IEEE802.3af POWER OVER ETHERNET:A RADICAL NEW TECHNOLOGY

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What’s all this Power Over Ethernet Stuff Anyway?
Power Over Ethernet technology allows IP telephones, wireless LAN Access Points and
other appliances to receive power as well as data over existing LAN cabling, without
needing to modify the existing Ethernet infrastructure.
It has just become an international standard, called IEEE802.3af, as an extension to the
existing Ethernet standards. The freezing of the standard will allow an explosion of
Power Over Ethernet devices and installations. Power Over Ethernet is likely to be
ubiquitous in a few years, as the cost of adding the power supplies to the Ethernet
switches is going to be small. Indeed, it offers the first truely international standard for
power distribution.


The Motivation for Power over Ethernet
Almost all appliances require both data connectivity and a power supply. In a familiar
example, telephones are powered from the telephone exchange through the same twisted
pair that carries the voice. Now we can do the same thing with Ethernet devices.


The Current State of the Standard
The IEEE began the standardization process in 1999. Early players were 3Com, Intel,
PowerDsine, Nortel, Mitel and National Semiconductor. They recognised that there was
a need to supply power over Ethernet cables, and a number of proprietary
implementations, but the lack of a standard was holding back the market.
The IEEE standardization process involved experts from many companies, and the
specification benefits from this, as the issues were examined from many different
perspectives. In addition, the draft specification was put to IEEE members for their
approval in a ballot, at which stage any other objections could be raised and reconciled.



How Power is Transferred Through the Cable
A standard CAT5 Ethernet cable has four twisted pairs, but only two of these are used
for 10BASE-T and 100BASE-T. The specification allows two options for using these
cables for power:
• The spare pairs are used. Figure 2 shows the pair on pins 4 and 5 connected
together and forming the positive supply, and the pair on pins 7 and 8 connected and
forming the negative supply. (In fact, a late change to the spec allows either polarity
to be used).