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Power over Ethernet

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Not to be confused with power line communication or Point-to-Point Protocol over Ethernet.
"PoE" redirects here. For other uses, see Poe (disambiguation).
Wireless LAN access point, powered by a PoE splitter

Power over Ethernet or PoE technology describes a system to pass electrical power safely, along with data, on Ethernet cabling. PoE requires category 5 cable or higher for high power levels, but can operate with category 3 cable for low power levels.[1] Power can come 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[2] PoE standard provides up to 15.4 W of DC power (minimum 44 V DC and 350 mA[3][4]) to each device.[5] Only 12.95 W is assured to be available at the powered device as some power is dissipated in the cable.[6]

The updated IEEE 802.3at-2009[7] PoE standard also known as PoE+ or PoE plus, provides up to 25.5 W of power.[8] Some vendors have announced products that claim to comply with the 802.3at standard and offer up to 51 W of power over a single cable by utilizing all four pairs in the Cat.5 cable.[9] Numerous non-standard schemes had been used prior to PoE standardization to provide power over Ethernet cabling. Some are still in active use.

Advantages over other integrated data and power standards

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.

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 modular connector, whereas there are numerous types of USB connectors.

PoE is presently deployed in applications where USB is unsuitable and where AC power would be inconvenient, expensive[note 1] 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.

Uses for PoE

File:1140E-7.JPG
PoE 1140E VoIP Phone

Uses for Power over Ethernet include:

Terminology

Power sourcing equipment

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's called an endspan. Otherwise, if it's an intermediary device between a non PoE capable switch and a PoE device, it's called a midspan.

Powered 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. The IEEE 802.3af standard specifies a minimum available power of 12.95 W.

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.[10]

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. 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.[11]

Standard implementation

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
Property 802.3af (802.3at Type 1) 802.3at Type 2
Power available at PD[note 2] 12.95 W 25.50 W per mode
Maximum power delivered by PSE 15.40 W 34.20 W per mode
Voltage range (at PSE) 44.0 - 57.0 V[12] 50.0 - 57.0 V[12]
Voltage range (at PD) 37.0 - 57.0 V[13] 42.5 - 57.0 V[13]
Maximum current 350 mA[14] 600 mA[14] 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, 10°C with two modes (four pairs) simultaneously active
Supported cabling Category 3 and Category 5[1] Category 5[1][note 3]
Supported modes Mode A (endspan), Mode B (midspan) Mode A, Mode B, Mode A and Mode B operating simultaneously

Powering devices

Two modes, A and B, are available. Mode A delivers phantom 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, not the powered device (PD), 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 kΩ resistor between the powered pairs. A major difference between IEEE802.3af and IEEE802.3at is that while IEEE802.3af clearly precluded collocating two PD interfaces on a single 8P8C connector, IEEE802.3at changes the definition of a PD, and therefore allows two PDs collocation, one mode A and the other mode B. 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 less than 400 ms since last use or else it will be unpowered by the PSE.[15]

There are two types of PSEs: 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.

Stages of powering up a PoE link
Stage Action Volts specified
[V]
802.3af 802.3at
Detection PSE detects if the PD has the correct signature resistance of 15 - 33 kΩ 2.7 - 10.0
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 > 37.2[16]
Normal operation Supply power to device 44 - 57 50 - 57[16]

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.[16]

Power levels available[17]
Class Usage Classification current
[mA]		 Power range
[Watt]		 Class description
0 Default 0 - 4 0.44 - 12.94 Classification unimplemented
1 Optional 9 - 12 0.44 - 3.84 Very Low power
2 Optional 17 - 20 3.84 - 6.49 Low power
3 Optional 26 - 30 6.49 - 12.95 Mid power
4 Reserved 36 - 44 12.95 - 25.50 High power

PSEs classify as Class 0[17]

For IEEE 802.3at (type 2) devices class 4 instead of Reserved has a power range of 12.95 - 25.5 W.[16]

Configuration via Ethernet layer 2 LLDP

LLDP-MED Advanced Power Management[18]
TLV Header MED Header Extended power via MDI
Type  
(7 bits) 		Length
(9 bits) 		TIA OUI  
(3 octets) 		Extended power via MDI subtype 
(1 octet) 		Power type 
(2 bits) 		Power source 
(2 bits) 		Power priority 
(4 bits) 		Power value 
(2 octets)
127 7 00-12-BB 4 PSE or PD Normal or Backup conservation Critical,
High,
Low 		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

[18]

Non-standard implementations

Cisco

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[19] and 6.3 W[20] 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.[21]

PowerDsine

PowerDsine, now a Microsemi brand, sold midspans since 1999 with its proprietary Power over LAN solution. Several companies like Level1, 3Com and Nortel followed PowerDsine's Power over LAN.

Notes

  1. ^ Mains wiring must often be done by qualified and/or licensed electricians for legal or insurance reasons.
  2. ^ Most switched power supplies within the powered device will lose another 10 to 25% of the available power.
  3. ^ 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).

Category 5 cable uses 24 AWG conductors, which can safely carry 360 mA at 50 V according to the latest TIA ruling.[citation needed] 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.

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)
  • Unexpected AC current flow due to faulty design of the PoE source, and/or power supplied to non-differential I/O signals such as RS232. The major cause of this problem is unaccounted for capacitance which can form a bridge to an AC wall source. Symptoms include electrical shock when touching the case, and failure to negotiate startup on some PoE sources, especially when non-differential I/O is connected prior to power up.

A partial solution to the input source 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 solution is to use an existing IEEE 802.3af compliant power supply chip, adapting its sample designs to your specific needs. The chip will handle negotiation, slow startup, multiple auxiliary sources, and possibly provide additional protection in the form of automatic shutdown. If possible use the fully isolated design, especially if there is exposed metal on the outer case. The drawback can be a high component count.

PoE compatible 8P8C connectors are available with internal magnetics, input diodes, minor capacitors, and LED indicators incorporated into the package. These can help reduce component count. Be careful when placing them anywhere but at the edge of a circuit board, as most are designed to support a dangling cable. If the cable has a boot protecting the end, it can press against the circuit board and produce an intermittent connection.

The 0.74 W 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.

802.3af Standards A and B
PINS on Switch     10/100 DC on Spares (mode B)         10/100 Mixed DC & Data (mode A)         1000 (1 Gigabit) DC & Bi-Data    
Pin 1 Rx + Rx +             DC + TxRx A +             DC +
Pin 2 Rx - Rx -              DC + TxRx A -              DC +
Pin 3 Tx + Tx +             DC - TxRx B +             DC -
Pin 4 DC + unused TxRx C +
Pin 5 DC + unused TxRx C -
Pin 6 Tx - Tx -             DC - TxRx B -             DC -
Pin 7 DC - unused TxRx D +
Pin 8 DC - unused TxRx D -

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.

When converting an existing ethernet design to accept PoE, verify that the input isolation transformer is rated to carry PoE currents.

See also

References

  1. ^ a b c IEEE 802.3at-2009, clause 33.1.1c
  2. ^ 802.3af-2003, June 2003
  3. ^ IEEE 802.3-2005, section 2, table 33-5, item 1
  4. ^ IEEE 802.3-2005, section 2, table 33-5, item 4
  5. ^ IEEE 802.3-2005, section 2, table 33-5, item 14
  6. ^ IEEE 802.3-2005, section 2, clause 33.3.5.2
  7. ^ 802.3at Amendment 3: Data Terminal Equipment (DTE) Power via the Media Dependent Interface (MDI) Enhancements, September 11, 2009
  8. ^ "Amendment to IEEE 802.3 Standard Enhances Power Management and Increases Available Power". IEEE. Retrieved 2010-06-24.
  9. ^ "802.3at-2009 Power over Ethernet (PoE) Plus Standard Ratified". Retrieved 2010-06-24.
  10. ^ National Semiconductor application note 1474: "The LM507X Family of PoE Devices: Frequently Asked Questions (FAQs)"
  11. ^ Powered Device Controller Supports Upcoming IEEE 802.3at PoE Standard, Akros Silicon AS1135
  12. ^ a b IEEE 802.3at-2009 Table 33-11
  13. ^ a b IEEE 802.3at-2009 Table 33-18
  14. ^ a b IEEE 802.3at-2009 Table 33-1
  15. ^ Banish Those "Wall Warts" With Power Over Ethernet
  16. ^ a b c d "LTC4278 IEEE 802.3at PD with Synchronous No-Opto Flyback Controller and 12V Aux Support" (PDF). 2010-01-11 cds.linear.com
  17. ^ a b IEEE 802.3-2005, section 2, table 33-3
  18. ^ a b "LLDP / LLDP-MED Proposal for PoE Plus (2006-09-15)" (PDF).2010-01-10
  19. ^ "Planning for Cisco IP Telephony > Network Infrastructure Analysis". 2010-01-12 ciscopress.com
  20. ^ "Power over Ethernet on the Cisco Catalyst 6500 Series Switch" (PDF). 2010-01-12 conticomp.com
  21. ^ "Understanding the Cisco IP Phone 10/100 Ethernet In-Line Power Detection Algorithm - Cisco Systems". 2010-01-12 cisco.com
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