Computing Fundamentals Revision Notes

Computing Fundamentals

OCR F451 Written Exam (January 2010)

:: Jack Bennett, 12F

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Components of a Computer System (3.1.1) Hardware: physical parts of a computer system

o Three categories: input devices, storage devices and output devices

All systems need instructions of some sort which generally come from the user.

Some systems need text, some high-quality images, and some video images.

Input devices are chosen for their quality and ease of use.

Used so that the computer can be told what to do and what data to use.

Storage devices are used so that the computer can remember what it is meant to do when it is switched back on after having been turned off, otherwise by the time you tried to give it a second instruction it would have forgotten the first.

An output device is any hardware used to communicate processed data to the user.

Used to display what the computer has just done or to control some device to do something useful.

Software: actual programs or data that a computer system uses (two types...) o Systems software: set of programs that organise, utilise and control hardware o Applications software: designed to make use of system for specific purposes

Software (3.1.2) The systems lifecycle is a set of rules written as a series of stages that need to be

followed in order to produce a new computer system.

It is a term used to describe the stages in an ICT project.

Commonly it is the process by which an existing system is replaced with another.

There are nine stages to the systems development lifecycle: 1. Definition of the problem 2. Feasibility study 3. Collection of information 4. Analysis 5. Design 6. Implementation 7. Evaluation 8. Installation 9. Maintenance

On the next page is an illustration of the systems lifecycle...




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Implementation: if a team of programmers have to be employed, problems can

occur such as multiple uses of the same variable but for different functions – an area

like this is where bugs can occur.

Installation: analyst must decide how to install system into business, alternatives...

o Switch off the old system and switch on the new.

o You run the old and new system in parallel for a time.

o You run only part of the new system (phased approach).

Problem Definition

•Define problem the system is to overcome

Feasibility Study

•Looks at alternative solutions

Information Collection

•Define how old system works and problems

Analysis

•Analyse how current system works - uses lots of diagrams and flowcharts

Design

•Exact details of requirements are defined

Implementation

•Establishing new system

Evaluation

•Does the finished solution meet its requirements?

•Does it solve the problem?

Installation

•How to install new system

Maintenance

•Problems cleared; tweaks to improve system; data backed up; peripherals upgraded; relocate system




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The waterfall model is one model of the cycle in which progress is seen as flowing

steadily downwards (like a waterfall) through the phases of:

1. Conception

2. Initiation

3. Analysis

4. Design (Validation)

5. Construction

6. Testing

7. Maintenance

The first formal description of waterfall model is often cited to be from an article

published in 1970 by Winston Royce.

Well suited to projects that have low risk in the areas of user interface and

performance requirements, but high risk in budget and schedule predictability and

control.

The spiral model is the most generic of the models.

Most lifecycle models can be derived as special cases of the spiral model.

Uses a risk management approach to software development.

Intended for large, expensive and complicated projects.

Generic applications software has a relatively large number of different uses.

o Word processors are an example of generic software.

o Tailored to user’s specific needs (rather than task-specific).

Software can be divided into two types:

o Off-the-shelf – available for immediate use

o Custom-written – specifically written to solve a particular problem

An example application might be software to control a specific process on a

production line, which therefore may well be the only one of its kind, and so would

not appear in a local computer shop.

Knowledge-based systems are software designed to be able to store vast amounts

of data about a particular topic.

This data can then be interrogated so that sensible information can be gleaned.

o Data is initially collected from specialist experts.

o The software is then used to replace the experts in conditions where the

expert is unavailable.

Typical uses would be to provide medical diagnoses or to interpret geological data.




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Operating systems (OS)... o Provide a platform on which applications can run o Control hardware o Allow communication with the outside world

Functions of an OS... o Provide and manage hardware resources (memory management etc.) o Provide Human Computer Interface between computer and user o Provide interface between applications and the machine itself o Provide security for the data on the machine (particularly for multi users) o Provide utility software for maintenance

With batch processing, jobs with similar needs are batched together.

They are run through the computer as a group.

Performance is increased by attempting to keep CPU busy at all times. o Buffering, offline operation, spooling and multiprogramming.

:: SIX TYPES OF OPERATING SYSTEM SINGLE USER – one user at a time. MULTI USER – data centrally stored, e.g. supermarket checkout system. MULTITASKING – allows multiple tasks to be run concurrently (or appear to). BATCH – from the 1950s, to stop people slowing down the few computers, similar tasks run sequentially. DISTRIBUTED – centrally located data which is downloaded or process distributed to local machines be worked on. REAL TIME – safety critical systems; responses within a critical timeframe; failsafe conditions.

Process of transferring data by

placing it in a temporary folder

where another program may

access it for at a later time.




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One of the purposes of the operating system is to provide a human-computer interface which lets users tell the computer what to do, and allows the computer to ask the user for input or to display results.

o GUI – Graphical User Interface o WIMP – Windows Icons Menus Pointers

MENU BASED – often confused with GUI (menus). The whole interface is run from

menus. Screens are in hierarchical layers so are often intuitive and simple.

FORM BASED – DVD film control. Multiple options displayed as forms where there

are buttons to move forms.

GRAPHICAL USER

Great for beginners.

GUIs tend to be intuitive.

Context sensitive help.

Add-ons can cause actual software to be quite slow because of processing time.

Many commands are hidden so not to confuse the user.

COMMAND LINE

Powerful and fast.

Simple prompt screen.

Good example is MS-DOS®.

Quick and flexible.

Only useful if you understand how the system works.

Otherwise can cause drastic problems.

COMMON INTERFACE

Many Windows applications use a common interface so that they all become easier to use.

For example, they all have similar File and Edit menus and some shortcut keys do the same in all applications.

NATURAL LANGUAGE

Designed to understand human phrases.

Input may be through human speech.

Easy to use interface.

Intensive processing (makes slow).

People use different phrases to mean the same thing (ambiguity).

Good interface design is important because it will...

o Be easy to use for inexperienced users

o Give information about what processing is happening

o Be consistent so that the user becomes familiar with it quickly

o Have facilities for more experienced users to customise the interface

o Always do as the user expects

o Be clear (everything should be obvious to the user)

o Reduce the possibility of mistakes by checking the user’s input

o Not require the user to remember many commands




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System utilities are programs that perform specific functions (generally

maintenance).

Some tasks that system utilities carry out include...

COMPRESSION System utility programs convert data into a format that takes up far less memory space. Communication of data is faster when file size is reduced, and it is therefore particularly useful when data is being sent from one computer to another.

FILE CONVERSION File extensions are related to specific applications. A utilities program enables applications to open a file with a different extension. e.g. MS Word can open a .wps file created by the word processor in MS Works even though its own files have an extension of .doc(x).

KEEPING CONFIGURATION FILES These contain information on system parameters. When a program is run, it may need to look at configuration files to see which conditions it should adapt to.

Data: Its Representation, Structure and Management in

Information Systems (3.1.3) We generally use the decimal (or denary) system as our number system in everyday

life as humans.

However computers find it easy to work with a number system based on 2 because only the digits 0 and 1 are used – this is called a binary system.

To convert a number from our system to binary (base 2), we use a column diagram to simply puts 1s in each column that is needed to make the number, for example...

128 64 32 16 8 4 2 1 172

1 0 1 0 1 1 0 0

128 64 32 16 8 4 2 1 75

0 1 0 0 1 0 1 1

o Therefore, taking denary 75, its binary form is 01001011.

If the column headings are changed so that we have base 8 numbers, then in the

case of 75, we have 0113 – this is called the octal system.

512 64 8 1 75

0 1 1 3




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Some information is stored in computers as numbers in base 16.

This is called hexadecimal (or just hex).

Principles are exactly the same as for denary, binary, octal or any other base.

We have ten of them (from 0 to 9) but we need six extra ones.

They are the capital letters from A to F.

A stands for the denary value ten B stands for eleven C stands for twelve D stands for thirteen E stands for fourteen F stands for fifteen

...and then 10 (in hex) stands for sixteen (in denary).

If the column headings are changed so that we have base 16 numbers, then in the

case of 75, we have 4B.

256 16 1 75

0 4 11

Binary coded decimal (BCD) is another representation which is similar to binary.

It simply represents the different digits in the number separately using four binary

digits for each denary digit.

for example, 7 in binary is no eights, 1 four, 1 two and 1 one – i.e. 0111 5 in binary is no eights, 1 four, no twos and 1 one – i.e. 0101

so 75 in BCD is 01110101 (just pit the two lots of binary together)

We cannot store negative numbers as there is nowhere to put the minus sign.

Therefore we put a 1 in front of the number to stand for a – sign.

If you have to be able to count from 0 to fifteen before going back to the start and putting a 1 in front of the 0 to stand for sixteen it means we have to have sixteen digits.




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Using -75, it goes from 01001011 (in binary) to 11001011 as the first bit changes its

meaning – i.e. it no longer stands for “128” but instead “+/-”.

This is called sign/magnitude representation because the byte is in two parts:

o The sign and

o The size of the number

There are two PROBLEMS with sign and magnitude:

1. The biggest number that can be represented is now half what it was

2. The binary now contains two types of data: a sign and a value

This makes it very difficult to do arithmetic because different bits mean different

things.

Another way of doing negative numbers which gets round the problem of having a

bit that is no longer a number is to use a system called 2s complement.

In 2s complement the first bit stands for -128 instead of just a minus sign...

-128 64 32 16 8 4 2 1

Now, -75 must start with a 1 because it is the only place where it can get the minus

sign from, but -128 is 53 too many.

So as well as 1 lot of -128 we will need +53 to get back to -75.

To obtain +53...

-128 64 32 16 8 4 2 1 -75

1 0 1 0 1 1 0 0

So -75 in 2s complement binary is 10110101.

Addition: with binary there are only 4 possible sums:

o 0 + 0 = 0

o 0 + 1 = 1

o 1 + 0 = 1

o 1 + 1 = 0, carry 1




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-128 64 32 16 8 4 2 1 0 1 0 0 1 0 1 1

75

0 0 0 0 1 1 1 0

14

0 1 0 1 1 0 0 1

=89

CARRY ==>

1 1 1

Subtraction: easy if using 2s complement because 75 – 14 is the same as 75 + (-14)

-128 64 32 16 8 4 2 1 0 1 0 0 1 0 1 1

75

1 1 1 1 0 0 1 0

-14

0 0 1 1 1 1 0 1

=61

1

1 <== CARRY

A character set consists of a code that pairs each character from the alphabet with a

sequence of natural numbers, in order to store text on a computer.

ASCII is a binary code used to handle text using the English alphabet.

Stands for ‘American Standard Code for Information Interchange’.

ASCII uses 7 binary digits to represent characters:

o 1000001 represents the upper-case letter A;

o 1000010 represents B;

o 1000011 represents C and so on in sequence

EBCDIC is an 8-bit character encoding used on IBM machines primarily.

Stands for ‘Extended Binary Coded Decimal Interchange Code’.

Based on BCD – coding done in groups of 4.

Unicode is very similar to ASCII, except it contains far more characters.

The sum here is 1 + 1 and then a carry is added.

Think of it as 1 + 1 = 10 and then add the carry making

11, which is 1 down and carry 1 again.




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Designed to include all the symbols that are used throughout the world including all

Arabic, Chinese and Japanese characters.

It is constantly growing in size as more and more characters are accepted.

Data can be collected in a manual way – i.e. not directly connected to a computer –

such as via questionnaires, and then it can be typed into a computer using a

keyboard.

o For this reason, forms must be designed in such a way that they are not at all

ambiguous in gathering the required data.

The person entering data from the forms can make typos in processing so much

data, taking the date of birth 1st February 2003 (01/02/03) as an example.

The typist may accidently key in 41/02/03 – they have broken a rule, as there are at

most 31 days in a month.

As long as the computer knows the rules, it can tell the operator their mistake.

This is known as data validation.

This type of data validation is known as a range check because it checks that the

data is within a certain range, in this case between 1 and 31 (or 1 and 28 if it’s a bit

more sophisticated!).

Other validation checks include:

o FORMAT CHECK – 01/02/003 would be rejected because the computer knows

the format should be dd/mm/yy and will not accept anything else.

o LENGTH CHECK – 01/02/003 will be rejected because knows to expect 8

characters and there are 9.

o CHARACTER CHECK – a1/02/03 will be rejected because the computer knows

the first character has to be a digit.

o PRESENCE CHECK – the typist missed it out, the computer will report an error

because it was expecting data.

Data verification is the process of determining the correctness of the data.

It tests against specifications – i.e. is the data present rather than is it correct.

Data can equally be collected in an automatic way.

Speech input and temperature sensors are examples of automatic inputs.




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Barcode readers read the light and dark parallel lines on many products and devices,

but the barcode has had to be created first.

Dark lines are read in pairs and correspond to digits that combine to form the

barcode that will convey information about the article to the computer.

Optical Mark Recognition (OMR) is the process of capturing human-marked data

from forms such as surveys and tests. It is the use of shading in of boxes or specific

areas of a form to represent data – it’s the positioning of the shading that matters,

hence the paper needs to be positioned precisely.

Optical Character Recognition (OCR) is the reading of the individual characters by

the computer. These shapes are compared to a standard set of character shapes

stored in the computer’s memory.

Magnetic Ink Character Recognition (MICR) is a character recognition technology

used primarily by the banking industry to facilitate the processing of cheques. The

technology allows computers to read information off of printed documents.

Unlike barcodes or similar technologies these three codes can be easily read by

humans.

A typical computer game such as a car racing game will have many outputs:

o Video – realistic view of the race on screen

o Sound – atmosphere and clues as to the engine performance

o Graphical images – map of the track (position), speedometer, time etc.

o Tabular information – show salient points from last lap

o Text – report any problems that arise

o Some form of alarm to warn when fuel is low




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THE SYLLABUS STATES THAT IN THE EXAM YOU MAY NEED TO...

Describe possible forms of output such as graphs, reports, interactive presentations, sound, video, images, animations, stating the advantages and disadvantages of each with reference to the target audience.

Data needs to be backed up.

There are lots of ways data can be lost...

o Hard drives can fail

o Viruses can destroy or corrupt data

o Computer systems can be damaged in fires or floods

o Files might be accidentally deleted

GLOBAL BACKUPS All data files are copied to a second storage medium. Quick to restore data. Backing up can be slow – problem if backups need to be regular.

INCREMENTAL BACKUPS Only copies files that have changed since last backup. Only backing up altered files makes it much quicker. Restoring can take a long time as data needs to be rebuilt from separate backups.

MIRRORED HARD DRIVES A second hard disk keeps an exact mirrored copy of the main hard disk. If main disk fails, mirror is immediately available and fully up-to-date. Mirroring systems can be very expensive.

Archiving is the storing of data that is not being actively used, but may be needed at

some point in the future.

Data is copied to a suitable storage medium, but then original data is removed.

MAIN PURPOSE of archiving is that it frees storage space on main system.

However archived data is still available should it ever be needed.

Hardware (3.1.4) The CPU (Central Processing Unit) is the central part of a computer.

o Consists of special registers – ALU, CU and IAS.

Control Unit manages execution of instructions: fetches the instruction, decodes it

and synchronises its execution. It then sends control signals out to rest of processor

(in order of instructions) using pathways called busses. There are different types of

busses depending on the type of data. The CU uses a control bus.




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The Arithmetic Logic Unit is where any arithmetic instructions or logical comparisons

are carried out. It also acts as the gateway to the processor.

All inputs/outputs pass through it before being directed to other parts of the system.

The Immediate Access Store is simply the main memory of the computer.

Special registers keep a check on the progress of the instructions and data as they

move around the processor...

PC

•PROGRAM COUNTER: keeps a check on whereabouts the next instruction is in the memory – after one instruction has been carried out the PC will always be able to tell the processor where the next instruction is.

MAR•MEMORY ADDRESS REGISTER: this is where the address that was read from the PC is sent.

MDR

•MEMORY DATA REGISTER: the memory is searched to find the address being held in the MAR and whatever is in that address must be the instruction – a copy of it is placed in the MDR.

CIR

•CURRENT INSTRUCTION REGISTER: the instruction that is now in the MDR is copied into the CIR. When it is in the CIR, the instruction can be split into its parts.

•One part is sent to the computer to be decoded so that the processor can send signals to the relevant parts of the processor to carry out the instructions.

•Another part is an address that tells the processor whereabouts in the memory the data is that is to be used.

•So if the instruction is ADD 20, it will be split up, the control units works out how to do an 'add', and the 20 is where the processor will find the data that has to be added.




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The address part (20 in the example) is sent back to the MAR.

The memory is then searched.

o Whatever is in the address 20 is copied into the MDR.

The value in the MDR can then be used according to the instruction in the CIR.

If the instruction is to do some arithmetic or logical comparison the data is sent to

the Accumulator that will carry out the task.

Control bus sends instructions from control unit to different parts of processor.

Data bus carries data from one register to another.

Address bus carries location address to which the data is going.

As they travel around the processor...

o The various localities look at the address as it passes,

If it matches then the data is grabbed.

Two characteristics of computer memory are ROM and RAM.

They describe the different characteristics required perfectly.

o Read Only Memory: data held here cannot be altered. Data is not erased

when power is switched off, making it ideal to store instructions that a

computer needs to ‘boot up’ – this is called the boot (or bootstrap) program.

o Random Access Memory: data wiped clear when power is switched off. The

quality of being erasable when power is lost is called being ‘volatile’ while

ROM therefore is ‘non-volatile’.

Primary memory is the only storage directly accessible to the CPU.

CPU continuously reads instructions stored there and executes them as required.

There are more sub layers of the primary memory, besides main large-capacity RAM:

o Processor registers are located inside the processor.

o Processor cache is an intermediate stage between ultra-fast registers and

much slower main memory (increases performance of the computer).

Secondary storage media is basically external memory.

o Can be categorised into three types – magnetic, optical and solid state.

Magnetic devices are typified by the hard disk that is part of most computers.

Misnamed because it will consist of series of disks stored on top of each other.

Data is stored as small amounts of magnetism on surface of disks can be read by

heads and can then be interpreted as standing for a 0 or a 1.

Diagram to show components of a hard disk...




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Optical devices are typified by CDs, DVDs and BDs (which are similar to hard disks).

They store data as pits (little indentations) on the surface of a reflective disc.

Data is read by moving a laser beam across the surface of the disc and reading the

change in position of the reflected beam.

Solid state storage is typically found in flash memory (USB pen drive).

No moving parts = no mechanical process involved.

Data is stored in a thin layer of oxide between non-conductive layers.

Main disadvantage compared to magnetic media is that they are more easily lost.

Buffers are small areas of memory that act to temporarily store data while it is

waiting to be used or to be sent somewhere else.

A printer will have a buffer whose job it is to store work sent from the processor while

the printer prints it out – this lets the processor get on with something else and not

have to wait until the printout is finished before it can carry on.

Interrupts are messages sent from other places to the processor.

A message needs to be sent to the processor when the printer runs out of data in the

buffer and wants some new data sent so that it can carry on.

Disc refers to an optical media whereas

disk refers to a magnetic media.




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Object Features Advantages Disadvantages

Barcode Reader

Uses a laser beam to read the code.

Fast and reliable. Can even read curved surfaces or upside down. Check digit that is included in the code makes it good at rejecting errors.

Input is limited to a number code. An extra layer of packaging is needed for multipack goods (different barcode to the individual items).

MICR Uses special ink which contains iron particles (can be magnetised) and then read when numbers pass through MICR machine.

Provides a high level of security and automation to the processing of cheques.

Systems are expensive and can only accept a few characters.

OCR OCR software converts a scanned image of the text into an actual text file by recognising each character.

Can convert large amounts of text into a digital form that can be edited in a word processor etc.

Recognition can be poor, particularly with low quality originals and unusual fonts.

OMR Uses infrared light to scan marks on prepared forms such as multiple-choice examination answer sheets or lottery tickets.

Quick and reliable way of inputting large amounts of simple data.

Limited to fairly simple data capture such as selections from a menu or marking things present or absent. Cannot be used for text and number input.

Scanner Uses reflected light to read data from a paper document.

Once an image is digitalised then it can be enhanced with graphics software.

Images can take a lot of memory space. Quality can be lost in digitalising process.

Touch Screen

Devices which enable a use to control a computer by touching an area of the screen.

Software can alter the screen while it is running making it more flexible than a concept keyboard.

Not suitable for inputting large amounts of data. Selecting detailed objects can be difficult with fingers

Dot Matrix Printer (for example)

Printers produce a hard copy on paper. Different printers are suited to different purposes.

Low setup and maintenance costs (robust machines). Print quickly as produces duplicate copies of printouts at once.

Low quality – produce black and white printing and can only print low resolution graphics. Can be quite noisy because of their design.

Plotter A drum plotter prints by moving a pen sideways over the surface of a

Accurate and can produce far larger printouts than standard

Slow and relatively expensive compared to printers. Cannot print




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sheet of paper. printers. photographic images. Can only fill solid blocks using closely hatched lines.

Speaker These transducers convert an alternating electrical current into sound.

Cheap and widely available. Capable of producing very high quality sound.

Analogue device so the digital sound signal has to be converted (by a sound card).

Microphone Digital audio can be saved for playback. Voice recognition can also be used in security systems.

Voice recognition software can be used to convert your voice into text or to control menu options etc.

Stored audio files can take up a lot of memory. Voice commands can be difficult to recognise.

Sensor Detect changes in the physical or chemical environment and convert them into electrical signals.

Huge range of possible sensors. Switch sensors can detect angles of tilt or whether something is open or closed.

Most sensors need an interface to convert analogue signals into the digital signals that a computer can understand.

Actuator Used to control physical movement.

Electricity is easily routed to actuator. Easily controlled. Faults easy to diagnose.

Can be expensive as complicated intricate mechanisms are required.

Actuators are output devices that control physical movement.

o Three main types – motors, hydraulic and pneumatic.

Motors are powered by electrical signals from the computer.

o Stepper-motors are ones where the signal moves the motor in a series of tiny

but accurate steps (as in flatbed scanners).

o Servo-motors are where the signal enables the motor to move continuously

at high speed (as in computer-operated drills).

Hydraulic actuators are powered by fluid pressure.

Controlled by computer.

Slow but very powerful (useful for heavy lifting).

Pneumatic actuators are like hydraulic ones but are powered by air pressure.

Therefore less powerful but more responsive.

Often used to power robots on fast moving automated production lines.




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Data Transmission (3.1.5) A Local Area Network (LAN) is a collection of computers and peripherals in one

building or site which are connected together using cable or wireless connection.

A defining characteristic is their higher data-transfer rates.

LANs are usually linked using cables or fibre-optic backbones for fast links.

A Wide Area Network (WAN) connects computers over a large distance to other

towns or countries using telecommunication links, e.g. the internet.

There has been a large increase in the number of WANs in recent years, due to the

reduced costs of transferring data and the demand for more instant communication.

WANs use long distance communication technologies.

HARDWARE Each computer needs a network interface card (NIC) and a connection to the network. There’s usually a main central computer (server) which often stores shared programs and individual files. To connect to a WAN, like the internet, a modem or gateway/router will be needed.

SOFTWARE The server itself needs a network operation system and software. Network software will usually allow data compression, encryption and error correction. Each machine will need a browser to view the internet/intranet pages. Email software will be needed to communicate across the LAN. If an internet connection is required then the server will need connection software and software to stop unauthorised access from outside the LAN (a firewall).

If data is going to be transmitted from one computer to another, then the bits have

to be sent from one machine to another.

Using the 8-bit byte as an example...

1 0 0 1 1 0 1 1




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One way of doing the transmission would be to send the byte, one bit at a time,

through a single wire (connection) – this is known as serial data transmission.

Another way would be to use a number of wires, one for each of the bits in the byte,

this is known as parallel data transmission.

Parallel transmission is much faster than serial transmission.

But serial transmission is safer because it is difficult to keep all the bits together.

Bits naturally transmit at slightly differing rates.

Simplex data transmission: one way data communication (e.g. radio station).

Duplex transmission: bidirectional data communication (e.g. telephone).

Half-duplex: data only in one direction at a given time (e.g. walkie talkies).

The more wires connecting devices, the more data that can be transmitted in a given

period of time – the number of bits that can be transmitted within a timeframe is

called the bit rate (measured in units called baud).

The number of bauds that can be transmitted per second is called the baud rate.

Low and high bit rates are of importance and are both preferable in different

situations.

For example, the transmission of a video from one device to another...

1 0 0 1 1 0 1 1

1 0 0 1 1 0 1 1

1 0 0 1 1 0 1 1

1 0 0 1 1 0 1 1




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Errors can occur in data transmission.

o There are three methods – echoing, checking sums and parity checks.

Echoing back: if a set of data is transmitted from one device to another, one

checking mechanism is to send it back to the sending device. When data gets back to

the sending device it is compared to what was sent, and if the two sets of data

match, we can assume that it go there OK. However, if it is different, an error as

occurred somewhere, so the original data is sent again. This process is then repeated

until the ‘echo’ matches the original.

Check sums: data that is sent is made up of binary numbers. Numbers can be added

together. When a set of data is sent the original bytes are added together and the

answer is then sent with the data. When the data bytes get to the destination they

are added together and their checksum is worked out. If the calculated checksum is

the same as the one that has been sent, it is assumed that the data has not been

corrupted. If it is different, then data needs to be resent.

Parity check: data is sent in fixed byte sizes (normally 8 bits). The data being sent

would be in the first 7 bits of the byte, the eighth bit being reserved as the parity bit.

IMMEDIATE THOUGHTS...

Video files are very large.

Must require high bit rate for transmission. HOWEVER THIS IS NOT NECESSARILY TRUE...

If the recipient is not going to watch the video until next week then it can take days to download onto the recipient’s hard drive.

When it is needed, it can run straight from the hard drive.

If the recipient wants to actually watch the video as it arrives (called streaming) the data must arrive quickly enough to stop pausing or breaking up of the picture or sound.

o This effect can sometimes be seen on a network if all the machines on a network are being used to exhibit the same screen as it is being streamed around the network – if the picture starts freezing then the bit rate is not high enough to cope.

The problem is not caused by the size of the files being sent, but rather by the way that the files are being used.

These files are said to be time sensitive because they lose their value unless they arrive in a particular time period.




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Imagining that the data is 0110110, and there is one bit left over which is used as the

check, the parity check works by making the number of 1s in a byte either always

odd or always even. This is something that has to be sorted out during the first

contact between the devices, whether they will agree to use odd or even parity

checking. In our example, there are four 1s in the data at the moment – four is an

even number, so if the devices are using even parity the last bit will be set to 0. On

the other hand, if odd parity is being used then we have to have an odd number of

1s in the byte. The 7 bits we already have cannot be changed because that would

change the data! But the eighth bit can be set to 1, giving five 1s, which is odd. So

the data above would be sent as 01101100 in a system with even parity and

01101101 in an odd parity system. Whichever parity, the data stays the same and it

is the final bit that changes (called the parity bit).

Packet switching: simply transmission across a network.

Groups data – irrespective of content, type, or structure – into suitably-sized blocks,

called packets.

Packets can be sent by any route, and are rebuilt by the receiver.

Multiple messages can be sent simultaneously on an ad hock basis.

Circuit switching: another form of transmission across a network.

Uses a dedicated route reserved for whole message.

The major advantage is that there are very fast rates.

Although only a few messages can be sent simultaneously.

Eavesdropping is also far easier than in packet switching.

WHAT’S THE DIFFERENCE?

In circuit switching, the path must be decided before data transmission starts, whereas with packet switching, there is no need

to worry about establishing a route.

Completion of packet switching method is more reliable because of the ability to send data packets over any route.

However, the completion can sometimes be delayed as packets may have to be received over different routes (and must always be

reassembled in the correct sequence).

Although if the connection breaks with circuit switching, so does the transmission of data.




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A protocol is the language used by computers while talking with each other.

In its simplest form, a protocol sets the rules used in communication between

computers in the transmission of data.

There is no point in one device transmitting at a different rate than the other can

receive, because the data pulses will get confused.

Part of the protocol must be an agreement over bit rates.

Another part will be parity – if the data is sent as having even parity and the

receiving device is expecting odd parity, messages will never be accepted.

The same goes for echoing back – if one device is set to send back data and the other

is not, then the data will never be checked.

Other important aspects to the protocol would be what character set is used.

Rules that go to make up the protocol are divided into two:

o Those parts that are logically based and physically based.

The logical parts can generally be thought of as applying to the data.

The physical parts can be thought of as being applied to the methods used for

communication (e.g. are we going to use cable or wireless?).

Different parts of the protocol are arranged in layers.

Each layer contains different information.

Different types of information can be thought of as being reliant on each other.

A simple example being that the choice of the physical means of communication will

dictate whether it is possible to use serial or parallel transmission of the data.

Because there is a sensible structure of one thing having to be decided before going

onto the next, it allows us to put the various layers in order and it means that

changes to the protocol can be made easily by altering just one layer (and the links

to the other layers it is in contact with), not the whole protocol.

This has meant a standardisation of the components of a computer system.

Implications of Computer Use (3.1.6) THE SYLLABUS STATES THAT THIS TOPIC COVERS...

Economic, social, legal, ethical and environmental implications of computer use.

THIS SECTION IS FAIRLY OBVIOUS (AS IT IS OPINIONATED), AND KNOWLEDGE CAN BE SUMMARISED... ICT has changed national patterns of employment (economic).

ICT services have changed how businesses work.

Social issues have been created (affecting the way society is organised).




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There are five main social issues:

1. Information rich and information poor (two-tier society; money)

2. Internet’s changing how people interact (home shopping; less physically fit)

3. Overreliance on technology (hackers exploit weaknesses)

4. Impact on literacy (text language and automatic spell check)

5. Faster pace of life (increased stress levels)

Three main ethical issues:

1. Unrestricted internet access

2. Ease of copying (copywritten) computer files

3. Increased government surveillance (“Big Brother”)

Environmental issues are generally one-sided...

Working from home saves petrol; less pollution

Teleconferencing saves international travel

Shop online so companies do not need to have ‘real’ shops

Insistence on always upgrading mobile phones (landfill sites)

Privacy and confidentiality of data held in computer systems is of vital importance.

o Folders can be set to read-only.

o Discs can be write-protected.

o Data can be encrypted (applying algorithms).

o Presence of a firewall.

o Different users can have different permission levels.

o Software logs can be used to look at patterns of access.

o Security software (virus, antispyware etc.) must be kept up-to-date.

o Passwords need to be kept secure:

Use a mixture of numbers and letters.

Don’t use a recognised word (from the dictionary).

Change password regularly (every month recommended).

Never use the same password more than once.

Large organisations that store people’s personal data are controlled in what they can

and cannot store by a law called the Data Protection Act.

This lays down rules about what data can be stored and for how long.

It also states that organisations must declare who within the organisation can be

allowed to see personal data.

The main problem with legislation is that policing it is almost impossible!

Documents

Computing Fundamentals Revision Notes