4.5.5 Information Encoding — AQA 7517

ASCII — American Standard Code for Information Interchange

ASCII assigns a unique 7-bit code (0–127) to 128 characters including uppercase/lowercase letters, digits, punctuation, and control characters. Extended ASCII uses 8 bits (256 characters).

Character	Decimal	Binary (7-bit)
A	65	1000001
a	97	1100001
0	48	0110000
Space	32	0100000

Note: uppercase letters are 32 less than their lowercase equivalents. The digit '0' = 48, '1' = 49, etc.

Unicode

Unicode is an international standard that assigns a unique code point to every character in every writing system worldwide — over 140,000 characters including emoji.

Encoding	Bits per code unit	Characters
UTF-8	Variable (8–32 bits)	Backwards compatible with ASCII; widely used on web
UTF-16	Variable (16–32 bits)	Used internally in Windows, Java
UTF-32	Fixed 32 bits	Simple but wastes space; all code points as fixed width

Why Unicode matters: ASCII only covers English. Unicode enables multilingual software, supporting Chinese, Arabic, emoji, mathematical symbols, etc. UTF-8 is backward compatible with ASCII — the first 128 code points are identical.

Error Detection

Data transmission and storage can introduce errors (bit flips). Error detection techniques identify when corruption has occurred.

Parity Bit

An extra bit added to make the total number of 1s either even (even parity) or odd (odd parity). The receiver checks parity to detect single-bit errors.

Data (7 bits)	Count of 1s	Even parity bit	Transmitted (8 bits)
1010011	4 (even)	0	10100110
1110001	4 (even)	0	11100010
1011100	5 (odd)	1	10111001

Limitation: Parity can only detect an odd number of bit errors — if two bits flip, parity appears correct and the error is missed.

Majority Voting

Each bit is sent three times; the receiver takes the majority value. Can correct single-bit errors per group of three. Used in space probes and safety-critical systems.

Example: send 1 as 111 — if received as 101, majority is 1 → corrected.

Checksums

A value calculated from a block of data; the same calculation is performed at both ends and results compared. More powerful than parity — detects many patterns of errors. Used in TCP/IP, file downloads.

Exam tip: Know ASCII code values for key characters (A=65, a=97, 0=48). Understand why Unicode was needed (international character support). For parity, be able to add a parity bit and detect errors. Know the limitation: parity cannot detect two simultaneous bit errors. Compare ASCII (7-bit, 128 chars) with Unicode (variable, 140,000+ chars). AQA may ask you to compare methods or explain why error detection matters.

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Worksheet — 4.5.5 Information Encoding

8 questions · instantly marked · AQA 7517 standard

Q1State the decimal ASCII values for: (a) 'A', (b) 'a', (c) '5'. Explain the relationship between uppercase and lowercase letters.[4]

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A = 65 [1]; a = 97 [1]; '5' = 53 (48 + 5) [1]; lowercase letters have ASCII values 32 greater than their uppercase equivalents — e.g. 'a' − 'A' = 97 − 65 = 32 [1].

Q2Explain why Unicode was developed and state two advantages it has over ASCII.[3]

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Unicode developed because ASCII only supports English/basic Latin characters — inadequate for globalised software [1]; advantage 1: supports over 140,000 characters from all world writing systems [1]; advantage 2: includes emoji, mathematical symbols, and is backward compatible with ASCII (first 128 code points identical) [1].

Q3A system uses even parity. The data byte (7 bits) is 1001101. What parity bit is appended, and what is the full transmitted byte?[2]

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Count of 1s in 1001101 = 4 (even); parity bit = 0 (already even) [1]; transmitted byte = 10011010 [1].

Q4A receiver using even parity receives 10110011. Has an error occurred? Explain your answer.[2]

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Count of 1s: 10110011 has five 1s (odd) [1]; this does not match even parity, so an error is detected [1].

Q5State one limitation of using a parity bit for error detection.[2]

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A parity bit can only detect an odd number of bit errors [1]; if two bits are corrupted, the parity appears correct and the error goes undetected [1].

Q6Explain how majority voting works for error correction, and state one application.[3]

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Each bit is transmitted multiple times (typically three) [1]; the receiver takes the majority value — e.g. 110 → 1 (two 1s and one 0), so it can correct single-bit errors [1]; application: space probes / satellite communications / safety-critical systems (any one) [1].

Q7Compare UTF-8 and UTF-32 in terms of storage efficiency and compatibility. Which is preferred for web content?[3]

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UTF-8 is variable-length (1–4 bytes per character), more storage-efficient for ASCII text, backward compatible with ASCII [1]; UTF-32 is fixed 4 bytes per character — wastes space for ASCII/Latin text but simplifies indexing [1]; UTF-8 is preferred for web content (HTML, JSON) due to storage efficiency and backward compatibility [1].

Q8A file is downloaded over the internet. Explain how a checksum can be used to detect transmission errors.[3]

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The sender calculates a checksum value from the data (e.g. sum of all bytes, modulo some value) and transmits it alongside the file [1]; the receiver independently calculates the same checksum from the received data [1]; if the two checksums differ, a transmission error is detected; the file can then be requested again [1].

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ASCII — American Standard Code for Information Interchange

Unicode

Error Detection

Parity Bit

Majority Voting

Checksums

Worksheet — 4.5.5 Information Encoding

Mini Test — Information Encoding