Ethernet IEEE 802.3 Includes:<\/strong> Power over Ethernet, PoE, sometimes also called Power over LAN, PoL \nis defined under IEEE 802.3af and 802.3at and is a very convenient \nmethod of powering remote Ethernet linked devices using the Ethernet \nline.<\/p>\n\n\n\n Often small items like routers, hubs and other devices need power and\n many times they each require a small power supply. To avoid the use of \nthese supplies, and also provide functionality where there may not be a \nconvenient power source, PoE, Power over Ethernet provides an ideal \nsolution.<\/p>\n\n\n\n Originally the concept was defined under IEEE 802.3af, but after its \ninitial introduction the standard was refined and released with many \nenhancements as IEEE 802.3at.<\/p>\n\n\n\n With Ethernet now an established standard, one of the limitations of \nEthernet related equipment was that it required power and this was not \nalways easily available. As a result some manufacturers started to offer\n solutions whereby power could be supplied over the Ethernet cables \nthemselves. To prevent a variety of incompatible Power over Ethernet, \nPoE, solutions appearing on the market, and the resulting confusion, the\n IEEE began their standardisation process in 1999.<\/p>\n\n\n\n A variety of companies were involved in the development of the IEEE \nstandard. The result was the IEEE802.3af standard that was approved for \nrelease on 12 June 2003. Although some products were released before \nthis date and may not fully conform to the standard, most products \navailable today will conform to it, especially if they quote compliance \nwith 802.3af.<\/p>\n\n\n\n A further standard, designated IEEE 802.3at was released in 2009 and \nthis provided for several enhancements to the original IEEE 802.3af \nspecification.<\/p>\n\n\n\n The standard allows for a supply of 48 volts with a maximum current \nof 400 milliamps to be provided over two of the available four pairs \nused on Cat 3 or Cat 5 cable. While this sounds very useful with a \nmaximum available power of 19.2 watts, the losses in the system normally\n reduce this to just under 13 watts.<\/p>\n\n\n\n The standard Cat 5 cable has sets of twisted pair cable, and the IEEE\n standard allows for either to be used for 10Base-T and 100Base-T \nsystems. The standard allows for two options for Power over Ethernet: \none uses the spare twisted pairs, while the second option uses the wires\n carrying the data. Only one option may be used and not both.<\/p>\n\n\n\n When using the spare twisted pairs for the supply, the pair on pins 4\n and 5 connected together and normally used for the positive supply. The\n pair connected to pins 7 and 8 of the connector are connected for the \nnegative supply. While this is the standard polarity, the specification \nactually allows for either polarity to be used.<\/p>\n\n\n\n When the pairs used for carrying the data are employed it is it is \npossible to apply DC power to the centre tap of the isolation \ntransformer that are used to terminate the data wires without disrupting\n the data transfer. In this mode of operation the pair on pins 3 and 6 \nand the pair on pins 1 and 2 can be of either polarity.<\/p>\n\n\n\n As the supply reaching the powered device can be of either polarity a\n full wave rectifier (bridge rectifier) is used to ensure that the \ndevice consuming the power receives the correct polarity power.<\/p>\n\n\n\n Within the 802.3af standard two types of device are described:<\/p>\n\n\n\n It is useful to have the connections for the power on an Ethernet cable or conenctor for using PoE.
\nEthernet introduction<\/a> \nStandards<\/a> \nEthernet data frame structure<\/a> \n100 Base T<\/a> \nGigabit Ethernet, 1GE<\/a> \nEthernet cables<\/a> \nHow to buy Ethernet cables<\/a> \nPower over Ethernet, PoE<\/a> \n<\/p>\n\n\n\n
\n\n\n\nPoE Development<\/h2>\n\n\n\n
PoE overview<\/h2>\n\n\n\n
Power over Ethernet connections<\/h2>\n\n\n\n
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