22Jun ARPANET

The ARPANET (Advanced Research Projects Agency Network) created by ARPA of the United States Department of Defense during the Cold War, was the world’s first operational packet switching network, and the predecessor of the global Internet. Packet switching, now the dominant basis for both data and voice communication worldwide, was a new and important concept in data communications. Previously, data communication was based on the idea of circuit switching, as in the old typical telephone circuit, where a dedicated circuit is tied up for the duration of the call and communication is only possible with the single party on the other end of the circuit. With packet switching, a system could use one communication link to communicate with more than one machine by disassembling data into datagrams, then gather these as packets. Not only could the link be shared (much as a single post box can be used to post letters to different destinations), but each packet could be routed independently of other packets. A form of packet switching designed by Lincoln Laboratory scientist Lawrence Roberts underlay the design of ARPANET.

By mid-1968, a complete plan had been prepared, and after approval at ARPA, a Request For Quotation (RFQ) was sent to 140 potential bidders. Most regarded the proposal as outlandish, and only 12 companies submitted bids, of which only four were regarded as in the top rank. By the end of the year, the field had been narrowed to two, and after negotiations, a final choice was made, and the contract was awarded to BBN Technologies on 7 April 1969.

BBN’s proposal followed Taylor’s plan closely; it called for the network to be composed of small computers known as Interface Message Processors, what are now called routers. The IMPs at each site performed store-and-forward packet switching functions, and were connected to each other using modems connected to leased lines (initially running at 50 kbit/second). Host computers connected to the IMPs via custom serial interfaces to connect to ARPANET.

BBN initially chose a ruggedized version of Honeywell’s DDP-516 computer to build the first-generation IMP. The 516 was originally configured with 24 kB of core memory (expandable) and a 16 channel Direct Multiplex Control (DMC) direct memory access control unit. Custom interfaces were used to connect, via the DMC, to each of the hosts and modems. In addition to the lamps on the front panel of the 516 there was also a special set of 24 indicator lights to show the status of the IMP communication channels. Each IMP could support up to four local hosts and could communicate with up to six remote IMPs over leased lines. The BBN team of initially only seven people were considerably helped by the detail into which they had gone to produce their response to the RFQ and quickly produced the first working units. The entire system, including both hardware and the world’s first packet switching software, was designed and installed in nine months.