What does the data transmission and representation part of Eduqas Component 2 cover?

It covers data transmission (serial versus parallel, packet switching, network protocols and the TCP/IP stack), data representation (binary, denary and hexadecimal conversion, sign and magnitude and two's complement, binary arithmetic and overflow, floating-point representation and normalisation, and representing text, images and sound), compression, encryption and error checking, and the organisation and structure of data (files, relational databases, primary and foreign keys, normalisation to third normal form, and SQL).

How do you represent a negative number in two's complement?

Write the positive value in binary, invert every bit, then add one. For example, in 8 bits, plus 12 is 0000 1100; inverting gives 1111 0011; adding one gives 1111 0100, which is minus 12. Two's complement has a single zero and an 8-bit range of minus 128 to plus 127, and it is preferred because ordinary binary addition and subtraction work directly on it.

How is the file size of an image and a sound clip calculated?

For a bitmap image the size in bits is width multiplied by height multiplied by the colour depth (bits per pixel), then divided by 8 for bytes. For a sound clip the size in bits is the sample rate multiplied by the duration in seconds multiplied by the bit depth, multiplied by the number of channels, then divided by 8 for bytes. Higher resolution, colour depth, sample rate and bit depth give better quality but larger files.

What is the difference between symmetric and asymmetric encryption?

Symmetric encryption uses the same single key to encrypt and decrypt, which is fast but the key must be shared securely. Asymmetric encryption uses a key pair: a public key to encrypt that can be shared openly, and a private key to decrypt that is kept secret, which solves the key-distribution problem. Real systems often use asymmetric encryption to exchange a symmetric key, then symmetric encryption for speed.

How should I revise data transmission and representation for Eduqas?

Drill binary, hexadecimal and two's complement conversions and arithmetic until automatic, since there is no calculator. Practise evaluating and normalising floating-point numbers and the image and sound file-size calculations. Learn the lossy versus lossless and symmetric versus asymmetric distinctions, the error-detection methods, and be able to write SQL queries and normalise a table to third normal form.

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Eduqas A-Level Computer Science data transmission and representation: binary, floating point, media, compression and databases made exam-ready

A deep-dive Eduqas Component 2 guide to data transmission and representation (sections 3.2 to 3.4). Covers serial and parallel transmission with TCP/IP, binary, hexadecimal and two's complement, floating-point and normalisation, representing text, images and sound, compression, encryption and error checking, and relational databases with SQL and normalisation.

Generated by Claude Opus 4.815 min readEduqas-A-Level-CS-3.2-3.4Updated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

What this section actually demands
Transmission and number representation
Media, protection and databases
How this section is examined
Check your knowledge

What this section actually demands

The data transmission and representation part of Component 2 (specification sections 3.2 to 3.4) is the most calculation-heavy of the course, and it is sat without a calculator. Eduqas rewards accurate, shown working: conversions, two's complement arithmetic, floating-point evaluation and normalisation, file-size sums, and SQL queries. Every numeric answer should show the method, not just the result.

This guide walks through the topics in order and sets out the exam patterns Eduqas repeats. Each topic has a matching dot-point page with practice; this overview ties them together.

Transmission and number representation

Data transmission covers serial versus parallel (serial for long distances, parallel for short), packet switching and packet structure, network protocols and the four-layer TCP/IP stack. Number representation covers binary, denary and hexadecimal conversion, sign and magnitude versus two's complement, binary addition and subtraction with overflow detection, and floating-point representation with a mantissa and exponent plus normalisation and the range-precision trade-off.

Media, protection and databases

Representing text, images and sound covers ASCII and Unicode, bitmaps (resolution, colour depth, the file-size calculation) and sampled sound (sample rate, bit depth, the file-size calculation). Compression, encryption and error checking covers lossy versus lossless (run-length and dictionary methods), symmetric versus asymmetric encryption, and parity, checksums and check digits. Organisation and structure of data covers files and access methods, relational databases with primary and foreign keys, normalisation to 3NF, and SQL.

How this section is examined

A typical Eduqas profile for sections 3.2 to 3.4:

Transmission. Serial versus parallel; packet structure; the TCP/IP layers.
Number representation. Convert between bases; two's complement negation and addition; evaluate and normalise floating point.
Media. Calculate image and sound file sizes; ASCII versus Unicode.
Protection and databases. Lossy versus lossless; symmetric versus asymmetric; parity and checksums; write SQL and normalise a table to 3NF.

Check your knowledge

A mix of recall and calculation questions covering the section. Attempt them under timed conditions, then check against the solutions.

Convert $1010\,1100$ to denary. (1 mark)
Represent $-20$ in 8-bit two's complement. (2 marks)
State the leading bits of a normalised positive mantissa. (1 mark)
Calculate the file size in bytes of a $100 \times 50$ pixel image at $4$ bits per pixel. (2 marks)
Give one situation where lossless compression must be used. (1 mark)
Write an SQL statement to select the Name field from a Student table where Year is $13$ . (1 mark)

Solutions

Six questions covering data transmission, number representation, media, and databases.

Step 1: Q1 - Convert $1010\,1100$ to denary

Each bit position in a binary number represents a power of 2, starting from $2^7 = 128$ on the left. Add the powers of 2 where a $1$ appears: positions for $128$ , $32$ , $8$ and $4$ are set.

128 + 32 + 8 + 4 = 172

Final answer: $172$ .

Step 2: Q2 - Represent $-20$ in 8-bit two's complement

To negate a number in two's complement, first write the positive value in binary, then invert every bit, then add one. The method works because adding the result to the original gives zero in fixed-width arithmetic.

+20 = 0001\,0100

\text{Invert:}\ 1110\,1011

\text{Add 1:}\ 1110\,1100

Final answer: $1110\,1100$ .

Step 3: Q3 - State the leading bits of a normalised positive mantissa

Normalisation removes leading zeros so that the mantissa carries the maximum number of significant bits. For a positive number the bit to the left of the binary point must be $0$ (to show it is positive) and the first bit after the point must be $1$ (so there are no wasted leading zeros).

Final answer: A normalised positive mantissa starts $0.1\ldots$ (the first bit after the point is $1$ ).

Step 4: Q4 - Calculate the image file size

The number of bits in a bitmap image is width times height times the colour depth (bits per pixel). Dividing by $8$ converts bits to bytes.

100 \times 50 \times 4 = 20\,000\ \text{bits}

20\,000 \div 8 = 2\,500\ \text{bytes}

Final answer: $2\,500$ bytes.

Step 5: Q5 - State a situation where lossless compression must be used

Lossy compression permanently discards some data, which is acceptable for images or audio where small quality reductions are tolerable. For files where every bit of the original must be recovered exactly, lossless compression is required.

Final answer: Compressing a text document or program executable; any file where losing data would corrupt it.

Step 6: Q6 - Write an SQL SELECT statement

The SELECT statement names the field to retrieve, FROM names the table, and WHERE filters rows by a condition. The field name, table name and condition value must all match the schema exactly.

SELECT Name FROM Student WHERE Year = 13;

Final answer: SELECT Name FROM Student WHERE Year = 13;

Sources & how we know this

WJEC Eduqas GCE AS/A Level Computer Science specification (from 2015) — Eduqas (2015)