Terabits-Per-Second Data Rates Achieved at Short Range

Using the same kind of techniques that allow DSL to transmit high-speed Internet over regular phone lines, scientists have transmitted signals at 10 terabits per second or more over short distances, significantly faster than other telecommunications technologies, a new study finds.

Digital subscriber line (DSL) modems delivered the first taste of high-speed Internet access to many users. They make use of the fact that existing regular telephone lines are capable of handling a much greater bandwidth than is needed just for voice. DSL systems leverage that extra bandwidth to send multiple signals in parallel across many frequencies.

Using megahertz frequencies, current DSL technologies can achieve downstream transmission rates of up to 100 megabits per second at a range of 500 meters, and more than 1 gigabit per second at shorter distances. (DSL signal quality often decreases over distance because of the limitations of phone lines; telephone companies can boost voice signals with small amplifiers called loading coils, but these do not work for DSL signals.)

This new study began with a call "out of the blue" from John Cioffi, "the father of DSL," says study senior author Daniel Mittleman, a physicist at Brown University. Cioffi, who is now chairman and CEO of Internet connectivity firm ASSIA in Redwood City, California, wanted to see if recent advances in gigahertz transmitters might boost the data rates of DSL a thousandfold, Mittleman says.

To explore this possibility, the scientists experimented with sending a continuous 200-gigahertz signal through a setup that emulated the metal-sheathed twisted pairs of telephone cables typically used for DSL service. This consisted of two half-millimeter-wide copper wires (the most common gauge used in telephone cable) running parallel inside a wide stainless steel pipe. The metal sheath was designed to enclose the energy of the signals and eliminate any losses that might come from any bending of the wires.

When the researchers analyzed the output port, they found the energy of the signals was distributed across space in a manner that confirmed it was divided across multiple channels. They found their system could support a data rate of roughly 10 terabits per second at a distance of 3 meters, dropping to 30 gigabits per second at a range of 15 meters.

"Theoretically, this is faster than any channel that you can imagine-even fiber optics can't reach 10 terabits per second," Mittleman says.

Ultimately the channel was limited in range due to energy lost due to resistance from the metallic sheath. "If it had been possible to send signals at terabit-per-second speeds over hundreds of meters, the idea would've been worth billions, which would've been nice," Mittleman says.

Still, this work might find use in applications that require huge amounts of data to move quickly over short distances, such as between racks in a data center or for chip-to-chip connections, Mittleman says. "People have previously talked about terahertz signals in data centers, but a lot of those conversations centered around the idea of doing so wirelessly," he notes. "There may be good reasons to use waveguides rather than wireless, if the waveguides don't leak."

Future research can investigate how to extend the system to a larger range by reducing the amount of energy lost due to resistance from the metallic sheath. "One could reduce the total amount of metal in the system, but then the waveguide becomes a leaky structure," Mittleman says. "We've discussed the possibility of removing the metallic sheath in areas where the propagation is straight and only keeping the metallic sheath in areas with curves, but I don't know how effective that strategy might be. My intuition is that we might get some improvement, but probably not an order-of-magnitude difference."

Mittleman, Cioffi, and their colleagues detailed their findings online on 31 March in the journal Applied Physics Letters.