References 3.4 Transmission control protocol (TCP) As I outlined in the previous section, peer entities in clients and servers exchange HTTP protocol data units when they wish to transfer a resource over the web. I gave very little detail about this because I wanted to focus on the general features of protocols in the application layer of the TCP/IP model. The HTTP protocol data units are transferred from the sender host to the receiver host by calling on the services of the transport layer. In the case we are considering, the transport layer 3.2 Domain name system Applications use easy-to-remember names for hosts on the Internet, but before sending any data to a host an application in the source host must translate its name for the destination host to the numerical network address. The Internet is divided into domains, and an authority in each domain is responsible for allocating names. However, the domains may be divided into sub-domains and the responsibility of allocating sub-domain names may be delegated to other authorities. In this way the 3.1 What does TCP/IP protocol architecture do? The Internet is a worldwide public internetwork, which allows computers to communicate with each other even though they may have different manufacturers and different operating systems. The origins of the Internet lie in a project of the US Defense Advanced Research Project Agency in the 1970s, where it was intended to foster communication between research institutions rather than operate for profit. However, a substantial amount of traffic carried by the Internet is now related to com 2.1 Layers of communication An internetwork is a network of networks, composed of terminals, switches and communication media. The overall objective of an internetwork is to allow communication between two (or more) networks. This simple description hides the complications that arise in real networks, in which the types of medium vary, transmission errors occur, transmission links fail, switches fail or become congested, equipment is produced by different manufacturers, networks are owned and maintained by differ 5.3 Regaining meaning Suppose for a minute that the numbers I presented above were generated by a scanner as it produced a bitmap of a photograph. Clearly, the machine on which they are stored will have to get the image back to us by means of a device that can render it into a form meaningful to the human eye – an output device. I shall shortly review such devices. However, there is still work to be done before the computer can pass digitally-encoded data to such a device. For a start it will need to have 3.1 Ghosts of departed quantities They are neither finite quantities, or quantities infinitely small, nor yet nothing. May we not call them the ghosts of departed quantities? (Bishop G. Berkeley, The Analyst) This section follows up the ideas presented in and aims to: define the terms analogue, discrete and digital; look briefly at the human perceptual system, which e 2.4.4 Manipulation Suppose I take a digital photograph of myself for my website. Horrified by my wrinkled, baggy appearance, what can I do? Actually, with the right software I can do more or less anything I like: I can smooth out the wrinkles; I can restore the grey hair to its former splendour; I can even put in a background of books to give me a scholarly appearance. In fact, I can so improve the picture that if you met the real me you probably wouldn't recognise me. ‘Massaging’ my photographic imag 8.3 The AND operation The AND operation combines two binary words bit by bit according to the rules 0 AND 0 = 0 0 AND 1 = 0 1 AND 0 = 0 1 AND 1 = 1 In other words, only when both bits are 1 is the result 1. You may find it helpful to think of it this way: when one bit is one and the other bit is 1 the result is 1. 7.3.1 Finding the 2's complement In Section 2.4 you saw how to find the 2's complement representation of any given positive or negative denary integer, but it is also useful to be able to find the additive inverse of a 2's complement integer without going into and out of denary. For instance, 1111 1100 (−4) is the additive inverse, or 2's complement, of 0000 0100 (+4), but how does one find the additive inverse without converting both binary integers to their denary equivalents? The answer is that the additive inve Acknowledgements 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 17.2 The checkout terminal The first computer block represents the checkout terminal. The processes at the checkout (receiving, storing, retrieving, manipulating and sending data to the user), are the same as I described earlier. However, the checkout terminal also sends data via the supermarket's network. 16.2.2 Storing and retrieving data As each item is scanned, the checkout computer looks up its price. The running total for each customer's purchases is stored temporarily in the checkout terminal. Other data may also be stored, such as the amount of money that has been taken at that checkout during the day. 13.2 Magnetic storage As I mentioned earlier, your computer has a hard disk which provides a permanent storage area for your computer's programs and the files you create. When you save files to your computer's hard disk, you are using a magnetic storage medium. Data stored in magnetic form can be changed once it has been stored, so if you run out of space you can delete some files to make room or, if you want to edit a file, you can make the necessary changes and then save it again. At the time of writing, a mediu 2.2 The characteristics of text Text has a number of technical advantages over other media. Text files are small. Images, sound, animation and video all produce large computer files, even when compressed. These files can be slow to load from CD-ROM or to transmit via the Internet; this can reduce the usability of the UI. Text files are much smaller. Text can be manipulated very easily. For example, it is possible to search for words. If you are visiting a 7.4 Using flowcharts to describe a task (contd) Now consider what happens when you are weighing, for example, flour on a set of scales. You slowly add more flour to the scalepan until you reach the desired weight. As you do this the display constantly changes, showing the weight increasing as you add more flour. To do this, the scales’ computer must repeatedly examine the input and update the display each time it does so. The flowcharts in figures Author(s): 2.3 Computer systems So far, I have introduced the major components of a computer, namely a processor along with input and output devices, plus main and secondary memory. I now want to explore three of these components a little further, starting with input devices. Input devices have to collect some information from outside the computer and present it to the computer as data which is in a form the processor can work with. (Strictly speaking, ‘data’ is the plural of the Latin word ‘datum’. But in the 2.3 Synchronous and asynchronous message passing Synchronous message passing involves one entity (usually a client) in the message passing process sending a message and a second entity (usually a server) receiving it, carrying out some processing and then sending back some response which the first entity processes in some way. While the second entity is carrying out the processing the first entity pauses waiting for the response. In asynchronous message passing each entity in the process does not have to wait for the next part 6.2.15 Free products and services It might seem paradoxical to include sites which provide free products or services under the category of business models. Typical sites which come under this category include gaming sites where users can play computer games using their browser, sites which run free raffles and sites which offer free software. Such sites do not earn any revenues from the products or services they offer; revenue is earned indirectly, for example by means of banner adverts or by receiving revenue from site 2.5.2 Link checking sites The World Wide Web contains millions of web pages. Many of these pages are impossible to read, even though many existing web pages will reference them: your browser will usually return with some message such as ‘Error 404 Page not Found’ when you try to access them. Error 404 is a standard message returned by web servers when a non-existent page is accessed. It is also the telephone area code for Atlanta in the United States; you will occasionally hear technical staff referring to non-exi
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Coffman, K. G. and Odlyzko, A. (1998) ‘The size and growth rate of the Internet’, First Monday, Vol. 3, Issue 10, http://firstmonday.org
ITU-T 1–150 (1999) B-ISDN Asynchronous Transfer Mode Functional Characteristics, ITU-T.
ITU-T X.200 (1994) Open Systems Interconnection – Model and Notation, ITU-T. (Also known as ISO/IEC 7498–1.)
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