The concrete and plaster in his Idaho, house blocks Wi-Fi signals. But computer consultant Marc Schoenberg found a way to network the six devices in his house without stringing Ethernet cables: He uses powerline adapters.

"I use them with wireless access points to fill in the various dead spots around the house," Schoenberg said. "Or when the weather is nice, I use them to provide connectivity outside so I can lie in a hammock and, ummm, work."

Indeed, the average house has about 40 power outlets, and with powerline adapters, any of them can be turned into a data port. No additional wiring is necessary.

Powerline adapters were previously limited by interference from the electrical noise generated by appliances and household gadgets using the same circuit, but the latest generation appears to have largely overcome that problem. Also, the adapters offer a theoretical speed of about 200Mbit/sec., which is enough to handle digital video signals, even when actual throughput is less than half the theoretical speed (as is common with Ethernet).

There are three competing (and largely incompatible) technologies on the market: the HomePlug AV standard from the HomePlug Powerline Alliance, the Universal Powerline Association (UPA) standard, and Panasonic's High-Definition Powerline Communications (HD-PLC) specification.

Besides somewhat comparable speed, all of them offer configurable encryption, both to prevent eavesdropping and to avoid crosstalk with other networks of powerline adapters that might reside on the same circuit. (All powerline adapters downstream from the power company's transformer can hear one another.) If they come with any software at all, it is intended primarily to set the encryption key. They support at least 16 units on a circuit, with the units automatically configuring themselves into a network.

With all three technologies, the powerline adapters are a little bigger than a mobile phone. Each has prongs for a power outlet as part of the unit or at the end of an extension line, and each also has an Ethernet port. The user plugs the unit into a power outlet and attaches one end of an Ethernet cable to the Ethernet port and the other end of that cable to a computing device.

After the user does the same with a second adapter and a second computing device (typically a router), the two devices are connected as if via an Ethernet cable. There are usually indicator lights on the adapter to show that it's functioning.

With all three technologies, individual adapters cost less than US$100. Of course, you need at least two. Beyond retail distribution, several power utilities have begun trialling or offering broadband Internet connectivity over power lines, typically using some form of the UPA or HomePlug technologies, and adapters are included with the service.

HomePlug standard

"Data over power line has been kicking around for more than 15 years, but by 2000 nearly all the interference issues had been resolved," explained Matt Theall, Intel's powerline initiative manager and president of the HomePlug Powerline Alliance. The alliance was formed in 2000, and 14Mbit/sec. HomePlug 1.0 adapters began shipping in early 2002, he said. About three years ago, an intermediate Turbo specification offering 85Mbit/sec. reached the market.

"That enabled a new class of applications, not just for data networking, but for things like audio, streaming video and voice over IP," he recalled. "Then, in November 2006, we began shipping the 200Mbit/sec. HomePlug AV standard, geared for video."

Currently, 37 vendors are shipping HomePlug adapters, Theall said, adding that product returns have been less than 1%. Sales of HomePlug units of all sorts have been almost exactly doubling from year to year, and he expects that 11 million units will have shipped during the year ending in October.

In the future, he expects to see them built into or used for televisions, DVD players, speakers, home automation gear, smoke detectors and docking stations to connect iPods to sound systems. A 1Gbit/sec. version is also under development.