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The NPL network, or NPL Data Communications Network, was a local area computer network operated by the National Physical Laboratory (NPL) in London that pioneered the concept of packet switching.
Based on designs conceived by Donald Davies in 1965, development work began in 1966. Construction began in 1968 and elements of the first version of the network, the Mark I, became operational in early 1969 then fully operational in January 1970. The Mark II version operated from 1973 until 1986. The NPL network was the first computer network to implement packet switching and the first to use high-speed links. Its original design, along with the innovations implemented in the ARPANET and the CYCLADES network, laid down the technical foundations of the modern Internet.
A written version of the proposal entitled A digital communications network for computers giving rapid response at remote terminals was presented by Roger Scantlebury at the Symposium on Operating Systems Principles in 1967. The design involved transmitting signals ( Network packet) across a network with a hierarchical structure. It was proposed that "local networks" be constructed with interface computers which had responsibility for multiplexing among a number of user systems (time-sharing computers and other users) and for communicating with "high level network". The latter would be constructed with "switching nodes" connected together with megabit rate circuits (T-carrier links, which run with a 1.544 Mbit/s line rate). 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". (2025). 9781474602778, Hachette UK. ISBN 9781474602778
Davies' original ideas influenced other research around the world. Larry Roberts incorporated these concepts into the design for the ARPANET.; The NPL network initially proposed a line speed of 768 kbit/s. Influenced by this, the planned line speed for ARPANET was upgraded from 2.4 kbit/s to 50 kbit/s and a similar packet format adopted. Louis Pouzin CYCLADES project in France was also influenced by Davies' work. These networks laid down the technical foundations of the modern Internet.
The team worked through 1967 to produce design concepts for a wide-area network and a local-area network to demonstrate the technology. Construction of the local-area network began in 1968 using a Honeywell 516 node. The NPL team liaised with Honeywell in the adaptation of the DDP516 input/output controller, and, the following year, the ARPANET chose the same computer to serve as Interface Message Processors (IMPs).
Elements of the first version of the network, Mark I NPL Network, became operational in early 1969 (before the ARPANET installed its first node). The network was fully operational in January 1970. The local-area NPL network followed by the wide-area ARPANET in the United States were the first two computer networks that implemented packet switching.; The network used high-speed links, the first computer network to do so.
The NPL network was later interconnected with other networks, including the Post Office Experimental Packet Switched Service (EPSS) and the European Informatics Network (EIN) in 1976.
In 1976, 12 computers and 75 terminal devices were attached. The following year there were roughly 30 computers, 30 peripherals and 100 VDU terminals all able to interact through the NPL Network. The network remained in operation until 1986.
The Mark II version, which operated from 1973, used such a "layered" protocol architecture.(source: Roger Scantlebury - p.201)
The NPL team also introduced the idea of protocol verification. Protocol verification was discussed in the November 1978 special edition of the Proceedings of the IEEE on packet switching.
Davies proposed an adaptive method of congestion control that he called isarithmic.
Barber was appointed director of the European COST 11 project and played a leading part in the European Informatics Network (EIN). Scantlebury led the UK technical contribution, reporting directly to Donald Davies. The EIN protocol helped to launch the INWG and X.25 protocols. INWG proposed an international end to end protocol in 1975/6, although this was not widely adopted. Barber became the chair of INWG in 1976. He proposed and implemented a mail protocol for EIN.Barber, D. L. A. and Laws, J. (February 1979). “A basic mail scheme for EIN,” International Network Working Group (INWG), Note no. 192.
NPL investigated the "basic dilemma" involved in internetworking; that is, a common host protocol would require restructuring existing networks if they were not designed to use the same protocol. NPL connected with the European Informatics Network by translating between two different host protocols while the NPL connection to the Post Office Experimental Packet Switched Service used a common host protocol in both networks. This work confirmed establishing a common host protocol would be more reliable and efficient.
Davies and Barber published Communication networks for computers in 1973 and Computer networks and their protocols in 1979.; They spoke at the Data Communications Symposium in 1975 about the "battle for access standards" between and , with Barber saying the "lack of standard access interfaces for emerging public packet-switched communication networks is creating 'some kind of monster' for users". For a long period of time, the network engineering community was polarized over the implementation of competing protocol suites, commonly known as the Protocol Wars. It was unclear which type of protocol would result in the best and most robust computer networks.
Beyond NPL, and the designs of Paul Baran at RAND, DARPA was the most important institutional force, creating the ARPANET, the first wide-area packet-switched network, to which many other network designs at the time were compared or replicated. The ARPANET's routing, flow control, software design and network control were developed independently by the IMP team working for Bolt Beranek & Newman. The CYCLADES network designed by Louis Pouzin at the INRIA in France built on the work of Donald Davies and pioneered important improvements to the ARPANET design.
Moreover, in the view of some, the research and development of internetworking, and TCP/IP in particular (which was sponsored by DARPA), marks the true beginnings of the Internet. The adoption of TCP/IP and the early governance of the Internet were also fostered by DARPA.
NPL sponsors a gallery, opened in 2009, about the "Technology of the Internet" at The National Museum of Computing at Bletchley Park.
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