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What's all this Power Over Ethernet stuff anyway?There is no doubt that Power over Ethernet is going to transform the way electrical equipment is powered. 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 Power over Ethernet 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 truly international standard for power distribution. Fig 1 shows a typical system. In the wiring cabinet, existing Ethernet switch equipment is retained and a midspan power source injects power into the twisted pair LAN cables. At the other end of the cables, the power is used to run phones, wireless access points, cameras and other appliances. An Uninterruptable Power Supply (UPS) can optionally support the installation in the case of power failures. In Fig 1, the Midspan Hub is the Power Sourcing Equipment, and the VoIP Phone, Wireless Access Points, and Network Camera are the Powered Devices. Newer Ethernet Switches include the PSE function internally, so the Midspan Hub is not required. The voltage is nominally 48V, and about 13W of power is available at the Powered Device. An isolated DC-DC converter transforms the 48V to a lower voltage more suitable for the electronics in the Powered Device, while maintaining 1500V of isolation for safety reasons. What is 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 (see Table 1). Recognised needThe IEEE began the standardisation 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 standardisation 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. The IEEE802.3af standardization process is now complete. It was formally approved by the IEEE Standards Board on 12 June 2003. Many products are available that claimed compliance with the standard before it was formally in existence. Since the draft specification was changing over the months prior to June 2003, and some last minute changes were made to the spec, it is possible that some products on the market don't actually comply with the final standard. Readers will have to form their own view on products that claimed compliance to a non-existent standard, and treat them with some caution. In general, products designed reasonably recently are likely to be reasonably compliant with the final standard. Preventing damageAn obvious requirement of the spec is to prevent damage to existing Ethernet equipment. A "discovery process", run from the Power Sourcing Equipment (PSE), examines the Ethernet cables, looking for devices that comply with the specification. It does this by applying a small current-limited voltage to the cable and checks for the presence of a 25kilohm resistor in the remote device. Only if the resistor is present is the full 48V applied, but this is still current-limited to prevent damage to cables and equipment in fault conditions. The Powered Device must continue to draw a minimum current. If it does not (for example, when the device is unplugged) then the PSE removes the power and the discovery process begins again. As an optional extension to the discovery process, a Powered Device may indicate to the Power Sourcing Equipment its maximum power requirements. The Power Sourcing Equipment may optionally provide a level of system management, using, for example, the Simple Network Management Protocol (SNMP). This allows for management of actions such as devices to be powered off at night, or remotely reset. A new web site, www.PowerOverEthernet.com, dedicated to IEEE802.3af Power over Ethernet technology and sponsored by DSP Design, provides those interested in this new technology with a forum to exchange all types of information and knowledge.
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