Cambridge International Catalogue 2024 - Final - Flipbook - Page 46
CAMBRIDGE IGCSETM
Further knowledge
and advice boxes are
included throughout
the Student’s Book
2.1 Types and methods of data transmission
2 DATA TRANSMISSION
sequence.
Computer ‘B’ will now have to reassemble the packets into the original
Activity 2.1
in two different
1 Suppose a video conference is taking place between delegates
data
countries. Packet switching is being used to send video and sound
between the delegates:
Computer ‘B’
Computer ‘A’
Delegate 2 in India
Delegate 1 in USA
R1
Video and sound sent between
delegates using packet switching
R3
R4
R2
Router ‘B’
▲ Figure 2.7
R5
Router ‘A’
Describe:
i any potential problems with sound and video quality
ii how these problems could be caused.
web page
2 Explain how packet switching could be used to download a large
from a website.
3 a The trailer in a packet will use one form of error checking.
R7
R6
R10
R8
R9
each packet
▲ Figure 2.5 Typical network showing possible paths taken by
Find out more
Another method of
sending packets is
called circuit switching.
Find out how this
differs to packet
switching, and then
re-draw Figure 2.5
showing the route the
packets take when
using 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-routing
11110000 10000011 00110011 00111111 11111110 11100011
packets
» it is relatively easy to expand package usage
» a high data transmission rate is possible.
the
Use this data to show how the receiving computer can verify that
received payload was error-free.
transmission
4 a Explain how it is possible for packets to be lost during their
across a network.
and
b Describe how it is possible for a system to deal with lost packets
prevent 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-sent
a
» 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 transmission
around
Sometimes it is possible for packets to get lost because they keep ‘bouncing’
the
from router to router and never actually reach their destination. Eventually
up,
network would just grind to a halt as the number of lost packets mount
used. A
clogging up the system. To overcome this, a method called hopping is
reduced
hop number is added to the header of each packet, and this number is
by 1 every time it leaves a router (Figure 2.6).
Hop number:
Router 3
Router 5
Router 8
Router 11
Router 15
Router 21
10
9
8
7
6
5
Data transmission can be either over a short distance (for example, computer
another
to printer) or over longer distances (for example, from one computer to
in a global network). Essentially, three factors need to be considered when
transmitting data:
one
» the direction of data transmission (for example, can data transmit in
direction only, or in both directions)
at the
» the method of transmission (for example, how many bits can be sent
same time)
data is
» how will data be synchronised (that is, how to make sure the received
in the correct order).
▲ Figure 2.6 Hop numbers between routers
Advice
Hopping is not included
on the syllabus but
is included here for
completeness (to
help understand how
packets 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 number
is
reaches zero, and the packet hasn’t reached its destination, then the packet
be flagged
deleted when it reaches the next router. The missing packets will then
be made.
by the receiving computer and a request to re-send these packets will
7 THE SYSTEMS LIFE CYCLE
7
48
49
Once 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 is
signed
off and handed over to the management team. This whole process is called
the
systems life cycle.
The systems life cycle
In this chapter you will learn about the:
★ analysis stage:
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Navigate
the syllabus
con昀椀dently with
introductions
to each topic
outlining the
learning objectives
There are many stages in the systems life cycle, which have been summarised
in
Figure 7.1.
– methods of analysing the current system
– identifying the inputs, outputs and processing in the current
system
– hardware and software requirements for the new system
ANALYSIS
★ design stage:
★
★
★
★
EVALUATION
– file structures, input and output formats
– use of data capture forms
– screen and report layouts
– validation routines
development and testing stage:
– test strategies and test plans
– use of test data (including live data)
implementation stage:
– direct changeover
– parallel running
– pilot running
– phased implementation
documentation stage:
– technical documentation
– user documentation
evaluation stage:
– evaluation of new solution
– compare solution to original system.
DESIGN
DOCUMENTATION
DEVELOPMENT/
TESTING
This is a two-way process,
since development and
testing may uncover a need
to redesign the solution
several times
IMPLEMENTATION
▲ Figure 7.1 Systems life cycle
7.1 Analysis
The first stage in the process is the analysis of the current system. Figure
7.2
shows the stages in analysis.
Research the current system
Identification of input and output
In this chapter we will consider the stages and processes involved
when
replacing computer systems, from analysis of an existing system
right
through to evaluation of the new system.
What processing takes place?
ANALYSIS
Problems with the current system
Suppose a company has been using an existing computer system for a
number
of years. They have decided to replace the current system with a new
one. After
a number of years of successful operation, what would initiate this need
to
upgrade?
User requirements for the new system
What are the information requirements?
» The existing computer equipment is now obsolete (it cannot be repaired
anymore).
» 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 stage
7.1.1 Analyse the current system
The changes will involve an ICT solution, which means a systems analyst
needs to be brought in to oversee the whole upgrade process. Their first
task
will be to analyse the existing (current) system, and then suggest a number
of improvements that can be made. All these improvements need to be
costed
and their advantages over the current system need to be reported back
to the
company’s management team.
There are four methods used to research the current system. The four methods
used are:
»
»
»
»
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observation
questionnaires
interviews
examination of existing documents.
170
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