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GCE A Level Computer Science 9608 Notes
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Notes for GCE A Level Computer Science 9608. I got A* using these notes. A2 notes are in red. AS notes are in black.
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Information representation Show understanding of the basis of different number systems and use the binary, denary and
hexadecimal number systems
Convert a number from one number system to another
Express a positive or negative integer in two’s complement form
o First digit is negative, every other digit is positive
o Getting two’s complement of a number is swap bits and plus 1
Show understanding of, and be able to represent, character data in its internal binary form
depending on the character set used (Candidates will not be expected to memorise any
particular character codes but must be familiar with ASCII and Unicode.)
o ASCII is 8 bits (1 byte) but MSB is always 0
o Unicode is 2 bytes and is U+XXXX
Express a denary number in Binary Coded Decimal (BCD) and vice versa
Describe practical applications where BCD is used
o In BCD, each digit is stored as a binary number
o Used in calculators and representation of currency
o Adding BCD is adding normally but each time there is a “carry over” add 0110 to current
position
Show understanding of how data for a bitmapped image is encoded
o A file header which defines the resolution of the image and the coding scheme for the
pixel colour, and a matrix of pixels which is defined by its position and its colour.
Use the terminology associated with bitmaps: pixel, file header, image resolution, screen
resolution
o Pixel: The smallest identifiable component of a bitmap image, defined by just two
properties: its position in the bitmap matrix and its colour
o File header: Defines the resolution of the image and the coding scheme for the pixel
colour
o Image resolution: The width and height of the image in pixels
o Screen resolution: The width and height of the screen in pixels
Perform calculations estimating the file size for bitmapped images of different resolutions
o File Size = Width * Height * Colour Depth (In bits)
o 8 bits = 1 bytes
o 1024 bytes = 1 kibibytes (KiB)
o 1024 kibibytes = 1 mebibyte (MiB)
o 1024 mebibytes = 1 gibibyte (GiB)
Show understanding of how data for a vector graphic is represented and encoded
Use the terminology associated with vector graphics: drawing object, property and drawing list
o Each drawing object is defined by geometric formulae and associated properties, such
as line colour and style, and added to the drawing list.
Show understanding of how typical features found in bitmapped and vector graphics software
are used in practice
, Justify where bitmapped graphics and/or vector graphics are appropriate for a given task
o Vector graphics can be scaled without becoming pixelated, can group elements but
requires rasterising to be displayed.
o Bitmap images can be created and captured easily, uses less processing power, and are
typically larger but can be compressed more significantly.
Show understanding of how sound is represented and encoded
Use the associated terminology: sampling, sampling rate, sampling resolution
Show understanding of how file sizes depend on sampling rate and sampling resolution
o Sound is converted using an ADC. The sound is sampled at a certain rate (at least twice
the highest frequency) where the amplitude and frequency are captured.
o Sampling is the measurement of the value of an analogue signal at regular time intervals
o Sampling resolution is the number of distinct values available to represent the sample
and is specified by the number of bits used to store each sample.
Show understanding of how typical features found in sound editing software are used in practice
o Combining different sources
o Fading in or out
o Removing noise and other imperfections
Show understanding of the characteristics of video streams:
- the frame rate (frames/second)
- interlaced and progressive encoding
- video interframe compression algorithms and spatial and temporal redundancy
- multimedia container formats
o Frame rate is the number of frames displayed per second
o Interlaced encoding is when the video swaps between the odd lines and even lines to
give the appearance of a “doubled” frame rate
o Progressive encoding is when every line in every frame is displayed.
o Interframe compression decreases the file size by removing neighbouring frames which
are similar
o Spatial redundancy is the redundancy within a frame and can be compressed like an
image
o Temporal redundancy is the redundancy between frame which records the differences
between frames
o Multimedia container formats contain multiple forms of media, e.g video and sound.
Show understanding of how digital data can be compressed, using either ‘lossless’ (including
run-length encoding – RLE) or ‘lossy’ techniques
o Lossless compression are coding techniques that allow subsequent decoding to recreate
exactly the original file
1. Huffman coding: most common characters are given shorter codes
2. Run-length encoding: a lossless form of compression which converts a string of
repeating characters (a run) into two values, the character in the run and the
number of times it occurs.
, o Lossy compression are coding techniques that cause some information to be lost so that
the exact original file cannot be recovered in subsequent decoding.
1. Removing information that human ears and eyes can’t perceive.
Communication and Internet technologies
Explain the client-server model of networked computers
o Client-server architecture is established using a web server and a client browser
Give examples of applications which use the client-server model
o Ecommerce, email, online banking, proxy
Describe what is meant by the World Wide Web (WWW) and the Internet
o World Wide Web: A system of interlinked hypertext documents that use the HTTP
protocol to transmit data
o Internet: Interconnected network of computer devices and runs on TCP/IP protocol
Explain how hardware is used to support the Internet: networks, routers, gateways, servers
o Local Area Network (LAN): A group of connected devices over a small geo area
o Wide Area Network (WAN): Multiple LANs connected together
o Router: A device that acts as a node on the Internet
o Gateway: A device that connects networks of different underlying technologies
o Server: A device that provides services via a network
explain how communication systems are used to support the Internet: The Public Switched
Telephone Network (PSTN), dedicated lines, cell phone network
o Public Switched Telephone Network (PSTN): The telephone network around the world
which allows dialup internet.
o Dedicated line: Link between two points that is continuous.
o Cell Phone Network: Mobile devices connect to cell towers which connect with each
other and can access the internet.
explain the benefits and drawbacks of using copper cable, fibre-optic cabling, radio waves,
microwaves, satellites
o Copper Cable: low cost, low bandwidth, high attenuation and interference
o Fibre-optics: high cost, high bandwidth, low attenuation and interference
o Radio waves: low cost, lower bandwidth, interference
o Microwaves: low cost, higher bandwidth, interference
o Satellites: low cost to use, high cost to set up, interference
show understanding of bit streaming (both real-time and on-demand)
o Bit streaming is a continuous sequence of bits transmitted serially.
o Data is streamed to a buffer which is refilled whenever possible to prevent buffering
o On-demand streaming is when the server sets up a bit stream with the client.
o Real-time streaming is when the event is captured, encoded and then sent to the client
show understanding of the importance of bit rates/broadband speed on bit streaming
o Greater bit rates and broadband speed are required for better quality streams
explain the format of an IP address and how an IP address is associated with a device on a
network
, o An IP address is a numeral label assigned to each device that connects to the internet.
o Is stored as IPv4 (32 bit) or IPv6 (128 bit).
explain the difference between a public IP address and a private IP address and the implication
for security
o Public IP is provided by ISP and is unique
o Private IP is provided by router and is only unique across the network. This cannot be
accessed from the internet
explain how a Uniform Resource Locator (URL) is used to locate a resource on the World Wide
Web (WWW) and the role of the Domain Name Service
o A URL (Uniform Resource Locator) identifies a web page.
o URL format: sub-domain.domain.top-level-domain
o The role of the DNS (Domain Name Service) is responsible for mapping a domain name
to an IP address
describe the sequence of events executed by the client computer and web server when a web
page consisting only of HTML tags is requested and displayed by a browser
- Client-side
o recognise and identify the purpose of some simple JavaScript code
o describe the sequence of events executed by the client computer and web server when
a web page with embedded client-side code is requested and displayed by a browser
o show understanding of the typical use of client-side code in the design of an application
- Server-side
o recognise and identify the purpose of some simple PHP code
o describe the sequence of events executed by the client computer and web server when
a web page with embedded server-side code is requested and displayed by a browser
o show understanding that an appropriately designed web application for accessing
database data makes use of server-side scripting
1. User opens browser on client
2. User types URL
3. Browser asks DNS for IP
4. Browser connects to IP and requests web page
5. Page is sent to browser by server
6. Browser displays page
Hardware
identify hardware devices used for input, output, secondary storage
show understanding of the basic internal operation of the following specific types of device:
- keyboard
o Uses switches and circuits to translate keystrokes into signals the computer can
understand
o The key matrix is a grid of circuits
o Each circuit is broken beneath the key
o When a key is pressed, a circuit is completed and a signal is sent
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