Power over Ethernet
Power over Ethernet or PoE describes any of several standardized or ad-hoc systems which pass electric power along with data on twisted pair Ethernet cabling. This allows a single cable to provide both data connection and electric power to devices such as wireless access points, IP cameras, and VoIP phones.
There are several common techniques for transmitting power over Ethernet cabling. Two of them have been standardized by IEEE 802.3 since 2003. Only two of the total four twisted pairs in an Ethernet cable are used for data in 10BASE-T or 100BASE-TX Ethernet; thus, power can be transmitted on the unused two pairs. In IEEE standards, this is referred to as Alternative B. It is a common PoE technique because it separates data and power conductors, making troubleshooting easier. 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 condenser microphones. In the IEEE standards, this is referred to as Alternative A.
In addition to standardizing existing practice for spare-pair (Alternative B) and common-mode data pair power (Alternative A) 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 25.5 W is available for a device.
- 1 Standards development
- 2 Comparison with other integrated data and power standards
- 3 Uses
- 4 Terminology
- 5 Power management features and integration
- 6 Standard implementation
- 7 Non-standard implementations
- 8 Power capacity limits
- 9 Pinouts
- 10 References
- 11 External links
The IEEE standards for PoE require category 5 cable or better for high power levels but allow using 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. A midspan power supply, also known as a PoE power injector, is an additional PoE power source that can be used in combination with a non-PoE switch.
The original IEEE 802.3af-2003 PoE standard provides up to 15.4 W of DC power (minimum 44 V DC and 350 mA) on each port. Only 12.95 W is assured to be available at the powered device as some power dissipates in the cable.
Standards body IEEE is currently looking at ways of increasing the amount of power transmitted. The upcoming IEEE 802.3bt aka 4PPoE (4 Pair Power over Ethernet) standard will introduce two new levels of power: 55 W (Level 3) and 90-100 W (Level 4). Each twisted pair can handle a current of up to one ampere. Additionally, support for 2.5GBASE-T, 5GBASE-T and 10GBASE-T is planned. This development opens the door to new applications and expands the use of applications such as operating high-performance wireless access points and surveillance cameras.
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 support long cable runs e.g. 100 m (330 ft) and deliver 12 W of galvanically isolated power. PoE-plus provides even more power.
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 or power line communication may be more economical than running a data cable for the device.
When data rate and power requirements are both low, other approaches may be viable. Some security alarm systems use battery-powered sensors that communicate wirelessly. Remote weather sensors for personal weather stations use very low data rates, so batteries (sometimes supplemented with solar power) and custom wireless data links are used.
Depending on the application, advantages with PoE over other technologies include:
- Inexpensive cabling carries both data and power
- Power to equipment can be remotely cycled
- Fast data rate
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 network 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.
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
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 can be justifiable. Power can always be introduced on the device end of the Ethernet cable (radically improving efficiency) where AC power is available.
Switch power features
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.
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 W 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 12.95 W per port) to one with devices of up to 25.5 W, the direct current (DC) power losses over Ethernet cables increased exponentially. Approximately 4.5 W/port of power is wasted on a CAT5, CAT5e, CAT6 or CAT6A cable...after 100 m... EEE typically saves no more than 1 W per link, so addressing the 4.5 W per link loss from PoE transmission inefficiency would provide much more incremental savings. New energy-efficient PoE (EEPoE) technology can increase efficiency to 94% while transmitting over the same 25 ohm 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 25.5 W per port), more than 50 W 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.
|Property||802.3af (802.3at Type 1) "PoE"||802.3at Type 2 "PoE+"|
|Power available at PD[note 1]||12.95 W||25.50 W|
|Maximum power delivered by PSE||15.40 W||30.0 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 (9 °F) 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|
- 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 Ω for Category 3 versus 12.5 Ω 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 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.
Configuration via Ethernet layer 2 LLDP
|TLV Header||MED Header||Extended power via MDI|
|Extended power via MDI subtype
|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:
- 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.
A proprietary high-power development called LTPoE++ using a single CAT-5e Ethernet cable is capable of supplying varying levels at 38.7 W, 52.7 W, 70 W, and 90 W in addition to being backwards compatible with IEEE 802.at.
PowerDsine, acquired by Microsemi in 2007, has 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.
The common 100 Mbit/s 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) and data on 1-2 and 3-6. 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 voltage or 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 (330 ft), 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 W per 100 m cable) limits the total number of cables in a bundle to be 100 cables at 45 °C (113 °F), according to the TIA. This can be somewhat alleviated by the use of Category 6 cable which uses 23 AWG conductors.
The ISO/IEC TR 29125 and Cenelec 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: 1.) bundles heating up from the inside to the outside, and 2.) bundles heating up from the outside to match the ambient temperature. The second scenario largely depends on the way that the cable bundle has been installed, whereas the first is solely influenced by the physical make-up of the cable. In a standard U/UTP 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. Put another way, the temperature increases by twice as much in a U/UTP cable bundle than in a comparable bundle of S/FTP cables. This is because the metal in the shield helps to transport heat from inside the bundle to the outside.
- IEEE 802.3at-2009, clause 33.1.1c
- 802.3af-2003, June 2003
- IEEE 802.3-2005, section 2, table 33-5, item 1
- IEEE 802.3-2005, section 2, table 33-5, item 4
- IEEE 802.3-2005, section 2, table 33-5, item 14
- IEEE 802.3-2005, section 2, clause 220.127.116.11
- 802.3at Amendment 3: Data Terminal Equipment (DTE) Power via the Media Dependent Interface (MDI) Enhancements, September 11, 2009
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- Clause 33.3.1 stating, "PDs that simultaneously require power from both Mode A and Mode B are specifically not allowed by this standard."
- IEEE 802.3-2012 Standard for Ethernet, IEEE Standards Association, December 28, 2012
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- IEEE 802.3at-2009 Table 33-18
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