Describe methods of data transmission

IGCSE Computer Science – Core Computer‑Systems (Topics 1‑6)

1. Data Representation

1.1 Number Systems

• Binary (base‑2) – digits 0 and 1.
• Denary (decimal, base‑10) – digits 0‑9.
• Hexadecimal (base‑16) – digits 0‑9 and A‑F.

Conversions

• Binary ⇄ Decimal – multiply each bit by 2ⁿ (from right‑most bit, n = 0) and add; or repeatedly divide by 2 and record remainders.
• Binary ⇄ Hexadecimal – group binary bits in sets of four (starting at the right) and replace each group with its hex digit.

Two’s‑complement (signed integers)

1. Positive numbers: same as ordinary binary.
2. Negative numbers: invert all bits (1→0, 0→1) then add 1.
3. Range for an n‑bit word: –2ⁿ⁻¹ … 2ⁿ⁻¹ – 1.

Shift operations

• Logical left shift – multiplies by 2 (fills with 0 on the right).
• Logical right shift – divides by 2 (fills with 0 on the left).
• Arithmetic right shift – divides by 2 while preserving the sign bit.

Key formulas for the exam

• Binary addition: add column‑wise, carry = 1 when sum ≥ 2.
• Overflow detection (two’s‑complement): overflow occurs when the carry into the sign bit differs from the carry out of the sign bit.
• Percentage / ratio: percentage = (part ÷ whole) × 100 %.
• Rounding to n significant figures: keep the first n digits, then round the (n+1)th digit.

1.2 Text, Sound and Images

Data type	Typical representation	Key parameters	Example calculation
Text	ASCII (7‑bit) or Unicode (8/16‑bit)	Characters per byte, encoding scheme	“Hi” → 2 bytes (ASCII) or 4 bytes (Unicode)
Sound (PCM)	Samples of amplitude taken at regular intervals	Sampling rate × Bit depth × Channels	5 s of 44 kHz × 16‑bit mono: 44 000 × 16 × 1 × 5 = 3 520 000 bits ≈ 440 KB
Images (bitmap)	Pixel grid, each pixel stores colour	Resolution × Colour depth	1024 × 768 × 24‑bit: 1024 × 768 × 24 ÷ 8 = 2 359 296 bytes ≈ 2.3 MB

1.3 Data Storage & Compression

• Units of storage – bit, byte (8 bits), KB = 2¹⁰ B, MB, GB, TB, PB, EB.
• File‑size calculation – multiply the number of units by the size of each unit (e.g., image size = width × height × colour‑depth ÷ 8).
• Lossless compression – original data can be perfectly restored (e.g., ZIP, PNG, Run‑Length Encoding). Used for text, spreadsheets, some images.
• Lossy compression – some data is permanently discarded to obtain higher compression (e.g., MP3, JPEG). Acceptable for audio/video where exact fidelity is not required.

2. Data Transmission

2.1 Classification of Transmission Media

Media are divided into guided (wired) and unguided (wireless) types.

Guided (Wired) Media

Media	Typical bandwidth	Maximum distance (per segment)	Advantages	Disadvantages
Twisted‑pair (UTP / STP)	up to 1 Gbps (Cat 6a)	100 m (no repeaters)	Low cost, easy to install	Susceptible to EMI, limited distance
Coaxial cable	up to 10 Gbps (modern)	≈ 500 m (with amplifiers)	Better shielding than UTP	Bulkier, more expensive than UTP
Fiber‑optic cable	10 Gbps – 100 Tbps	Several kilometres (single‑mode)	Very high bandwidth, immune to EMI, secure	High installation cost, fragile

Unguided (Wireless) Media

Media	Frequency range	Typical bandwidth	Typical range	Common uses
Radio waves	30 kHz – 300 GHz	up to several hundred Mbps (Wi‑Fi, LTE)	Up to 100 km (cellular towers)	Mobile phones, Wi‑Fi, Bluetooth
Microwave	1 GHz – 30 GHz	up to 1 Gbps (point‑to‑point)	1 km – 50 km (line‑of‑sight)	Backbone links, satellite uplink/downlink
Infrared (IR)	300 GHz – 400 THz	up to 10 Mbps	Few metres (line‑of‑sight)	Remote controls, short‑range links
Satellite (geostationary)	Ku/Ka bands (12‑40 GHz)	up to 100 Mbps (consumer)	≈ 36 000 km (space‑to‑earth)	Global broadcasting, remote‑area connectivity

2.2 Direction of Data Flow (Transmission Methods)

1. Simplex – one‑way only (e.g., keyboard → computer).
2. Half‑duplex – two‑way but not simultaneous (e.g., walkie‑talkies).
3. Full‑duplex – simultaneous two‑way communication (e.g., telephone, Ethernet).

2.3 Transmission Modes (How Bits Are Sent)

• Serial transmission – bits travel one after another over a single channel (USB, Ethernet, most long‑distance links).
• Parallel transmission – several bits travel at the same time on separate wires (inside a computer, e.g., the system bus).
• Synchronous transmission – sender and receiver share a clock; data sent as a continuous stream.
• Asynchronous transmission – data sent in characters/bytes framed by start and stop bits; no shared clock needed (e.g., RS‑232 serial ports).

2.4 Packet Structure

A typical data packet consists of three parts:

1. Header – source & destination addresses, protocol information, sequence number, length.
2. Payload (data) – the actual user information being transferred.
3. Trailer (footer) – error‑checking bits such as a CRC.

Example – Simplified Ethernet frame

| Destination MAC | Source MAC | Type/Length | Payload (46‑1500 B) | CRC |

2.5 Error‑Detection & Error‑Control Methods

Method	How it works (brief)	Typical use
Parity bit (even/odd)	Add one extra bit so the total number of 1s is even (or odd).	Small data units, e.g., early RAM, simple serial links.
Checksum	Sum all data words (often using 1’s‑complement); receiver recomputes and compares.	TCP/IP headers, many file‑transfer protocols.
CRC (Cyclic Redundancy Check)	Treat data as a binary polynomial, divide by a generator polynomial; remainder placed in trailer.	Ethernet, USB, storage devices.
ARQ – Automatic Repeat reQuest	Receiver asks sender to retransmit if error detected. Variants: Stop‑and‑Wait, Go‑Back‑N, Selective Repeat.	Reliable protocols such as TCP.

Worked example – 8‑bit even parity

• Data byte: 1010 0110 → number of 1s = 4 (already even) → parity bit = 0.
• Transmitted word: 1010 0110 0.
• If a single‑bit error flips the third bit, receiver sees 1000 0110 0 (three 1s) → detects error.

2.6 Factors Influencing Choice of Transmission Method

• Bandwidth (B) – wider frequency range → higher possible data rate.
• Signal‑to‑noise ratio (S/N) – determines how much error‑control is required.
• Distance – longer links need repeaters/amplifiers; attenuation grows with length.
• Cost – cabling, equipment, installation and maintenance.
• Security – wired media are harder to intercept than wireless.
• Mobility & flexibility – wireless media support portable devices.

2.7 Theoretical Maximum Data Rate – Shannon Capacity

Shannon’s theorem gives the upper bound on reliable data transmission:

C = B log₂(1 + S/N)

• C – channel capacity (bits per second).
• B – bandwidth (Hz).
• S/N – signal‑to‑noise power ratio (unitless).

3. Hardware

• CPU (Central Processing Unit) – executes instructions; consists of control unit, ALU, and registers.
• Fetch‑Decode‑Execute cycle – fetch instruction from memory, decode it, perform operation, store result.
• Registers – tiny, fast storage inside the CPU (e.g., accumulator, program counter, instruction register).
• Buses – pathways for data, addresses, and control signals (data bus, address bus, control bus).
• Cache memory – high‑speed memory close to the CPU; reduces average access time.
• Multi‑core processors – two or more independent cores on one chip; enable parallel processing.
• Embedded systems – computers built into appliances, cars, medical devices; usually dedicated to a single task.

4. Software

4.1 System vs. Application Software

• System software – operating system (OS) that manages hardware, file systems, security, and provides a platform for applications.
• Application software – programs that perform specific user tasks (e.g., word processors, browsers, games).

4.2 OS Functions (key for the syllabus)

• Process management (multitasking, scheduling).
• Memory management (allocation, swapping, virtual memory).
• File management (creation, deletion, directories, permissions).
• Device control (drivers).
• Security & user authentication.

4.3 Interrupts

An interrupt is a signal that temporarily halts the CPU’s current task so it can service an event (e.g., keyboard press, I/O completion). After handling, the CPU resumes the interrupted task.

4.4 Programming Concepts (required for Topics 7‑10)

• Variables & data types – integer, real, Boolean, character, string.
• Control structures – sequence, selection (if‑else, switch), iteration (while, for, repeat‑until).
• Arrays – ordered collection of same‑type items; indexed from 0 or 1 depending on language.
• File handling – open, read/write, close; text vs. binary files.
• Pseudocode & flowcharts – exam‑approved conventions (see Section 7).

4.5 Boolean Logic & Logic Gates

Gate	Symbol	Truth table (inputs A, B)
AND	&	0 0 → 0 0 1 → 0 1 0 → 0 1 1 → 1
OR	|	0 0 → 0 0 1 → 1 1 0 → 1 1 1 → 1
NOT	¬	0 → 1 1 → 0
NAND	↑	inverse of AND
NOR	↓	inverse of OR
XOR	⊕	1 when inputs differ

4.6 Simple Database (SQL) Basics

• One‑table design – each row is a record, each column a field.
• Primary key – unique identifier for each record.
• Common commands (exam‑style):

  SELECT column1, column2 FROM table WHERE condition ORDER BY column;
  INSERT INTO table (col1, col2) VALUES (val1, val2);
  UPDATE table SET col1 = newVal WHERE condition;
  DELETE FROM table WHERE condition;
          

5. Cyber Security (Topic 5.3)

• Confidentiality – keeping data secret (e.g., encryption, passwords).
• Integrity – ensuring data is not altered without authorisation (e.g., checksums, digital signatures).
• Availability – data and services are accessible when needed (e.g., backups, anti‑DoS measures).
• Authentication – verifying identity (usernames/passwords, biometrics, two‑factor).
• Firewalls & filters – control inbound/outbound traffic based on rules.
• Malware types – viruses, worms, trojans, ransomware.
• Best practices for students – strong passwords, regular updates, avoid suspicious links, use reputable antivirus.

6. Emerging Technologies (Topic 6)

• Digital currency & blockchain – decentralized ledger, cryptographic hashing, mining, public vs. private keys.
• Internet of Things (IoT) – everyday objects with sensors/network connectivity (e.g., smart home devices).
• Cloud computing – delivery of services (storage, processing) over the internet; SaaS, PaaS, IaaS.
• Artificial Intelligence & Machine Learning – algorithms that learn from data; basic concepts of training vs. inference.
• 3‑D printing – additive manufacturing; converts digital models to physical objects.

7. Assessment Support

7.1 Flowchart Symbols (exam‑approved)

Symbol	Name	Purpose
⧈	Process	Indicates an operation or instruction.
◇	Decision	Yes/No or True/False branching.
⭮	Loop/Iteration	Repeats a set of steps (while, for).
▶︎	Start/Stop	Marks the beginning or end of the algorithm.
⬭	Input/Output	Read from or write to the user/computer.

7.2 Pseudocode Conventions

• All statements in UPPERCASE.
• Indentation shows structure (e.g., inside IF, WHILE).
• Keywords: IF … THEN … ELSE … ENDIF, WHILE … DO … ENDWHILE, FOR i ← 1 TO n DO … ENDFOR, READ, WRITE, SET, CALL.
• Variables are named meaningfully (e.g., totalScore).

7.3 Command Words & Assessment Objectives (AO)

Command word	What the examiner expects	Relevant AO
Describe	Give a brief, factual account.	AO1 – Knowledge and understanding.
Explain	Give reasons or mechanisms.	AO2 – Application of knowledge.
Compare	Identify similarities and differences.	AO2 & AO3 – Analysis and evaluation.
Evaluate	Make a judgement, weighing advantages/disadvantages.	AO3 – Critical analysis.
Calculate	Perform a numerical operation using the correct formula.	AO1 & AO2.

7.4 Quick‑Reference Cheat‑Sheet (exam‑friendly)

• Binary ↔ Decimal: 2ⁿ values, e.g., 1011₂ = 1·2³ + 0·2² + 1·2¹ + 1·2⁰ = 11₁₀.
• Hex ↔ Binary: 0 = 0000, 1 = 0001, … F = 1111.
• Two’s‑complement (8‑bit) negative example: –45 → 45 = 0010 1101 → invert = 1101 0010 → add 1 = 1101 0011.
• File size (image): width × height × colour‑depth ÷ 8 = bytes.
• Audio file size: sampling rate × bit depth × channels × seconds ÷ 8 = bytes.
• Shannon capacity: C = B log₂(1 + S/N).
• Parity check: even parity → total 1s must be even.
• Checksum: 1’s‑complement sum of 16‑bit words; add overflow back into low-order bits.
• CRC: Use generator polynomial (e.g., CRC‑32) – remainder appended to data.