Electric power lines crisscross continents and thread their way through buildings of all kinds. Using them for communications purposes has long been the logical next step—and a necessity for turning dumb grids into smart ones. But how do you give power lines, which were never designed for communications, the bandwidth to handle local area networks, factory control systems, streaming video, telephony, and other potential applications—sometimes over long distances? How do you keep power-line noise from corrupting or drowning out data signals? And how do you reconcile the requirements of smart-grid signaling with existing and future systems?
The new IEEE 1901 Broadband over Power Line (BPL) standard is a good place to start. Sponsored by the IEEE Communications Society, the standard covers communications over AC or DC power lines of any voltage, using protocols and modulation techniques also applicable to wireless communications. The standard, more than 1500 pages long, also covers network architecture, security, and provisions for the simultaneous operation of multiple communication systems on a single line.
Backward-compatible with millions of power-line communications devices already in use, IEEE 1901’s physical-layer technology provides for data delivery at rates up to 500 megabits per second (Mbps) in LAN applications under ideal conditions—which translates to about 100 Mbps in a real environment, using the normal electric wiring found in houses, apartment buildings, hotels, and businesses, says the working group’s chairman, Jean-Philippe Faure.
And because it is compatible with DC as well as AC wiring, it can be used in vehicles and to carry data to and from the passenger seats of airplanes, trains, buses, and subways. Streaming data and passenger-entertainment signals over the vehicle’s internal power grid instead of over dedicated cables saves weight.
The IEEE Standards Association considers IEEE 1901 a key enabling technology, not only for the smart grid but also for transportation, LANs, and more.
The standard might help in implementing the U.S. government’s $7.2 billion stimulus appropriation to bring broadband Internet access to underserved areas, most of which already have power lines. By carrying multimedia data over the electric wiring in multistory buildings, for example, it should offer enhanced Internet access at less cost.
FIVE CHALLENGING YEARS
The working group that developed the standard had many hurdles to overcome. Unlike other forms of wired communications, power-line communications systems work over existing networks built for other purposes, with unpredictable topologies that can change each time a user plugs in or unplugs a device or flicks a switch. Devices plugged into the lines can contribute unpredictable amounts of noise interference. So can devices like power drills, hair dryers, and vacuum cleaners operating near the lines that are unshielded. Yet communications signals carried by those unshielded lines must not interfere with wireless devices such as routers and cellphones.
Those signals also must not interfere with one another or with other data, multimedia, or services that might be carried on the line.
Further, a practical power-line communications system must be able to interact seamlessly with the ubiquitous IEEE 802-compliant wired and wireless LANs. And, because electrical systems are designed for easy plug-in, provision for robust security that prevents intrusion and assures privacy is a must.
Meeting all the requirements took five years of effort, during which nearly 100 companies, trade groups, and universities from around the world assisted in the standard’s evolution. The industries included electric utilities and companies involved with consumer electronics, computers, networking, semiconductors, aircraft, and telecommunications.
The modulation systems for encoding the data signals specified by IEEE 1901 were chosen both for their technical advantages and for their compatibility with power-line communications systems already in wide use. Both are forms of orthogonal frequency-division multiplexing: with FFT for fast Fourier transforms (which is compatible with the popular HomePlug Alliance standard) and wavelet (widely used in Japan’s HD-PLC high-speed power-line communication system).
“We have developed a mechanism to ensure there’s no collision between these modulation systems,” Faure says. “Both can coexist on the same power line without conflict. The standard is very flexible. You can implement wavelet or FFT, or implant both on the same chip so they can interchange data.”
Faure considers the IEEE 1901 standard’s mandatory provisions for interference-free coexistence essential. This applies not only to the two modulation systems specified in the standard but also to the G.hn system espoused by the International Telecommunication Union.
“We had in mind the coexistence of multiple systems on the line and how to coordinate, for example, Internet, streaming TV, and so on in a way that met the varying bandwidth needs of all these different services,” he says. “Under IEEE 1901, there can be different communications standards operating on a power line, but there is a coexistence scheme that makes everything work together.”
At the Consumer Electronics Show held in January less than 90 days after the standard’s adoption, about a dozen companies—some of them new to power-line communications—were already showing devices and systems for power-line transmission of multimedia signals and computer data, with more certain to come by next year’s show.
“There is an increasing demand for bandwidth inside the home,” says Faure, who points out that the demand has been addressed not only by IEEE 1901 but also by wireless systems such as Wi-Fi (based on the IEEE 802 family of standards) as well as Ethernet, television cable, and even telephone lines. “None of these systems are perfect,” he notes. “Each has advantages and disadvantages. So they must work together. The IEEE 1901 standard helps ensure this.”