The requirement for data communication is closely linked to the invention and development of the digital computer. The first commercially viable computers emerged during the late 1950s and were relatively simple devices, accessed by a single terminal directly associated with the computer hardware. However, the data handling capacity of computers was soon realised and in the early 1960s an airline seat reservation scheme was introduced which required access to a central computer from terminals situated remotely from the computer mainframe. Thus data transmission was born. In the ensuing 30 years there has been an enormous growth in computing power and facilities and in data processing applications. The provision of data communication facilities has, therefore, likewise increased beyond the wildest imaginations of those of us involved with data transmission at its inception in the 1960s. In the earliest days of data transmission, the only readily available medium with easy access to almost any location in the world was the telephone network. Although not designed to convey digital signals, its very ubiquitousness made it a prime candidate for the transmission of data. Despite its limitations, it served well as the almost exclusive carrier of data signals for a decade or more. In order to interface with the analogue environment of the telephone network, modems were designed to convert the digital signals into signals with a spectral content more like that of telephone speech signals. The first modems appeared in public service in the UK in 1965 and offered data throughput rates of 1200 bit/s half-duplex or 200 bit/s full-duplex over the switched telephone network. Since that date more sophisticated modems have been developed using complex modulation schemes and incorporating adaptive equalisation. These have extended the data rates available over the telephone network to a rate of 9600 bit/s. This seems to be an upper limit and even this requires specially selected lines.

This chapter has reviewed the requirements for the communications subsystem in each of a set of interconnected computers that enables them to communicate in an open way to perform various distributed application functions. The philosophy behind the structure of the ISO Reference Model for open systems interconnection has been presented and a description of the functionality of the seven layers that make up the reference model described. Finally, a selection of the ISO/CCITT standards that have been defined have been identified.

In 1992, there were 960,000 channels of access to Integrated Services Digital Networks (ISDNs) in Western Europe. It has been estimated that, within ten years, 28 million channels, or half of all business exchange lines in Europe, will be connected to an ISDN. In December 1993, the European Commission produced a white paper on the European economy; the paper recommended the building of a Europe-wide ISDN initiative at a cost of ECU 15 billion, targeted at the medium and small business market, and residential users. However, the main driving force behind ISDN has been technological. ISDN is the natural progression resulting from the modernisation of the public switched telephone networks (PSTN) from an analogue network to a more flexible and 'future-proofed' digital network. Most major network providers are now looking towards ISDN as being the only way to access their networks in the future, a single interface offering the user combined voice, data, text, video and still image applications, in a simple, convenient, pre-provisioned and user controlled manner. In the United Kingdom, modernisation started with the introduction of pulse code modulation (PCM) into the main trunk network and the replacement of analogue switching with digital switching to form an integrated digital network (IDN). The final stage to reach an end-to-end digital system was the extension of the digital capability into the local loop all the way to the user. All that remained then for the launch of an ISDN was to ensure that ISDN terminals were available, viable in cost and application, and capable of interworking both nationally and internationally.

The purpose of a local area network (LAN) is to provide interconnection between a variety of computing systems within a building or small site, typically within an area of up to 5 km diameter. This interconnection helps users to communicate effectively (by sending messages or formal documents) and to share resources (by sharing printers, file storage, databases etc.). LANs are now commonplace in many organisations as cheaper, smaller and more powerful systems find their way onto an increasing number of desks. This chapter is constructed as follows. The remainder of this section presents an overall introduction to the characteristics of LANs and to the principles under which they work. The second section discusses standardisation of LANs. This is followed by a description of the three principal LAN standards: CSMA/CD, the token passing bus and the token ring. This chapter concentrates on each of the three LAN technologies in turn.

In this chapter, we shall discuss the support of inter-site corporate data communications across the wide area. An introductory section sets the scene by indicating the trends that are creating the requirements for increased performance from managed data services in wide area networks. Next, we see how the traditional vehicle for inter-site networking the leased line is failing to meet the growing needs for performance and management at affordable costs. Two services that are designed to meet the above needs are then discussed. The first frame relay provides an HDLC-based inter-LAN communications service. The second switched multimegabit data service is a high speed connectionless data service that anticipates the B-ISDN. Finally, the concluding section shows how data internetworking can be provided by both frame relay and connectionless data services in a B-ISDN/ATM environment.

The growth of cellular radio has enabled organisations to extend their everyday office based operations to the mobile environment. Inevitably, this has included data and facsimile. The problem with 'data' (excluding facsimile) is that there are too many standards to promote its widespread adoption even in the fixed networks. Although there has been some growth in electronic mail, its use is generally confined within a particular organisation. It is not surprising therefore that the use of 'data' in cellular environments is small. Other than speech, facsimile is the most commonly used method of communications worldwide. The reason for the success of facsimile is its ease of use, primarily because it is standardised as an end to end service. The cellular environment allows the use of facsimile machines at remote locations not served by telephone land lines. However, the performance of facsimile machines under certain conditions is unpredictable, both in the TACS and GSM environments, and in neither case is the attachment to a cellular telephone simple or cheap. However, the investment in facsimile is now so large that it seems unlikely that even the much needed promise of the ISDN to provide an alternative for national and international communications will make a significant penetration in the foreseeable future. Until there is an incentive to change attitudes towards data communications in the fixed network, it is unlikely that we shall see any significant growth in data communications in the cellular network. The use of data over cellular radio is likely to remain, therefore, confined to the specific needs of organisations and niche markets for a long time to come.

With the widespread adoption of personal computers, the end user has become much more aware of the need to communicate between such devices and the problems that such communications introduce. Despite this awareness the user is becoming more and more demanding in terms of movement of computer generated information. As a result any excuse as to why the movement of such information is difficult generally falls on deaf ears. It is therefore important that telecommunications experts take steps to understand fully users' total communications needs. In order to understand how a user perceives electronic communications it is necessary to appreciate the history of information processing and in turn differentiate between the various types of information. In this latter regard it is only proposed to consider two broad types of information, i.e. verbal and image, and for the purpose of this chapter image type information will be assumed to consist of all types that are not audible.