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Jfopt Location: Jfopt > News > Fiber knowledge > Power Over Ethernet(POE) - (Part I)

Power Over Ethernet(POE) - (Part I)

Edit: Jfopt   Date: 16-04-07

●Summarize of POE
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.


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 25.5 watts is available for a device.

Directions of POE offering power

1.Power sourcing equipment(PSE)

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

2.Powered device(PD)

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.

3. offering power method

(1) PI  (Alternative A) :wire of 12、36 offering power, that is signal wire offering power

PI(Alternative B): wire of 45、78 offering power, that is free wire offering power

(2)The security directions

The Voltage of POE offering is under 57V, belong to safe voltage,fit for the demand of safety.

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