In association with heise online

05 February 2008, 17:50

Powering this LAN

Benjamin Benz

Up to now, wireless access points have been anything but wireless; they not only need to be connected by a cable to the LAN, but also need a power cable. But IEEE standard 802.3af has made the latter redundant. PoE can thus supply both data and power over one cable to PoE consumers in current installations.

Wireless base stations and surveillance cameras are often placed in inaccessible locations, making it difficult and expensive to provide proper 230 V power connections. While a power pack is not really troublesome for IP telephones, the phones need to work during power outages, and a central uninterruptible power supply (UPS) costs far less than buying one for each desk. Furthermore, administrators can save themselves a lot of walking around to turn devices on and off by supplying power over the data cable.

The main advantage of Power over Ethernet (PoE) is that it uses existing network cables, usually CAT5 twisted pair cables. The PoE standard IEEE 802.3af takes advantage of the physical limits of twisted pair cables, which have relatively high electrical resistivity because of their small diameter. This means they're only suitable as power supply sources for devices that need 15 W or less.

Old and new

The idea behind 802.3af is not completely new. Some proprietary solutions used to provide power via Ethernet. Cisco had its own system for supplying power to IP telephones, while other companies implemented various drafts of the IEEE standard. A number of manufacturers of inexpensive hardware even offered power packs that applied voltage to the 4/5 and 7/8 wires in 10BaseT and 100BaseT.

Finally, after several years of debate the Institute of Electrical and Electronics Engineers (IEEE) adopted an expansion of standard 802.3, which specifies Ethernet, in August of 2003. The standard now provides a detailed description of the conditions under which a specified amount of current can flow through the network cable both to power devices (PDs) and from power source equipment (PSE). Special attention is paid to the protection of old devices that do not comply with 802.3af so that they are not damaged by PoE current.

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