User:Dkhydema/PoE

This is a draft rewrite for the PoE article. Copy made 27Jan10.



Power over Ethernet or PoE describes networking technology to safely pass electrical power, along with data, on Ethernet-type network cabling. Its goal is to power network-attached devices -- commonly IP phones, wireless access points, video surveillance cameras -- using the data connection to these devices, thus avoiding a second wired connection just to power to the same devices. PoE was developed in the early years of the 2000 decade. It was first standardized as IEEE 802.3af (ratified June 2003), and then updated in IEEE 802.3at (ratified September 2009).

Powered Device (PD)
This is the device being powered by PoE. Terminology is from the IEEE 802.3af standard. Common PoE-compatible PDs include IP Phones, wireless access points, and IP cameras. Many powered devices have another connector for an optional auxiliary power supply. If used, this gives backup power to the device if the power to the Ethernet connector is inadequate or suddenly fails.

Power Sourcing Equipment (PSE)
This is the device putting power on the Ethernet cabling, power intended for downstream Powered Devices. Terminology is from the IEEE 802.3af standard. Common PoE-compatible PSEs include switches and devices purpose-built for "injecting" power onto Ethernet cabling. There are two types of PSEs specified by IEEE 802.3-2008: 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 Mbit/s to 1 Gbit/s 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.

IEEE 802.3af Standard
The IEEE 802.3af PoE standard (ratified June, 2003) provides up to 15.4 W of DC power (minimum 44 V DC and 350 mA ) to Powered Devices. Only 12.95 W is assured to be available at the powered device as some power is dissipated in the cable.

Power over Ethernet is usually implemented following the specifications in '''IEEE std. 802.3af-2003''' which added clause 33 to the IEEE 802.3 standard. It allows the powering device to use a voltage between 44–57 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 10–350 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 to allow the powered pairs to 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.

IEEE 802.3at Standard
The IEEE 802.3at PoE standard, sometimes called "POE+", (ratified September 11, 2009), provides up to 25 W of power. Some vendors have announced products that claim to comply with the new 802.3at standard and offer up to 51 W of power over a single cable by utilizing all 4 pairs in the Cat.5 cable. The newly released IEEE std. IEEE 802.3at-2009 amendment enhanced Power over Ethernet Category 5 cable to dynamically provide between 0.1–25 W of power.

Cabling
IEEE 802.3af Power over Ethernet specifies Ethernet category 5 cable or higher. Ethernet cabling has multiple wires. PoE standards using 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 (only half of which are used for power) and therefore the absolute maximum power transmitted using direct current is 50 V × 0.360 A × 2 = 36 W. Considering the voltage drop after 100 m, a PD would be able to receive 31.6 W. The additional heat generated in the wires by PoE at this current level (4.4 watts per 100 meter cable) limits the total number of cables in a bundle to be 100? at 45 °C, according to the TIA.

Power Characteristics
This varies by implementation / standard.

The IEEE 802.3af standard specifies a maximum power continuous output power per PSE is 15.40 W, and power usage maximum allowed by PD is 12.95 W.

How Power Is Delivered
Two modes, A and B, are available as defined by IEEE 802.3af.

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 PSE decides whether power mode A or B shall be used, not the powered device (PD). PDs that implement only Mode A or Mode B are specifically not allowed by the standard.

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 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 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.

Stages of powering up a PoE link
IEEE 802.3at capable devices are also referred to as "type 2". An 802.3at PSE may also use layer2 communication to signal 802.3at capability.

Power levels available
PSEs classify as Class 0

For IEEE 802.3at (type 2) devices class 4 instead of Reserved has a power range of 12.95 - 25.5 W.

Configuration via Ethernet layer 2 LLDP
The setup phases are as follows:
 * PSE (provider) tests PD (consumer) physically using 802.3af phase class 3.
 * PSE powers up PD.
 * PD sends to PSE: I'm a PD, max power = X, max power requested = X.
 * PSE sends to PD: I'm a PSE, max power allowed = X.
 * PD may now use the amount of power as specified by the PSE.

The rules for this power negotiation are:
 * PD shall never request more power than physical 802.3af class
 * PD shall never draw more than max power advertised by PSE
 * PSE may deny any PD drawing more power than max allowed by PSE
 * PSE shall not reduce power allocated to PD, that is in use
 * PSE may request reduced power, via conservation mode

Non-Standard / Pre-Standard Implementations
Numerous non-standard schemes of Power over Ethernet have been marketed. Some are still in active use. There have been pre-standard versions of both IEEE 802.3af and, more recently, of IEEE 802.3at.

Cisco
Cisco's original PoE equipment was manufactured many years before there was an IEEE standard for delivering PoE. Cisco's original PoE equipment was capable of delivering up to 10 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 proprietary Cisco Discovery Protocol (CDP). CDP is also responsible for dynamically communicating the Voice VLAN value from the Cisco switch to the Cisco IP Phone.

The PSE (switch) will send a Fast Link Pulse (FLP) on the transmit pair. The PD (device) connects the transmit line to the receive line via an low pass filter. And thus the PSE gets the FLP in return. And a common mode current between pair 1 and 2 will be provided resulting in 48 V DC and 6.3 W default of allocated power. The PD has then to provide Ethernet link within 5 seconds to the auto-negotiation mode switch port. A later CDP message with a type-length-value tells the PSE it's final power requirement. A discontinued link pulses shuts down power.

Cisco manufactured 13 different devices, like, WLAN access points and IP phones that were not compliant with the IEEE 802.3-2005 Clause 33.

Cisco pre-standard IP phones
7985G, 7960G, 7940G, 7910G, 7910G + SW, 7912G, 7905G, 7902G, 7970G

Cisco IEEE 802.3af IP phones
7961G, 7961G-GE, 7971G-GE, 7931G, 7937G, 7941G, 7941G-GE, 7945G, 7965G, 7975G, 7985G

Cisco IEEE 802.3af and pre-standard IP phones
7970G, 7961G, 7906G, 7941G, 7911G, 7962G

The Cisco 7936 Conference Phone does not support any LAN based power and requires a Cisco power injection adapter. Cisco's original PoE implementation is not software upgradeable to the IEEE 802.3af standard.

3Com
Sold a midspan solution called "Ethernet Power Source" in 2000 - 2004 used with then current 3Com NBX phones, Access points, and Network jack switches. It measured a capacitance signature, and then provided -24 V DC.

Benefits of Power over Ethernet
This technology is especially useful for powering IP telephones, wireless LAN access points, cameras with pan tilt and zoom (PTZ), remote Ethernet switches, embedded computers, thin clients and LCDs. It has been proposed as a long term replacement for the MIDI cabling standard for music devices.

All these require more power than USB offers and very often must be powered over longer runs of cable than USB permits. In addition, PoE uses only one type of connector, an 8P8C (RJ45), whereas there are four different types of USB connectors.

PoE is presently deployed in applications where USB is unsuitable and where AC power 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. However, even where USB or AC power could be used, PoE has several advantages over either, including the following:
 * Cheaper cabling — even category 5 cable is cheaper than USB repeaters, and the task of meeting building code requirements to run AC power cable is eliminated.
 * A Gigabit of data per second to every device is possible, which exceeds 2009 USB and the AC powerline networking capabilities.
 * Global organizations can deploy PoE everywhere without concern for any local variance in AC power standards, outlets, plugs, or reliability.
 * Direct injection from standard 48 V DC battery power arrays; this enables critical infrastructure to run more easily in outages, and make power rationing decisions centrally for all the PoE devices.
 * Symmetric distribution is possible. Unlike USB and AC outlets, power can be supplied at either end of the cable or outlet. This means the location of the power source can be determined after cables and outlets are installed.

Power management feature and integration
Most advocates expect PoE to become a global longterm DC power cabling standard and replace "wall wart" converters, which cannot be easily centrally managed, waste energy, are often poorly designed, and are easily vulnerable to damage from surges and brownouts. A combination of G.9960 networking on existing AC power lines to an outlet where a PoE router is plugged in is capable of moving a gigabit per second to every device, with minimal wiring and participating fully in both AC and DC device power demand management.

Integration with the IEEE 802.3az standard, the energy management capabilities of the combined standard are expected to be formidable. However, that integration has not yet occurred.

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



Drawbacks of IEEE 802.3af are:
 * Excessive voltage with a peak at 60 V (many standard 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) (The diode bridge will waste 0.74 W at 25.5 W operation)

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.

The 0.74W waste in the diode bridge, above, assumes the use of standard rectifier diodes. If Schottky diodes are used, the waste will be half that much. In either case, the waste is much less than the losses in the DC-DC converter that must be used to convert the power to the low voltages used in the PD logic circuits.

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.