4.2.1 ATM physical layer The ATM physical layer is divided into two sub-layers: the transmission convergence sub-layer and the physical medium sub-layer. Functions of the transmission convergence sub-layer include generating and receiving cells, and generating and verifying the cyclic redundancy check in the header error control field. For correct interpretation of ATM cells it is important to identify the beginning of a cell. In theory, if ATM cells are transmitted as a continuous stream of bits,
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
3.3 Hypertext transfer protocol (HTTP) In this section, I shall look at one example of an application of the TCP/IP protocol suite – sending hypertext pages over the world wide web (WWW or simply the web). However, first I shall very briefly summarise the main features of the web that are relevant to this discussion. There are many sources of information about the web on the web itself for those who want to know more. In very basic terms, the web is an application of the Internet for accessing resources where
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
2.4 Examples of layer functions There are several functions that can be performed at one or more of the OSI layers. Some of the more common ones are discussed below.
Connection control
For connection-oriented services, a connection must be established between peer entities. A connection has three phases: connection set-up, data transfer and connection clear. If the peer protocol supports connections, each protocol data unit type corresponds to a primitive type; for instance, a connection request primiti
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
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
1.2 Protocols in multi-service networks: introduction Early automatic telephone networks were built to carry only voice traffic and to provide a very simple telephone service – now called plain old telephone service (POTS). When computer networks started to appear, either they were separate from telephone networks or the data carried between computers was a small proportion of the traffic on the telephone network. There are various estimates for the growth of voice and data traffic, and various dates have been given for when data traffic will
Introduction People have always communicated with each other – initially by face-to-face communication through gestures and sounds, then over a distance through written messages and signals in the form of fires, lights or flags. Technology, for instance in the form of electrical signals, has reduced many of the limitations of distance. Communication networks have become very important, and modern society depends on them for the smooth operation of economic and social activities. In this unit we regard a
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.
5.5.2 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
4.2.4 Keyboards Every computer comes with a keyboard. They are still the main way of taking text across the boundary into the computer. The one I'm using to type this unit has 109 keys. Under each key is a pressure sensor that detects when the key has been pressed and sends an electronic signal into the computer. There, a small program called the BIOS (Basic Input/Output System) translates the signal into the appropriate numeric code. Other software stores that code in a suitable place in the memory.
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, whic
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
Acknowledgements All materials included in this unit 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 2.0 Licence 1. Join the 200,000 students cur
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. 8.2 The NOT operation The NOT operation (note that, as with all logic operators, NOT is always written in capital letters) acts bit by bit on a single binary word according the following rules: NOT 0 = 1 NOT 1 = 0 In other words, all the 1s in the word are changed to 0s and all the 0s are changed to 1s. Hence, for example, NOT 1101 1011 = 0010 0100 As you saw earlier, the term complement or 1's complement is sometimes used for the result of the NOT operation. In f 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. 7.4 Multiplying 2's complement integers Multiplication can be thought of as repeated addition. For instance, in denary arithmetic 7 × 5 can be thought of as 7 + 7 + 7 + 7 + 7 There is therefore no need for a new process for the multiplication of binary integers; multiplication can be transformed into repeated addition. In multiplication the result is very often much larger than either of the two integers being multiplied, and so a multiple-length representation may be needed to hold the result of a mu 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
