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4.2.2 ATM layer

The primary functions of the ATM layer are associated with the routing and switching of ATM cells. Because ATM cells are packets, the switches are packet switches and the switching operation can be called forwarding, but by convention, because the ATM layer provides a connection-oriented service, the term ‘forwarding’ is generally not used.

The path cells take and the resources allocated to them depend on their service category. This is determined when a virtual connection is
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3.5 Internet protocol (IP)

At the time of writing (2002), two versions of IP are available: versions 4 and 6. In this section I shall describe version 4, which is abbreviated to IPv4, as it is the more widely available version. Version 6 may eventually replace version 4 because it has some additional features that may prove essential for multiservice networks.

IPv4 is the main TCP/IP protocol in the internetwork layer of the TCP/IP reference model. It supports a connectionless service between hosts in an interne
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2.2 Vertical communication

Figure 6 shows the OSI view of adjacent layers. The interface between two layers in the same system is called a service access point (SAP). One of the features of a service access point is that it has an identifier, or an address, which allows each communication between adjacent layers to be uniquely identified. The processes that communicate across the interface are called entities. These are typically software routines, but may also be hardware components. The notation in Figu
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6.7 Summary

This section has looked at simulations, in which digital models of key aspects of the real world can be manipulated by programs. The examples included models of the world's climate, the early cosmos, stock markets, biological evolution and fantasy worlds and personalities. I've offered the view that simulation has far reaching implications for science, politics and society and will invite you to question that view in the final section.


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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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5.5.2 Printers

Colour models were dealt with in Subsection 4.7.

You probably also own a printer. Many computers now come with them as part of a package. There are two main types in use today: inkjets and lasers.

InkJet printers work, as their name suggests, by firing tiny droplets of ink at the p
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5.2 A conundrum about meaning

SAQ 19

Look at the following set of binary numbers:

00011010 00100011 10001001 10011100 10100011 01001101 10000011 01010100 10001000 00010001 10000110 11110010 …

which we may imagine are stored in the memory of a computer.
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4.13 Sound capture devices

In the past, the work of recording sound and music was carried out by professional recording studios. Before digital technology arrived, recordings were made by picking up sounds on a microphone which converted them to an analogue electrical signal. This signal was then transferred to another analogue medium, such as the grooves of a vinyl record or the changing patterns of metallic atoms on a magnetic tape.

At the start of the digital revolution, analogue to digital conversion, and the
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4.2.5 Scanners and OCRs

A better solution is to get some electronic help. A page of text is placed in a scanner, which produces an image of the page using techniques that I will discuss shortly. The image is passed to a computer program called an optical character recogniser (OCR), which detects each letter on the page in turn and transforms it into its digital code. This recognition is an immensely difficult task, requiring very sophisticated software, so OCRs are generally only partially effective.

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3.11 Summary

In this section I examined the terms analogue, discrete and digital and illustrated their correct use through examples and brief definitions.

I raised the familiar idea of the five human senses which enable us to perceive our analogue world.

Finally I focused on the digital world of counting and representing numbers, and in particular the binary system used in the inner world of the computer.


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3.7 How we work with numbers

Most civilisations have had to face the problem of counting and recording numbers. Our own culture has adopted the so-called Arabic system of numbers. This system is now used more or less worldwide. In this section I will look very briefly at some of its key features.

We have an infinity of numbers at our disposal. If we start counting from 1, we can in theory go on for ever. But although there is an infinity of numbers, we only have a very small, fixed number of digits to
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3.5 Digital things

The terms ‘discrete’ and ‘digital’ are often used interchangeably. For example, The New Penguin Dictionary of Computing contains the following definition.

Digital. Any communication or computing technology whose data may only have a finite number of discrete values.

However, I want you to be clear about the strong association between a digital thing and a number
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2.8 The price

But using computers to acquire, store, exchange and manipulate data comes at a price. By this, I don't mean that the technology is expensive, although this may be an issue. Rather it's the fact that the quality of the information computers give us can often be suspect. More worrying still are the questions of privacy, liberty and security that are raised. The computer gives ordinary people unprecedented access to information. But it also gives people that might not wish us well – gov
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2.6 Going back

Capturing bits of reality and transferring them to a computer would be a pointless exercise if they stayed locked in the digital world. We want access to what we've captured. We want to see the results. In particular, we may want to look at our captive in a different form. For instance, suppose we input the series of temperature readings shown in Author(s): The Open University

1 Aims

In this unit, I want to be more specific and look at the way computers represent and handle data. The unit aims to:

  • broaden the definition of a computer and explain the concept of crossing the boundary between the computer's world and our own

  • explain the digital nature of the computer's world and contrast it with our analogue world of sense and motion

  • describe in detail how to transform features of our world i
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Introduction

This unit introduces the important distinction between our analogue world of colour, sound, taste and touch and the computer's peculiar binary world of digital entities. Concepts of the analogue universe in which we live and the digital world we create are explained. The way in which information, in the form of text, still and moving images, and sound can cross the boundary from the analogue universe into a digital world is explored.

This unit is from our archive and is an adapted extr
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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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8.1 Introduction

Study note: You may like to have the Numeracy Resource to hand as you study Section 15. It offers extra practice with the logic operations, and you may find this useful.

Please click on the 'View document' link below to read the Numeracy Resource.


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