Making Connections

“Making Connections”
Communications of the ACM, June 2023, Vol. 66 No. 6, Pages 8-10
News
By Neil Savage

“Bob Metcalfe, recipient of the 2022 ACM A.M. Turing Award for his role in the development of Ethernet, briefly considered a career in tennis.”

When he was a student at the Massachusetts Institute of Technology (MIT), Ethernet inventor Bob Metcalfe briefly considered pursuing a career in tennis. He was captain of the 1968–1969 MIT tennis team, which had a record of 15 wins and 4 losses, and he was ranked sixth in New England in doubles, even while taking classes and holding a programming job at defense contractor Raytheon. That, unfortunately, was not enough to make a go of it.

“There’s playing pros and there’s teaching pros,” Metcalfe says. “I could easily be a teaching pro, but that just seemed boring. And for being a playing pro, I wasn’t good enough.”

The tennis world’s loss was the computer world’s gain, however, as Metcalfe went on to become an Internet pioneer, develop Ethernet, and help get it named a networking standard, actions that earned him the 2022 ACM A.M. Turing Award on the 50th anniversary of the invention of the technology.

Metcalfe was working at Xerox’s Palo Alto Research Center (PARC) in 1973, having just earned his Ph.D. from Harvard University. At that time, the standard model for computer users was to have dumb terminals with no processing power at their desks, all linked to a central computer. PARC scientists decided to build a personal computer so users could run software at their desks, but needed a way to network the machines. At the same time, Xerox was building laser printers, which could handle input at a rate of 20Mb/s; the dumb terminal network operated at only 300b/s.

Metcalfe teamed up with electrical engineer David Boggs, who died last year, to come up with a way to build a fast, reliable local area network. Their only constraint was that it had to fit on a card that held 60 medium-scale integration (MSI) chips, each of which held hundreds of transistors, a much smaller number than today’s computer chips.

The pair set out to design a system that could use as few wires as possible—preferably zero. At the time, though, radios were too bulky to make wireless networking feasible, so they settled on one connection, a coaxial cable. They chose the name “Ethernet” to represent the idea that the system would be agnostic about the medium that carried the signal. In the 19th century, physicists had believed that light traveled through a medium they called “the luminiferous ether,” but in the early 20th century the Michelson-Morley experiment proved that no such thing existed. That left the term “ether” up for grabs, and Metcalfe grabbed it.

The original Ethernet worked on three basic technologies, none of which are still in use. One was the Jerrold tap, a method used by the cable television industry to tap into a coaxial cable. The second was Manchester encoding, in which each bit cell was either 1-0 or 0-1 transition. That had the advantage of embedding a clock into each data packet. Boggs and Metcalfe still needed a way to get nodes of the network to take turns retransmitting packets in the event two of them interfered with each other.

Metcalfe’s friend Steve Crocker suggested he look at the retransmission scheme developed by ALOHAnet, a radio-based network created by the University of Hawaii to connect computers on different Hawaiian islands. Metcalfe spent a month in Hawaii studying their system and determined that randomized retransmission was the right method to use when modified for cable.