1. INTRODUCTION

A definition of LANs

Local area network (LAN) is not a precise term but one definition could be as follows.

“A local area network provides a system for inter-communication between computer terminals, PCs and related equipment operating within the same general area.”

The initial introduction of LANs was based on the sharing of information and resources within a local work group or department. While this still continues to be the main use for LANs, it is perhaps better to regard them as a tiered system of work group networks. For example, where a small department has a LAN to connect its PC, printers and fileservers, they may also wish to share information with users in another department, connected on another LAN. Therefore a backbone LAN links these, and possibly other departmental LANs together. In addition, it may still be necessary to access some larger, mini computers or central computers. This can also be achieved using the backbone LAN, allowing any user, on the backbone or any departmental LAN access if so authorised.

During the late 1980’s and early 1990’s LANS were more complex and it was not simply a matter of choosing the right cable to be able to inter-work. Equipment protocols and even applications needed to be carefully chosen to ensure that not just compatibility, but also optimum network performance, was achieved. Over the last few years for better or worse the world has gone towards the Internet Protocol (IP), making the issue of protocol selection almost redundant. The drive to IP was largely due to the fact it was supplied free with Unix, and more powerful and complex protocols, such as OSI, fell by the wayside.

A brief history of LANs

In recent years a major change has occurred in computer use. In the first half of the 1970s in the major industrialised countries there were more companies than computers. By the middle of the 1980’s there were more computers than companies. This phenomenon was due to the advent of the small Personal Computer (PC).

What the PC did was to change the perspective of the manager, from using a central computer, designed for a specific set of jobs, towards using a desktop tool assimilating information, supporting decisions and, more recently, improving the quality of personal output and hence productivity.

The advent of department computers, or distributed processing, generated new needs in computer communications. Previously these had centred on attaching terminals to a large central computer (mainframe), often over large distances. The advent of the PC began to introduce needs for sharing information and resources, such as high quality printers and shared servers, within the local area, and usually involved operation at higher speeds than had been traditional in data communications.

This need drove the development of the local area network – a term which was originally coined in America by the Xerox Corporation. In fact the word ‘Local’ is a misnomer as LANs can operate between buildings and even internationally.

Initially it was envisaged that a LAN would extend across one floor of a building or possibly throughout a building. Since that time, and in response to the meteoric growth in LAN implementation, many new capabilities have been developed. These have extended the operation of LANs to the level of one of the most sophisticated transportation methods available today.

Unfortunately, as with many developments, market growth left standards organisations lagging behind the development programmes of commercial organisations. As a consequence, in the earlier days of LANs several different types appeared which could not be linked to one another, thereby creating confusion in the marketplace.

Fortunately the standards organisations managed to recoup much of the lost ground using the Open Systems Interconnection (OSI) reference model; the majority of the necessary standards were produced and implemented by most suppliers.

Some confusion still remains, however, as there are several distinct types of standards networks and protocols. These are explained in detail in the following section.

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