Topic 1 Data representation
1.1 Number systems
1. Understand how and why computers use binary to represent all forms of data
- Data is processed in a computer by using logic gates
- that only have two states (0 or 1)
Forms of data binary represents in a register:
- Data
- Instructions
- Number
- ASCII/Unicode value
2. (a) Understand the denary (base 10), binary (base 2) and hexadecimal (base 16) number systems
(b) Convert between positive denary, binary and hexadecimal
3. Understand how and why hexadecimal is used as a beneficial method of data representation
How hexadecimal is used:
- Colour codes
- MAC addresses
- IPv6 addresses
- Error codes
- Assembly language
- URL
Why hexadecimal is used (Benefits):
- Easier for humans to understand
- Takes up less screen space
- Easier to spot errors
4. (a) Add two positive 8-bit binary integers
(b) Understand the concept of overflow and why it occurs in binary addition
- The value is larger than 255
- So all bits required to represent the value cannot fit in the predefined 8 bit register
5. Perform a logical binary shift on a positive 8-bit binary integer and understand the effect this has on the
positive binary integer
- A shift to the left by n places multiplies the binary number by 2^n. 2 shifts = 4, 3 shifts = 8 etc.
- Likewise, A shift to the right divides the binary number by 2^n
- Effect: the value is incorrect because bits are lost
6. Use two’s complement to represent positive and negative 8-bit binary integers
- The leftmost bit is used as a sign bit to represent positive (0) or negative (1)
- The maximum range for two’s complement in 8-bits is -128 to 127 because of the lost sign bit
,1.2 Text, sound and images
1. Understand how and why a computer represents text and the use of character sets, including American
Standard Code for Information Interchange (ASCII) and Unicode
A character set is all the characters and symbols that can be represented by a computer system (1), each
character and symbol is assigned a unique value (2).
- ASCII is a character set that consists of 128 characters, using 7 bits to uniquely represent them
- Extended ASCII uses 8 bits rather than 7 so up to 256 characters can be represented
- Unicode is encoded using more bits per character, allows for a greater range of characters and symbols
than ASCII, including different languages and emojis but it takes up more storage space. (16 bits per
character)
2. Understand how and why a computer represents sound, including the effects of the sample rate and
sample resolution
- Analogue sounds must be digitally recorded in binary
- A sound wave is sampled at regular intervals for sound to be converted to binary using an analogue to
digital converter, which is processed by a computer
- The sample rate is the number of samples taken in a second
- The sample resolution is the number of bits per sample
- Effect: The accuracy of the recording and the file size increase as the sample rate and resolution increase
3. Understand how and why a computer represents an image, including the effects of the resolution and
colour depth
- An image is a series of pixels that are converted to binary, which is processed by a computer
- The resolution is the number of pixels in the image
- The colour depth is the number of bits used to represent each colour.
- Effect: The file size and quality of the image increases as the resolution and colour depth increase
,1.3 Data storage and compression
1. Understand how data storage is measured
- A byte is a unit of data 8 bits
2. Calculate the file size of an image file and a sound file, using information given.
- Must use 1024 for measurements, be careful of units e.g. divide by 8 for bits to bytes
- Image file size = image resoluaon (pixels) x colour depth (bits)
- (Mono) Sound file size = sample rate (Hz) x sample resoluaon (bits) x duraaon of sample (seconds)
- Stereo sound file = mono sound file size mulaplied by 2
3. Understand the purpose of and need for data compression
- Compression exists to reduce the size of the file
Benefits of data compression:
- Less bandwidth required
- Less storage space required
- Shorter transmission ame (quicker to upload/download)
- Smoother streaming/listening experience
4. Understand how files are compressed using lossy and lossless compression methods
Lossy compression
- A compression algorithm is used
- Reduces the file size by permanently removing unnecessary data
- E.g. reduces colour depth, sampling resoluaon, sampling rate, sounds humans cannot hear
- (for music files) Perceptual music shaping is used
- The original file cannot be reconstructed once it has been compressed
File formats: JPEG, MP3, WMV, MPG
Advantages: gives smallest file sizes, so it requires less storage space/bandwidth/transmission ame etc.
Disadvantage: reduced quality, original file cannot be restored
Lossless compression
- A compression algorithm is used
- Reduces the file size without permanent loss of data, original file can be restored
- Repeated words and paeerns are idenafied
- …, indexed and put into a table
- …and are replaced with their index
- …their posiaons
- …and the number of ames the word/paeern appears is stored in the table
- E.g. Run length encoding (RLE)
-
File formats: TIF, PDF, GIF, PNG, ZIP
Advantages: gives highest quality while being compressed, no data is lost so can restore original file
Disadvantage: Less significant reducaon in file size (larger file size) so takes longer to upload/ take up more
storage space compared to lossy. Slower streaming speed affects listening experience (scenario)
,Topic 2 Data Transmission
2.1 Types and methods of data transmission
1. (a) Understand that data is broken down into packets to be transmieed
(b) Describe the structure of a packet
- Packet header: desanaaon IP address, packet number, sender’s address
- Payload: the data in the packets
- Trailer: marks the end of the packet
(c) Describe the process of packet switching
Packet switching is the process of sending packets over the internet.
- Data is broken down into packets and sent
- Each packet takes a different route
- A router controls the route it takes, selecang the fastest available route
- Packets arrive out of order
- Once the last packet has arrived, packets are reordered using the packet numbers
- If packet is missing, it is requested to be sent again
2. (a) Describe how data is transmieed from one device to another using methods of data transmission
(b) Explain the suitability of each method of data transmission for a given scenario.
Serial: Data is sent over a single wire one bit at a ame
Advantages: cheaper to produce/setup, less chance of interference, more reliable over long distances
Disadvantage: slower transmission speeds than parallel
Parallel: Several bits are sent over mulaple wires simultaneously
Advantages: faster transmission speed, can send data in both direcaons at the same ame
Disadvantages: more expensive to setup, more prone to interference, limited distance, skewing may occur
Simplex: Data can only be transmieed in one direcaon. (computer to monitor, radio, TV)
Half duplex: Data can be transmieed in both direcaons, but only one at a ame (walkie-talkie)
Duplex: Data is transmieed both direcaons simultaneously (internet connecaons, phone calls)
3. Understand the Universal Serial Bus (USB) interface and explain how it is used to transmit data
- Industry standard method of transmitting data
- Uses serial transmission (data sent one bit at a time over a single wire)
- Can be half/full duplex
Benefits:
- It is industry standard, so it is likely to be compatible with the device
- It can only be inserted one way, so it cannot be inserted incorrectly
- Supports different transmission speeds, so allows high speed transmission
- Automatically recognises device, so driver is automatically loaded up/downloaded
- Backward compatible
Drawbacks:
- Slower than ethernet limited speed
- Limited to 5m cable length without a hub
, 2.2 Methods of Error Detection
1. Understand the need to check for errors after data transmission and how these errors can occur
- Errors can occur during data transmission due to interference, e.g. data loss, data gain or data change
2. Describe the processes of the error detection methods: parity check, checksum and echo check
Parity Check
- A parity bit is added to the parity byte
- The parity can be odd or even
- The receiver checks the number of 1s
- If the parity is incorrect, an error is detected
Parity byte: contains parity bit from vertical parity calculation
Parity block: the number of 1s are totalled horizontally and vertically
Checksum
- A value is calculated from the data
- Using an algorithm
- The value is sent with the data
- The receiver recalculates the value
- The values are compared
- If the values do not match, an error is detected
Echo check
- The receiver immediately sends a copy of the data back to the sender after receiving it
- The sender compares the returned data with the original data
- If there are no errors, the data is sent again
- If the data do not match, an error is detected
3. Describe how a check digit is used to detect errors in data entry and identify examples of when a check
digit is used, including international standard book numbers (ISBN) and bar codes.
Check Digit
- A validation method used to check data entry
- A digit is calculated from the data
- Using an algorithm
- The data is input with check digit
- The digit is recalculated on the input data
- The digits are compared to the stored value
- If it matches data entered is correct
- If digits do not match, an error is detected
- Uses E.g. Modulus 11 for product codes, barcodes, ISBN numbers
4. Describe how an automatic repeat query (ARQ) is used to establish that data is received without error
Automatic Repeat Query (ARQ)
- ARQ is an error control protocol
- Check (parity check/check digit/checksum/echo check) is performed on the received data
- If an error is detected, a request is sent to resend data
- Resend request is repeated until the data received is correct
- A positive acknowledgement that data is received is sent
- If no acknowledgement is returned within a time period, a timeout occurs, and data is resent
