All houses are already wired for electricity, why not just use those same wires for computer networking? Sounds simple, but there are major obstacles to be overcome, as W. Wayt Gibbs writes in The Network In Every Room, an article in the February issue of Scientific American. Imagine the convenience! Plug your computer in anywhere in the house, then just plug in your printer anywhere else and the two are joined by a network connection. Add your cable modem or DSL (digital subscriber line) router to the mix, and you can access the Internet from any room in the house. Transfer information between any two computers in the house, or control network enabled devices like burglar alarms or climate control devices, just by plugging them in.. In the future appliances may be network-aware and as easily accessed. All these devices need electricity to run, so why not let the power-line do double duty?

Existing home networking methods come in three basic flavors. You can connect each device to a computer, and connect the computers with ethernet cables -- this is the method used by 80 or 90% of the some 5 million American homes already networked. Or you can rely on existing telephone wiring (HomePNA), or use radio waves (Wi-Fi) to connect your computers. Ethernet is about 10 times faster than either HomePNA or Wi-Fi, but requires new wires be run for each computer or network device. HomePNA is limited by the number of phone jacks available -- and that can be a major limit in older homes. Wi-Fi tends to be expensive, and users worry about security -- although encryption is usually available, many users don't know how to work it. There are existing standards for information transmission over power lines (X-10, CEBus, LonWorks, etc.), but they are terribly slow, just 1/1,000th the speed of HomePNA or Wi-Fi and 1/10,000th the speed of ethernet. A co-operative group of some 90 electronic and computer companies formed the HomePlug Power Alliance to overcome the limitations of existing systems.

Line noise is a primary problem, but not the only one. There are also dampening fields to be overcome, changing configuration problems (each time a switch is thrown the shape of the network changes), and radio frequency interference and transmission concerns.

Line noise comes from the devices attached to the power system, all those electric motors in vacuum cleaners, blenders, etc. To control the static they create there is a dampening effect created by the division of the power grid into multiple circuits at the breaker box, but that dampens data signals as well. Newer homes have dual-phase power, so a data message would have to travel all the way out of the house and up to the transformer hanging on a nearby utility pole, then back again -- a trip that can sap the strength out of the data signal.

The configuration of the network changes constantly as different devices or extension cords are plugged in or unplugged from the system. Sudden changes in resistence, such as that caused by unused outlets, can create an echo effect as the data signal bounces off the obstacle. The long looping electrical wires of your home can act like antennas, both receiving and transmitting radio waves. If the data transmission is boosted to a high power level to overcome all the noise problems, it can generate radio waves that cause static in your stereo, radio or TV.

To engineers developing a new standard for data transmission, these were all just problems to be overcome. First, the chose to transmit the data at a very high frequency -- above four megahertz. Appliances generate less noise at those frequencies, and the signal bypasses the dampening effect of circuit breakers and distribution transformers more smoothly than lower frequencies. Next, they decided to use a wide band of the spectrum -- from 4.5 megahertz all the way up to 21 megahertz -- and they broke that band down into 84 different channels. This allows signals to be sent at lower power levels. Creating less powerful radio waves causes less interference in other devices. Eight of the 84 channels were found to overlap with frequencies used by amateur radio operators so they were not used, leaving 76 active channels for communication.

To overcome the noise problem they relied on a system developed in Europe for digital television broadcasting, called orthogonal frequency division multiplexing (OFDM). Under this system, the two devices attempting to communicate with one another first send a test signal on each of the 76 channels, then block out any channels that are too weak or noisy. These test signals are repeated every few seconds to adjust to changing line conditions..The digital data being sent is divided into packets of information of fixed length. These are padded with a guard interval that allows echos to die down between packet transmissions. Also, each packet contains extra error-correction data, so that the receiving device can be sure the data has not been corrupted, and can even reconstruct small errors and compensate for them. For security reasons, 56 bit encryption is built-in to the system -- sufficiently secure to thwart the casual hacker or prevent accidental interception by neighbors served by the same transformer.

All of this complex signal processing takes a lot of computing power, but modern microchips are efficient and compact processors. Intellon of Ocala, Florida has developed a dedicated chip to control this process that is nearly as complex as the first Pentium chip.

So does it work? According to Gibb's article it passed with flying colors when tested in 25 companies and 500 homes of various sizes and vintages around the world. Late in 2001 Linksys and Phonex Broadband became the first companies to announce retail devices using this system. Others will surely follow. For now, prices are similar to Wi-Fi, but due to the simpler circuitry required by these devices, prices are expected to fall as volume increases.