the powered pairs may also carry data. This permits its use not only with 10BASE-T and 100BASE-TX, which use only two of the four pairs in the cable, but also with 1000BASE-T (Gigabit Ethernet), which uses all four pairs for data transmission. This is possible because all versions of Ethernet over twisted pair cable specify differential data transmission over each pair with transformer coupling; the DC supply and load connections can be made to the transformer center-taps at each end. Each pair thus operates in "common mode" as one side of the DC supply, so two pairs are required to complete the circuit. The polarity of the DC supply may be inverted by cross cables; the powered device must operate with either pair: spare pairs 4,5 and 7,8 or data pairs 1,2 and 3,6. Polarity is required on data pairs, and ambiguously implemented for spare pairs, with the use of a bridge rectifier.

The standard describes two types of devices: Power Sourcing Equipment (PSE) and Powered Devices (PD). Power Sourcing Equipment provides power to the Powered Devices.

Powering devices

Two modes, A and B, are available.

Mode A has two alternate configurations (MDI and MDI-X), using the same pairs but with different polarities. In mode A, pins 1-2 (pair #2 in T568B wiring) form one side of the 48 V DC, and pins 3-6 (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 for patch cables and automatic RX/TX detection.

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 "spare" pairs in 10BASE-T and 100BASE-TX. Mode B, therefore, requires a 4-pair cable.

The power sourcing equipment (PSE) decides whether power mode A or B shall be used, not the powered device (PD).

The PSE can implement mode A or B or both (but must not supply power in both modes at the same time). A PD indicates that it is standards-compliant by placing a 25 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 IEEE 802.3af. An optional "power class" feature allows the PD to indicate its power requirements by changing the sense resistance at higher voltages. To stay powered, the PD must continuously use 5–10 mA for at least 60 ms with no less than 400 ms since last use or else it will be unpowered by the PSE.

There are two types of PSEs specified by IEEE 802.3-2005: endspans and midspans. Endspans are Ethernet switches that include the Power over Ethernet transmission circuitry. Endspans are commonly called PoE switches. Midspans are power injectors that stand between a regular Ethernet switch and the powered device, injecting power without affecting the data. Endspans are normally used on new installations or when the switch has to be replaced for other reasons (such as moving from 10/100 to 1 gigabit or adding security protocols), which makes it convenient to add the PoE capability. Midspans are used when there is no desire to replace and configure a new Ethernet switch, and only PoE needs to be added to the network.

Stages of powering up a PoE link

Stage Action Volts specified
per 802.3af
Volts managed
by chipset (LM5071)
Detection Measure whether powered device has the correct signature resistance of 15–33 kΩ 2.7-10.0 1.8–10.0
Classification Measure which power level class the resistor indicates (see below) 14.5-20.5 12.5–25.0
Startup Where the powered device will startup >42 >38 (LM5072)
Normal operation Supply power to device 36-57 25.0–60.0

Power levels available

Class Usage Maximum Power Levels
at Input of Powered Device
[Watt]
0 Default 0.44 to 12.94
1 Optional 0.44 to 3.84
2 Optional 3.84 to 6.49
3 Optional 6.49 to 12.95
4 Reserved (PSEs classify as Class 0)

Under development extension (IEEE 802.3at)

A future standard, commonly referred to as PoE+, is being developed by the IEEE 802.3at task force, which officially began work in September 2005. The draft standard describes extending the IEEE Power over Ethernet by using two pairs of standard Ethernet Category 5 cable to provide up to 24 W of power. The higher power available with this future standard should make self-powered equipment with higher power requirements such as WiMAX transmitters, pan-tilt-zoom cameras, videophones and thin clients possible.

The 802.3at Task Force objectives are along the following lines:

  • 802.3at should operate on Cat.5 and higher infrastructure, unlike 802.3af, that had take into account the Cat.3 limitations.
  • 802.3at should follow the power safety rules and limitations pertinent to 802.3af
  • A 802.3at PSE must be backwards compatible with 802.3af, being able to power both 802.3af and 802.3at PDs.
  • 802.3at should provide the maximum power to PDs as allowed within practical limits, at least 24 W per 802.3at task force draft 3.0.
  • 802.3at PDs, when connected to a legacy 802.3af PSE, will provide the user an indication that a 802.3at PSE is required.
  • Research the operation of midspans for 1000BASE-T
  • Research the operation of midspans and endspans for 10GBASE-T

Non-standard implementations

Cisco

Measure returned LinkPulse, then provide 48V DC. Capable of delivering a maximum of 7.6 W. Requires special PHY. Filter will only couple LinkPulse not ordinary packets. Cisco manufactured 13 devices, WLAN access points and IP phones that were not compliant with the IEEE 802.3-2005 Clause 33.

3Com

Measure capacitance signature, then provide -24 V DC.

Homebrew

A simple method of homebrew PoE involves wiring the spare pairs 4-5 (positive) and 7-8 (negative) to an appropriate DC power source. Wire resistance and current limits must be taken into account. In many countries, voltages above 50 V are subject to special regulations; additional precautions may be required. Example: A Linksys WRT54G (12V, 1A) fed over 10 meter Cat 5 cable (AWG 24, 0.2 mm²). The cable resistance will be 0.0875 Ω/m, resulting in a voltage drop of 0.0875 ohm/m * 10m * 1A = 0.875V (U=R*I). The resulting voltage at the end of the cable 11.1V will then be adjusted inside the unit to 5V. The cable loss of 0.875 V should be acceptable. If a 12V 10% (common) supply margin is acceptable, devices needing 12V specifically may also be used.

Notes

Category 5e cable uses 24 AWG conductors, which can safely carry 360 mA at 50 V according to the latest TIA ruling. The cable has eight conductors and therefore the absolute maximum power transmitted using direct current is 50 V × 0.360 A × 4 = 72 W. Considering the voltage drop after 100 m, a PD would be able to receive 59 W. The additional heat generated in the wires by PoE at this current level limits the total number of cables in a bundle to be 100 at 45 °C, according to the TIA.

Drawbacks of IEEE 802.3af are:

* Excessive voltage with peak of 60 V (many components are limited to ~30 V).
* Undefined polarity (requires a diode bridge which causes a voltage drop and require more board space and components).
* Undefined wire pairs (multiple configurations must be handled which requires more board space and components).

A partial solution to the drawbacks of IEEE 802.3af is to assume pin 4 + 5 as positive (+) and pin 7 + 8 as negative (-). This would not be standards compliant but will make PD implementation easier and not damage anything. Any incompatibilities with IEEE 802.3af will only result in an unpowered device.
Another modification is to limit voltage from the PSE to 30 V and thus enable the use of standard components. But this may destroy the PD if it is connected to a PSE that isn't modified to keep the voltage low enough. It also limits the amount of power that can be used.

Terminology

Power Sourcing Equipment (PSE)

Power Sourcing Equipment is a device (switch or hub for instance) that will provide power in a PoE setup. Maximum allowed continuous output power per such device in IEEE 802.3af is 15.40 W.

When the device is a switch, it's called an endspan. Else, if it's an intermediary device between a non PoE capable switch and a PoE device, it's called a midspan.

Powered Device (PD)

A powered device is a device powered by a PSE and thus consumes energy. Examples include wireless access points, IP Phones, and IP cameras. The IEEE 802.3af standard specifys a maximum power usage of 12.95 W.

See also

References

External links

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