Power over Ethernet

Encyclopedia : P : PO : POW : Power over Ethernet

Power over Ethernet or PoE technology describes any system to transmit electrical power, along with data, to remote devices over standard twisted-pair cable in an Ethernet network. This technology is useful for powering IP telephones, wireless LAN access points, webcams, Ethernet hubs, computers, and other appliances where it would be inconvenient or infeasible to supply power separately. The technology is somewhat comparable to POTS telephones, which also receive power and data (although analog) through the same cable. It works with an unmodified Ethernet cabling infrastructure.

Note on Terminology: There are several general terms used to describe this feature. The terms Power over Ethernet (PoE), Power over LAN (PoL), Power on LAN (PoL), and Inline Power are synonymous terms used to describe the powering of attached devices via Ethernet ports. Additional terms, such as Powered Device (PD) and Power Source Equipment (PSE) are terms that are not synonymous with PoE, but are directly related to the feature

There are several PoE implementations, ad-hoc techniques, but the PoE supplying power according to the IEEE standards are strongly recommended.

A PoE injector

PSE puts power into the cable and a PoE power splitter PD draws power from the cable. The two devices actually deliver PoE according to the safe standard IEEE 802.3af.

Contents

0.1 PoE: Non-standard
0.2 PoE: IEEE 802.3af
0.3 PoE: Future standard IEEE 802.3at

1 Notes
2 See also
3 External links

PoE: Non-standard

Cisco

Measure returned LinkPulse, then provide 48 volt DC. Requires special PHY. Filter will only couple LinkPulse not ordinary packets.

3Com "EPS" (3C10220 & 3C10222 ?), Designed and Manufactured by PowerDsine, Inc (www.powerdsine.com)

Measure capacitance signature, then provide -24 volt DC.

Homebrew

Commonly just wires the spare pairs 4-5 (positive) and 7-8 (negative) to an appropriate DC power source. Issues like wire resistance and maximum current has to be calculated. There is usually regulations for voltages above 50V in many countries as well.

Example: Linksys WRT54G (12V, 1A) fed over 10m Cat5 cable (AWG 24, 0.2mm^2). The cable resistance will be 0.8Ω, the resulting voltage drop 0.8V. As the Linksys coverts the voltage to 5V internally this drop is not critical and the installation will work fine.

PoE: IEEE 802.3af

See main article: IEEE 802.3af

IEEE 802.3af provides 48 volts DC over two out of four available pairs on a Cat.3/Cat.5 cable with a maximum current of 400 mA for a maximum load power of 19.20 watts, although, after counting losses, only about 12.95 watts are available. Most switched power supplies will lose another 10-20% of the available energy. A "phantom" technique is used so that the powered pairs may 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 is unspecified; the powered device must operate with either polarity or pair 45+78 or 12+36 with the use of a bridge rectifier.

Alcatel

Power over Ethernet (PoE) is supported on OmniSwitch 6600 switches Series and provides inline power directly from the switch’s Ethernet ports. Standards supported: IEEE 802.3af DTE Power via MDI; IEEE 802.3u 10BASE-T/100BASE-TX; IAE RFCs 826 and 894

PoE: Future standard IEEE 802.3at

See main article: IEEE 802.3at

As of May 2005 there is discussion about increasing the amount of power available on the cable. This may be done by sending power through all four pairs of wires, which would double the amount of power. Other discussions include increasing the amount of current. The new standard has been designated IEEE 802.3at.

Notes

It should be noted that Cat.5 uses 24 AWG which should be able to handle 800 mA and 48V. The cable has 8 wires and therefore the absolute maxpower is 48 * 0.8 * 4 = 153.6W. The additional heat generated in the wires by PoE or resistance in the RJ-45 connectors will reduce the amount of net power available, particularly in crowded cable closets with a lack of proper ventilation. Due to the limited conductive area of the cable the highvoltage-SMPS approach is needed.

External links

[Net-powered computer goes on show] - BBC article on a computer powered entirely by PoE
[Poweroverethernet.com]
[Info on some proprietary PoE wiring]