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
4.2.1 First computer (your computer) In the block diagram, the computer receives data from the user and sends it into the network. It will manipulate and also store and retrieve data. If you send a message to a FirstClass conference, your computer receives the message from you as data via the keyboard. The computer manipulates the data into a form that can be sent into the network, in this case the internet via your internet service provider (ISP). Your computer will also store or retrieve relevant data, such as details of
1.3.4 How to use colour to good effect The effective use of colour is a complex and technical area. In Table 2 we have listed some general guidelines. 3.3 Antivirus software In conjunction with these patches, you should also install antivirus software. This is because patches and updates are not always issued quickly enough to protect your system from new viral attacks. In addition, patches only stop the types of malware that exploit errors in the program code. Antivirus software is updated as new malware is found, so it can offer a higher level of protection than just relying on manufacturers’ updates. Antivirus software is specifically designed to catch 2.1 Introduction In this section we will cover the following topics: browsing for information on the Web; searching for information on the Web; using search engines; bookmarking websites; finding images on the Web; how to reference sources. Some of the material in this section has been drawn from Safari, an interactive website provided by the Open University Library. Unit summary In this unit, the emphasis has been on devices communicating with each other in networks. You were introduced to some general principles about signals and networks, and the differences between wired and wireless networks. You met some of the network technologies in common use (Ethernet, WiFi and Bluetooth), before looking more closely at specific applications (smart homes, RFID systems) for networked devices. But we have barely had time to scratch the surface of what these technologies offer 2.2 Memory You should now be beginning to build up a picture of what a computer is: you know it needs input and output devices to communicate with the world outside and a processor to carry out the instructions that are programmed into it. But where are these instructions stored within the computer? The answer is that they are stored within what is called the computer's main memory, along with any data needed to carry them out. However, the main memory in computers like PCs is much too smal 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 Introduction Models are mechanisms for communication. This unit looks at what a model is and what the process of modelling is about. The techniques discussed here are applicable to a wide range of systems and have one thing in common: they are all commonly used diagramming techniques. The five techniques are: data flow diagrams, use case modelling, activity diagrams, entity–relationship diagrams and state machines. This unit is an adapted extract from the Open University courseAuthor(s): 4.3 A commercial implementation In order to conclude this section I shall describe a commercial implementation of an object bus. It has been developed by a company known as SoftWired Ltd and is known as iBus. It is based on TCP/IP rather than UDP. The facilities offered by the iBus API provide developers with the facilities to construct objects which can subscribe to channels and to transmit any Java object to a channel. The code for a transmitter is shown below; the import statements are not shown. In Learning outcomes After studying this unit you should be able to: describe some of the architectural and programming paradigms used in distributed system development; describe message passing and the role of protocols within a message passing paradigm; introduce the concept of a distributed object; describe how event-based architectures are used within distributed system development; introduce one implementation of an event-based archite Acknowledgements The following material is Proprietary (not subject to Creative Commons) and used under licence (see terms and conditions). Grateful acknowledgement is made to the following for permission to reproduce material: Ince, D. Developing Internet Applications, chapters 1 and 4, published by Pearson Education Limited in collaboration with The Open University, © Pearson Education Limited, 2002, 2003. This publication forms part of an Open University course M360 Developing 7.2.10 B2B exchanges A B2B exchange is a website or collection of websites which make the process of carrying out business to business transactions much easier. Under this banner comes sites which enable multiple companies to procure services and products from each other; help businesses form temporary alliances to carry out activities such as joint marketing or project bidding, and enable a marketplace in raw materials to function. 1.6 Implementation Implementation involves the construction of a database according to the specification of a logical schema. This will include the specification of an appropriate storage schema, security enforcement, external schema, and so on. Implementation is heavily influenced by the choice of available DBMS, database tools and operating environment. There are additional tasks beyond simply creating a database schema and implementing the constraints – data must be entered into the tables, issues relating Screening for genetic defects Now that scientists have mapped the human genome, computers can be used to detect genetic defects. Screening for genetic diseases existed before the application of computers. Family histories were used, together with a knowledge of inheritance patterns and statistics, to determine the likelihood of a couple having offspring with genetic disorders such as sickle cell anaemia. Some genetic disorders such as phenylketonuria have had simple chemical detection tests available for some 3.3 Data and information This unit is also about information, which in Subsection 2.1 was distinguished from data. Whereas data is a discrete item like a price or the name of a product such as milk, information links two or more items of data to give knowledge: e.g. the price of milk is 50p. To give a simple example, if I said to you that I was standing at approximately 1 degree 40 minutes and 20 seconds longitude west (written 1°40′20″W), 55 degrees, 4 minutes and 57 seconds latitude 2.3 Summary This section showed that computers pervade our daily lives, but that many of them are invisible to us. It investigated the information requirements of certain individuals, such as shoppers and doctors. You learned that their requirements can range from the simple and obvious to the complex and not so obvious. You also learned that it is not just individuals who require information: it is also essential to the operation of organisations. The example of loyalty cards was used 2.1.1 A day in my life I wake to a radio-alarm. It's controlled by a small computer that lets me set the time I want to wake up and the radio programme that will wake me. I prepare breakfast on a cooker which has a small computer that controls the clock, timer, and other functions such as oven temperature. I take my dog for a walk. She has a ‘microchip’ (i.e. a very small simple computer) implanted under her skin that will enable her to be traced if she is lost or stolen. I take my son to his A Why use algebra? We've seen examples in Section 2 where the quantities in problems are always related in a particular way. For example, V = u + at is a general expression connecting initial and final velocity (u and V), constant acceleration (a) and time (t). This single formula allows you to calculate final velocity, 5.2.1 A relaxation exercise This is an exercise that can be done during an exam without drawing attention to yourself, or disturbing others.
Table 2: Making e
1
Pull in stomach muscles tightly. Hold for a count of five. Relax.
2
Clench fist tightly. Hold for a count of five. Relax.
3
Extend yo
