It's not a wireless technology, but Power over Ethernet is at the heart of most Wi-Fi installations in big offices. It's also a key element of virtually every product aimed at enterprise Wi-Fi. So if you are putting in am 802.11 wireless network across a big office, you need to know about Power over Ethernet.

What is it and why do it?
Devices plugged into a LAN usually get their power from a separate plug. This is fine for a desktop PC or a wiring closet switch - there's usually a plentiful supply of power sockets to plug those into. It's not so convenient if the device is somewhere out of the way.

This very definitely applies to Wi-Fi base stations. The best place to put them is usually on a wall or in the ceiling cavity, where there are no power sockets. If you have to have an electrician in to add power wiring outlets to the ceiling ducts, the budget for putting in wireless LANs will go through the roof (sorry).

Luckily, the Power over Ethernet standard specifies how to deliver DC power over normal Cat 5 Ethernet cable, alongside the data signal - just right for powering base stations in a ceiling.

Other candidates for Power over Ethernet include security cameras and sensors, but the main user so far is voice systems such as IP-based telephones. POE devices were originally created as parts of proprietary IP phone systems.

The technology has been standardised by the IEEE's 802.3af standard, and is now an addition to the Ethernet family of standards. It is ready for use and mainstream products are emerging.

Running POE also has major benefits in managing your access points. The power can be controlled by the SNMP management control, and the device can be powered down and re-booted remotely over the network - much more easy than taking a ladder to get to each access point, and press a re-set switch.

What does POE do?
Most POE devices currently in place are add-on boxes (so-called "mid-span" devices) which sit in the switch rack in the wiring closet. The LAN port connect to the POE box, which adds power, and a cable then takes the power-plus-Ethernet on to the final device.

Since the 802.3af standard was completed, switches with built in power have appeared (so-called "end-span" devices). Products have begun to appear very rapidly from companies including Hewlett-Packard and NetGear, and 3Com. The price is expected to tumble.

The University of New Hampshire's Interoperability Testing Laboratory is working on making sure that POE equipment works together, under the banner of the POE Consortium.

POE goes a distance of about 100m, in other words roughly the same as the Ethernet signal, so it can be used more or less wherever Ethernet is used. The power is provided at 48V, and up to around 13W.

Because existing Ethernet devices were created without any expectation that there would be DC voltages on the Ethernet cable, the standard has had to be designed to protect existing equipment. The power supply in the wiring closet sends a small voltage first, to check whether the device is able to take power from Ethernet - if it is, it will have a 25 kOhm resistor. Mid-span devices can only be used on Fast Ethernet, because they rely on the fact that 802.3af allows power to be delivered on "spare" pairs of wires in the cable.

Gigabit Ethernet has no "spare" pairs, but it is still possible to run power over it, since the DC voltage is just an offset. However, this can only be done using a purpose built power-over-Gigabit Ethernet switch. We are not aware of any such switches at the moment, but their arrival is only a matter of time, even though none of the current POE uses requires Gigabit Ethernet.

"Power over Gigabit is a high priority for us," says Brice Clark, worldwide director of strategy for HP's ProCurve networking business. "The port of the future will be powered Gigabit, with intelligence up to Layer 4."

No access point requires a Gigabit connection, so you need not worry about this for Wi-Fi installation.

What do you need?
Any self-respecting enterprise Wi-Fi access point will accept power over Ethernet. That is really point one of the definition of an enterprise Wi-Fi access point.

To power it, you must have a powered Ethernet switch (again, most purpose-built Wi-Fi switches should deliver power over Ethernet) or a mid-span power hub.

If you really want to power a non-POE device over your Ethernet, you can get a splitter which takes the DC power off the Ethernet cable, and delivers it to a power port. However, this will add to the cost of what would otherwise be a cheap access point.

Check the overall power capacity of the powered switch you have, and the requirements of your access points. Wireless access points usually have a power requirement of around 13W, so you may find that you can only use some ports on your powered switch. For example, although each of the 48 ports of an HP 2650PWR can deliver up to 15W of power, the total power available is around 300W, so you can't run 48 access points at the same time off it.

But wait - there's more!
There is a lot more to POE than simply powering access points and phones. It could radically change the way you operate your whole network.

The POE community has a jokey slogan: "Power over Ethernet is the first truly global standard for power distribution". If powered Ethernet switches are widely available, then in future, travellers could use Ethernet to charge their laptops, and save carrying power-bricks around with them.

Powering more devices in the office over Ethernet could have a downside - more heat produced in the wiring closet - but also a very positive set of benefits. Power delivered through the wiring closet can be managed effectively, devices will be inherently protected against surges they might get through direct connection to the mains. It will be possible to deliver power backed by a UPS anywhere in the building, and select the devices that require it.

Moving devices will be easier as it will not require you to move power sockets. Panels under desks will not need so many power outlets, and users will not drape power adapters around. Fewer AC sockets in an office will make it safer.

POE Consortium