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Power over Ethernet or PoE describes any of several standardized or ad-hoc systems which pass electrical power along with data on Ethernet cabling. This allows a single cable to provide both data connection and electrical power to devices such as wireless access points or IP cameras. Unlike standards such as Universal Serial Bus which also power devices over the data cables, PoE allows long cable lengths. Power may be carried on the same conductors as the data, or it may be carried on dedicated conductors in the same cable.
There are several common techniques for transmitting power over Ethernet cabling. Two of them have been standardized by IEEE 802.3. Since only two of the four pairs are needed for 10BASE-T or 100BASE-TX, power may be transmitted on the unused conductors of a cable. In the IEEE standards, this is referred to as Alternative B. Power may also be transmitted on the data conductors by applying a common-mode voltage to each pair. Because Ethernet uses differential signalling, this does not interfere with data transmission. The common mode voltage is easily extracted using the center tap of the standard Ethernet pulse transformer. This is similar to the phantom power technique commonly used for powering audio microphones. In the IEEE standards, this is referred to as Alternative A.
In addition to standardizing existing practice for spare-pair and common-mode data pair power transmission, the IEEE PoE standards provide for signalling between the power source equipment (PSE) and powered 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. Up to a theoretical 51 watts is available for a device, depending on the version of the standard in use and the vendor of the hardware.
The IEEE standard for PoE requires category 5 cable or higher for high power levels, but can operate with category 3 cable if less power is required. Power is supplied in common mode over two or more of the differential pairs of wires found in the Ethernet cables and comes from a power supply within a PoE-enabled networking device such as an Ethernet switch or can be injected into a cable run with a midspan power supply.
The original IEEE 802.3af-2003 PoE standard provides up to 15.4 W of DC power (minimum 44 V DC and 350 mA) to each device. Only 12.95 W is assured to be available at the powered device as some power is dissipated in the cable.
The updated IEEE 802.3at-2009 PoE standard also known as PoE+ or PoE plus, provides up to 25.5 W of power. The 2009 standard prohibits a powered device from using all four pairs for power. Some vendors have announced products that claim to be compatible with the 802.3at standard and offer up to 51 W of power over a single cable by utilizing all four pairs in the Category 5 cable.
PoE provides both data and power connections in one cable, so equipment doesn't require a separate cable for each need. For equipment that does not already have a power or data connection, PoE can be attractive when the power demand is modest. For example, PoE is useful for IP telephones, wireless access points, cameras with pan tilt and zoom (PTZ), and remote Ethernet switches. PoE can provide long cable runs e.g. 100 m (330 ft) and deliver 12 W of galvanically isolated power. PoE-plus provides even more power.
There are competing data and power technologies. The Universal Serial Bus (USB) provides both data and power, but it is designed for short cables with a maximum length of 5 m (16 ft) and provides less than 2.5 W of non-isolated power. It is less expensive than PoE, and works well for low power peripherals such as a computer mouse, a headset/microphone or a serial port. Some peripherals, such as speakers, scanners and printers, need more power than USB can provide. IEEE 1394 (FireWire) is similar to USB but can provide substantially more power (45 W) at a distance of 4.5 m. On the other hand, USB peripherals can operate using very little power, but maintaining an Ethernet connection uses a significant amount of power. Thunderbolt specifies up to 10W per port.
If a device already has power available but no data link, then PoE may not be attractive. A wireless data connection such as IEEE 802.11 may be more economical than running a data cable for the device. Alternatively, there are power line communication technologies that can use power cables for transmitting data. Using some power line modems may be more economical than running a cable.
When data rate and power requirements are both low, other approaches may be viable. Mobile phones, for example, use batteries for power and antennas for communication. Remote weather sensors use very low data rates, so batteries (sometimes supplemented with solar power) and custom wireless data links are used.
Depending on the application, some of the advantages with PoE over other technologies may be:
Some types of devices powered by PoE include:
Power sourcing equipment (PSE) is a device such as a switch that provides ("sources") power on the Ethernet cable. The maximum allowed continuous output power per cable in IEEE 802.3af is 15.40 W. A later specification, IEEE 802.3at, offers 25.50 W.
When the device is a switch, it is commonly called an endspan (although IEEE 802.3af refers to it as endpoint). Otherwise, if it's an intermediary device between a non PoE capable switch and a PoE device, it's called a midspan. An external PoE injector is a midspan device
Many powered devices have an auxiliary power connector for an optional, external, power supply. Depending on the PD design, some, none, or all power can be supplied from the auxiliary port, with the auxiliary port sometimes acting as backup power in case of PoE supplied power failure.
Most advocates[who?] 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.
Critics of this approach argue that DC power is inherently less efficient than AC power due to the lower voltage, and this is made worse by the thin conductors of Ethernet. A typical 48-port Ethernet switch has a 50 W to 80 W power supply allocated for the traditional Ethernet switch and transceiver IC. Over and above this it requires typically a 740 W (for 802.3af) to 1480 W (for 802.3at) power supply allocated solely for PoE ports, permitting a maximum draw on each. This can be quite inefficient to supply through long cables. However, where this central supply replaces several dedicated AC circuits, transformers and inverters, and prevents expensive human interventions (AC installations) the power loss of long thin DC cable is easily justifiable. Power can always be introduced on the device end of the Ethernet cable (radically improving efficiency) where AC power is available.
The switches themselves often contain "active", "smart", or "managed" power management features to reduce AC draw of all devices involved.
By late 2011, some of the energy management features are proprietary. Advertising for power-over-Ethernet devices usually cites its "green" features including less packaging and improvements over previous models.
After integration with the IEEE 802.3az Energy-Efficient Ethernet (EEE) standard, the energy management capabilities of the combined standard are expected to be good. Pre-standard integrations of EEE and PoE (such as Marvell's EEPoE outlined in a May 2011 white paper) claim to achieve a savings upwards of 3 watts per link, extremely significant across the tens of millions of new links shipped each year. These losses are especially significant as higher power devices come online. Marvell claims that:
Standards-based power over Ethernet is implemented following the specifications in IEEE 802.3af-2003 (which was later incorporated as clause 33 into IEEE 802.3-2005) or the 2009 update, IEEE 802.3at. 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 crossover 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 diode bridge.
|Property||802.3af (802.3at Type 1)||802.3at Type 2|
|Power available at PD[note 1]||12.95 W||25.50 W|
|Maximum power delivered by PSE||15.40 W||34.20 W|
|Voltage range (at PSE)||44.0–57.0 V||50.0–57.0 V|
|Voltage range (at PD)||37.0–57.0 V||42.5–57.0 V|
|Maximum current||350 mA||600 mA per mode|
|Maximum cable resistance||20 Ω (Category 3)||12.5 Ω (Category 5)|
|Power management||Three power class levels negotiated at initial connection||Four power class levels negotiated at initial connection or 0.1 W steps negotiated continuously|
|Derating of maximum cable ambient operating temperature||None||5°C with one mode (two pairs) active|
|Supported cabling||Category 3 and Category 5||Category 5[note 2]|
|Supported modes||Mode A (endspan), Mode B (midspan)||Mode A, Mode B|
Two modes, A and B, are available. Mode A delivers power on the data pairs of 100BASE-TX or 10BASE-T. Mode B delivers power on the spare pairs. PoE can also be used on 1000BASE-T Ethernet, in which case there are no spare pairs and all power is delivered using the phantom technique.
Mode A has two alternate configurations (MDI and MDI-X), using the same pairs but with different polarities. In mode A, pins 1 and 2 (pair #2 in T568B wiring) form one side of the 48 V DC, and pins 3 and 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 PoE to accommodate for crossover cables, patch cables and auto-MDIX.
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 can implement mode A or B or both. 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 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 more than 400 ms since last use or else it will be unpowered by the PSE.
There are two types of PSEs: endspans and midspans. Endspans (commonly called PoE switches) are Ethernet switches that include the power over Ethernet transmission circuitry. 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.
|Detection||PSE detects if the PD has the correct signature resistance of 19–26.5 kΩ||2.7–10.1|
|Classification||PSE detects resistor indicating power range (see below)||14.5–20.5|
|Mark 1||Signals PSE is 802.3at capable. PD presents a 0.25–4 mA load.||—||7–10|
|Class 2||PSE outputs classification voltage again to indicate 802.3at capability||—||14.5–20.5|
|Mark 2||Signals PSE is 802.3at capable. PD presents a 0.25–4 mA load.||—||7–10|
|Startup||Startup voltage||> 42||> 42|
|Normal operation||Supply power to device||37–57||42.5–57|
|1||Optional||9–12||0.44–3.84||Very Low power|
|4||Valid for 802.3at (Type 2) devices,|
not allowed for 802.3af devices
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 considered non-compliant and, instead, will be treated as a Class 0 device.
|TLV Header||MED Header||Extended power via MDI|
|TIA OUI |
|Extended power via MDI subtype |
|Power type |
|Power source |
|Power priority |
|Power value |
|127||7||00-12-BB||4||PSE or PD||Normal or Backup conservation||Critical,|
|0–102.3 W in 0.1 W steps|
The setup phases are as follows:
The rules for this power negotiation are:
Cisco manufactured WLAN access points and IP phones 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 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.
Under Cisco's pre-standard scheme, 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 a 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 its final power requirement. A discontinued link pulses shuts down power.
PowerDsine, now a Microsemi brand, have been selling midspan power injectors since 1999 with its proprietary Power over LAN solution. Several companies such as Polycom, 3Com, Lucent and Nortel utilize PowerDsine's Power over LAN.
Most passive applications use the pinout of 802.3af mode B - with DC plus on pins 4 and 5 and DC minus on 7 and 8 (see chart below). Data is then on 1-2 and 3-6. This limits operation to 100Mbit/s. Gigabit passive injectors use a transformer on the data pins to allow power and data to share the cable and is typically compatible with 802.3af Mode A. In the common "passive" PoE system, the injector does not communicate with the powered device to negotiate its wattage requirements, but merely supplies power at all times. Passive midspan injectors up to 12 ports simplify installations. Devices needing 5 Volts cannot use PoE at 5 V on Ethernet cable beyond about 15 feet (4.6 m) due to IR loss, so a 24 V or 48 V to 5 V DC-DC converter is required at the remote end. Passive DC-to-DC injectors also exist which convert a 9 V to 36 V DC input power source to a stabilized 24 V 1 A or 48 V 0.5 A PoE feed with '+' on pins 4 & 5 and '−' on pins 7 & 8. These DC-to-DC PoE injectors are used in various telecom applications.
Category 5 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 cables at 45 °C, according to the TIA. This can be somewhat alleviated by the use of Category 6 cable which uses 23 AWG conductors.
|PINS on Switch||T568A Color||T568B Color||10/100 DC on Spares (mode B)||10/100 Mixed DC & Data (mode A)||1000 (1 Gigabit) DC & Bi-Data (mode B)||1000 (1 Gigabit) DC & Bi-Data (mode A)|
|Rx +||Rx + DC +||TxRx A +||TxRx A + DC +|
|Rx -||Rx - DC +||TxRx A -||TxRx A - DC +|
|Tx +||Tx + DC -||TxRx B +||TxRx B + DC -|
|DC +||unused||TxRx C + DC +||TxRx C +|
|DC +||unused||TxRx C - DC +||TxRx C -|
|Tx -||Tx - DC -||TxRx B -||TxRx B - DC -|
|DC -||unused||TxRx D + DC -||TxRx D +|
|DC -||unused||TxRx D - DC -||TxRx D -|