2.6 Modifying the medium to carry the message Earlier we said that we can think of a signal as a deliberate variation in some property of the medium used to convey the data. Such variation needs to be done in a meaningful way. For example, think of the way pulses of light could be used to convey Morse code. (Morse code is a code in which letters and numbers are represented by groups of short dots and long dashes.) The light could be switched on and off so that a short light pulse could represent a dot and a longer pulse a dash. The same
5.4 Conclusion You have seen that although the three products you have looked at are very different types of computer, they all embody the same basic functionality and a version of Figure 3 can be drawn for each product to illustrate this. One feature of the PC is the range of forms of secondary memory it can use, and
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General principles In principle, online shopping is simple and involves the following stages: You find a site that sells what you want (possibly using a search engine, but more frequently going to a site which you know (from personal experience, word of mouth or advertising) sells the desired goods. Having found on the site, you then locate – by searching or browsing – the goods in its online catalogue. You then fill in an online fo
6.1 Reliable information Information is worthless if you have no trust in it. This has always been the case, but there are issues of trust that arise specifically in the context of modern information and communication technologies. Think about the following: You do a search on the Web and get results from several different sites. Do you trust the information in them all? How do you decide which are the most trustworthy? You get an email, a letter or a phone c
4.3 Recorder The descriptions of newsgathering in the extracts by Taylor and Higgins make reference to videotapes because, until recently, tape was the main storage medium used for video. Camcorders had a built-in VTR (videotape recorder) and in the first camcorders the video was recorded as an analogue signal. Camcorders used for ENG (electronic news gathering) now store the video as a digital signal, whatever medium is used for recording. Camcorders are now appearing which can store the digital video on
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4.1 Introduction Working with others online requires some skills that are not always obvious when you first start using email and computer conferencing. Here are some basic rules of good practice about Computer Mediated Communication (CMC) that we recommend to all students when they begin using conferencing. These are collectively known as ‘netiquette’.
2.1 Introduction In this section you will be spending time exploring a number of resources on the Web. As someone who is interested in ICTs, it is important that you feel confident about keeping up to date with technological developments. The Web is one of your best sources of information and news in this subject area, but you will need to develop a critical eye in order to sift through and analyse the material that you find online. 1.2 Guidance on note taking When taking notes the first thing you have to consider is what the notes are going to be used for, since notes taken for one purpose may not be suitable for another. Here are some possibilities: to help you understand the information; to help you remember the information; to help you explain the information to someone else; to highlight the points that will be useful in an assignment; 9.1 What is a state machine? An event is an occurrence of a phenomenon at a certain moment in time. The occurrence of the event itself is assumed to have no duration. Typically, when an event occurs, it affects the state of an object. A state machine is a model of the behaviour of a single object over time and helps you to understand how that object's state affects its reactions to events. Figure 18 shows a state machine diagram (known as a statechart diagram in the UML) relating to the occupancy of a room in a hot 7.2 Exercises Draw an activity diagram for the main success scenario for the check out guest use case. 7.1 Activity diagrams Each use case in a requirements model represents a unit of functionality or, more simply, a task initiated by an actor. Within each task there may be a number of identifiable actions that the software system should perform in order to complete that task successfully. In addition, it is necessary to identify what an actor does in unusual circumstances. To model these aspects of a system you can use activity diagrams. In the UML, an activity diagram is used to model the coordinatio 6.2 Actors Iteration is a natural part of the modelling process. It does not matter whether you start by looking for the actors or the use cases. We have chosen to begin with the actors, since it is a way of expressing the system boundary implicitly and identifying the different views that need to be taken into account. In practice, you are likely to find that the actors are to be found in the roles that people play as employees in the problem domain, such as the hotel's receptionist or manager. A Learning outcomes Having studied this unit you should be able to: explain why modelling plays a key role in eliciting requirements; identify the different kinds of model used in eliciting requirements; explain the need for modelling languages; interpret a data flow diagram describing a simple process; interpret a use case diagram describing a system's response to a business event; interpret an activity diagram describing t References 5.1 Introduction The final approach to developing distributed systems is based on a radical view of such systems. The approach is based on work carried out by two American academics, Nicolas Carriero and David Gelerntner. These two academics developed a language known as Linda in the 1980s. The language, and its associated technology, has always been thought of highly by other academics within the distributed systems area, but has never taken off in terms of commercial use. However, in the late 1990s Sun deve 4.2.1 Hub and spoke architectures There are two main types of bus architectures. These are hub and spoke architectures and multicast bus architectures. Figure 4 shows a typical hub and spoke architecture. 2.1 Protocols
Message passing is the simplest form of development paradigm. For example, the way that a client running a browser communicates with a web server is via message passing. Message passing is based on the idea of a protocol: a language which embodies the functions required by one entity in a distributed system (usually a client) which another entity provides (usually a server). As an example of a protocol consider Table 1. It shows the protocol associated with a naming servi 3.5.1 Anonymous remailers An anonymous remailer is a website which enables you to send an email anonymously to some recipient. The main reason for this is to do with something known as spam. This term describes unsolicited email which tries to sell the recipient something. Throughout the internet you can
Exercise 5
Answer
Spam
