ALOHAnet Introduced Random Access Protocols to the Computing World

Photo: Norm Abramson/University of Hawaii at Mnoa This user is operating the ALOHAnet hub machine, which used a random access protocol to transmit packets over a shared channel.

THE INSTITUTE Until the 1970s, far-flung computers generally connected to one another through telephone networks. In 1968 researchers at the University of Hawaii began to investigate if radio communications could be used to link multiple computers at once.

The team introduced its Additive Links On-line Hawaii Area network, ALOHAnet, in June 1971. The network used a random access protocol, which allowed computers to transmit packets over a shared channel, as soon as they had information to send. ALOHAnet was the first use of wireless communications for a data network. Its protocol is now widely used in nearly all forms of wireless communications.

We [the team] thought that what we were doing would be important, but I don't think any of us thought it would be as important as it turned out to be," IEEE Life Fellow Norman Abramson, who led the team, said in a 2009 interview about ALOHAnet in IEEE Communications Magazine. It exceeded my wildest expectations."

ALOHAnet is now an IEEE Milestone. Its nomination was sponsored by the IEEE Hawaii Section. Administered by the IEEE History Center and supported by donors, the Milestone program recognizes outstanding technical developments around the world.

The dedication ceremony, originally planned for June 2020 at the University of Hawaii at Manoa, in Honolulu, was postponed until next year due to the COVID-19 pandemic.

ESTABLISHING A CONNECTION

The University of Hawaii used ALOHAnet to connect its campuses to one another. Each campus had a small interface computer-a hub machine-that used two distinct radio frequencies: an outbound channel and an inbound channel. In order to connect, one hub machine broadcasted packets to another computer on the outbound channel, and that computer sent data packets to the first hub machine on the inbound channel.

If data was successfully received at the hub, a short acknowledgment packet was sent back. If an acknowledgment was not received by the computer, it would automatically retransmit the data packet after waiting for a randomly selected amount of time. The mechanism detected and corrected collisions that were created when the machine and the computer attempted to send a packet at the same time, according to the Engineering and Technology History Wiki entry about the Milestone.

Computer networks were not well understood at the time, and it took several years for the researchers to perfect their design.

In a sense, [the acknowledgement mechanism is] an obvious thing to do," Abramson said in the article. But when you start off on this kind of research project, some of the obvious things don't appear as obvious as they do a little later."

ALOHAnet was connected to ARPANET via satellite in December 1972 under the guidance of the U.S. Defense Advanced Research Projects Agency. The connection allowed for reliable computer communications throughout the United States, according to the Wiki entry.

ALOHAnet used a VHF transponder in 1973 to connect to an experimental NASA satellite in order to demonstrate PacNet, an international satellite data network. The demonstration connected the NASA facility in California with five universities in Australia, Japan, and the United States, the Wiki entry says.

The Milestone plaque is to be displayed at the entrance of Holmes Hall at the University of Hawaii at Manoa, which was where the technology was developed, tested, and demonstrated. The plaque reads:

In June 1971 the ALOHA packet radio data network began providing inter-island access to computing facilities at the University of Hawaii. ALOHAnet was the first to demonstrate that communication channels could be effectively and efficiently shared on a large scale using simple random access protocols. It led directly to the development of Ethernet and personal wireless communication technologies.

This article was written with assistance from the IEEE History Center, which is funded by donations to the IEEE Foundation's Realize the Full Potential of IEEE campaign.