The NPL Network or NPL Data Communications Network was a local area computer network operated by a team from the National Physical Laboratory in Teddington outside London that pioneered the concept of packet switching. Following a pilot experiment during 1967, elements of the first version of the network, Mark I, became operational during 1969 then fully operational in 1970, and the Mark II version operated from 1973 until 1986. The NPL network, followed by the wide area ARPANET in the United States, were the first two computer networks that implemented packet switching, and were interconnected in the early 1970s. The NPL network was designed and directed by Donald Davies.
In 1965, Donald Davies, who was later appointed to head of the NPL Division of Computer Science, proposed a national data network based on packet switching in Proposal for the Development of a National Communications Service for On-line Data Processing. After the proposal was not taken up nationally, during 1966 he headed a team which produced a design for a local network to serve the needs of NPL and prove the feasibility of packet switching. The design was the first to describe the concept of an "Interface computer", today known as a router.
The next year (1967) a written version of the proposal entitled NPL Data Network was presented by Roger Scantlebury at a conference at Gatlinburg of the proceedings of the Association for Computing Machinery, which described how equipment (nodes) used to transmit signals (packets) would be connected by electrical links to re-transmit the signals between and to the nodes, and interface computers would be used to link node networks to so-called time-sharing computers and other users. The interface computers would transmit multiplex signals between networks, and nodes would switch transmissions while connected to electrical circuitry functioning at a rate of processing amounting to mega-bits. In Scantlebury's report following the conference, he noted "It would appear that the ideas in the NPL paper at the moment are more advanced than any proposed in the USA".
The first theoretical foundation of packet switching was the work of Paul Baran, in which data was transmitted in small chunks and routed independently by a method similar to store-and-forward techniques between intermediate networking nodes. Davies independently arrived at the same model in 1965 and named it packet switching. He chose the term "packet" after consulting with an NPL linguist because it was capable of being translated into languages other than English without compromise. Packet switching was used to produce an experimental network using a Honeywell 516 node. According to Zakon, NPL under Davies was the earliest organisation that created a packet switching network.
Following a pilot experiment during 1967, Davies gave the first public demonstration of packet switching on 5 August 1968. Elements of the first version of the network, Mark I, became operational during 1969 then fully operational in 1970, and the Mark II version operated from 1973. The NPL team also carried out simulation work on the performance of packet networks. The local area NPL network and the wide area ARPANET in the United States, were the first two computer networks that implemented packet switching.
The NPL network was later interconnected with other networks, including the ARPANET in 1973. The NPL network used a line speed of 768 kbit/s in 1967. Influenced by this, the proposed line speed for ARPANET was upgraded from 2.4 kbit/s to 50 kbit/s and a similar packet format adopted. In 1976, 12 computers and 75 terminal devices were attached, and more were added. The network remained in operation until 1986, influencing other research in the UK and Europe. Alongside Donald Davies, the NPL team included Derek Barber, Roger Scantlebury, Peter Wilkinson, Keith Bartlett, and Brian Aldous.
Then in June 1966, Davies wrote a second internal paper, "Proposal for a Digital Communication Network" In which he coined the word packet,- a small sub part of the message the user wants to send, and also introduced the concept of an "Interface computer" to sit between the user equipment and the packet network.
the ARPA network is being implemented using existing telegraphic techniques simply because the type of network we describe does not exist. It appears that the ideas in the NPL paper at this moment are more advanced than any proposed in the USA
they lacked one vital ingredient. Since none of them had heard of Paul Baran they had no serious idea of how to make the system work. And it took an English outfit to tell them.
Roger actually convinced Larry that what he was talking about was all wrong and that the way that NPL were proposing to do it was right. I've got some notes that say that first Larry was sceptical but several of the others there sided with Roger and eventually Larry was overwhelmed by the numbers.
Although he was aware of the concept of packet switching, Roberts was not sure how to implement it in a large network.