Digital communications
Optical-fibre communications became commercially viable in the 1970s and innovation continues today. This free course, Digital communications, will illustrate how very high data rates can be transmitted over long distances through optical fibres. You will learn how these fibres are linked, examine the technology used and assess the future direction of this continually developing area of communication.
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Introduction This key skill develops your information technology (IT) skills in your studies, work or other activities over a period of time. To tackle all of this key skill, you will need to plan your work over at least 3–4 months to give yourself enough time to practise and improve your skills, to seek feedback from others, to monitor your progress and evaluate your strategy and present outcomes. Skills in information technology cover a broad range, from using software unitages to developing a c
Managing complexity: A systems approach – introduction
Do you need to change the way you think when faced with a complex situation? This free course, Managing complexity: A systems approach introduction, examines how systemic thinking and practice enables you to cope with the connections between things, events and ideas. By taking a broader perspective complexity becomes manageable and it is easier to accept that gaps in knowledge can be acceptable.
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Design thinking
Are you ever frustrated with something that you thought you could design better? Design thinking can structure your natural creativity to come up with solutions to all kinds of problems, and have fun in the process too! First published on Thu, 22 Dec 2011 as Author(s):
4.3 Character code functions Many programming languages provide two functions associated with the character codes (see Table 2). We shall call these functions ASC and CHR. ASC takes a character as input, and returns the integer giving the ASCII code of the input character. CHR returns the character whose ASCII code is the input integ
4.1 Functions A function is a process that, when given an input of a specified type, yields a unique output. This is a key idea in providing a precise, mathematical, description of processes in computing. To describe a particular function, we first give the set from which the input will be drawn and the set from which the output is drawn. This information is called the signature of the function. An example will make this clearer. Author(s):
Objectives for Section 3
After studying this section you should be able to do the following.
Recognise and use the terminology: disjoint union; power set (of a set); representation (of a data abstraction). Use and interpret the notation: Â
X
3.2 Combining data structures In Section 2, we introduced the notation SeqOfX for the set of all sequences whose members come from the set X. In Section 2, we looked only at sequences whose members were of one of the primitive forms of data (integers, characters or Booleans). We can have sequences whose members are themselves data with a more complicated form. For example, suppose that Jo is working at the till T1 and is replaced by Jessica. We might represent this handover by the 3-tuple (Jo, T1, Jes
Module team David Reed (Chair and author) Jill Alger (Editor) Chris Bissell (Critical reader and author) Philippa Broadbent (Print buyer) David Chapman (Author) Daphne Cross (Assistant print buyer) Glen Darby (Graphic designer) Donna Deacon (Course secretary) Alan Dolan (Course manager) Roger Jones (Author) Jo Lambert (Learning projects manager) Roy Lawrance (Gra
The T822 course team
4.2.2 ATM layer The primary functions of the ATM layer are associated with the routing and switching of ATM cells. Because ATM cells are packets, the switches are packet switches and the switching operation can be called forwarding, but by convention, because the ATM layer provides a connection-oriented service, the term ‘forwarding’ is generally not used. The path cells take and the resources allocated to them depend on their service category. This is determined when a virtual connection is
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.
