Welcome to the subject of premises data communications. In this book we will explore its many facets, expose the various problems encountered in this area of interest and illustrate the way that these problems can be solved. The approach is very "results oriented." The view taken is not that of the academic network architect who has the opportunity and freedom of designing and specifying everything in advance by "starting with a blank page." Rather, for the most part, it is from the perspective of the person who has a premises data communications capability and for some reason or other is unhappy with it. It has problems which need to be dealt with. Either something needs to be fixed, expanded or protected. This book is a "Physicianís Desk Reference" not a "Grayís Anatomy." Furthermore, we assume that our audience may or may not have a technical background in data communications. This book is written for the "hands on" user of data communication equipment, not the experienced design engineer.
It is best to begin by describing the setting for the communications problems of interest. Premises data communications deals with data communications in the office building, factory, manufacturing plant site or campus. An example of this environment, the small office building, is shown in Figure 1. The distances involved may vary from several inches to several thousands of feet. This is the world of what is commonly called short haul, limited distance or local area data communication. It is necessary to emphasize that this is not the world of long haul communication or Wide Area Networks-the world usually associated with long distance telephone dial-up circuits, leased lines, ISDN, packet switching, ATM or frame relay. Both data communication environments are important, but, the economics, technical problems and solutions differ.
Within the premises data communications setting there are data producing and using devices which need to be connected in order to support the modern automated office, factory or campus. These devices, these data sources and sinks, may be computers, computer terminals, badge readers, bar code readers, printers, plotters or other peripherals. These may even be the communication interfaces to the outside world, devices like high speed dial-up modems or packet switches.
The most commonly encountered situation in the premises data communications environment is that of the multi-user computer placed at one location e.g. the computer room, with terminals scattered throughout the rest of the building.
Figure 1: Typical Premises Data Communications Environment
In a sense from a user perspective it is a much "cozier" environment than that of the wide area network. The user can often "see," "touch" and play with all elements of the data communication system. The user has a closer connection to the data communication devices supporting the applications.
The subject of premises data communications deal with the methods and equipment needed to accomplish and support the connections of the data devices in this setting. While we said that our view is not "architectural" we must mention that there are three(3) architectural alternatives to solving this connection problem.
The first is the extreme of not really needing data communications at all due to the proliferation of self-contained work stations in the limited environment. All of the equipment needs of such a work station (both data processing and data communications) are contained within it and supplied by the same manufacturer or integrator. Here the problem of needing connection across the office building, factory or campus for the most part presents few challenges. We will touch upon these from time-to-time, but only in passing.
The second alternative is that of accomplishing the connectivity through the use of a Local Area Network (LAN). Here all data equipment resources "tap onto" the LAN. True distributed computing, sharing of all data resources, is then possible. This is the approach of choice when one has the freedom and the money to design a system starting essentially with a "blank page." We will deal with this in some limited depth towards the end of this book in Chapter 8.
The third alternative is the central focus of our attention. Here premises data communications is accomplished across limited distances using an empirical approach. There is no "grand architectural" design. Rather, the needed capabilities evolve in a haphazard manner driven by the demand of an operating business to have something "up and running." Devices are connected in the most straightforward manner as they are purchased, dealing with problems as they arise, on a case by case basis. The premises data communications capability in this environment usually grows like "Topsy" in "Uncle Tomís Cabin." It is driven by the need to service user connections of the type illustrated in Figure 2. Here we show terminals which need to be connected to a multi-user minicomputer or now a 286/386/486 PC running UNIX/XENIX with many users or some equivalent system.
The problems that are encountered come under a number of different headings: Equipment Interfaces, Wiring Alternatives, Extending Long Distance, Protection, Resource Sharing and Testing.
In the following chapters we will deal with these subjects. The most commonly encountered situations will be discussed. We will illustrate how the problems can be dealt with and when appropriate indicate which Telebyte products can be used as solutions.
Figure 2: Typical Multi-User PC Environment