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 twisted-pair 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 sourcing 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 25.5 watts is available for a device.
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 dissipates 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.
Both of these amendments have since been incorporated into the IEEE 802.3-2012 publication.
Comparison with other integrated data and power standards
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.
Universal Serial Bus (USB) and IEEE 1394 (FireWire) both provide data and power over limited distances. USB and FireWire are a good choices for connecting peripherals to a PC.
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:
- Inexpensive cabling carries both data and power
- Power to equipment can be remotely cycled
- Fast data rate
An IP camera powered by Power over Ethernet.
Avaya 1140E IP-Phone with PoE support
Some types of devices powered by PoE include:
- VoIP phones
- IP cameras including pan–tilt–zoom cameras
- Wireless access points
- IPTV decoders
- Network routers
- A mini network switch installed in distant rooms, to support a small cluster of ports from one uplink cable. (Mini-switches do not usually provide PoE on their output ports.) (Many modern VoIP phones include a two-port mini-switch that can provide a network connection for a nearby computer.)
- Network Intercom / Paging / Public address systems and hallway speaker amplifiers
- Wall clocks in rooms and hallways, with time set using Network Time Protocol
- Outdoor roof mounted radios with integrated antennas, 802.11 or 802.16 based wireless CPEs (customer premises equipment) used by wireless ISPs.
- Industrial devices (sensors, controllers, meters etc.)
- Access control and Help-points (intercoms, entry cards, keyless entry, etc.)
- Lighting controllers and LED Lighting fixtures
- Stage and Theatrical devices, such as networked audio breakout and routing boxes
- Remote Point of Sale (POS) kiosks
- Physical Security devices and controllers
- Inline Ethernet extenders
Power sourcing equipment
Power sourcing equipment (PSE) is a device such as a switch that provides (or 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.
A powered device (PD) is a device powered by a PSE and thus consumes energy. Examples include wireless access points, IP Phones, and IP Cameras.
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.
Power management features and integration
Avaya ERS-5520 switch with 48 Power over Ethernet ports
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.
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. 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 issue of heat generation typically generated remotely at the end devices is now transferred into the DataRoom Switch with increased heat dissipation within the datacenter room altering the BTU cooling requirement specifications as well as the power consumption.
Switch power features
The switches themselves often contain "active", "smart", or "managed" power management features to reduce AC draw of all devices involved.
Multi-protocol teaming standards (G.9960, G.hn, and IEEE P1905) and handoff standards (IEEE 802.21) generally rely on simulating Ethernet features in other media.
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.
Integrating EEE and PoE
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:
"With the evolution of PoE from a fairly low power source (up to 12.95W per port) to one with devices of up to 25.5W, the direct current (DC) power losses over Ethernet cables increased exponentially. Approximately 4.5W/port of power is wasted on a CAT5, CAT5e, CAT6 or CAT6A cable...after 100m... EEE typically saves no more than 1W per link, so addressing the 4.5W per link loss from PoE transmission inefficiency would provide much more incremental savings. New energy-efficient PoE (EEPoE) technology can change increase efficiency to 94% while transmitting over the same 25ohm cable, powering IEEE 802.3at-compliant devices in synchronous 4-pairs. When utilizing synchronous 4-pairs, powered devices are fed using all the available wires. For example, on a 24-port IEEE 802.3at-2009 Type 2 system (delivering 25.5W per port), more than 50W are saved."
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.
Standard PoE parameters and comparison
- Most switched power supplies within the powered device will lose another 10 to 25% of the available power.
- More stringent cable specification allows assumption of more current carrying capacity and lower resistance (20.0 Ohms for Category 3 versus 12.5 Ohms for Category 5).
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 (power sourcing equipment), not the PD (powered device), decides whether power mode A or B shall be used. PDs that implement only Mode A or Mode B are disallowed by the standard.
The PSE can implement mode A or B or both. A PD indicates that it is standards-compliant by placing a 25 k? 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.
Stages of powering up a PoE link
|PSE detects if the PD has the correct signature resistance of 19–26.5 k?
|PSE detects resistor indicating power range (see below)
|Signals PSE is 802.3at capable. PD presents a 0.25–4 mA load.
|PSE outputs classification voltage again to indicate 802.3at capability
|Signals PSE is 802.3at capable. PD presents a 0.25–4 mA load.
|Supply power to device
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
|Very Low power
|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.
Configuration via Ethernet layer 2 LLDP
LLDP-MED Advanced Power Management:8
|Extended power via MDI
|Extended power via MDI subtype
|PSE or PD
|Normal or Backup conservation
|0–102.3 W in 0.1 W steps
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:10
Some Cisco manufactured WLAN access points and IP phones supported a proprietary form of PoE 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.
In 2014, Cisco created another non-standard PoE implementation called Universal Power over Ethernet (UPOE). UPOE can use all 4 pairs, after negotiation, to supply up to 60 W.
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 typically use PoE at 5 V on Ethernet cable beyond short distances (about 15 feet (4.6 m)) as the voltage drop of the cable becomes too significant, 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.
Power capacity limits
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.
802.3af Standards A and B from the power sourcing equipment perspective
|Pins at switch
|10/100 mode B,
DC on spares
|10/100 mode A,
mixed DC & data
|1000 (1 gigabit) mode B,
DC & bi-data
|1000 (1 gigabit) mode A,
DC & bi-data
|TxRx A +
|TxRx A +
|TxRx A ?
|TxRx A ?
|TxRx B +
|TxRx B +
|TxRx C +
|TxRx C +
|TxRx C ?
|TxRx C ?
|TxRx B ?
|TxRx B ?
|TxRx D +
|TxRx D +
|TxRx D ?
|TxRx D ?