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
Acknowledgements The content acknowledged below is Proprietary (see terms and conditions). This content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 Licence Course image: S
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.
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.
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.
5.2 A conundrum about meaning 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 o 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 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. 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. 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 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 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 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): 1 Aims In this course, I want to be more specific and look at the way computers represent and handle data. The course 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 in Acknowledgements All materials included in this course are derived from content originated at the Open University. Except for third party materials and otherwise stated (see terms and conditions), this content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 Licence 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 16.2.1 Receiving data In a supermarket ICT system, there needs to be some way for the computer to receive information about the items a customer is buying. Think back to a recent visit to your local supermark 12.1 Introduction Data must be stored somewhere when it is not being manipulated. Modern ICT systems require increasingly large amounts of data to be stored for later use, and it is important that the data can be accessed quickly. Data may be stored on the stand-alone computer's hard disk in the form of files. You may want to move files from one stand-alone computer to another. In addition, you may want to move files from a device, such as a digital camera, to a computer. These activities require some fo 10.1 Introduction A stand-alone computer receives data from a user by means of input devices. The two most commonly used input devices are the keyboard and the mouse. A computer sends data to a user by means of output devices. Data may be output via devices such as a screen or a printer. There are many different ways of getting data into a computer. For example, a scanner converts images and texts into a format that can be processed by the computer and displayed on screen. Devices such as t 6.1 Networks Next I'll be looking more closely at the 'network' block in Figure 8, and in particular at the links that must be present before communication can take place. I'll introduce you to just a few of the forms that these links can take; links may be physical ones, such as cables, or they may be wireless, such as radio links. I'll also d
SAQ 19
Activity 13 (exploratory)
