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Rapid developments in computer and information technology continue to expose new requirements for switched data services, e.g. teletext, electronic funds transfer, electronic mail. The features of packet switching make it attractive for many of these new services, and many countries, including the UK, now operate or plan to introduce public packet-switched data networks. But packet switching is a relatively new art, particularly in the realm of public-switched networks, and is itself developing rapidly, driven both by the increasing demand for switched data services and by advances in semiconductor technology. It tends therefore to be the preserve of a relatively small band of specialists. This paper reviews the principles, origins and evolution of packet switching, with particular reference to public-switched networks. The picture emerges of well laid foundations and the prospect of rapid expansion in public packet-switched networks throughout the world. The development of large switches, such as those being developed at BTRL, will play an important part in this. IntroductionEarly demand for switched data services, during the 1960s and early 1970s, arose largely from the high cost of computers. To make most effective use of the expensive data processing equipment, time-sharing by remote terminals became an attractive choice for many users. The ubiquitous public switched telephone network (PSTN) was pressed into service to provide the necessary switched access. As computer technology developed, however, it became clear that the PSTN would not be adequate for many of the new applications and that dedicated switched data networks would be required.Today computer-based equipment is cheap and very widespread and data communication is motivated more by the need to share information than to share equipment, an emphasis reflected in the term 'teleformatics', now more commonly telematics. Indeed, during the last twenty years the cost of computer equipment has fallen considerably faster than transmission costs, and continues to do so. For many of the emerging applications this changing balance of costs increasingly favours packet switching. 2.Packet switching principles Store-and-forward switchingCommunication between computers or between computers and terminals usually involves the transfer of 'blocks' of data. Packet switching exploits the fact that data blocks may be transferred between terminals without setting up a continuous end-to-end connection. Instead they are transmitted on a link-by-link basis, being stored temporarily at each switch en route where they are queued for transmission on an appropriate outgoing link. Routeing decisions are based on control information contained in a 'header' prefixing each data block. The term 'packet' refers to the header plus data block.In fact the idea of store-and-forward switching is older than packet switching. It had been used in a variety of forms to provide message switched services, in which users could exchange complete messages -often very long messages -with the advan...

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