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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. ==Standard development== 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.〔IEEE 802.3at-2009, clause 33.1.1c〕 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 of DC power (minimum and 〔IEEE 802.3-2005, section 2, table 33-5, item 1〕〔IEEE 802.3-2005, section 2, table 33-5, item 4〕) to each device.〔IEEE 802.3-2005, section 2, table 33-5, item 14〕 Only is assured to be available at the powered device as some power dissipates in the cable.〔IEEE 802.3-2005, section 2, clause 33.3.5.2〕 The updated IEEE 802.3at-2009 PoE standard also known as PoE+ or PoE plus, provides up to of power.〔(【引用サイトリンク】title=Amendment to IEEE 802.3 Standard Enhances Power Management and Increases Available Power )〕 The 2009 standard prohibits a powered device from using all four pairs for power.〔Clause 33.3.1 stating, "PDs that simultaneously require power from both Mode A and Mode B are specifically not allowed by this standard."〕 Both of these amendments have since been incorporated into the IEEE 802.3-2012 publication. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Power over Ethernet」の詳細全文を読む スポンサード リンク
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