computing academy ebook

8/10/2019 Computing Academy eBook

1/214


2/214


3/214

Terms

This publication may only be used in accordance with the terms set out at:

http://www.computingacademy.org.uk/terms. By continuing to access this publication you

agree to be bound by these terms.

Computing Academy makes every reasonable e!ort to ensure the quality and accuracy of

this publication, however errors and omissions do occur. Computing Academy does not

guarantee that this publication is accurate, suitable or relevant for a particular syllabus or

examination board. You are responsible for ensuring that this resources meets your needs

and the requirements of the syllabus you are following; we will accept no responsibility or

liability for any mistakes, errors or omissions in this publication but we will endeavour to

correct any errors brought to our attention within a reasonable amount of time.

ii
http://www.computingacademy.org.uk/termshttp://www.computingacademy.org.uk/terms


4/214

Acknowledgements

All images, photographs, diagrams and text are the original works of the author, with the

exception of those acknowledgments made below.

Every e!ort has been made to trace all copyright holders, but if any have beeninadvertently overlooked please do contact us.

Photo Credits

p6http://en.wikipedia.org/wiki/File:Charles_Babbage_-_1860.jpg; p10 The University of

Manchester - used wi th permiss ion ; p27 Paul Wootton Graphics

(www.graphicnet.co.uk); p43Gnome 3.8 CC BY-SA 3.0; p46 Google Inc; p51 Google

Inc; p123-138 Apache OpenO"ce Screenshots used with permission from Apache.

Apache, Apache Foo, Foo, and the Foo logo are trademarks of The Apache Software

Foundation. No endorsement by The Apache Software Foundation is implied by the use of

these marks; p140-145 Josh Tynjala, Logicly- used with permission; p150 London Eye:

http://www.sxc.hu/photo/616616;

iii
http://www.sxc.hu/photo/616616http://www.sxc.hu/photo/616616http://www.sxc.hu/photo/616616http://www.graphicnet.co.uk/http://www.graphicnet.co.uk/http://en.wikipedia.org/wiki/File:Charles_Babbage_-_1860.jpghttp://en.wikipedia.org/wiki/File:Charles_Babbage_-_1860.jpg


5/214

Foreword

Computer Science isnt a new subject.

The science of sequencing instructions

and developing algorithms has beenaround since the 17th Century with

Gottfried Wilhelm Leibnizs Stepped

Reckoner; a mechanical counting machine

built in 1672, Johann Helfrich von Mllers

Di!erence Engine built in 1786 and the first

programmable computer built by Charles

Baggage in 1812.

Much has changed since then. Computers

no longer fill entire buildings; indeed it is

often the case today that we dont even

notice computers at work.

Computers have become so embedded in

our everyday lives and so easy to use that

weve stopped thinking about how they

actually work; what goes on inside that

box?

This book aims to guide you through the

fundamental principals of computer

science, get you thinking about how to use

computers to solve problems and get you

excited about creating and running your

own computer programs and games rather

than just using existing programs and

playinggames.

Students and teachers can use this book

as a companion for any Computer Science

course: although we do not endorse any

particular examination board, we have paidclose attention to the examination board

specifications and believe that this text will

prove to be an invaluable resource.

iv


6/214

v


7/214

Contents

1. Computer Systems ............................................................................ 8

What Are Computer Systems 9

Input, Process, Output 11

Using Computer Systems 13

Standards 18

Further Considerations 21

2. Hardware ............................................................................ 24

Permanent Storage 26

Random Access Memory 31

ROM and Virtual Memory 33

Central Processing Unit 35

3. Software ............................................................................ 41

What Is Software? 42

Operating Systems 43

Drivers 49

Applications 50

Utilities 53

4. Data Representation ............................................................................ 54

Binary 55

Hexadecimal 71

ASCII 81

Images 85

Sound 90

vi


8/214

5. Networking ............................................................................ 95

What Are Networks? 96

Network Classification 101

Network Infrastructure 103

Network Types 107

Network Topologies 111

Network Protocols 114

The Internet 117

6. Databases ............................................................................ 128

Thinking About Data 129

Database Management Systems 133

Relational Databases 138

Queries 145

7. Algorithms ............................................................................ 148

Logic 149

Sequencing 156

8. Programming ............................................................................ 163

Programming Languages 164

The Basics 174

Variables 176

Data Types 182

Operators 186

IF Statements 199

Loops 206

Arrays 208

Documenting Code 211

vii


9/214

Computer Systems

1Computer systems take a s

of inputs, process them an

create a set of outpu


10/214

What Are Computer Systems?

Computers today are all around us in one form or another. Take a quick look around a

typical home and youre sure to find many di!erent types of computers.

There are obvious examples such as desktop computers, laptop computers, tablet

computers and mobile phones. There are also computer systems which we take for

granted and probably dont even think of as being computers such as alarm clocks,

watches, washing machines and microwaves.

The first electronic programmable computer, built in 1943 was called Colossus, but as

Colossus could not store programs the operators had to enter the program from scratch

each time they turned the machine on.

9


11/214

The first computer able to store a program was built at Manchester University in 1948; it

was called the Small Scale Experimental Machine, or Manchester Baby.

The first program Baby

ran took 52 minutes tocomplete; a modern

computer would take

less than a second to

complete the same

calculation.

Computers can be

dedicatedwhich means

that they have been

designed for a specific

purpose such as a satellite or terrestrial television recorder, or a burglar alarm. These

dedicated computer systems are designed to perform a specific function and nothing else.

Computers may also be general purpose which means they do not have one main

function, for example desktop computers and laptop computers.

Although computers come in all shapes and sizes they share a common principle:

Key Point

Computers consist of a set of parts which work together to achieve acommon purpose. All computer systems take a set of inputs, process them

and create a set of outputs. This is done through a combination of

hardware and software.

10


12/214

Input, Process, Output

The following diagram shows a computer system in its most basic form.

Computer systems have one or more inputswhich sense, detect and collectdata. This

data is then processed, which can include sorting, searching andcalculating. The result

11


13/214

of this processing is then output or stored for outputting later. Outputs may include

displaying data to a screen, printing a hard copy (paper copy), or playing a sound.

In order to be clear about whether a device is an inputor an outputdevice consider:

Is data flowingINTOthe computer from the outside world...?

...or is data flowing OUTfrom the computer to the outside world?

Here a few examples of computer hardware that you will be familiar with, separated into

inputand outputdevices.

Input Output

Keyboard Monitor / Screen

Mouse Printer

Cashless Reader / NFC Projector

Keypad Speakers

TaskWrite down 3 more input and 3 more output devices that might be used as

part of a computer system.

12


14/214

Using Computer Systems

Why are Computer Systems Important?

Think about how many computer systems you might interact with during a typical day:

checking text messages or email in the morning; microwaving a bowl of porridge; setting a

burglar alarm; using a swipe card to enter a building; being observed by a CCTV security

system...........and thats all before you even arrive at work or school.

All day, every day, computer systems are at work; here are just a few examples.

13


15/214

Description Input Process Output

Cash MachineUser inputs their pin

numberSystem checks that the

pin number is valid

Options (such as checkbalance and withdraw

cash) output to user viathe screen

Online Cinema Booking User selects filmSystem checks

availability

Available seats output touser in a diagrammaticrepresentation of the

cinema

Contactless PaymentUser touches contactlesscard or chip to the card

reader

System looks up theusers account and

checks to see whetherthey have su"cient funds

- if so, the account ischarged

A message is displayedto the user stating thateither the transaction

was successful or thatthe transaction was

declined

Voice-activatedSmartphone

User speaks into themicrophone (recognised

instructions)

The system determineswhat has been said using

speech recognitionalgorithms

Depending on thecommand input, thephone may dial a

number, perform aGoogle search, speak a

response etc.

Here are some further examples of where computer systems can be found.

Retail

Ecommerce / online shopping websites, automatic stock ordering systems, logistics /

delivery tracking

Communication

Email, social networks, video calling, telephone calls, text messages

Education

Virtual Learning Environments (VLEs), interactive learning activities, quizzes and

educational games

Science

Simulations and data processing / number-crunching

Travel

Flight booking and tracking, mobile app bus timetables, GPS navigation, train signalling,

aircraft guidance 14


16/214

Reliability

We rely on computer systems an awful lot. Air tra"c control systems and hospital life

support systems are just two examples of where we quite literally put our lives in the

hands of computer systems. We therefore need to know that they are going to be reliable.

System designers, and the companies and individuals buying computer systems talk

about reliability using two main measures.

Availability

Ideally, a computer system will be available all the time, 24 hours each day, 365 days per

year. In reality that might not be the case due to system failure and the need to temporarily

take a system o$ine for maintenance. Availability is one measure used to judge the

reliability of a system. Over a period of 30 days, if the system was available without issue

for 30 days the system availability would be 100% for this period. If the system had a fault

and as such was out of action for 1 out of the 30 days, the system availability would be

97% for this period ([29 / 30] * 100).

MTBF

Another measure of reliability is the Mean Time Between Failure (MTBF) which is a figure

provided by hardware manufacturers who test their products and generate a MTBF value

before they are sold. The MTBF will be the average length of time, usually in hours, that a

particular hardware component is predicted to operate for without failure.

Testing

Software engineers will write millions of lines of programming code, and whilst they are

often highly skilled, even the most experienced programmers sometimes make mistakes.

A program to control the operation of a washing machine may seem simple when

compared with the program used to manage and track aircraft, but even the software

controlling that washing machine will consist of thousands of lines of code.

Software engineers need to test their code in order to catch any errors they may have

made. Testing is a time consuming and therefore expensive process that requires testers

15


17/214

to think about every possible way a system could be used; in the case of the washing

machine this could mean testing every possible combination of buttons that a user could

press, testing what happens if the door isnt shut when the start button is pressed, testing

how the system recovers if the power is cut during a wash cycle etc.

Extensive testing should take place before software is released, whether that software is

an air tra"c control program or just a new app for a smartphone.

Once the software engineers are happy with the software they may first release this to a

limited group of beta testers who may find errors also known as bugs that were missed

by the developers.

After a piece of software is released and put to use it will need maintenance andsoftware updates. This is necessary to fix further bugs found after release or to ensure

the software continues to function correctly if the systems around it change. When

smartphone operating systems receive updates, app developers sometimes need to

perform updates of their own app in order to ensure that it will continue to run correctly on

the updated smartphone operating system.

Failure

System failure could have catastrophic consequences so its important for organisations to

be prepared, and protect against or prepare for system failure. In the context of computing

Redundancymeans spare, excess or additional; if a computer system has redundant

storageit means that rather than just one hard drive, it has two or three.

High profile ecommerce and social networking websites would lose millions of pounds if

their systems failed, so they use redundancy. If one hard drive fails, a redundant hard drive

will take over. If an entire server fails, a redundant backup server will automatically take

over.

In addition to using redundant hardware, organisations who rely on hardware and data for

their very survival will have prepared a disaster recovery plan.A disaster recover plan is

just what it sounds like - a plan of what to do in case of a disaster such as fire, theft, flood,

power loss, terrorist attack etc. Disaster recovery plans set out which individuals need to

16


18/214

do what and when in the event of a disaster. Some individuals may need to make a

telephone call to a di!erent city or country asking them to bring a redundant server online;

others might need to unplug backup drives and get these to a safe location.

TaskThink about a computer system that would be used in each of the

following categories:

Retail, Communication, Education, Science, Travel

For each computer system, write down the inputs, processes and outputs

taking place.

17


19/214

Standards

Standards

In order to reduce system failure and make systems more reliable, developers adhere to

standards.

Heres an example of two systems trying to work together that dont use the same set of

standards.

18


20/214

Example

Imagine that System Ahas been designed so that code #105means yes,everything is fine. System Asends out this code to System B to let it know.

The problem is, System Bis using a di!erent set of standards. In System B

code #105means please send me some data. System Bnow starts

sending lots of data to System A.

System Awasnt expecting this data so sends code #993to System B.

The set of standards used by System Asay that code #993means stop, but

the set of standards used by System Bsay that code #993means can you

send the data faster please.

Systems need to operate using an agreed, common set of standards otherwise they justwont work very well (or at all) together.

Proprietary Standards

Proprietary standards are owned and controlled by companies. The .doc and .docx file

formats are owned by Microsoft and these file formats use a particular standard way of

saving and presenting word processed documents. Every computer which has Microsoft

Word installed will be able to follow this standard in order to open a .doc or .docx file

properly.

Similarly, Pages, the word processor created by Apple uses a di!erent standard. Microsoft

Word does not use the same standard as Apple Pages, so you cannot open Apple Pages

documents using Microsoft Word.

19


21/214


22/214

Further Considerations

Environmental Considerations

Each year the computer systems we use consume millions of Gigawatt Hours (GWh) of

energy, and millions more GWh of energy are used on air conditioning in order to ensure

that those systems stay cool.

The average lifespan of a computer system - whether that be a smartphone, laptop, tabletor desktop - is 3 years. These computer systems contain lots of toxic materials such as

mercury and radioactive isotopes, all of which ends up in landfill.

Power consumption and waste due to computer systems is a big problem for which we do

not yet have a solution

21


23/214

Ethical Considerations

While legalities refer to what you can or cant do under law, ethics is concerned with

fairness; what it right and wrong. Doing something that is ethically wrong does not

necessarily mean that it is against the law.

Should you wish to, you could setup a business o!ering to design websites for customers

even though you dont really know how to create websites. Ethically, you should not do

this as its wrong and you wont be able to produce good quality websites for your

customers; yet while ethically wrong, this would not be illegal.

Legal Considerations

As our use of computer systems increases, so does the range of laws and legislation to

control and govern our use of these systems. The Data Protection Act (1998) is a law in

place to protect peoples privacy and state what can and cannot be done with data

collected about individuals. The Computer Misuse Act (1990) concerns unauthorised

access to and improper use of computer systems.

Task 1Think about 3 more activities or actions involving the use of computer

systems that could be considered ethically wrong.

22


24/214

Task 2Find out more about the Data Protection Act and the Computer Misuse

Act.

What can and cant organisations do with data they collect under the Data

Protection Act?

What exactly is and isnt allowed under the Computer Misuse Act?

23


25/214

Hardware

2Hardware describes t

physical components of

computer syste


26/214

Hardware

Hardware vs Software

Computer systems consist of both hardwareand software working together.

Software

describes the sequences of instruction, code and programs that computers execute.

While software may be stored on a CD, DVD or USB drive these devices themselves arenot software. Software cannot be physically touched.

Hardware

describes the physical components of a computer system anything that can be seen

or touched which can include input, output, storage and processing devices.

25


27/214

Permanent Storage

Computers need to be able to rememberlots of di!erent things.

Each time you login to your computer, you expect your documents, music and photos to

be ready and waiting for you - the computer needs to remember them all.

Each time you advance to the next level in a computer game, you expect to earn points or

additional lives - the computer needs to be able to remember this data both during thegame, and next time you play the game.

When using computers today, we seem to be performing lots of di!erent tasks at the

same time: we check our emails while having a video call with family, while browsing on

the Internet for a new pair of shoes, while streaming a new album.

26


28/214

Computers use several di!erent types of memory to ensure that they dont forget all of this

data.

Permanent Storage / Secondary Storage

Hard Disk Drives (HDDs) and Solid State Drives (SSDs) are used in modern computers as

permanent, secondary or persistent storage. When you save a piece of work or copy

photos from your digital camera to

your computer they will be stored on

a HDD or SSD.

HDD

A HDD is a stacked set of magneticplatters or plates. These platters spin

up to 10,000 revolutions per minute

(rpm), and mechanical arms seek

backward and forwards across the

spinning platter in order to read or

write data to a particular sector of

the disk.

Data is stored to a HDD by

manipulating and adjusting the

magnetic charge into one of two

directions. Each direction represents

0 (o!) or 1 (on).

HDDs are a type of secondary or permanent storage. They are persistent which means

that the data will remain intact even when the power is turned o!.

In 1956 when IBM invented the first HDD it cost around $65,000 and stored about 5MB.

Today, 100 will buy you a 5TB HDD (which is 5,242,880MB).

27


29/214

As well as making it very a!ordable to store large amount of data, HDDs are fairly reliable.

Often, when HDDs fail it is either because the disk has been corrupted due to interference from

other magnetic sources (such as speakers), or because the mechanical parts have failed.

Advantages Disadvantages

Large Capacity Relies on moving parts

Faster than optical disks e.g. DVDs Disk surface can become damaged

Persistent storage Heavy power consumption

Easily replaced / upgraded Noisy

Slower read / write speed than RAM

SSD

A SSD can be used as an alternative to a HDD.

Rather than using spinning platters and mechanical arms there are no moving parts in

SSDs. SSDs use microchips to store data using electricity rather than magnetism. Special

transistors are used which can retain their 1 (on) or 0 (o!) state even when there is no

power.

SSDs have very fast read / write speeds as there is no need to wait for a mechanical disk

to spin and an arm to move into position.

As there are no moving parts SSDs use much less power than HDDs so they are often

used in laptop computers in order to extend battery life.

As of 2013, SSDs are still a relatively new technology and as with all new technologies,

when they first emerge the price is extremely high. Due to the high costs involved, the size

of a SSD in a laptop is usually only around 300GB, compared with laptops which use

HDDs which can be over 2TB.

28


30/214


Fast read / write access Small capacity

Robust as no moving parts Very expensive

Low power consumption Limited life (100,000 writes)

Silent operation

Persistent (not volatile)

Cloud Storage

People like to be able to access their files, music, photos and more from anywhere at any

time. Rather than using secondary storage such as HDDs or SSDs to store these types offiles, people are increasingly turning to cloud storage.

Cloud storage may sound very science fiction, but all this means is that rather than storing

your data on your own HDD or SSD, you instead upload it to the Internet where it is stored

on a HDD or SSD in another computer or server.

In order to access your files you can now simply login to your cloud storage system from

any computer or device as long as you have an Internet connection. At the time of writingpopular cloud storage solutions include: Google Drive, Dropbox and iCloud.

TaskInvestigate the di!erent types of permanent storage available. What is the

typical size and cost for a HDD or SSD in a computer today?

29


31/214

30


32/214


33/214

webpage you were on when you switch to your email and then back to the web browser; it

remembers what you have written in a word processed document, even though it hasnt

been saved yet. Think of RAM as short term memory.

The more RAM a computer has, the better it can perform, especially when multitasking.Lets say that you open up a music player, a photo editing program, a web browser, a

social networking website and a word processor - all of the data to make the programs

work needs to be loaded from permanent storage into RAM.

RAM is known as volatile memory; when you shut down your computer and turn the

power o!, the contents of RAM is wiped clean. All of the data is lost.


Fast read / write access Small capacity

Robust as no moving parts Very expensive

Low power consumption Volatile

Silent operation

32


34/214

ROM and Virtual Memory

Read Only Memory (ROM)

Every time you switch on your computer it performs a sequence of tasks such as checking

that all of the hardware is present, checking that a keyboard and mouse is connected and

checking to ensure that the HDD or SSD contains the operating system; this is known as a

boot sequence.

We know that RAM is volatile, so once the computer is turned o!, everything in RAM is

wiped. Part of the boot sequence involves checking the HDD or SSD and we know that

these storage systems are too slow, so the instructions necessary to perform the boot

sequence cant be stored here.

33


35/214


36/214

Central Processing Unit (CPU)

The CPU is the brain of the computer where the sorting, calculating and searching takes

place. Although it only looks like one chip, the CPU consists of several components:

%Arithmetic and Logic Unit (ALU)

% Program Counter (PC)

% Control Unit

% Cache

% Buses

35


37/214

ALU

The ALU conducts the arithmetical and logical

operations such as comparing values,

addition, subtraction, multiplication etc.

Program Counter

The PC keeps count of which instructions

have been executed and which one is next.

The PC passes this information to the control

unit.

Control Unit

The control unit keeps things ticking over

nicely within the CPU. It uses electrical signals

to ensure that the correct instructions are

executed at the correct time in the correct

order.

Cache

Cache is another type of volatile memory that is very quick (and very expensive).Instructions and data are loaded into cache from RAM before being passed to the CPU

Buses

A bus is a pathway between di!erent components of the CPU. Instructions, data and

control signals travel through buses within the CPU to the di!erent components.

Fetch-Decode-Execute Cycle

Programs and data need to be fetched and loaded into RAM before they can be

executed by the CPU. Once loaded into RAM, the processor needs to decode the

string of binary digits being fed to it and determine whether it is receiving an

instruction such as ADD or a data value. The CPU can then executethe instructions

and perform the necessary calculations on the data it has been given.

36


38/214

Fetch

Get location of next instruction from Program Counter (PC), Get instruction and data by

going to specified location in register, Update (+1) to Program Counter (so the CPU knows

the address of the next instruction it needs to fetch)

Decode

The CPU now identifies the

operation code (op-code), part

of the instruction that has

been fetched so it knows

which operation to perform.

The second part of theinstruction will contain the

data to be operated upon.

Execute

The instructions and the data

are fed to the Arithmetic and

Logic Unit (ALU) which then

performs the calculation. The

result is saved back to a

register and then back to RAM

to be output to the user.

Executing Instructions

Whether its playing the latest computer game, editing an image or simply browsing the

Internet, all of those complicated tasks are broken down into a series of simplemathematical calculations to be processed by the CPU.

The CPU runs incredibly quickly but it doesnt store data, it simply crunches numbers. We

know that data is stored using secondary storage such as a HDD or a SSD, but these

devices are not quick enough to keep supplying data to the CPU at the rate at which it can

37


39/214

process this data. Computer Systems therefore make use of RAM, and an even quicker

type of memory called cache.

This constant stream of data from the HDD (or SSD) into RAM and then into Cache means

that there will always be data ready to pass to the CPU for executing.

As cache is so fast and so expensive, computer systems do not have very much cache.

There are multiple levels of cache (depending on the brand and type of CPU).

% Level 1 (L1) cache is always built into the CPU.

% Level 2 (L2) cache is usually either built into the CPU or on the motherboard between the

CPU and main memory (RAM)

% Level 3 (L3) cache is usually built into the motherboard

Each level of cache is increasingly quicker (L1 being the quickest) but with a smaller

capacity

38


40/214

Registers

Each CPU core executes just one instruction at a time. A typical instruction for a CPU to

execute might be:

ADD 8, 4

Right before these values are passed to the ALU, they are moved from L1 cache into the

CPU registers. Registers are very fast, very small segments of memory. Registers are

used to temporarily store the very next instruction and data to be processed. Once the

ALU has performed a calculation, the result of that calculation is then stored back to a

register, before moving back through memory into cache, then into RAM.

Hit or Miss

If everything goes to plan, the CPU will always have enough data to process. When the

CPU can find the next instructions in cache and load these into the registers ready to

execute this is known as a cache hit; the data and instructions are ready and the CPU

does not have to wait around.

If the instructions are not ready as they are still being loaded into cache from RAM (or even

from the HDD or SSD) the CPU will have to wait, this is known as a cache miss.

Clock Speed

In the human body, the beating of a heart is controlled by the Sinoatrial Node (SAN); some

individuals with heart conditions need to have a pacemaker fitted in order to replicate the

functionality of the SAN. The SAN (or pacemaker) causes the heart to beat and at that

moment oxygenated blood is pumped out of the heart and around the body, and

deoxygenated blood is pumped out of the heart into the capillaries surrounding the lungs

39


41/214


42/214

Software

3Software describes t

sequences of instructio

code and programs th

computers execu


43/214

What Is Software?

We know that Computer systems consist of both hardware and software working together

and weve explored hardware (the physical components of a computer system anything

that can be seen or touched which can include input, output, storage and processing

devices). So what is software?

Software describes the sequences of instruction, code and programs that computers

execute; the computer games, smartphone apps, operating systems and applications that

run on hardware.

There are several types or categories of software.

42


44/214

Operating Systems

An operating system (OS) is a collection of programs that control the system hardware

resources. The OS manages the flow of data between permanent storage (HDDs and

SSDs), main memory (RAM) and the CPU. The OS also controls communication with

peripherals connected to the computer system such as the keyboard and mouse.

An OS can be found running on almost all computer systems from digital television

recorders and smartphones to games consoles and web servers. Other software such as

word processors and web browsers rely on the OS and could not function without it.

43


45/214

SecurityPart of managing the system involves protecting users, system resources and files. There

is a danger when passing data between permanent storage, main memory and the CPU

that data could be lost, corrupted or overwritten - it is the job of the OS to ensure that this

does not happen and prevent conflicts.

One computer is not necessarily used by just one person. In schools, businesses and

even at home, computers need to manage several user accounts and ensure that the

correct files and access permissions are granted to the correct users.

Graphical User Interface (GUI)

When we use computer systems we manipulate windows, click on buttons, and scrollthrough menus.

We use a mouse to control a cursor on the screen and navigate this cursor to the icon we

wish to click on. This type of interface is called a Graphical User Interface (GUI). GUIs

44


46/214

allow users to interact with the system using graphical icons and visual indicators; today

this is the most common way for home users to interact with computer systems.

The way we interact with computer systems continues to change and develop. We are

increasingly using touch, voice and even gesture commands to control our computersystems.

Command-Line InterfaceBefore GUIs were developed, computer users had to type in each command they wanted

to give to the computer using a command-line interface. While today home users find

using a GUI easier, many systems administrators, programmers and expert users still

prefer to use the command-line as it can allow greater control and speed up certain

processes.

Windows

Developed by Microsoft, the Windows series is still the most popular operating system for

personal computers. The first version of this proprietary commercial OS was released in

1985 and called Windows 1.0. The latest version, Windows 8, was released in 2012.

45


47/214

Mac OSThe first version of Mac OS was released in 1984 and was simply known as the system.

It was not until 1996 when the software became known as Mac OS. The latest version is

known as OS X Mountain Lion and was released in 2012.

LinuxAlthough not as popular as an OS for home PCs, most of the websites that we visit and

the online services that we use are hosted on computers that run a Linux OS. As Linux is

open source, any developer can freely access the source code and modify it as they

wish; this has led to the development of many di!erent versions or distributions of Linux

including Debian, Ubuntu, Fedora and Red hat.

Linux is a popular choice for embedded systems and can be found controlling many

modem routers, digital and satellite television recorders and smartphones. In fact, almost

30% of smartphones in use today run a version of Linux. Did you know that Android is a

modified version of Linux?

Mobile Operating SystemsMobile devices are designed to be used di!erently to home

PCs. They need to consume less power as they run on a

battery, the screen is smaller so information needs to be

laid out appropriately, and today, most smartphones accept

touch or voice commands rather than input from a

keyboard. As such, standard operating systems were not

suitable to be used on mobile devices and developers had

to build specialist platforms suitable for this type of device.

Palm developed Palm OS and Windows developed

Windows CE in 1996, Nokia launched S40 OS (Java ME) in

1999 and Symbian brought out their OS in 2000.

Blackberry brought out their OS in 2002 and this provided

extremely popular with business users before Apple

launched iOS on the first iPhone in 2007. Google released

46


48/214


49/214

Task

Conduct some research using the Internet to find an example of acomputer system controlled by touch, voice and gesture.

48


50/214

Drivers

While the OS takes on much of the responsibility for controlling and managing peripherals

such as the mouse, keyboard and printer, it needs a little help. There are thousands of

manufacturers, producing thousands of di!erent products that you might wish to connect

to your computer system; many of these peripherals may have been designed after the OS

was released - so there is no way that an OS can possibly be prepared to communicate

with and control peripherals without some additional help.

Device Drivers are small programs written by the hardware designers specifically for a

piece of hardware. When you purchase a new printer or webcam it normally comes

packaged with the drivers, or instructions about where on the Internet you can download

the drivers. A driver recognises signals sent from a piece of hardware and translates these

so that the OS understands.

49


51/214


52/214

CustomSometimes, o!the shelf software is too generic and doesnt quite work in the way youd

like it to. Some companies have very specific requirements and need software to work in a

very specific way, look a certain way or integrate with existing computer systems in a

certain way. If o!the shelf software does not give businesses the control they need over

the software, they can commission a developer to build a new, custom written application.

There is often a significant cost involved in this, and as the software needs to be designed,

built and tested - it is not available immediately.

Software as a Service (SaaS)Traditionally, if you wanted an application to perform a certain job you would go to a high

street or online store and buy it. Once installed on your computer it was yours to use. Aftera period of time a new version of the software with new features might have been released

and you could choose to go and

buy the latest version, or

continue to use the (slightly out

of date) version you already

owned.

Increasingly, users are now

turning to Software as a Service

(SaaS) as an alternative. SaaS

invo lves users access ing

applications via the Internet

(usually using a web browser).

The software is not installed on

the users computer but isinstead hosted centrally on a

server in the cloud.

Unlike traditional software where

t h e u s e r p u r c h a s e s t h e

application once, SaaS is usually sold on a subscription basis with users paying a monthly

51


53/214

fee to access this on-demand software. Although some users may prefer to own

software rather than rent it, SaaS does mean that users always have access to the latest

version of the software and can access it from any computer with an Internet connection.

Some SaaS is actually free; examples include Google Drive and Apple iCloud which allowusers to access word processing, spreadsheet and presentation applications over the

Internet which are not installed on their home computers.

SaaS does rely on a stable, reasonably fast Internet connection and as such has only

become a possibility in recent years as higher bandwidth broadband services have

become available.

Proprietary vs Open SourceProprietarysoftware is developed commercially to be sold in order to make money. The

developer owns the software and they allow customers to use it by selling them a license.

Only the owner of the software - the intellectual property - has the right to sell it and

modify it.

As proprietary software is sold, customers can expect a reasonable level of service and

support; there is usually a help website, telephone number or email address.

Open source software is not owned by anyone. Developers write the software either

individually or as part of a community and make it freely available. Anyone can then use

the software as it is, or edit and the source code to make new versions of the software.

52


54/214

Utilities

We know that the operating system (supported by device drivers) take care of the

hardware in a computer system and provide a platform on which applications can run. In

addition to application software, computer systems often run small maintenance programs

or utilities.

Anti-virus software, firewall software, disk management software and backup software are

examples of utilities. Often these programs run in the background and perform their jobs

without us even knowing that they are there.

53


55/214

Data Representation

4Representing data with

computer systems is done

turning switches on and o


56/214

Binary

What is Binary?

We need to be able to store data within a computer system and transfer it from one

computer to another, for example when sending emails, storing images, and playing

sound.

Electronic devices such as computers consist of thousands of tiny electronic switches.

The basics of any electronic device means either electricity is flowing or it is not i.e. a

switch is either on or o!. Representing data within a computer system is done by turning

switches on and o!.

How on earth can we represent letters, reports and essays, videos and sounds with just 1

and 0? Thats exactly what computers do and its called Binary.

55


57/214

Example

So, data is passed through and stored in computers by turning on and o!

lots of tiny switches.

Switches are either ON (1) or OFF (0) therefore data in a computer can

only be represented by two numbers...1 and 0.

The normal number system that we use everyday and is very familiar to us has 10 di!erent

symbols 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. We use these symbols to write numbers, so 629means 6 lots of 100, 2 lots of 10 and 9 1s. In primary school we may have seen this

written in a table:

Hundreds (100) Tens (10) Units (1)

6 2 9

When learning basic maths at school, decimal tables are sometimes used like this one

(below). If we had one unit and we multiply it by 10 we get one ten. If we multiplied one ten

by 10 wed end up with one hundred and so on.

Thousands Hundreds Tens Units

56


58/214

The column values are 10 times the value in the previous column moving from right to left.

This is called the base 10 system, or the denaryor decimalsystem.

Computers do not use the decimal system. They use a system based on just two

numbers: 1 and 0. This is called the binaryor base 2 system.

In the same way that we multiply by 10 in the denary system to move across from right to

left, when using the binary system, multiplying by 2 moves across to the left.

See how we move across from right to leftwith 1, 2, 4, 8, 16, 32, 64, 128.

128 64 32 16 8 4 2 1

Binary Units

When using the denary (base 10) system we assign certain numbers special names, for

example 10 & 10 & 10 is 1000 which we call a thousand. 1 thousand multiplied by 1

thousand is called 1 million etc.

Binary also has some special names to refer to certain values.

The basic unit is 0 or 1 this is a binary digitor bit.

When talking about storing and transferring data within a computer system we talk about

bits and bytes. Bit is short for Binary Digit and a byte contains 8 bits. A nibble is

half of one byte (4 bits).

57


59/214


60/214

Denary to Binary

Performing conversation from denary to binary is also really simple.

Lets say wed like to convert the denary number 37 into binary. First well need our binary

column headers.

128 64 32 16 8 4 2 1

Now, looking at our target number of 37 we can ask ourselves, do we need 128 to get to

37?

The answer is of course, no. 128 is too big. We can put a 0 down in the 128 column.

128 64 32 16 8 4 2 1

0

We can then ask, do we need 64 to get to 37? The answer is again no as 64 is bigger than

36. We put down another 0 in the 64 column.

128 64 32 16 8 4 2 1

0 0

Next, we have 32. Can we use 32 to get to 37? Yes! 32 is smaller than our target of 37 so

we can use it. Well put a 1 in the 32 column to show that were using it.

128 64 32 16 8 4 2 1

0 0 1

We wanted 37, so far we have 32, so were still in need of 5. Can we use 16 to get 5? No:

too big. Can we use 8 to get 5? No: too big. We can put 0 down in the 16 and the 8

column.

59


61/214


62/214


63/214

Task

Here are some more binary numbers for you to convert to decimal but itsa little more tricky as you dont have the headings to show you what each

bit represents. The first one has been done for you.

Binary Decimal

1 10000000 128

2 00101001

3 10101010

4 1000011

5 11111111

TaskWork out what numbers are being represented here in decimal and write

down the binary equivalent. The first one has been done for you.

Decimal Binary

1 1 00000001

2 6

3 24

4 9

5 131

62


64/214


65/214

8 4 2 1

0 1 0 1

+ 0 1 1 1

1. To add these two numbers, we first consider the 1 column and calculate 1 + 1 which

equals 2. In binary 2 is written as 10. We carry the 1 to the 2 column and leave the 0

in the 1 column.

8 4 2 1

0 1 0 1

+ 0 1 1 1

1 0

2. Moving on to the 2 column, we calculate 1 + (0 + 1), which again in binary is 10. We

carry the 1 to the 4 column and leave the 0 in the 2 column.

8 4 2 1

0 1 0 1

+ 0 1 1 1

1 0 0

3. We now look at the 4 column and calculate 1 + (1 + 1). This equals 3 which in binary is

written as 11. We carry 1 to the 8 column and leave 1 in the 4 column.

8 4 2 1

0 1 0 1

+ 0 1 1 1

1 1 0 0

64


66/214

4. Looking at the 8 column we see that there are no further 1s to add together which

leaves us with an answer of 1100, or 1 lot of 8, 1 lot of 4, 0 lot of 2 and 0 lot of 1. 1100 is

the binary equivalent of 12.

8 4 2 1

0 1 0 1

+ 0 1 1 1

1 1 0 0

Here is another example, this time adding together the binary numbers:

0110101 plus 0001101.

64 32 16 8 4 2 1

0 1 1 0 1 0 1

+ 0 0 0 1 1 0 1

1 0 1 + 1 = 10 (carry the 1)

1 0 1 + (0 + 0) = 1

1 0 1 0 1 + 1 = 10 (carry the 1)

1 0 0 1 0 1 + (0 + 1) = 10 (carry the 1)

1 0 0 0 1 0 1 + (1 + 0) = 10 (carry the 1)

1 0 0 0 0 1 0 1 + (1 + 0) = 10 (carry the 1)

1 0 0 0 0 1 0 1 + (0 + 0) = 1

So the answer is 1000010 (the binary equivalent of 66).

65


67/214


68/214


69/214


70/214


71/214

If we convert the result back to decimal form....

128 64 32 16 8 4 2 1

0 0 1 0 1 0 0 0

we have 1 lot of 32 and one lot of 8 which is 40.

Our original problem was 49 - 9 which is 40. So it worked!

70


72/214


73/214

Binary to HexadecimalThe binary number for 143 is: 10001111.

128 64 32 16 8 4 2 1

1 0 0 0 1 1 1 1

This uses 8 bits or 1 byte of data. In order to covert this binary number into hex, we first

split the 1 byte into 2 nibbles (each of 4 bits).

8 4 2 1 8 4 2 1

1 0 0 0 1 1 1 1

Notice that we also change the column headers so that each nibble uses. 8, 4, 2, 1. We

can now work out the value of each nibble.

8 4 2 1 8 4 2 1

1 0 0 0 1 1 1 1

= 8 8 + 4 2 + 1 = 15

The symbols used in hexadecimal are 0-9 and then A-F. So, any number larger than 9 will

be represented by a letter. 15, is therefore represented by F.

8 4 2 1 8 4 2 1

1 0 0 0 1 1 1 1

8 F

10001111 in binary is 8F in hex. Here is another example.

72


74/214

The binary number for 111 is: 01101111

128 64 32 16 8 4 2 1

0 1 1 0 1 1 1 1

8 4 2 1 8 4 2 1

0 1 1 0 1 1 1 1

4 + 2 = 6 8 + 4 + 2 1 = 15 (F)

01101111 in binary is 6F in hex

TaskConvert the following binary numbers to hexadecimal. The first one has

been done for you.

Decimal Binary Working Out... Hex

54 00110110

0011

3

0110

6 36

195 11000011

99 01100011

101 01100101

200 11001000

73


75/214


76/214

8 4 2 1 8 4 2 1

0 0 1 1 1 0 1 0

2 + 1 = 3 which gives is 0011. In hex, A represents 10 which we get from 8 + 2. 3A in hex

is therefore 00111010 in binary.

TaskConvert the following hexadecimal numbers to binary. The first one has

been done for you.

Hex Working Out... Binary

9D

9

9

1001

D

13

1101

10011101

77

F2

A0

13

75


77/214

Hexadecimal to DenaryRather than using the column headers for denary:

1000 100 10 1

...or for binary:

128 64 32 16 8 4 2 1

...we use just:

16 1

If we then wish to convert the hex number 9F into denary we can note this down below the

column headers.

16 1

9 F

We now need to multiply each valueby the column headeras follows

16 1

9 F

9 x 16 = 144 F x 1 = 15

9 x 16 = 144, and F (which represents 15) x 1 = 15. We then add together the two values:

144 + 15 = 159. So, 9F in hex is the equivalent of the denary number 159.

76


78/214

Here are some more examples:

16 1

5 D

5 x 16 = 80 D x 1 = 13

5D in hex = 93 in denary

16 1

7 6

7 x 16 = 112 6 x 1 = 6

76 in hex = 118 in denary

16 1

C 3

12 x 16 = 192 3 x 1 = 3

C3 in hex = 195 in denary

77


79/214


80/214


81/214


82/214

ASCII

In 1960 the American Standards Association developed a standard set of codes to be

used in telegraph systems to represent the main characters used in communications; this

set of codes is known as ASCII (American Standard Code for Information Interchange).

ASCII allows us to represent all alphabetic upper and lower case characters, all numeric

characters, 32 punctuation and other symbols including space and 32 reserved non-

printable codes.

81


83/214

ASCII Table

ASCII Char ASCII Char ASCII Char ASCII Char

0 NULL 32 65 A 97 a

1 SOH 33 ! 66 B 98 b

2 STX 34 67 C 99 c

3 ETX 35 # 68 D 100 d

4 EOT 36 $ 69 E 101 e

5 ENQ 37 % 70 F 102 f

6 ACK 38 & 71 G 103 g

7 BEL 39 72 H 104 h

8 BS 40 ( 73 I 105 i

9 HT 41 ) 74 J 106 j

10 LF 42 * 75 K 107 k

11 VT 43 + 76 L 108 l

12 FF 44 , 77 M 109 m

13 CR 45 - 78 N 110 n

14 SO 46 . 79 O 111 o

15 SI 47 / 80 P 112 p

16 DLE 48 0 81 Q 113 q

17 DC1 49 1 82 R 114 r

18 DC2 50 2 83 S 115 s

19 DC3 51 3 84 T 116 t

20 DC4 52 4 85 U 117 u

21 NAK 53 5 86 V 118 v

22 SYN 54 6 87 W 119 w

23 ETB 55 7 88 X 120 x

24 CAN 56 8 89 Y 121 y

25 EM 57 9 90 Z 122 z

26 SUB 58 : 91 [ 123 {

27 ESC 59 ; 92 \ 124 |

28 FS 60 < 93 ] 125 }

29 GS 61 = 94 ^ 126 ~

30 RS 62 > 95 _ 127 DEL

31 US 63 ? 96 ` 128

64 @

82


84/214

While ASCII was useful for textual messages, it did not allow non-English language

characters or mathematical symbols to be represented. Today we use the Unicode system

which allows us to encode all characters. ASCII is now a subset of Unicode, so 78

represents the letter N in ASCII and in Unicode.

When typing the words She"eld Wednesday into a computer, the following sequence of

ASCII codes is generated.

S h e f f i e l d W e d n e s d a y

83 104 101 102 102 105 101 108 100 32 87 101 100 110 101 115 100 97 121

Remember, that computers cannot deal with decimal numbers, so these ASCII codes then

need to be converted into binary to be processed by the CPU or stored.

83


85/214

Character ASCII Binary Character ASCII Binary

S 83 01010011 W 87 01010111

h 104 01101000 e 101 01100101

e 101 01100101 d 100 01100100

f 102 01100110 n 110 01101110

f 102 01100110 e 101 01100101

i 105 01101001 s 115 01110011

e 101 01100101 d 100 01100100

l 108 01101100 a 97 01100001

d 100 01100100 y 121 01111001

Space 32 00100000

TaskWrite a phrase or short sentence and convert this into ASCII codes. Pass

your code (without the original) to a friend and ask them to convert this

back into English.

You could even convert the phrase or sentence into ASCII codes and then

into binary and ask a friend to work out what the original message said.

84


86/214


87/214


88/214

This photograph of the Sacr-Cur Basilica de Montmartre in Paris originally used 293KB

of space to store. The colour depth of the image has been reduced below. Notice what

happens to the image and the amount of memory used to store the image.

1 bit colour (2 colours) [12KB] 2 bit colour (4 colours) [18KB]

3 bit colour (8 colours) [28KB] 8 bit colour (256 colours) [95KB]

The more colours used, the greater the colour depth, and the more space required to store

the data.

87


89/214


90/214

If we zoom in on the 2 bit (4 colour) version of the Sacr-Cur Basilica de Montmartre

image we can clearly see the 4 colours used.

The computer uses 2 bits to store the four colours; each colour is assigned a binary value.

00 01 10 11

As images are stored as 1s and 0s, the computer needs to know how to interpret thesebinary numbers and display the image. Metadata means data about the data and is

included with each image. The metadata specifies the colour depth (e.g. 2 colours, 4

colours, 256 colours etc.) and the height and width of the image in pixels

89


91/214


92/214


93/214

X (Time) Y (Frequency)

0 25

1 30

2 20

3 15

4 30

5 5

6 10

7 5

8 25

9 0

10 10

Once a sound wave has been sampled and the tone recorded as a sequence of number

values, it is easy to convert these values into binary for processing and storage.

If we increase the sample frequency and record the values of the curve more often the

digital representation of the audio will sound much closer to the original analogue version

as more tones (high and low sounds) can be recorded as illustrated in the following chart.

92


94/214


95/214

Recording 16 bit audio sampled at 44,100Hz means that the audio will play at

1,411.2kbps; this is standard CD quality sound

300kbps MP3s are compressed audio files with a reduced bit rate and reduced sample

frequency. MP3s will have a reduced range of tones due to the lower sample rate, and areduced range of pressures due to the reduced bit rate.

94


96/214


97/214

What Are Networks?

Stand-alone computers are devices that are not connected to any other computer

system. A user can interact with a stand-alone computer and input data; the computer

may then process the data and output information back to the user, but no data is

exchanged with any other systems.

While stand-alone computers were once extremely useful, computer scientists wanted to

coordinate and pass information between computer systems in di!erent locations and so

built networks; today we could not live without them.

96


98/214


99/214

The network was designed to allow communications between military installations around

the the United States; data on the network did not have to travel one particular route from

source to destination; it could instead travel via any other nodes on the network. This was

important, as ARPANET was designed to still function even if large portions of it were

destroyed.

It is said that ARPANET was designed to ensure messages could be sent even if some

areas of the country were destroyed due to nuclear war. Considering the previous map, if,

for example, HARVARD needed to send a message to UTAH, this could go from HARVARD

-> BBN -> MIT -> ILLINOIS -> UTAH. In the event that ILLINOIS was destroyed, however,

the message could instead take the route: HARVARD -> BBN -> RAND -> SDC -> UTAH.

Although originally conceived for military purposes, computer scientists quickly

recognised non-military application for computer networks. Today, we use networks on a

daily basis; examples include mobile phone networks, home computer networks and of

course, the Internet.

98


100/214

Advantages of Networks

BackupBacking up data is much easier when all files are stored centrally on a

networked file server.

Communications

Whether its email, video conferencing, instant messaging or file sharing -

it wouldnt be possible without networks. The physical infrastructure

(cables and hardware) allow us to communicate instantly with people

around the world. Without networks communications would have to take

place face-to-face which would be much more costly and time consuming.

Device Sharing

Businesses often need to print and scan documents, but not every single

computer in the office needs a printer and a scanner. These peripherals

can be connected to the network so that any computer on the network can

access them.

Distributed Systems

When connected via a network it is possible to use several computers to

process the same data in order to speed up the process. The Search for

Extraterrestrial Intelligence (SETI) project distributes massive amounts of

data collected by radio telescopes via the Internet to the computers of 3

million volunteers for processing.

File / Database Sharing

Networks allow files and databases to be shared and accessed by more

than one person in more than one location. All users access the same,up-to-date / live version of the data which makes collaboration possible.

Security

Networks usually have a central server which is responsible for managing

permissions and access rights. The network administrator can allow and

revoke access to files and resources and easily review server logs should

a problem or security breach occur. It is much easier to investigate a

problem when this can be done from the central server without needing to

visit several stand alone computers.

Software Sharing

When several computers require the same software (such as word

processing software and an email client), networks make it possible to

remotely install this software from the server. It is not necessary to go to

each computer individually to install the software, Maintaining the

software and installing updates is also far easier over the network.

99


101/214


102/214

Network Classification

Local Area Network (LAN)A LAN is a collection of computer systems (and peripherals such as printers) connected

together. A LAN will link computer systems within a building and even across several

buildings on the same site (within the same geographical region), for example, the

computer systems within a university campus (that has several buildings) will be

connected together using a LAN. The organisation, in this case the university, isresponsible for installing and managing the network and the network infrastructure

(including all of the cabling).

Wide Area Network (WAN)While a LAN is a collection of computer systems connected together on a single site, a

wide area network (WAN) is a collection of computer systems connected using additional,

101


103/214


104/214

Network Infrastructure

In order to build computer networks, we need infrastructure including servers, clients,

network interfaces and cables.

ServersServers are powerful computers that serve smaller client computers on the network. They

provide and manage services which may include printing (print servers), access to files (file

servers), websites (web servers) etc. One server may provide services to thousands of

client computers.

ClientsThe computers most of us use on a daily basis are clients. These are less powerful than

servers and rely on servers to provide certain services. If, for example, you wanted to print

103


105/214

a document to a networked printer, your computer (client) would send the request to a

print server; the print server would be receiving lots of requests from other clients too and

would need to manage these in a print queue. When you wish to view a webpage, the web

browser application on your computer (client) would connect to the web server and

request a webpage; the web server would be receiving lots of requests from other clients

too and would need to manage these.

Network Adapter / Network Interface CardIn order to be able to connect to a network, your computer must have a network

interface card (NIC) or adapter. A NIC is a special piece of hardware built into most

modern portable computer systems and can be a wired NIC or a wirelessNIC. NICs can

be purchased and added to desktop computer systems if they do not already have one.

HubsIf you have several computer systems that youd like to connect to a network, you can

connect them all to a hub. When a packet of data arrives into a hub it is sent out to all

connected clients.

104


106/214

SwitchesOften, you dont want to send data to every client on a network. A switch is an intelligent

hub that can filter the data it receives and only send it on to the intended recipient(s).

Switches manage the flow of packets of data sent across networks and ensure that data

gets to the right place.

ModemsA modulator-demodulator (Modem) is used to convert the digital information used by

computer systems into analog signals that can be transmitted across the telephone

network.

RoutersRouters are used when connecting together more than one network; they ensure that

network tra"c is transmitted and received properly from one network to the next. Routers

may be used when connecting together two local area networks (LAN), a LAN to a wide

area network (WAN), or a LAN to the Internet.

IP Addresses

In order to successfully send a message from one computer to another, we need to knowwhere to send it. When computers connect to a network they are assigned an Internet

Protocol (IP) address; this is a unique address which is di!erent to every other address

on the network.

IP addresses consist of 4 groups of decimal numbers such as: 74.125.224.72

Each decimal number represents a binary byte and computer systems will communicate

using binary - we however, write IP addresses in decimal form as its easier for us tounderstand.

The IP address: 74.125.224.72would be transmitted and read by computer systems as:

01001010 01111101 11100000 01001000

105


107/214

IP addresses can be static, so that a computer will always have the same address, or

dynamic, so that a computer may be allocated a di!erent address each time it connects

to the network.

MAC AddressesUnique Media Access Control (MAC)addresses are allocated to every Network Interface

Card (NIC) by the manufacturer. Every network interface has a di!erent MAC address, so a

laptop with a wired network connection and a wireless network connection will have 2

di!erent MAC addresses.

MAC addresses consist of six pairs of hexadecimal numbers such as: C1:B3:57:12:C1:F2

BandwidthBandwidth describes how much data can be transmitted through a medium - which could

be wired or wireless - at one time. If a network is being used by several users at the same

time - all of whom are transmitting or receiving data - the network may slow down as there

is not su"cient bandwidth.

Bandwidth is measured in bits per second (bps).

106


108/214

Network Types

Peer-to-peerIn a peer-to-peer network there is no central control or administration; all computers

connected to the network are equal. Files, software, email and printers are managed by

each user individually on their own computer.

While a peer-to-peer network is easy to configure (you just need to connect each

computer to a hub), they do have limited use because there is no central control so each

user is responsible for installing their own software updates, managing their own security,

performing their own backups etc.

107


109/214


110/214


111/214

along to the next node. If one node is busy, or o$ine, the packet can be routed via a

di!erent node.

Packets that make up the same file, may travel to their destination via di!erent routes and

may arrive at their destination in any order. The Transmission Control Protocol (TCP) isresponsible for ensuring that all expected packets have been received and that they are

then reassembled in the correct order.

110


112/214

Network Topologies

There are several ways of configuring a computer network, each with benefits and

drawbacks. An organisation looking to setup a network will need to consider costs and

performance when choosing which topology (configuration) to use.

111


113/214

BusBus networks feature a central backbone cable to which all computer nodes including

servers and printers are connected.

When any computer

on the network needs

to pass data to another

computer, the packet

of data is sent down

t h e b u s i n b o t h

directions to every

computer node on thenetwork unt i l the

intended recipient is

found. If more than

one node attempts to

send data down the bus at the same time collisions occur and as such bus networks do

not cope well will lots of tra"c; they are only suitable for domestic home use, or for small

organisations with few computer nodes.

A bus network only features one main cable and and as such is relatively inexpensive to

setup.

RingRing networks are setup in a circular configuration and

data travels only one way around the ring passing

from one computer to the next with each computeracting as a repeater.

Every node in a ring network is critical; if one of the

computers is broken, it cannot pass on the data to the

next and the network will fail.

112


114/214

StarEvery node on a star network is connected to a central

server or switch; this is the most common network

infrastructure. Star networks take much longer to

setup and configure and and more expensive than the

alternatives as it is necessary to run a cable from every

computer back to the central server (which might be

quite far away). Despite these drawbacks, the

performance of star networks is much greater than the

alternatives and if any node breaks, it does not a!ect

any other computer on the network. It is also very

simple to add additional nodes to the network withoutdisrupting existing nodes.

113


115/214


116/214


117/214

HTTPHypertext Transfer Protocol (HTTP) is the standard protocol used for transmitting

webpages (built using Hypertext Markup Language - HTML) over the Internet. Although

modern web browsers do this for us if we forget, when visiting any website, the domain

name is also preceded by the http:// prefix to let the browser know that this protocol

needs to be used.

FTPViewing websites is handled using HTTP, but in order to copy those website files to the

web server (so that they become live on the Internet), we need to use the File Transfer

Protocol (FTP).

116


118/214

The Internet

The Internet is a network of networks. The Internet consists of private LANs connected by

making use of infrastructure owned and managed by third parties such as the telephone

network and fibre optic cables; the Internet is therefore a very large WAN.

Key PointThe Internet and the World Wide Web (WWW) are not the same.

The Internet is the infrastructure of cables and routers that connect

computer systems. The World Wide Web is a collection of webpages and

resources hosted (stored) on the Internet.

117


119/214


120/214


121/214

Router

Routers are used to connect one network to another. When we connect to the Internet, we

are essentially connecting our home network to another network (the Internet), so it is

necessary to use a router. Routers ensure that packets of data leaving one network arecorrectly passed to another network so that they reach the intended destination. It is

common today to buy a single box that is both the modem and router in one. This box

also often acts as a wireless access point too which allows our computers to access the

Internet wirelessly.

Hypertext Markup Language (HTML)HTML is the standard adopted for the creation and interpretation of webpages. Even the

most complex modern webpages make use of HTML.

HTML is a text based language that uses tags to define the content of webpages. Tags

work in pairs, so it is necessary to have an opening and a closing tag. To create a large

heading, for example, we would write:

This text is our heading

Notice that we have used the opening tag and the closing tag . Some of the

most commonly used tags include:

... Contains details about the webpage, such as the author of the page and the title.

... The title will appear at the top of the web browser when viewing this page.

... The content appearing between these tags forms the main page content.

...
Paragraphs are defined using these tags. Each paragraph will begin on a new line.
... Heading tags define a title or heading. is the largest heading tag.

This tag inserts an image and tells the web browser the location of the image file.

... This tag defines a hyperlink.

See some of these tags in action in the following webpage written in HTML.

120


122/214

Sheffield Wednesday Football Club

Welcome to the Unofficial Sheffield Wednesday Football Club

Website

A history of the club

Sheffield Wednesday Football Club is a football club based in

Sheffield, South Yorkshire, England. They are one of the oldest

professional clubs in the world and the third oldest in the English

league.

The Wednesday, as they were named until 1929, were founding

members of The Football Alliance in 1889, and its first champions

that inaugural season. The Wednesday joined The Football League

three years later when the leagues merged. Sheffield Wednesday were

also one of the founding members of The Premier League in 1992.

For more information about Sheffield Wednesday visitthis link

121
http://www.swfc.co.uk/http://www.swfc.co.uk/http://www.swfc.co.uk/http://www.swfc.co.uk/


123/214


124/214

The following CSS code is responsible for transforming the original webpage into the

styled version.

h1{ font-family: sans-serif; font-size: x-large; color: blue; text-align: center;}

h3{ font-family: sans-serif; font-size: large; color: purple;}

p{ font-family: sans-serif; font-size: medium; color: black;}

a{ text-decoration: none;}

123


125/214

CompressionEverything we do on the Internet involves uploading data from our computers, or

downloading data to our computers. When listening to music using an online streaming

service or downloading a video file a significant amount of data needs to be downloaded.

Faster Internet connectivity means that we do not now need to wait as long for data to

upload and download, but compressing data so that it takes up less space can also

speed up this process.

Lossy Compression

Removing some of the data from a file can reduce the file size and this makes transfer

over a network faster. Removing

data, means removing detail so its

important not to remove too much

detail otherwise the quality of the

file being compressed will become

unsatisfactory.

W e sa w i n t h e D a t a

Representation chapter of this

book that reducing the quality

(compressing) a photograph from

24 bit (16,777,216 colours) to 8 bit

(256 colours) means that the image takes up just 95KB of space rather than the original

293KB of space. Although a lot of detail has been lost, it is not noticeable, as illustrated in

this 8 bit image of the Sacr-Cur Basilica de Montmartre in Paris.

124


126/214

Lossless Compression

Encoding data can allow us to reduce the file size, without losing any data or quality.

Peter Piper is a nursery rhyme and well known tongue twister:

Peter Piper picked a peck of pickled peppers. A peck of pickled peppers Peter Piper

picked. If Peter Piper picked a peck of pickled peppers, Where's the peck of pickled

peppers Peter Piper picked?

Currently this text consists of 34 words. In order to compress this section of text, we could

encode each word with a number.

peter 1

piper 2

picked 3

a 4

peck 5

of 6

pickled 7

peppers 8

if 9

wheres 10

the 11

Now, rather than 34 words, we only need 11 numbers to represent the original text as

some of the words are repeated.

1 2 3 4 5 6 7 8. 4 5 6 7 8 1 2 3. 9 1 2 3 4 5 6 7 8, 10 11 5 6 7 8 1 2 3?

We dont need to store the same word more than once and this allows us to save space.

125


127/214


128/214


129/214


130/214

Thinking About Data

What is a Database25, She"eld and Volkswagen are pieces of data, but on their own lack meaning and

organisation. Does 25 represent age? house number? miles? We need to organise this

data so that we can understand it; then it can become useful.

Key PointA database is a persistent store of related data that is organised in such a

way as to make it useful.

129


131/214

Databases are not temporary deposits of information they are persistent or permanent

collections; as such they are stored in computer systems on persistent or permanent

storage (HDDs or SSDs) rather than volatile memory. Databases contain related data; all

the information about one person or one thing, for example, a company might have an

employees database which contains all the related data about employees such as name,

age, address etc.

TaskFor each of the organisations listed below, list the data that they would

need to collect, store, organise and search through.

$ the police

$ utility companies

$ hospitals

$ supermarkets

$ banks

A hospital deals with thousands of patients each day and its important that they have lots

of information about each person they treat in order to provide the best possible care.

Here is some information a hospital may collect about a patient.

Asthma 100813 020688

325434

Chandos Steven

Hardman Earp

Does all this make sense? Can you understand what this data means?

130


132/214


133/214


134/214

Database Management Systems

Organisations use Database Management Systems (DBMSs) to store, organise, add, edit,

delete and search through records. A DBMS is a piece of software that allows us to create

and work with databases. Just like a word processor allows us to create letters, a DBMS

allows us to create databases. DBMSs also manage:

%Automatic Backups

% Concurrency Control

% Data Integrity

% Data Redundancy

133


135/214

Automatic BackupsMany DBMSs allow you to schedule automatic backups. Consider the implications to a

hospital, popular social networking website, or bank of losing all of their data. It could cost

these organisations millions of pounds and even endanger lives. Its therefore critical that

organisations backup their data.

Concurrency ControlWhile a simple database might only need to be accessed by one person at once, some

databases need to be accessed concurrently by multiple users. At the time of writing the

most popular social networking website in the world has over 526 million active users

each day. This website, like many others, is driven by databases, and with 526 million

active users each day, the chances are that more than one person will be accessing theirdatabases at the same time.

DBMSs need to manage and control concurrency to avoid conflicts. Consider a ticket

sales website allocating seats and taking payment for sporting fixtures; many users may

be viewing and selecting seats at the same time. What is to stop two people choosing and

paying for the same seats at the same time? Who would the seats belong to?

134


136/214


137/214

When entering data into a computer system to be stored in a DBMS validation checks can

be performed on the data to assure the integrity of the data.

Validation Check Description

Format

Date of Birth is usually entered as DD / MM / YYYY.

Entering 01 / CB / QM34 would cause the format

validation check to fail as only numbers were

expected.

Length

UK telephone numbers are no longer than 11 digits

long. Entering 12 or more digits for a telephone

number field would cause a length check to fail.

Lookup

When entering a car registration, for example, a

lookup check might be performed to check the

registration against a list to ensure that the

registration actually exists.

Presence

This check looks to see if each field has been

completed. If a telephone number or email address

field has been left blank, the presence check will fail.

Range

A range check ensures the data entered is within a

valid range. A valid range for age might be 1-110.

Somebody inputting their age as 130 is likely to be

entering invalid data.

Type

A telephone number will have up to 12 digits and no

letters. Entering letters in a telephone number field

will cause the type check to fail.

Verification is another method of ensuring data integrity and is often used along with

validation. Typing in the same piece of data twice (such as when confirming a password) is

an example of verification - its there to make sure you dont accidentally press the wrong

key.

Data RedundancyWithin organisations there are often several departments: in a hospital for example, there

are wards specialising in the heart, the brain, fractures and bones etc.

If a patient visited several wards in a hospital and each ward created a new database

record for the patient data would be duplicated. Even if the patient only even visited one

136


138/214

ward, but attended several times, multiple records for that patient would be created as

illustrated below where David Jones has attended several times on di!erent dates.

Having redundant (duplicated) data is a problem. Should the patient change their name,

move house or get a new telephone number, each department would then need to update

every single occurrence of the patients details in their records; if some departm

computing academy ebook

Documents