New technology soon could enable high-speed Internet access through power lines, easily reaching rural areas and providing an alternative to existing service options.
Until recently, most devices developed have been based on a capacitor making an electrical connection between the power line and the data transmission device, but a new approach modifying the current by magnetic induction shows promise.
Inductive couplers may work better at higher frequencies, and they are more resistant to power surges. Different forms of the technology are being tested at 12 sites around the country.
"We've seen positive results so far," said Alan Shark, President of the Power Line Communications Association.
Power lines offer the least expensive method of providing high-speed Internet access to rural areas. The infrastructure is already in place, and the signal can be preserved with repeaters at intervals along the lines. By contrast, DSL only can reach 18,000 feet from the central office; other options such as cable, broadband or satellite are geographically limited or too expensive. "The service would be positioned [in the market] as a low-cost alternative," said Shark.
The concept of data transmission through power lines first emerged in the 1920s, and since the 1970s utilities have used small-scale techniques to monitor remote stations. However, only recently have transmission speeds high enough to be marketable become possible.
"Five or six years ago, some companies experimented with power line communications and were not successful," Shark said. "Through miniaturization and trial and error, engineers have developed a better understanding."
He added that the biggest remaining obstacle is getting a useful signal through a transformer, where the voltage is reduced from medium, where the signal is injected, to the 110V delivered to users. This can be overcome by routing the signal around the transformer or by broadcasting it from the medium-voltage line using existing wireless networking techniques. Since no industry standard has emerged, different companies are experimenting with different approaches.
The data is transmitted in analog form, using the same physical medium as electrical power, but oscillating at a much higher frequency.
"[The signals] share the available bandwidth, much like AM radio and FM radio use different parts of the wireless frequency spectrum so that they can operate simultaneously without interfering with each other," computer science Professor Tommy Guess said.
Because the signal is subjected to much outside interference as it travels, it is a noisy transmission medium.
"Devices such as vacuum cleaners with motors generate a lot of high-frequency static that makes its way back into the outlet," Guess said.
However, the reverse is not true. The power of the data signal is too small to interfere with appliances. There are several approaches to data transmission. Some narrowband techniques switch between two different frequencies representing 0 and 1, and more complex codes assign each symbol a sequence of 0s and 1s, spreading it over a wide frequency range to eliminate noise affecting only part of the transmission band.
Another is more adaptable: "Orthogonal frequency-division multiplexing transmits parallel streams of data on different frequency bands, with faster data rates on those bands that are less noisy," Guess explained.
Another potential problem is the radiation produced by the data transmission, which may interfere with other signals in the same airspace. Because the technology is so new, the FCC is still in the process of designing regulations.
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