CAMBRIDGE IGCSETMFurther knowledgeand advice boxes areincluded throughoutthe Student’s Book2.1 Types and methods of data transmission2 DATA TRANSMISSIONsequence.Computer ‘B’ will now have to reassemble the packets into the originalActivity 2.1in two different1 Suppose a video conference is taking place between delegatesdatacountries. Packet switching is being used to send video and soundbetween the delegates:Computer ‘B’Computer ‘A’Delegate 2 in IndiaDelegate 1 in USAR1Video and sound sent betweendelegates using packet switchingR3R4R2Router ‘B’▲ Figure 2.7R5Router ‘A’Describe:i any potential problems with sound and video qualityii how these problems could be caused.web page2 Explain how packet switching could be used to download a largefrom a website.3 a The trailer in a packet will use one form of error checking.R7R6R10R8R9each packet▲ Figure 2.5 Typical network showing possible paths taken byFind out moreAnother method ofsending packets iscalled circuit switching.Find out how thisdiffers to packetswitching, and thenre-draw Figure 2.5showing the route thepackets take whenusing circuit switching.Explain what is meant by a cyclic redundancy check.b The payload contains the following data:The benefits of packet switching are:» there is no need to tie up a single communication line» it is possible to overcome failed, busy or faulty lines by simply re-routing11110000 10000011 00110011 00111111 11111110 11100011packets» it is relatively easy to expand package usage» a high data transmission rate is possible.theUse this data to show how the receiving computer can verify thatreceived payload was error-free.transmission4 a Explain how it is possible for packets to be lost during theiracross a network.andb Describe how it is possible for a system to deal with lost packetsprevent them from slowing down the transmission process.c Explain why you think packet switching might improve data security.The drawbacks of packet switching include:» packets can be lost and need to be re-senta» the method is more prone to errors with real-time streaming (for example,live sporting event being transmitted over the internet)» there is a delay at the destination whilst the packets are being re-ordered.2.1.2 Data transmissionaroundSometimes it is possible for packets to get lost because they keep ‘bouncing’thefrom router to router and never actually reach their destination. Eventuallyup,network would just grind to a halt as the number of lost packets mountused. Aclogging up the system. To overcome this, a method called hopping isreducedhop number is added to the header of each packet, and this number isby 1 every time it leaves a router (Figure 2.6).Hop number:Router 3Router 5Router 8Router 11Router 15Router 211098765Data transmission can be either over a short distance (for example, computeranotherto printer) or over longer distances (for example, from one computer toin a global network). Essentially, three factors need to be considered whentransmitting data:one» the direction of data transmission (for example, can data transmit indirection only, or in both directions)at the» the method of transmission (for example, how many bits can be sentsame time)data is» how will data be synchronised (that is, how to make sure the receivedin the correct order).▲ Figure 2.6 Hop numbers between routersAdviceHopping is not includedon the syllabus butis included here forcompleteness (tohelp understand howpackets can get lost).These factors are usually considered by a communication protocol.Each packet has a maximum hop number to start with. Once a hop numberisreaches zero, and the packet hasn’t reached its destination, then the packetbe flaggeddeleted when it reaches the next router. The missing packets will thenbe made.by the receiving computer and a request to re-send these packets will7 THE SYSTEMS LIFE CYCLE74849Once a new system is agreed and it has been fully tested, it is then installed.It then needs to be fully evaluated and any changes made where necessary.Therefore, a cycle of events take place until a fully-working system issignedoff and handed over to the management team. This whole process is calledthesystems life cycle.The systems life cycleIn this chapter you will learn about the:★ analysis stage:Pages takenfrom CambridgeIGCSE™ and OLevel ComputerScienceNavigatethe syllabuscon昀椀dently withintroductionsto each topicoutlining thelearning objectivesThere are many stages in the systems life cycle, which have been summarisedinFigure 7.1.– methods of analysing the current system– identifying the inputs, outputs and processing in the currentsystem– hardware and software requirements for the new systemANALYSIS★ design stage:★★★★EVALUATION– file structures, input and output formats– use of data capture forms– screen and report layouts– validation routinesdevelopment and testing stage:– test strategies and test plans– use of test data (including live data)implementation stage:– direct changeover– parallel running– pilot running– phased implementationdocumentation stage:– technical documentation– user documentationevaluation stage:– evaluation of new solution– compare solution to original system.DESIGNDOCUMENTATIONDEVELOPMENT/TESTINGThis is a two-way process,since development andtesting may uncover a needto redesign the solutionseveral timesIMPLEMENTATION▲ Figure 7.1 Systems life cycle7.1 AnalysisThe first stage in the process is the analysis of the current system. Figure7.2shows the stages in analysis.Research the current systemIdentification of input and outputIn this chapter we will consider the stages and processes involvedwhenreplacing computer systems, from analysis of an existing systemrightthrough to evaluation of the new system.What processing takes place?ANALYSISProblems with the current systemSuppose a company has been using an existing computer system for anumberof years. They have decided to replace the current system with a newone. Aftera number of years of successful operation, what would initiate this needtoupgrade?User requirements for the new systemWhat are the information requirements?» The existing computer equipment is now obsolete (it cannot be repairedanymore).» Changes to laws or taxes requiring radical overhaul of software.» More suitable hardware is now available to improve efficiency and reliability.» There is a need to expand the company.There could be many more reasons.Identify hardware and software for the new system▲ Figure 7.2 Analysis stage7.1.1 Analyse the current systemThe changes will involve an ICT solution, which means a systems analystneeds to be brought in to oversee the whole upgrade process. Their firsttaskwill be to analyse the existing (current) system, and then suggest a numberof improvements that can be made. All these improvements need to becostedand their advantages over the current system need to be reported backto thecompany’s management team.There are four methods used to research the current system. The four methodsused are:»»»»169Flashcards and presentationsare included in our teachingand learning resources on theBoost platform44READY TO ORDER?hoddereducation ⸀com/igcse-mathseducation@hachette ⸀co ⸀ukobservationquestionnairesinterviewsexamination of existing documents.170Pages taken from CambridgeIGCSE™ Information andCommunication Technology
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