How data moves over wires

You've been warned since you were a child to keep fingers and toes away from light sockets and electric outlets. But despite its sharp bite, electric power can coexist quite well with data as long as they are separated into different frequencies.

Since the early 20th century, electrical power transmission has been standardized in this country at 60 cycles per second, or 60 hertz. This frequency is high enough to keep lights from flickering, but is low enough to be maintained over long distances. Power line communications systems use a much higher frequency to move data.

Interestingly, the electric power industry first standardized on 60-hertz transmission to provide a type of data service to their customers. Henry E. Warren, founder of the Warren Clock Co., in 1916 developed a master clock that power companies could use to keep their generators running at 60 cycles. This service allowed Warren to sell the first electric clocks that used this regular AC current to keep accurate time. Warren's Telechron clocks today still are keeping time on millions of mantels.

Modern power line data carrier frequencies are up to a half-million times higher than power transmission frequencies, ranging from 1 MHz to 30 MHz.

To minimize interference from electrical power spikes, data traffic can be separated into different channels, with signals routed to new channels when interference is detected. On outside power lines, some mechanism also is needed to mitigate interference with licensed radio operators. This is most commonly done by 'notching,' or lowering the power of offending frequencies.

For Internet access service provided over a power company's medium-voltage transmission lines, an Internet connection typically would be made at the power substation, the utility's last facility between the generators and the customer. For local networking over internal low-voltage wiring, Internet service is delivered to an electrical wiring interface on the customer premises.

Although data-over-power-line equipment is not standardized, a common set of elements are needed, whether for Internet access service or local networking:
  • A gateway (see diagram left) receives the data signal and converts it to a format used for power line transmission

  • A coupler, such as the Telkonet Coupler links the gateway to the electrical system

  • A converter separates the data signal and converts it to the appropriate format

  • Repeatersrepeat or amplify the data signal as needed. The higher the transmission frequency, the more rapid the attenuation or power loss and the more repeaters that are needed.

About the Author

William Jackson is freelance writer and the author of the CyberEye blog.

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