Before ARPANET, most computer systems consisted of a massive computer -- sometimes the size of an entire room -- with user terminals hardwired to it. A terminal was some form of user interface, often consisting of a keyboard or punch card reader. Multiple users could access the computer simultaneously, in a technique called timesharing. Other early networks required a direct connection between host computers, meaning that there was only one path for information to flow through. The direct connections limited the size of these computer networks, which became known as local area networks (LANs).
ARPA wanted to build a networked system that could stretch across the United States, linking governmental and scientific organizations in a way that had never been possible before. However, the first phase of ARPANET was much more modest: Four computer systems in different locations would link together using existing phone lines and four Interface Message Processors (IMPs).
ARPA chose the initial computer sites based on pre-existing research relationships with the United States government. Each site had its own team of engineers responsible for connecting the site computer to the ARPANET. The four host computers in the initial ARPANET structure included:
- UCLA's university computer, which was an SDS Sigma 7 running on the Sigma Experimental operating system
- Stanford Research Institute's SDS-90 Computer, which ran on the Genie operating system
- an IBM 360/75 running on the OS/MVT operating system at the University of California's Culler-Fried Interactive Mathematics center
- a DEC PDP-10 computer with the Tenex operating system at the University of Utah
In August 1969, the UCLA team hooked up its host computer to an IMP, a Honeywell DDP 516 computer, making it the first of the four sites to connect into ARPANET. Within a few days, the two computers could exchange information. In October, Stanford's team added the second IMP and host to the system. At 10:30 p.m. on October 29, the Stanford and UCLA computers communicated with each other over a 50 kilobit per second (kbps) phone line.
On the first attempt, the system crashed before UCLA could send a complete command to the Stanford computer. Fortunately, everything worked on the second try. The other two host computers joined the network before the end of 1969. For the first time, scientists could harness the power of multiple computers in remote locations.
In the next section, we'll look at the protocols the ARPANET team developed to make all of this possible.