How the Inventor of DSL Altered the Course of Connectivity

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When 7-year-old John Cioffi ran up to the Bell System pavilion at the 1964-1965 World’s Fair in New York City, he couldn’t wait to see the first telephone with video: the much-lauded Picturephone.

The boy had been disappointed that phone calls provided only audio. He gazed up at the Picturephone’s oval screen, with its grainy, black-and-white video images—the culmination of US $500 million in R&D by the telecommunications giant—and thought, Wow…that looks terrible!

John Cioffi




Professor of electrical engineering

Member grade

Life Fellow

Alma maters

University of Illinois Urbana-Champaign, Stanford

“That memory always stayed in the back of my mind,” Cioffi says. “As I went through my schooling and career, it seemed that the technology should be able to get there, and I was always curious about how we could make it happen.”

Nearly three decades later, at age 35, Cioffi developed the technology that would ultimately make possible video calls and much more including high-speed Internet. In 1991 he built the first asymmetric digital subscriber line (DSL) modem, which quickly replaced most dial-up connections. DSL meant a user could download data-heavy images and videos while simultaneously browsing the Internet and talking on the telephone, all from a single phone line.

DSL works by separating digital voice and data signals, then converting them into analog signals that can be sent far more quickly and easily over wires—typically the copper lines already found in landline telephones. Cioffi is known as the “father of DSL” not only because of his creation of the first such modem but also his work to commercialize and popularize the technology.

For his DSL efforts, Cioffi received a U.S. National Medal of Technology and Innovation, one of 12 bestowed in October by President Biden during a White House ceremony. The medal, the nation’s highest award for technological achievement, recognizes U.S. innovators whose “vision, intellect, creativity, and determination have strengthened the country’s economy and improved the quality of life,” according to the White House.

“I was awestruck and never imagined that they’d select me for this one, as there are so many [people] I can think of who’d be more deserving,” says Cioffi, an IEEE Life Fellow. “I came to learn that several [of my] former students—Dr. Krista Jacobsen, Professor Katie Wilson, and Dr. Pete Chow—were the nominators.”

The technology led to high-speed Internet, with data capacities and transmission rates that were unimaginable with dial-up systems. What’s more, DSL relied on the copper wires that phone companies insisted to Cioffi were passé, thereby unlocking a future forever altered by connectivity.

Fighting for copper in a fiber-obsessed world

Cioffi arrived at engineering by way of his love of mathematics. He had always been interested in pushing the boundaries of what was possible based on mathematical equations. After graduating in 1978 with a bachelor’s degree in electrical engineering from the University of Illinois Urbana-Champaign, he began working on data communications as a member of the technical staff at Bell Labs in Holmdel, N.J.

There he helped develop the first voice-band modem with echo canceling. It allowed high-speed voice data to be sent over a single telephone circuit—which permitted simultaneous transmission of both callers’ data without either disturbing the other. It was his first taste of maximizing what was possible over just one phone line.

His improvements to Bell’s modems got him noticed by top leadership. It was the early 1980s, and fiber-optic networks were seen as the future in telecommunications. The company already had digitized most of the process for connecting calls, but last-mile connectivity was still analog: that pair of copper phone lines twisted together. To digitize that last essential bit, Bell engineers were developing the Integrated Services Digital Network, a circuit-switched telephone system to send voice, video, and other data over digitized circuits.

In one meeting to discuss ISDN, Cioffi listened as senior, well-known Bell scientists and engineers talked about goals such as trying to send 150 kilobits of data per second to enable a few voice channels on a single line. He was befuddled by their approach and wondered why video wasn’t part of the conversation.

“We knew the judges wouldn’t select a little company’s technology unless it was really a slam dunk, and it was.”

He quickly did some back-of-the-envelope calculations and then interrupted the discussion. The system actually could handle 10 times as much data, he explained, so video calls were possible. His boss shot him a look to keep quiet.

Shutting down Cioffi’s suggestions became a theme at Bell, he says. The company was all in on a lower-speed ISDN, and it wasn’t interested in his ideas for the existing copper wires, which were predicted to be history soon. They said ISDN’s successor would be fiber to every home.

“The old way is dead. Everything will be fiber within a couple years,” Cioffi was told. “You need to think ‘infinite bandwidth.’ What can someone do with that?”

Cioffi says that despite the setbacks, he enjoyed his work at Bell, and the company paid the tuition for the Stanford master’s and Ph.D. degrees he pursued during paid leaves.

After he earned his doctorate in 1984, the U.S. government was in the midst of splitting up the Bell System, so he left the company to work for IBM in San Jose, Calif., as a research staff member. While there he developed technology that increased the capacity of storage disks by about 50 percent.

In 1986 Cornell approached the 30-year-old about teaching electrical engineering. Unsure if it was the right career move, Cioffi asked his Stanford advisor what to do. The advisor said Stanford itself had an opening for an EE professor—and Cioffi accepted the job at his alma mater.

Creating the first DSL modem

At Stanford, Cioffi and his graduate students worked on discrete multitone modulation, a technique for sending digital information over wires while adapting signals for efficiency. It was, he says, a necessary precursor to DSL.

Cioffi says he was energized by teaching advanced EE students and being free of the constant no’s he’d received in the corporate world. In 1987 he was given a Presidential Young Investigator Award, which provided financial support to help him advance his work: $312,000 (about $870,000 today) over five years.

By 1991, he was convinced he and his students had created the technologies needed to build a DSL modem. He took a leave of absence from Stanford to launch Amati Communications Corp. in Palo Alto, Calif.

two men standing smiling for the camera in suits against a yellow backgroundJohn Cioffi was presented with the U.S. National Medal of Technology and Innovation by President Biden during a ceremony held in October at the White House. Anna Moneymaker/Getty Images

Cioffi’s current and former students worked with him and other colleagues to build the first DSL modem: the Amati Prelude. It was revolutionary, transmitting about 6 megabits of data per second over more than 2,700 meters of telephone line: enough to support multiple live digital TV streams at the time.

Meanwhile a number of large companies were trying their own approaches to DSL, including two linked to Bell. In 1993 Bell Communications Research, known as Bellcore, sponsored a DSL competition. The Amati team entered Prelude, competing against AT&T, Broadcom, and Bellcore itself.

Amati’s modem sent data more quickly over greater distances while using much less power than the other entries. The competition, according to Cioffi, “wasn’t even close,” as Amati won the gold medal.

“We knew the judges wouldn’t select a little company’s technology unless it was really a slam dunk, and it was,” Cioffi says. “The rest is history.”

Dial-up modems indeed were history. DSL vastly reduced load times and eventually led to video calls, streaming video, and the rest of the modern Internet experience as we know it.

Meanwhile, building out fiber networks wasn’t moving nearly as quickly in the 1990s as the phone companies had predicted. (Decades later, the fiber buildout is still slow going.)

DSL powered millions of households worldwide for years—and though the technology is being phased out in favor of 5G and fiber in many areas, it remains the only source of broadband internet for Americans in rural communities and is still used in hundreds of millions of homes globally.

After the Bellcore contest win, Cioffi returned to teaching at Stanford while still participating in Amati, which went public in late 1995. In 1998 Texas Instruments bought the company for $440 million (the equivalent of about $854 million today).

With DSL technology proven, Cioffi’s interests turned to improving its performance. In 2003 he founded Adaptive Spectrum and Signal Alignment—ASSIA, a backronym for his wife and co-founder, Assia Cioffi—to achieve the goal.

The company employed about 170 people at its peak. Over the years, it evolved to largely licensing its intellectual property for Internet optimization techniques. Cioffi sold part of the business to DZS in 2021. He remains chief executive of the remaining business, which is dedicated to innovation and licensing in broadband connectivity improvement.

Cioffi continued teaching at Stanford full time until 2009, when he moved to the part-time status he maintains today.

Staying current and communicative with IEEE

Cioffi joined IEEE as a student member in 1976, and he has renewed his membership ever since.

“It’s been a good way to stay current, meet people, and get to know others with similar interests,” he says.

The organization has honored him for his work, as he received the 2010 IEEE Alexander Graham Bell Medal. He holds other top awards including the 2006 Marconi Prize and a Lifetime Achievement Award from the Broadband World Forum in 2014. He was named to the Internet Hall of Fame in 2014 and the Consumer Technology Association Hall of Fame in 2018.

Cioffi is still interested in teaching the next generation of communication engineers, he says. In his part-time work at Stanford he updates and teaches digital communications coursework for graduate students.

“I tell them that digital communications goes back to smoke signals, and even earlier than that,” he says. “If you look at the opening of Genesis in the Bible, it starts with this darkness and what God sees is not good. Then, God says, ‘Let there be light.’ And he sees that it is good. What’s light? It’s an electromagnetic wave that is the fundamental component of energy and communication.

“I also tell students, ‘You’re the custodians of God’s great gift to creation, and that’s why it’s immensely satisfying to work in communications.’”

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