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A Technical History of National Physical Laboratories (NPL) Network Architecture - A Technical Tour


Conventional, point-to-point communications networks like telephone networks had a reliability problem since they would lose the connection if any one link failed. And because there were necessarily more links in longer distance communications, long distance communications were proportionately less reliable and more expensive.

Donald Davies' proposed network architecture solves this problem by dividing each communication into individual, digital packets, and then using routing protocols to get the packets from one place to another. Each message is divided into many individual packets of information, and then each packet is sent to its destination independently. The network consists of nodes (similar to routers today) connected by digital links. The network carries short messages which are stored at each node through which they pass (called store-and-forward or message-switching systems). Each message enters the network in a well-defined format and includes some routing data. The responsibility for putting messages into this format belongs to the network, not the user. Between the network and the users are interface computers (similar to hosts or end systems today) each handling a mixed collection of subscribers in a geographical area. In Davies' architecture, time-division multiplexing would allow multiple users to take turns transmitting portions of their messages. If a user had a short message, such as a single command, the whole message could be sent in the first packet, while longer messages would take several time slots to transmit.

Packet switching, in Davies' view, was the communications equivalent of time-sharing: it would maximize access to a scarce resource in order to provide interactive computing. Davies thought the pressing need was for a network that could serve the users of commercial time-sharing services, and also remain simple and easy to use. His network architecture served the aim of building a commercial system mainly by bringing down the cost of data communications. Ultimately, Davies thought that his architecture using packet switching technology could be a commercial product that would contribute directly to the Ministry of Technology's plan to revitalize the British economy.

High level 
network with interface computers

Figure 1 (Davies)

The network is broken into two sections: a high-level network called the subnet and a local network built around the interface computer.(Campbell Kelly p229)


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Written by the THINK Protocols team, CS Dept, UT Austin
Please direct comments to Chris Edmondson-Yurkanan.

This document was last modified on Tuesday, 11-Jun-2002 10:18:02 CDT.