2.3 Horizontal communication

In the OSI reference model there is a clear separation of services and protocols, but this separation is not always evident in practical applications, so it is worthwhile spending some more time on the differences between them. A service is provided by one layer to the layer above, and the capabilities of a service are defined in terms of primitives and their parameters. A service relates to two adjacent layers in the same system. In contrast, a protocol defines the communication between two
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1.1 Abbreviations used within this unit

While reading this subject you may come across the following abbreviations.

AAL – ATM adaptation layer

API – application program interface

ARP – address resolution protocol

ATM – asynchronous transfer mode

B-ISDN – broadband ISDN

CLP – cell loss priority

DNS – domain name system

FTP – file transfer protocol

HTTP – hypertext transfer protocol


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8 Conclusions

In this course you have learned about the difference between the analogue world we inhabit and the digital world of the computer.

I've described how features of our world can ‘cross the boundary’ and be represented or modelled in the digital world, and then brought back across the boundary to us.

More excitingly, computer programs that manipulate digital representations of our world enable us to:

  • simulate physical and social processes;<
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5.5.5 Summary

In this section I've briefly considered the very contentious question of what digital representations mean, but this debate must be left to another course. I have also described some of the devices that take digital information back into the analogue world of sight and sound, presenting it in a form that is meaningful to human eyes and ears.


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5.5.4 Loudspeakers

Speakers also produce an analogue output. The audio program inside the boundary converts the digital encoding of the sound to a series of electrical pulses that are sent to the speaker, where they cause a cone of stiffened paper (or some synthetic material) to vibrate in and out. This makes the air vibrate in the characteristic sound wave.


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5.5.3 Plotters

A plotter is a special type of printing device mostly used by architects, engineers and map makers. Here the printed output is produced by moving a pen across the paper. Sometimes several differently coloured pens are available. Plotters are obviously most suitable for line drawings, which is why architects, for instance, use them. I've mentioned them here, however, because – in contrast with monitors and printers – they produce an analogue output directly.


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4.11.1 Digital still cameras and camcorders

These devices are now widely and (fairly) cheaply available. There is no film. You point your camera, take your shot and get a compressed digital image that can be transferred straight onto a computer, where it can be edited or printed. Digital still cameras usually compress their images into JPEG format and store them on a tiny, removable memory card inside the camera; the latest digital camcorders can record in MPEG format, stored on a special tape. Both devices work by means of an electron
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4.2.3 Text capture devices

Practically, how can we take text across the boundary?

SAQ 8

What are the main devices for transforming text into digital form inside the computer?


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3.9 A few more terms

Just to round off this description of the interior of the digital world, let me introduce and define a few more terms that you will come across again in this course and in any future studies of computers. Specifically, you may have heard the terms bits, bytes and words used in connection with computers. Now that we have taken a look at the binary system that underlies computer arithmetic, you will find there is no mystery in any of these three terms.

The word Bit is
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3.3.1 The human perceptual system

In order to survive, all living things have evolved some sort of ability to sense or perceive the world around them. Even the humble amoeba is sensitive to light. Complex animals have intricate perceptual systems that respond to many different features of their environment – insects, despite their impressive eyes, are most sensitive to trails of chemicals; bats are blind to light but responsive to sonar pulses; dogs and pigs depend more on smell than vision for sensing the world.

We w
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2.9 Summary

In this section, I started by emphasising the fact that the computer, which has become more or less omnipresent in modern society, is a tool like any other.

I went on to look at the special nature of that tool, establishing that its function is to capture, store, present, exchange and manipulate interesting aspects of the world.

I then introduced the idea of two contrasting realms: the analogue world we inhabit and the digital interior world of the computer. When we capture featur
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2.7 Manipulation

As I suggested above, we can change a digital version of some aspect of reality in any way we want. I've used the simple example of tinkering with a digital photograph, but the possibilities for transformation go far beyond this. We can set up elaborate replicas of real-world systems and inspect them in detail. We can establish digital models and run them forward in time to see what might happen in the future. We can even create completely imaginary digital worlds and then explore them as if
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2.5: Crossing the boundary

So computers are used to acquire, store and present, exchange, and manipulate interesting characteristics of the world. But this raises a serious problem: the world we inhabit and know so well and the world inside the computer are very different in kind. We live in an analogue world. The world of the computer is digital. The exact meaning of these terms may not be very clear to you at the moment. I will define them both in the next section. For the moment, the only point
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8.4 The OR operation

The OR operation (occasionally called the inclusive-OR operation to distinguish it more clearly from the exclusive-OR operation which I shall be introducing shortly) combines binary words bit by bit according to the rules:

  • 0 OR 0 = 0

  • 0 OR 1 = 1

  • 1 OR 0 = 1

  • 1 OR 1 = 1

In other words, the result is 1 when either bit is 1 or when both bits are 1; alternativel
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7.2 Adding 2's complement integers

The leftmost bit at the start of a 2's complement integer (which represents the presence or absence of the weighting −128) is treated in just the same way as all the other bits in the integers. So the rules given at the start of Section 7.1 for adding unsigned integers can be used.

Example 7

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6 Manipulating data in computers: introduction

Sections 1 to 5 of this course have shown that in a computer all types of data are represented by binary codes, and that programmers must make sure that the programs they write treat this data appropriately in any particular application: as text if it is intended to be text, as a binary fraction if it is intended to be a binary fraction, and so on.

Programmers must also ensure that the programs manipulate the binary codes in an appropriate way for the particular application. But what so
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4.3 Representing moving images

A moving image is simply a series of still images presented at sufficiently short time intervals that the eye smoothes over the change from one image to the next. In practice, this means the images must change at a minimum rate of around 20 per second; if the rate is lower then the moving image flickers or is jerky. Each still image that goes to make up a moving image is known as a frame.

So far as computers are concerned, moving images are of two types. One type is animations
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4.1 Introduction

Personal computers, or PCs, are very versatile computers and can perform a huge range of tasks. So whereas the uses of the kitchen scales and the digital camera indicate clearly what types of data are to be represented, the PC leaves the field very broad indeed. I have therefore chosen to consider some of the data representations used when families and friends use emails to keep in touch. Very conveniently, these lead to some different data representations.

At their simplest, emails are
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2.4 Representing numbers: negative integers

In Section 2.2 I showed you how integers can be encoded if they are known to be positive, treating the integers in the kitchen scales as if they were known to be positive. However, if the user invokes the ‘add-and-weigh’ function on the scales while there is an object in the scalepan and then removes t
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