Notes for F452
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Transcript of Notes for F452
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Computer science
3.1.1
1.
Hardware. "Hardware is the physical part of a computer system - theprocessor(s), storage, input and output peripherals".
2. Software. "Software consists of programs, routines and procedures (together
with their associated documentation) which can be run on a computer system".
3. Input device. An input device "is a peripheral unit that can accept data,
presented in the appropriate machine-readable form, decode it and transmit it
as electrical pulses to the central processing unit".
4. Storage device. A storage device is "a peripheral unit that allows the user to
store data in an electronic form for a longer period of time and when the
computer is switched off. The data can only be read by the computer and is not
in human-readable form".
5. Output device. An output device is "a peripheral unit that translates signals from
the computer into a human-readable form or into a form suitable for re-
processing by the computer at a later stage".
6. Peripheral. This "is a piece of equipment (or hardware) which can be connected
to the central processing unit. They are used to provide input, output and
backing storage for the computer system".
7. Computer- machine or device that contains and processes information
8. A PCis a microcomputer
9. Mainframe- one large computer that serves hundreds of terminals
10.Supercomputercomputer that has massive processing power and storage
capabilities@; may use multiple processors
11.Processor - A thin slice of silicon, containing millions of microscopic transistors and
circuitsruns in gigabytes (GB)
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Layers of software
Application software
- General purpose (word processor, databases etc.)
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Special purpose ( anti-virus software)- Bespoke ( personalised and made for you and your company )
System software
- Operating system software
- Utility programs
- Library programs
- Compilers, assemblers and interpreters
For general purpose programs in the exam use the general terms e.g. (word processor
instead of Microsoft word) or database instead of access.
Application software
The term applications software refers to all the programs used for a variety of purposes,
such as writing letters, creating webpages, and managing businesses.
General purpose software
The term general purpose software refers to applications with a number of common uses:
- Word processing packages
- Spread sheet packages
- Database packages
- Desktop publishing packages
- Presentation packages
- Graphics packages
These are often supplied as a software suite or integrated packages.
They are often better known by their brand names
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Special purpose software
- Special purpose software is designed to carry out a specific task rather than
providing a range of features for different uses.
- It is commonly used by organisations, businesses and in the home.
Business
Payroll systems, Financial accounting, Stock control
Home
Music composition, Photograph editing
Bespoke (custom) software
- The term bespoke software refers to software that is tailor made for you rather
than of the shelf
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It is usually for businesses which need specific programs- It may be created in- house or using programmers from a software house
- It can be time consuming and costly to produce, but it will do exactly what is
required
System software
The term system software refers to the software needed to set up and run the computer.
Operating system (OS)
The operating system software (OS) is a suite of programs that control software and
hardware
It acts as the interface between the user and the computer
The computer would not even turn on without an OS
OSs are better known by their brand names for example
- Windows
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Linux- Mac OS
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Utility programs(spell checkers, wizard programs, compression software, and the recycle
bin)
They are designed to make certain tasks more efficient and easier.
Library programs
- These are created for systems with multiple uses
- They are similar to utility programs although.
- There are usually a number of these available for a particular system
- It may work alongside other applications
- An example could be the science department of a university whose users may
need mathematical -library programs to carry out tasks on scientific data
Compilers, assemblers and interpreters
- These are used by programmers to convert the programme code into a form that
can be understood by the processor.
- A program is a series of instructions
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Compilers, assemblers and interpreters are used to turn these instructions into amachine code that the computer can carry out the instructions
Q1) the difference is that hardware is a physical component and software is a digital
component
Q2) Inputa device that helps enter information into the computer and complete a process
Output- a device that gives information out to the environment e.g. speaker and
monitor
Q3) peripheralsmouse, keyboard, speaker
Q4) OCR A-level computing book
Q5) Input device (keyboard, mouse, scanners)
Output device (monitors, speakers, printers,)
Storage device (usb, cd, DVDs, hard drive)
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3.1.2
System life cycle
- A process
- Used for developing ICT systems , also useful for other developments
- Each stage is dependent upon the previous stage
- Can return to previous stages for changes to be made
- Different personnel may be used at different stages
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Definition
- Identify the promlem
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May require- New system
- Upgraded system
Investigate
- The new requirements
- Current system procedures
Why develop a new system?
- Maintenance
- Out-dated system
- System is inefficient
Suitable problems for computerisation
- Repetitive tasks
- Need for precision
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Complex operations
- Dangerous situations (bomb disposal)
Decide whether to developed a new system
- is it feasible ( is it possible to make it and implement it is it affordable)
Some constraints might be: (money, time, software availability, hardware availability, personnel
availability)
Investigation and analysis
- An extension of the definition stage
- Only done if the feasibility answer is yes
The analyst must find out the following
- full details of the problem to be solved
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Methods and documentation
The following methods may be used
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Interviews
- Questionnaires
- Examination documents
- Observing user activities
User involvement helps to find and cater for the usersneeds. Also it will help the user find out the
way the new system works.
The system must meet the user requirement.
Design
The design must meet the userspreferences
It will be compared to the requirements at the end of the stage
The design stage may identify a problem in the requirements which will be fixed with the users
authority
Tasks in design
The design incudes
- Project planning and the Gantt chart
- Defining the data dictionary
- Tables, fields, data formats formulae
The Gantt chart tells us when the specialist people (electrician, programmer) comes in so as not to
waste their time waiting for others to finish their jobs so there part can be completed
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Importance of a good design interface
Clear
It has to be clear so that the user knows how the application works and so they will know where to
go and what to do. The user needs to be comfortable in what they doing, if the website is not clearthen the user would not get confused and frustrated.
Efficient
The interface has to efficient in a way that I does not crash and when going on to different functions
it does it fast and it doesnt require much effort in doing so. Also, it has to be efficient so that
everything that is needed on the website is there. It can also be efficient if people can access the
features easily.
Responsive
Responsiveness is an important feature for a good interface design, firstly because it needs to be fast
and if the website is unresponsive and the people have to wait for things to load, then the person
would get frustrated. When a user sees things load quicker than this person will feel more
comfortable.
Attractive
A good interface should be attractive because if it is attractive than user that it using will feel
satisfied in using this interface, therefore if it attractive than users would look forward to using,
whereas if it was dull than the user would just use it.
Forgiving
Forgiving is important as if the person deletes something by accident or does a mistake than the
interface can easily undo this error and the person would not lose the work, for example, if a person
by accident deletes a file, than you should be able to undo it.
Feedback
This is important because when the user presses a key the visual feedback will show them that theyhave successfully clicked the button properly. For example on Word processor if you press the undo
button it shows a border around the icon while the background changes colour.
Simplicity/complexity
This is a key feature because if someone starts using a computer/phone and sees that the interface
is too complex they wont be able to use it or know what ever button does. Furthermore if the
interface is the black screen where the user needs to type in their own command lines they may not
know any.
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Familiar
This is important because when the user looks at the interface and sees that it is familiar then he will
feel comfortable when using it, so they know what to expect and how to use it properly. So identify
similar things and integrate them into your user interface
Design 2
- Identifying the outputs e.g.
- Sketching output screens
- Sketching reports
- Identifying what data must be included
Defining what process that needs to take place
- Pseudo code
- Flow charts
- Descriptions
Test plan
- Identifying tests to be carried out and the method
- Identifying test data
- Identify expect results
- Each design sketch will include design specifications
Investigation Methods
Analyst must find out the following:
Full details of the problem to be solved
Current methods of carrying out tasks
Detailed requirements (user needs) for the new system
Methods that may be used by analysts:
Interviews
Questionnaires
Examining documents
Observing user activities
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These methods can be used depending upon:
- The information required by the analysts
- The people who need to be involved
- Availability of personnel
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What is permitted by the organisation?
Questionnaires
Useful for gathering information from lots of users
Questions need to be structured carefully:
Elicit the information required
Enable the analysts of the data effectively
Gather enough information without causing a large burden upon the users
Online questionnaires enable quicker analysts of data
A balance of questions is required:
Multiple choice- to make analysis of results more effective
Opinion ratingsto find out how effective the current system is
Open questionsto enable them to add opinions
Advantages:
Good for lots of users
Can elicit information from each user
Questions can be planned
Lots of responses can be analysed
Useful if its difficult to arrange appointments
Honest answer more likely due to anonymity
Disadvantages
Can appear to be impersonal
Questions need to be clear to elicit appropriate response
Cannot guarantee they will be returned
Cannot ask users to elaborate on the spot
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Interviews / Meetings
Used for individual users of small groups
Management
End users Customers
Can be used to elicit fuller responses than questionnaires
Need to be well planned:
Question should be planned
Potential answers should be considered
Possible follow up questions should be planned
Analyst can ask further questions of the user based upon user response
Interviews
Advantages
More detailed information can be obtained
The analyst can ask the users to elaborate upon their answers
Additional question can be asked
Opinions can be expressed informally
Body language can be read
Disadvantages
Time consuming
Not appropriate for lots of users
Does not provide statistical data analysis
Availability of the user
Lack of anonymity may lead to honest answers not being given
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Examining Documents
Collecting Documents used by the organisation:
Data collection forms
Invoices Receipts
Calendars
Purchase order forms
Business cards
Letter heads
Advantages
Analyst will identify how all the data used on the documents
Useful for identifying how data flows with an organisation
Disadvantages
Does not show all data used
Only shows formally used data
Not useful if documents are not used by the organisation
Observations
Observing users carrying out their normal everyday tasks
Can be done by:
Sitting with users ( questions be asked as you go along)
Using video camera (users may be less intimidated)
Users may try to do everything how it should be done rather than how they usually do it
Watching the real thing is better than descriptions
Very suitable where documentation is not available
Observations will include:
Tasks that are completed
How long each task is
Inputs and outputs used for each tasks
Inefficiencies with the tasks
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Advantages:
Can find out how tasks are performed
Can see inefficiencies in tasks
Useful if no documentation exists
Can share the experience of the users
Can asks users questions as they do their work
Disadvantages:
Users may be intimated being watched
Users may do things properly rather that usually when being watched
Takes up a lot of the analysts time
Only a snapshot is seen
Data Flow Diagram (DFD)
This is a diagrammatical way of showing how data moves around a system
It is not concerned with how the system processes the data- it only shows the flow
It uses four symbols:
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Structure Charts
Structure charts are also known as Jackson Structure Diagrams.
Structure charts shows a top-down approach
The top level shows major parts of the system
Each successive layer breaks the program down into smaller steps
They also work from left to right to shows how the system will work.
Flow charts
Flow charts use a recognised set of symbols to represent how the component parts of the system
will work
They are used to provide an overview of the system
Several may be needed to show a full system
Pseudo code
Pseudo code is lines of codes that show how the system will work
It does need to use the exact commands used in real programming language, as long as it shows how
the program will work
The programmer can later turn the Pseudo code into a real code
It allows programmers to plan out their ideas without getting bogged down in the techniques of
particular language
It can be used to write code in a very detailed way or just to provide an overview
It is important to be consistent when writing a pseudo code
You are inventing the code but you have standard way of writing the code
For example, you cant use the command output in one part of the code and then display
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Example of Pseudo Code:
This Pseudo code is designed to count all the elements in an array that that have a value greater
than 300
Set counter to 0
Set Found Count to 0
Testing Process
during design
Select data that will be used to test the system
Plan the tests
During Tests
Run the tests
Identify faults
Correct the faults
User testing
Purpose
Ensure system is free of errors
Although this cannot be guaranteed
Depends on the depth of testing
Ensure system ,athces requirements specification
To give users confidence on the new system
Test data
Test data is selected during the dosing stage
Data selected for input tests needs to cover the following types
Valid normal data
Data that is acceptable to the system
Invalid data
Data that is not acceptable for the system
Data that generates error messages
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Extreme valid data
data that is only just acceptable to the system
Extreme invalid data
data that is only just not acceptable to the system
Purpose of test data
Ensure that all possible types of data can be processed by the system:
Valid
Invalid
Extreme
Installation
Also known as changeover
Testing must be completed first
To ensure the system works
So customers get a quality product
Four main methods:
Parallel
Phased
Pilot
Direct (plunge)
Parallel running: The new system is run alongside the old system. Both systems operate together.
This allows the new system to prove itself before the old system is abandoned - data generated by
the new system can be compared to data generated by the old system. It also means that staff can
be trained and gain confidence in the new system. Of course, if you are running two systems
together, that means twice as much work for everyone for a short time
Pilot running:The new system is run alongside the old system, but only a portion of the data is
actually used in the new system. This method is less of a drain on resources. Data from part of the
new system can be checked with the old system, but you cannot check how the whole system will
react until you have got the whole system up and running.
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Direct changeover: The old system is stopped and the new system is started. This might happen over
a weekend, for example. If something goes wrong with the new system, then it has to be sorted out
because you cannot fall back on the old system. Staff training needs to take place in advance with
this method.
Phased implementation: Parts of a new system completely replace parts of an old system, whilst the
old system continues to be used as required. The part of the new system that has been installed can
be used for staff training and can prove itself before the next part of the installation takes place. This
method takes longer than the direct changeover method. A company with 10 branches may install a
new accounting system in one branch first, for example. They run it in the branch until it has proven
itself and possibly bring in staff members from other branches for training. Once the system has
proven itself in one branch, it can then be phased into the other branches
Iteration of the life cycle
Maintenances start the life cycle over again
When changes are made they need to be:
Identified (definition)
Specified (investigate & analysis)
Designed (design)
Developed (implementation)
Tested (testing)
Updated in the system (installation)
Documented (documentation)
Evaluate (evaluation)
Prototype
When a piece of software is being designed it is often useful to design a small part of it so that partcan be considered and decisions made about it without the rest of system getting in the way. This is
particularly important when designing parts of the system which the client/end user will be using. In
a data handling example it is important for the end user to be happy with the input and output
screens.
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An iterative process between
Design
Development
Effectively the system is designed as it is developed
In stages
User sees and evaluates each stage
- Gives feedback
- Changes are made
- These may include changes to requirements specification
Next stage is started
Usually starts with the user interface
User does not have to interpret designs
They see what it actually looks like
Feedback is therefore more effective
Prototype
userfeedback
changesimplemented
next stagedeveloped
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RAD essentials
Prototyping is the core method for design and development
Time boxing
- Each stage of development builds upon the previous stage
o
Requirements are at each stage of prototyping
o User and developing decide upon features to be added to next stage
o Each stage is evaluated against the agreed requirements
Design is created and made into a prototype
Requirements can be refined
RAD extras
RAD usually uses CASE (Computer aided software engineering) tools which are software tools to
assist with:
Feasibility
Gathering requirements
Design
Prototyping/ development
Testing
Documentation
Development is created automatically
An alternative approach - Rapid Application Development (RAD)
We will briefly mention here the RAD approach to project development as an alternative to the
traditional systems life cycle approach because it has become very important recently with the rise
of something called 'Object Oriented Programming'. The traditional waterfall approach has some
drawbacks. For example, it can take a while for customers to actually see the final product. Another
concern is that software produced using this approach in theory is easy to change but in practice is
difficult. The problems caused by the need to change software because of the Millennium Bug are
witness to this. The RAD approach is different to the classic systems life cycle. It involves building a
series of prototypes. After each one is built, the user is involved. They are asked to try out and
comment on features and test some of the functions. Their comments are then fed back into the
next prototype and a better one is produced. This process is repeated until the product is finished.
The product is in effect developed a little bit at a time but constantly involves the customer
throughout the development process.
It is also possible and common to combine the classic systems life cycle with a prototyping approach,
so that the waterfall model is followed, but the 'implementation stage' involves the user
commenting on a series of prototypes.
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The feasibility study
After a company has made contact with a Systems Analyst, the Systems Analyst will go into the
company and make a provisional study of the problem. They will do this to decide if a solution is
possible, how it can be done and whether it is a good idea to implement. This study is known as a
'feasibility study'. The deliverable for this stage in the systems life cycle is the Feasibility Study
Report.
Water fall model
Notice with this model that there is nothing stopping a stage feeding back into an earlier stage, so
that the project can constantly loop around until it is perfect.
An alternative approach - Rapid Application Development (RAD) - the Spiral Model
The traditional waterfall approach has some drawbacks:
It can take a while for customers to actually see the final product.
Another concern is that software produced using this approach in theory is easy to change
but in practice is difficult.
The RAD approach is different to the classic systems life cycle. It involves designing and building a
series of prototypes. After each one is built, the user is involved. They are asked to try out and
comment on features and test some of the functions.
Their comments are then fed back into the next design and prototype and a better one is produced.
This process is repeated until the product is finished. After each round, the product spirals closer to
the desired product. This process is sometimes called the Spiral Model. The product is in effect
developed a little bit at a time but constantly involves the customer throughout the development
process. It is very common to combine the classic waterfall model with a prototyping approach; the
waterfall model is followed, but the design and implementation stages involve the user commenting
on a series of prototypes.
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Advanced applications and effects
Expert Systems
An experts system is an application set up to allow users to benefit from the knowledge of
an expert or group of experts
It is also known as knowledge-based system (KBS)
Expert systems are designed to solve problems that would normally require a large amount
of human expertise
Many expert systems are used in the medical profession
For example a doctor might input the patients symptoms and the expert systems would
then diagnosis and suggest suitable treatments
Each expert system is created to solve a specific problem
Common features: Data
The data in an expert system is also known as the knowledge base
It is provided by an expert or group of experts
For example, medical experts will provide data on illnesses and treatments
The data must be updated regularly to take account of new information
For example, if new drugs are introduced to treat a particular problem, the knowledge base
will need updating
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Common features: Rules and the Inference Engine
The rules that apply to the data are also supplied by the expert
The rules need to be adjusted over time to ensure that the expert system is accurate
For example a medical expert might include a role that stipulates that one type of drug cannot be
used in conjunction with another
The inference engine is software that is used to search the data and the rules in order to make
conclusions
Common Feature: Interface
The programmer will create a human computer interface (HCI) that is appropriate for the
users
It will enable them to interrogate the data and make sense of the results
Users need to be able to follow different lines of enquiry
The expert system is designed to help the user to make decisions- not to replace them
completely
Creating an expert system
Expert systems are written using declarative programming language such as Prolog
The programmer inputs the data and the rules
The outcome of the program is not known, as there are many possible outcomes depending on what
the user wants
Expert system is a branch of Artificial Intelligence (AI)the system mimics the action of the human.
Example code
A typical algorithm for a fault diagnosis expert system might be:
IF the car will not start
AND the starter motor works
AND the battery is charged
THEN it is likely that
There is no petrol in the car
The alternator is broken
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The expert system starts with a goal, which is to find out why the car will not start
It examines the data and the rules and makes suggestions as the problem
There are hundreds of reasons why the car might not start and the expert system need to cover
them all
Uses of expert systems
The first expert system was called MYCIN and helped doctors diagnose and treat blood diagnose and
treat blood diseases and other medical expert systems have been created since
Falut diagnosis on equipment such as computers, cars, audio equipment and central heating systems
Oil prospecting
Tax return analysis for large businesses
Modelling military situations
Providing financial advice to businesses and investors
Helping social security benefit claimants
Identifying poisons in Accident and emergency
Operating systems (OS)
The operating system is a collection of software that acts as an interface between the hardware, the
software and the user
It hides the complexities of the computer from the user creating a virtual machine
The OS starts running at the pint when you switch on and is always operating, even if your machine
is apparently idle.
Common Operating system includes:
Windows
Linux
UNIX
MacOS
Modern operating systems are very large programs because they carry out many tasks
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Common OS tasks
Controls the start-up of the computer
Recognises that an input device (e.g. the mouse) is being used
Loads and uploads programs from the memory as needed
Allocates memory to programs
Attempts to cope with errors
Manages all the users on the network
Control print queues
Closes down the computer properly
Resource Management
The OS controls all of the resources on the computer for example:
The amount of processor time that will be spent on certain tasks
The amount of memory allocated to programs that are currently running
Access to secondary storage devices such as hard disks
Access peripherals such as printers
File management
The OS also controls the location of all the files stored on a computer
It uses directories (folder) to store file in and files are identified by the name given them by
the user and suffix that indicates the file type
For example, you might set up a folder called coursework and hen store coursework.doc
inside it
The user sees this as folder and is able to create, edit and delete folders into which to work
is stored and rename files
The OS stores folders as directories using a pathname
The pathname are stored on the drive that contains them (for example C drive is the hard
disk or D drive is the Cd disc)
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Pathnames
To take my coursework example, you might set up two folders under My Documents - one for
coursework and one for theory
You might then create a Word document that you save into the coursework folder calledcoursework.doc
The pathname for these files will be:
C:\MyDocument\coursework\coursework.doc
C is the hard disk (drive)
\ is used to separate the parts so there is a folder called my documents and within it another folder
called coursework
Coursework.docs the file itself
Classifying operating systems
There are four main types of operating system:
Batch (offline)
Interactive (online)
Realtime
Network
Batch (offline)
A number of instructions or tasks are passed to the OS
The OS then carries out the task without any need for further user intervention
For example, an electricity company prints out thousands of bills. The instructions for
printing them can be sent to the OS and the process for handling this request may take place
over the next few hours or overnight Where the OS is given several jobs, it will decide which jobs to do when
The main characteristics of a batch OS is that it is used for repetitive processes where an
instant response is not needed
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Interactive (online)
The user interacts with the OS
This means that the OS responds directly to the instructions it is receiving
This is the most common OS and is used on PCs at home and within organisations
For example:
You can select print and the computer sends your document to the printer
You select shut down and the computer shutdowns
Effectively, the user is having a dialogue with the computer
Real-time
The OS responds instantly to instructions that it receives
A feature of real time systems is that the input may be from, sensors rather than directly from the
user (e.g. a central heating system)
Real time systems can be further classified as:
- Critical: where an delay in response could cause damage or death (e.g. missile control, ABS
brakes)
-
Non-critical; where a delay in response would not be critical (e.g. computer controlledgreenhouse, central heating system)
Some real times systems do require human intervention (e.g. on-line bookings need to respond
instantly so that the same booking cannot be made twice from two different computers)
Network
Networks have multiple workstations and users so the OS needs to cope with this complexity
For example:
- Logging users on and off
- Allocating resources to users
- Controlling access right to data
- Handling data being transmitted between users
- Controlling shared peripherals such as printer
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Command Line Interface (CLI)
The user types in commands via the keyboard:
Advantages
The most direct way of controlling the computer
Can be quicker to operate once you know the commands
More commands are available than Graphic User Interface (GUI)
Disadvantages
Not user friendly
User has to remember text-based commands
The user has to know the syntax of the commands
Menu-driven interface
An interface which needs menus to run it
The user in presented with a list of options
Each option is selected may lead to another menu
The interface is driven by the mouse or keyboard
It is used on mobile phones, TV and cash machines
Advantages
The user is led through the appropriate sequence
There is more control over what the programmer lets the user see and do
Menus are more efficient way of organising software
Disadvantages
Commands are text based so the user many not understand the options
Menus take up a lot of the screen space
User may have to go through many menus to find what they want
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Natural language interface
This uses voice or keyboard to enter commandsprashes in common language
The interface interprets the spoken word or typed phrase and carries out the relevant action
Voice control is used on mobile phones to call stored numbers and alarm clocks to switch them off
Voice controlled applications on the PC are rare- voice activated word processing is available not
reliable
Some applications are for the disabled
Advantages
Fewer complexes for the user
Can be useful for the disabled
Disadvantages
Not reliable
Has limited applications
Phrases can be misunderstood
Graphical User Interface (GUI)
This uses mouse to point and click on icons
Icons (graphics) are used to make the interface less technical
This is also known as WIMP (window, icon, menu /mouse, pointer)
They have been around since the 1980s
GUI were popularised by windows
Advantages
Easy for non-specialist user
Interface can be customised
More intuitive than other methods
Disadvantages
Experienced user can carry out tasks quick using a CLI
Only a limited number of options can be displayed
Take up more memory and disk space
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Form Based
If the majority of the input to a system is of a standard type, in other words the computer known
what sort of input to expect, then a typical interface will produce a form on the screen to be filled in.
this sort of interface would be used where an operator is inputting information while asking a
customer questions over the telephone.
The interface:
Prompts the operator to ask each of the questions in turn
Makes the operator input the information in the correct order and ensures that nothing is
missed out
Ensures that the information is input in the correct format by having specific areas to input
the data
Makes the checking of the information easier
The characteristics of a form based interface are that:
It has specified areas for the data. For example. Boxes for input of coded material like the
date or the sex of the customer, and areas to be filled in with textual information
It has a cursor which moves to the next box to be filled in, sometimes the box is highlighted
to make it clear to the operator where the data is to be inserted
Some of the boxes are more important than others and the cursor will not move on until
some data has been supplied
It checks that what has been input is sensible for that box before moving on to the next
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3.1.3
Binary coded decimal
BCD represents denary integers using blocks of four binary digits. Each block of four is converted
and the denary values are then read off.
Uses of BCD
BCD enables fast conversion from denary to binary for applications such as pocket calculators.
Each digit on a calculator corresponds directly to a four bit block in BCD.
The Bit
The processor has millions of transistors which handle electricity in a relatively simple way
Either electricity is flowing or it is not
This means it is either on or off
- On = 1
- Off = 0
This is known as digital data, that is, data made up of series of zeros and ones
Computer process all data in digital form
Zeros and ones are also known as Binary Digits or BITS
Turning BITS into data
Computers use long strings of zeros and ones to represent text, number, sound, video and
everything else we use our computers for
We can programme the processor to recognise different patterns of on and off signals
For example, we could program it recognise on-off- off- off-off-off- off-on (1000001) as the
letter A on the keyboard
When we press A it sends a signal to the processor so that it knows the is pressed
In this example, we have used 8 bits (a byte) to represent one letter on the keyboard
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Bits and Bytes
8 bits is a useful number because it gives enough combinations to be able to represent every
character on the keyboard. A group of 8 bits is commonly referred to as a byte
As the number of bits used increases so does the range of different combinations
Number of bits Combinations No of combinations Also shown as
1 0 or 1 2
2 00, 01, 10 ,11 4 22or 2x2
3 000, 001 010 011 100
101 110 111
8 23or 2x2x2
Each extra bit increases the number of combinations available two-fold
This means that a 20-bit computer system is capable of producing for example, 16.7 million
different colours
The word
One byte (8 bits) is enough for 256 combinations. An 8-bit system therefore, could represent
256 different numbers or colours
This is not enough for modern computers, so bytes are grouped together to from words.
Two byte grouped together create a word length of 16 bits, which means that the computer
can handle twice as much data as an 8-bit system within the same time-frame
ASCII codes
A byte (8 bits) is sufficient to represent all of the keyboard characters
The American Standard Code for Information Interchange (ASCII) system defined a standard
8-bit code to correspond to every character on the keyboard
This means for example that capital A is always represented as 10000001 in every computer
system
Representing Numbers
Computers can handle different types of numbers in different ways, for example as
- Integers (whole numbers)
- Positive and negative numbers
- Real numbers to several decimal places
The process needs to know what kind of number it is handling, so different binary coding
systems are used to represent numbers
Humans use number base 10 (denary) and computers use number base 2 (binary) so the first
stage is to be able to convert one to another
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The Denary system
There are ten digits available 0 to 9
With one digit we can represent any number between 0 and 9
With two digits we can represent any number between 0 and 99
Every digit we add increases the range by a factor of 10 (because it is number base 10)
This is shown in simple mathematical terms as follows:
Thousands (1000) Hundreds (100) Tens (10) Units (1)
3 4 3 7
(3x1000) + (4x100) + (3x10) +(7x1) = 3437
Converting binary to denary
There are two binary digits available: 0 and 1
Every digit we add increases the range by a factor of 2 (because it is number base 2)
This can be shown in exactly the same way as denary: although the multiples now can increase by a
factor of 2 rather than 10
Denary 128 64 32 16 8 4 2 1Binary 1 0 0 1 0 1 1 1
The binary number 10010111 as shown above is the equivalent as (1x128) + (1x16) + (1x4) + (1x2) +
(1x1) which is 128 + 16 + 4+2+1= 151 in denary
Binary Coded Decimal (BCD)
BCD represents denary integers using blocks of four binary digits
Each block of four is converted and the denary values are ten read off:
8 4 2 1
0 0 1 0
0+0+2+0
2
8 4 2 1
0 0 1 1
0+0+2+1
3
8 4 2 1
0 1 1 0
0+4+2+0
6
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0010 + 0011 + 0110 = 236
BCD enables fast conversion from denary to binary for applications such as pocket calculators
Each digit on a calculator corresponds directly to a four-bit block in BCD
Representing characters
There are three main coding systems that provide conversions of keyboard characters into binary
EBCDIC
ASCII
UNICODE
EBCDIC
EBCDIC is pronounced eb-sid-ic and stands for Extended Binary Coded Decimal Interchange
Code
It is an extension of BCD which includes non-numeric characters, including all the keyboard
characters and special characters
It is commonly used to encode data onto magnetic type
ASCII (American Standard Code for Information Interchange)
It has been adopted as the industry standard way of representing keyboard characters as binary
codes
Every keyboard character is given a corresponding binary code
In initially a 7-bit extended ASCII now uses an 8-bit to provide 256 characters
Unicode
Unicode is the new standard to emerge that is replacing ASCII
It has been adopted by many of the big businesses in the computing industry
There are various versions, the smallest being 8-bit
It is designed to cover more of the characters that are found in languages across the world
It has become important due to the increased use of the internet as more data is being passed
around global
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Number Bases
A number base indicates how many different characters are available:
- Denary has ten: 0,1,2,3,4,5,6,7,8,9
-
Binary has 0 and 1
The size of the number base determines how many digits are needed to represent a number
For Example:
- 98 in denary takes to digits
- 98 in Binary takes is 1100010 which is seven digits
Computers often need to represent very large numbers which involve very long binary codes
Octal and Hexadecimal
Larger number bases allow fewer characters to be used when representing numbers. In computing,
two further numbers bases are used
Octal is number base 8 which uses the digit
- 0, 1, 2, 3, 4, 5, 6, and 7
Hexadecimal (Hex) is a number base 16 which uses the digits:
- 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B. C, D, E, and F
Identifying the number base
There is a scope for confusion as to which number base is being used
For example, is 101 a binary or denary number?
There is an accepted notation for showing which number base is used
The number base is shown in subscript after the number:
- 4310 is denary
- 10112is binary
- 1678is octal
- 2A716 is hexadecimal
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Hexadecimal
Hexadecimal is useful for representing large numbers, as fewer characters are required
- 21110 in binary is 11010011
-
In hex it is D3- Only two characters are needed to represent 8-bits
Hex is referred to as shorthand for binary
Two hex codes = one byte
Hex is used to represent memory addresses and colour codes
Convert denary to Hex
Convert denary to binary. For example 21110
128 64 32 16 8 4 2 1
1 1 0 1 0 0 1 1
Split the binary code into two groups of 4 bits and convert to hex:
8 4 2 1
1 1 0 1
1101 is D in hex (13 in denary) and 0011 is 3 in hex
Therefore 2110 equals D316
8 4 2 1
0 0 1 1
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Hex to denary conversions
The process is reversed, so the first stage is to convert the hex in binary, e.g. 2A316
Now convert the binary into denary in the usual way. On this occasion we need 10 bits excluding the
leading zeros
512 256 128 64 32 16 8 4 2 1
1 0 1 0 1 0 0 0 1 1
This converts to 512 + 128 +32+2+1= 67510
Integers and Real numbers
An integer is a whole number and can be positive or negative
Twos complementis a method of converting positive to negative integers into binary and
vice versa
The method is similar to converting positive integers
Converting binary to denary using twos compliment
To convert to denary, write out the multiples of two as before, for example, to convert 10011100
-128 64 32 16 8 4 2 1
1 0 0 1 1 1 0 0
With twos compliment, the MSB (most significant bit) becomes negative
Add the values: -128+16+8+4=-100
Using twos compliment:- A binary code starting with a 1is negative
- A binary code starting with 0 is positive
8 4 2 1
0 0 1 0
8 4 2 1
1 0 1 0
8 4 2 1
0 0 1 1
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Converting denary to binary
To convert -102 into binary, write out the binary equivalent of +102 in the usual way
128 64 32 16 8 4 2 1
0 1 1 0 0 1 1 0
Starting with the LSB, write out the number again up to and including the first 1
Then reverse all of the remaining bits: 0 becomes 1 and 1 becomes 0
The number becomes 10011010
Now convert this binary code using two complements
-128 64 32 16 8 4 2 1
1 0 0 1 1 0 1 0
Now add the numbers
-128+16+8+2=-102
Adding using twos compliment
Adding numbers in twos complement is the same as adding denary numbers
Therefore to add 01101100 and 10001000
01101100
10001000 +
11110100
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Subtracting using twos compliment
The computer cannot directly subtract binary codes
You need to convert the number to be subtracted into a negative value and then add it on
For example 20-13=7
The computer will handle this as 20+-13
The same method is used when subtracting binary codes
To carry out the same calculation 20-13:
Convert 20 into binary= 00010100
Convert -13 into binary = 11110011
Add 00010100 to 11110011
Check your answer by converting it back to denary
The final 1 is carried. This is called an overflow bit and is handled separately
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Input and output devices
Input devicesare devices that are used to enter information on a computer e.g. keyboard, scanner
etc. New input devices are being invented all the time. There is a misconception that other devices,
such as floppy disks (storage device) and modems (communication devices) are input devices, as
they are used to put data onto the computer. However, these handle data that is already in
computer format so cannot be classed as input devices.
Keyboards and Keypads
The standard QWERTY keyboard is the most common input device for PCs. Keypads are a variation
on the keyboard, but are used for specific devices, such as PDAs, calculators and cash machines.
Concept keyboards are touch sensitive and display icons which the user presses. They are typically
used in fast food restaurants where each icon represents a product.
Mouse
Along with the keyboard, this is the most common device for the PC and is very important within a
GUI environment. There are many variations, such as:
standard two button
three button (middle button accesses menus) and scrolling wheel
infra-red so no cable is needed
Mice that use light to record movement rather than a ball.
In addition, laptops have variations on the mouse:
tracker ball
mouse pad
Mouse nipple.
Scanners
Scanners take hard copy images and turn them into digital format to be used by the computer. Flat-
bed scanners are attached to PCs and used to scan in images and text up to A4 size. Hand-held
scanners are smaller and are physically moved across a document. Scanning technology is also used
for:
bar code scanning
biometric scanning
- OMR, OCR and MICR
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Bar code scanner
Bar code scanners are mainly used for inputting product details at checkouts. Shops often have a
scanner built into the checkout and the products are passed over it. Larger items are scanned using ahand-held scanner. The patterns of black and white lines store product details. There are different
bar code systems, but the most common in the UK is the European Article Number (EAN), which is
standard for food products and the bar codes on books that store the ISBN.
Biometric Scanner
Biometrics is physical characteristics that identify each individual. A biometric scanner scans part of
the body, such as the retina or fingerprint. The scanned image is compared to a database to confirm
identity. Its use is becoming more common and it is linked with the proposed National Identity Card.
Optical Mark Reader (OMR)
OMR scans and records marks made on a page. It is commonly used to input marks from teststhe
student shades in a box on a multi-choice answer and the paper is read by the OMR. It is also used:
to input your numbers on the national lottery
by meter readers, to input gas and electricity meter readings
- By teachers, when they record coursework marks and send them off to the exam board
Optical Character Recognition (OCR)
OCR scans written or printed text and converts it into digital form for further processing. OCR
software can be used with a flat-bed scanner to turn printed documents into word-processed
format. Another form of OCR uses a light pento write on a screenthe traces are recognised and
converted using software. This is a common feature of PDAs.
Magnetic character ink recognition (MICR)
The same technology as OMR and OCR is used, but it is used specifically to read characters printed
with magnetic ink. The most common application is to read the characters printed at the bottom of a
bank cheque. Multiple cheques can be handled at high speeds by passing the cheque through an
MICR reader.
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Microphone
Microphones are used for voice inputthe voice can be sampled and converted into digital form.
Voice-activated software is still a rarity due to the complexities of recognising different voices.
Specific applications include:
specialised software for disabled users
Voice-controlled telephone systems where the voice activates the options.
Touch Screen
Normally an output device, the touch screen also allows input and output. It is useful where a very
simple user interface is required, such as in:
tourist information offices, to display local information
estate agents, to show details and images of houses
Some cash point machines.
Magnetic stripe reader
Magnetic stripes are found on the back of plastic cards, such as credit cards. They are also used on
tickets as a means of entry/exit (e.g. train tickets, car parking tickets). The stripe contains data which
is read by the reader when the card is swiped. There are some concerns over the security of the
data, so many magnetic stripe cards are being replaced by smart cards.
Smart card reader
Smart cards look like normal credit cards but contain a microchip that stores the data. They are more
secure than the magnetic stripe and they can contain much more information on the card holder.
Banks have been keen to introduce them. Some countries use smart cards as driving licences. The
proposed national identify card would use a smart card which would store biometric data in addition
to text data.
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Sensors
Sensors are devices that read in physical data. Computer-controlled environments make extensive
use of sensors to record physical variables, for example:
Infra-red sensors in burglar alarms
Temperature sensors in central heating systems.
A number of physical factors can be input using sensors, for example:
Temperature
Light
Movement
Pressure
Acidity
Moisture
Strain
Voltage
Wind speed
Radiation.
Verification and validation
It is important to ensure that data entered into a computer system is as accurate as possible
Verification is the process of making sure that the data entered is the same as it appears on
the source document
Validation is the process of checking data against a set of rules as it is entered
Neither method ensures that the data is correct. They simply ensure that the data is thesame as it was on the source document and that it adheres to the set rules
For example, the source document might state that someone is 25 when they are actually
35. If it is wrong on the source document then it will still be wrong when inputted on the
computer system.
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Verification methods
There two methods:
Visual verification:
-
The data is visually checked for errors, which means comparing the data on the computer to
the data on the source document
- For example you are asked to verify the amount on a credit card receipt before signing it
Double entry verification
- The data is entered twice and the two entries are compared by two different people
- If the two sets of data are different then the data should be checked and entered again
- For example you are asked to enter your new password twice
Validation checks
Range check: data entered is within a set rangee.g. age between 0 and 120
Format check: data is entered in a set formate.g. data as 00/00/00
Type check: data entered is a specific typee.g. age is numeric
Presence check: data is entered into the field and cannot be left blank
Look-up check: data can be entered if it is in a pre-defined list - e.g. gender would have a
look up M or F
Data integrity
Data integrity is the process of ensuring that the data does not become corrupt
Errors can occur when data is served to another storage medium or when data is
transmitted around a network
Errors in data may not be immediately obvious but could have serious consequencese.g.
financial data that is corrupted during transmission
Checksum and control total
These methods also work including additional values added to the file when transmitted
A checksum is like a hash total, but only adds the values of the bytes held in a specified block
or file
A control total is like a hast total but uses a meaningful value such as the total value of sets
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Check digits
A check digit is a value that is added to the end of each item of data
The value is calculated from the data item so it can be re-calculated on receipt to see if its
correct
For example, you could have a data item 23045 and calculate a check digit by adding up the
numbers to make 14 and then adding these together
The data is sent therefore is 23045
The same calculation can be done on a receipt to seed if it is check digit is correct
Different forms of output
Graphs
These provide a pictorial representation of a set of numbers. They help a reader make sense
of data
A pie chart highlights the contribution a particular item makes to the whole
A bar chart shows how many occurrences there are of a set of items
A scatter graph is used to plot one variable against another variable to see if there is a
relationship
Reports
These are formal printed documents. Reports organise information in a clear way, they are
often used to present sets of number so that a skilled reader can look at them and then
interpret them
Interactive Presentations
The previous forms have relied on the format of the report being decided without the luxury
of being able to see what the figures look like in the first place. If the system allows the user
to decide the type and range of output required during the run, then there is some positive
user involvement leading to an interactive presentation where the user can adjust the
output to suit the example.
Sound
Sound can be used to output information. It can be used to highlight that a verification error
has occurred during data input by beeping the keyboard. It could be used to signal an
emergency situation, or could be used to read back textual information
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Video
Video can be used to output information. It takes large amounts of memory to produce
because the nature of the medium requires large quantities of pictures to produce the fell of
continuous motion. Video is useful for demonstration of techniques where there is little
value in pages of instruction if a simple video can illustrate something better
Text
So the reader can read about what is happening, and this will help if they do not want to see
pictures
Images
Images or pictures can be used to enhance understanding. These may be used to creategraphics packages, may be scanned into the computer or imported from a camera,
animations can be created
Multimedia output
Where it is required to present information in some way video, sound, animations, special
effects, text and pictures can all be combined using presentation software. Many packages
are very user-friendly, providing a user with templates to put together a presentation and by
providing on-screen help.
Animations
Provide a good stimulus for an audience and lead from one slide to another when making a
slide based presentation. Animation takes considerably less processing power than other
forms of motion, unless the images are animated is complex. Animations are used so often
that it can across as being a boring technique that has just been added for gloss.
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Backing up
A business or firm may back up:
- Details of each employee
- Who owes the company money?
-
Who needs to be paid?- Product designs and blueprints
- Legal documents
There are many ways that they can lose data:
- HDD/SSD failure (file corruption)
- Virus/Hacker
- Theft
- Misplacement
- Sabotage by employee/rival company
- Damage to the equipment (i.e. building fire, natural disaster)
In order to prevent this, companies need to have a back-up strategy.
There are 2 types of backups which may be done: Full backups, which back up every file, and
Incremental backups, which only back up the changed files since the last backup.
Full backups take a very long time, so they should be done occasionally, such as weekly,
whereas incremental backups are much quicker, but need to be done at a higher frequency,
such as each night.
These backups should be in a safe place, such as an offsite storage facility.
The company needs to have their backup policy clearly stated, which will include information such
as:
-
The hardware/software used- The personnel who deal with backing up the data
- The rate and times at which backups are made
- Location on backup
- Label/filenames of backups.
- How the company will ensure the system is working as it should.
Methods to store the data
- Since the backups will be very large in size, companies will need to use the appropriate
methods to store the data.
- Magnetic Tape is the most commonly used form of storage, as it can hold up to terabytes of
data. Files are also sometimes put on disks such as DVD's, and files are also sometimescompressed (using programs such as WinRAR or 7-Zip)
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Archiving data
Archiving data is transferring data to long term storage and it involves moving the data rather than
copying it. You may no longer need the data but it should still be kept in case you need it for the
future. An organisation must keep the data as the company is under a legal obligation to keep
records, so for example if a company sacks an employee, the employees details and profile must be
recorded somewhere or else they could be penalised. Archiving should take place at suitable time
intervals such as six months or every year, the data may also be compressed so you can fit more on
to the tape. Once the data is archived the original data can be deleted to save storage space.
Companies archive data to:
- To free up space on file
- To save data before it is deleted
- To keep details of completed orders for long-term analysis
-
For legal/taxation reasons- To clear up disputed orders
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3.1.4
Machine Architecture
The Central Processing Unit (CPU)
The CPU or processor is where all instructions are carried out
It is made up of a number of units
The control unit
The controls the fetch-decode-execute cycle
It ensures the data is being correctly routed around the processor
It ensures that the data passes through the appropriate register or section of memory
The arithmetic logic unit (ALU)
This carries out:
- the mathematical functions like add, subtract, multiply and divide
- less familiar processors such as shift operators
- logical operations such as AND, OR and NOT
It compares values to decide whether one is less than, greater than or equal to the other
Leads to true and false values being generated
It handles the operation code (instruction) and operand (data)
The clock
This generates a signal that synchronises the movement of data as part of the fetch-decode-
execute cycle
The speed of the clock is measured in megahertz (millions of cycles per second) or gigahertz
(1000 million cycles per second)
Clock speed can be used as a measure of the overall speed of a computer system
Bus width is equally as important as this dictates the amount of that can be processed
during one pulse of the clock
Registers
There are a number of storage locations on the CPU called registers
Registers are temporary storage locations for data being passed around the CPU
There are different registers which are used in different ways during the processing of an
instruction
Some registers are general purpose and some have a specific purpose
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Registers used as part of the fetch-decode-execute cycle
Current instruction register (CIR):stores the instruction that is currently being executed
Program counter (PC):stores the memory location of the next instruction needed/Contains the
address of the next instruction to be carried out
Memory data register (MDR):holds the data that has just been read from or is about to be written
to memory/ Contains the address of the next location to be accessed in the memory May be the
address of the next instruction/ copied from PC/ Address of next data item to be used copied from
address part of instruction held in CIR
Memory address registers (MAR): stores the memory location where data in the MDR is about to be
written in the register Contains the address of the next location to be accessed in the memory, May
be the address of the next instructions copied from PC/ Address of next data item to be used/ copied
from address part of instruction held in CIR
Status and interrupt
Status register: Stores the status of various parts of the computer for example if there is an
overflow
Interrupt register: a status register that stores details of signals generated from other
components attached to the processor such as the Input/output controller for the printer.
The accumulator
All data processed by the CPU passes through the accumulator
It is part of the ALU and contains the current data being processed
The results of all calculations will pass through the accumulator before being stored in a
memory location
The stored program of concept
Very early computers had no storage, so programs and data could not be stored at all
The invention of the microchip allowed programs to be stored for the first time
Programs are a combination of both instructions and data
Both are stored in memory which is the stored program concept
Instructions and data are passed between memory and the processor
Fetch-decode-execute cycle
The cycle which a processor runs through when a program is run
Fetch:the next instruction or data item need by the program is fetched from memory
Decode:the processor interprets the instruction by referencing the instruction set
Execute: the processor carries out the instruction which could involve reading an item from memory
performing a calculation or writing data back into memory
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Buses: are microscopic wires used it transmits data between these registers and between the
processor memory and the input/ output
The data bus: carries out instructions between the processor, memory I/O controllers and backagain it also carries data between registers
The address bus: carries the memory address of the next instruction needed by the processor or
the.
The control bus:Signals need to be sent around the computer to control when things happen. These
signals are sent along the control bus. The striped channel you can see in the diagram is the control
bus.
Hardwiring and Wireless connections
Interconnections considerations
- Bandwidth
- Environment
- Distance
- Security
- Cost
Type of connections
Unshielded Twisted Pair cable (UTP)
UTP cables are very light, flexible, cheap and reliable. It is used to connect up the telephone system.
It consists of pairs of conductors covered in insulation material and then twisted together. Within
one cable, you might have 4 pairs.
Coaxial Cable
It is made up of a central conduction core covered with some protective insulation. Wrapped around
the insulation is a thin metal sheath that provides the electrical interference protection. Finally, thecable has an outer covering. Coaxial cable is the cable used to connect your television aerial to the
television.
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Fibre optic cable
This consists of a glass core wrapped in protection within a cable. Data is sent down the fibre optic
cable as light, not electrical signals. The signals therefore dont suffer from electrical interference.
They are also resistant to the effects of moisture because they are non-metallic, unlike conventional
cables, which are metal-based. The cables themselves are very brittle when compared to
conventional metal-based cables and need to be well protected. Fibre optic is high bandwidth
compared to UTP. The price has dropped, but installing and repairs need specialist engineers, hence
it is expensive to install and modify
Wireless communication
Wireless communication is more popular now because it removes the restrictions placed on location
of devices because of the position of the cable. Wireless communication can be used anywhere
within the wireless footprint
Read Only Memory (ROM)
ROM is memory that cannot be altered. This means that even switching the computer off will not
affect the contents of the ROM
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3.1.5
Networks
Network - a number of computers connected together sharing processing power, storage
and resources Other devices such as printers, scanners can be part of the network
The connections can be made through various cable types or using wireless technology
Network vs stand-alone computer
A stand-alone computer has no connections to any other
They were common in businesses and other organisations during the 1980s and early 1990s
Many home computers are stand-alone though a lot of home users now use small networks
and by connecting to the Internet, your stand-alone computer becomes part of a global
network
Advantages Disadvantages
Easier communication between users Hardware and cabling costs are increased
Site licensing for software reduce the unit cost The operating system is more complex
Users can share peripherals Easier to hack into than stand-alone
Administrators have greater control If the main server or cable fail then the whole
network fails
Users can use any workstation Viruses are can be spread more easily
Information can be shared Some people dont like being monitored
Network adapter
In order to connect a LAN, the computer needs a network adapter or network interface card
(NIC)
It is like any other type of car such as graphics or sound card
The type of NIC dictates what type of network the computer can connect to and the speed
of transmission that can be achieved
Modem
Modem stands for modulator/demodulator
Modems are used to convert digital signals into analogue and vice-versa
Recent communications development such as ISDN and ADSL, many render the modem
obsolete
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Local Area Network (LAN)
In a LAN, number of computer are connected together over a small geographical distance e.g. one
building or site
LANS are usually made up of one or more servers.
A server is a high specification computer with large processing and storage capabilities
Standard PCs are connected to the server using a NIC, after which they are referred to as
workstations
LANS can be connected together in different layouts known as topologies
Wide area network (WAN)
In a WAN, a number of computers connected together over a large geographical distance typically
over 1Km
WANs use a variety of connections methods for example:
- Cables
- Telephone wires
- Satellite links
- Fibre optics
WANs dont normally have one central server like LAN
Instead they have distributed control which means that each separate location retains control
Uses of WANs
WANs are usually owned and operated by large operations, such as supermarkets, banks,
police force or the NHS
Supermarkets have Economic Point of Sale (EPOS) systems that record sales and stock
control - these are connected to the head office using a WAN
Financial transactions using credit or debit cards use Elect6ronic Funds Transfer at the Point
of Sale (EFTPOS) which also makes use of a WAN
The internet has also been described as a WAN, although as no single organisations owns it
this may not be accurate
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Networks topologies
Star Network
Main features of a star network:
Each workstation has a dedicated connection to the server
The server will be a high spec machine
The server holds the network operating system and all of the application software
The workstations may also hold a copy of the applications locally
All data passing around the network will go through the server
If the server is off then they wont work
Advantages
Fast connections as each connection has its dedicated connection
Will not slow down as much as the other topologies when more users are using it
Faults are easier to trace
Relatively secure connections
Easy to add new workstations
If the cable fails the other workstations will keep working
Disadvantages
Expensive to set up due to increasing cabling costs
If the cable fails there is no other way of sending/receiving data from that workstation
Difficult to install
Complex operating system
Server can get congested
Needs managing
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Bus Network
Features of a bus network
The server and workstations are connected through one main cable
All data is sent and received through this cable
It is the most common type of implementation and is used in Ethernet networks
Advantages
Cheaper to install, as there is only one main cable Easier to install
High transmission rates available
Easy to add new workstations
Multiple servers can be added easily
Disadvantages
Less secure as all data is transmitted down one cable
Transmission rates reduce when more users are on the network
If the main cable fails, all workstations fail
Less reliable to add new network due to the reliance of the main cable
More difficult to fault find
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Ring Network
Features of a ring network
It does not necessarily have one main server although a server can be used
Workstations pass data around the ring
Data has to pass through other workstations en-route to its destination
This is a common way of creating home networks
Advantages
The cheapest way to install a network as there is only one cable between each network
File server not necessary, which reduces cost
High speed can be achieved, as each workstation can boost the signal
Disadvantages
Cable failure can cause the whole network to fail
Less secure as data passes through other workstations
Adding new workstations affects the whole network during the installation process
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Baseband and broadband
Basebanddescribes communications that use a single frequency band in order to transmit data
High speeds can be achieved with baseband over shot distances, such as around a LAN
Broadband - describes communication that use a wide band of frequencies
Broadband increases the rate of transmission, which is becoming more important with the increased
use of the internet
Broadband communications can be achieved using various methods such as fibre optics, satellite,
ISDN, ADSL
Broadband uses a technique called Frequency Division Multiplexing (FDM)
Frequency Division Multiplexing (FDM)
A baseband communication could be represented as:
FDM splits the cable into different frequencies or channel and transmit data down each
channel at the same time
FDM is used for broadband internet access by using one channel for uploading, one for
downloading and one for voice data
Communication
Communication is the process of transmitting data and instructions between:
- The processor, memory and the I/O controllers
- The computer and the peripheral devices
- One computer and another computer on a local or global network
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Serial communication
Serial communication means sending data one bit at the a time in a sequence
It is used for most communications over 1 metre, including transmission of data around local
and global network
High transmission rates can be achieved by:
- USB, which provides high speed data cabling from peripherals
- Network cables, which transmit at 100Mbs (megabytes per second)
Parallel Communication
Parallel communication uses a number of parallel wires, so that more can be transferred
simultaneously It is only used over short distances, as the wires are expensive to produce, the signal
degrades over distance and synchronising the data down each wire can be complex
It is mainly used inside the computer in microscopic form in:
- Data, address and control buses
- The internet architecture of the processor
Serial and parallel compared
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Bandwidth
Bandwidth is the term that describes the amount of data that can be transmitted along a
communication channel
It relates to the range of frequencies that can be used to carry data
The greater the range of frequencies the greater the volume of data that can be transmitted within a
given time-frame
Bandwidth is measured in bits per second (bps), which is how many binary digits it can send each
second
Measurements are recorded in Kilobits per second (Kbps) or Megabits per second (Mps)
Bit rate
Bit rate is the term used to describe the speed at which a particular data transmission is
taking place
It is closely linked to bandwidth, as bit rate is limited to the bandwidth available
It is measured in bits per second 9bps) and is the actual of transmission
A 56K modem has a bandwidth of 56Kbps, but the bit rate of probably around 45Kpbs
This is because not all the bits are used to transmit rate datasome bits are start/stop bits
Baud rate
Baud rate is another measurement of the speed of data transmission
One baud represents on electronic state change in frequency on the carrier wave
Therefore, one baud roughly equates to one bit per second
Baud measurements were used before the widespread use of the internet
It is sufficient to state that one baud per second I the equivalent of one bit per second
Parity Checks
Number of ones in a bye is either always odd or always even
System has been initialised to always expect either odd or even number of ones in a byte
Each byte has one bit called the parity bit which does not contain data
It is set to 1 or 0 in order to maintain the agreed odd or even parity
When transmitted, the number of ones should match agreed parity
If not the byte contains an error
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Use of parity in data blocks to become self-correcting
Each byte has a parity bit
Each bit pace has also got a parity bit
All parity is checked
If there is an error in the parity for a byte and the parity for a place value
Then where they intersect will be the faulty bit
If it is 0, change it to 1
If it is 1, change it to 0
If more than one error in the block has occurred then data is re-transmitted
Check Sums
Block of bytes are added together before transmission This total is transmitted with the block
The same calculation is done on the data blocks at the destination
The result is compared with the transmitted value
If different, a transmission error has occurred
Echoing
When a set of data is transmitted from one device to another
The received data I retransmitted back to the sender
It is then compared to the original data Any differences will signify a transmission error
And the original data is retransmitted
Packet switching
File of data split into packets which are sent onto the network
Each packet I sent to the destination via different routes
Packets arrive at the destination at different times and in a wrong order
Packers must be reordered at destination
Circuit switching
Route is deserved before transmission
For the duration of the transmission
All packets follow the same route
Packets arrive at the destination in the correct order
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Packet switching Circuit switching
No established route Establishes a route along which to send packets
for duration of message
Packets sent on individual routes All packets follow same route
Secure becomes impossible to intercept all
packets
Message can be easily intercepted because all
packets are on the same routePackets need to be reordered/only as fast as its
slowest packet
Packets remain in correct order
Maximises use of network Ties up large areas of network
Protocol
A set of rules to govern communication between devices
A handshake signal is signal sent between two devices to sure that they are both ready to
communicate
A handshakes signal is sent from one device and acknowledged by the other device
This states that both devices are now ready for communication
Communication cannot be made, unless both devices follow the same rules
Physical protocols
Wireless/hardwired?
What frequencies?
Series of parallel?
Radio or microwaves?
Copper cable or fibre optic?
Logical protocols
Baud rate
Error correction technique
Routing
Flow control
Packet size
Compression type
File type
Layering
Protocols are arranged in layers because it allows different rules within a protocol to be set
up independently
Layers are ordered which simplifies the creation of protocol
