1.1 Data Representation

1.1.1 Binary Magnitudes — Binary vs Decimal Prefixes

Computers work in binary (base 2), so storage sizes use powers of 2. Humans prefer decimal (base 10). This causes a clash in prefixes:

1.1.2 Number Systems — Binary & Denary

There are three main number bases you need to master, plus BCD and two's complement:

Denary → Binary (Divide by 2 Method)

Binary → Denary (Place Value Method)

1.1.3 Hexadecimal System

Hexadecimal (base 16) is used as a shorthand for binary. One hex digit represents exactly four binary bits, making it much easier for humans to read long binary strings.

Binary ↔ Hexadecimal Conversion

Applications of Hexadecimal

1.1.4 Binary Coded Decimal (BCD)

In BCD, each denary digit is converted to its own 4-bit binary representation. The whole number is the concatenation of these 4-bit groups.

Applications of BCD

1.1.5 Binary Addition & Overflow

Binary Addition Rules

Overflow

1.1.6 Two's Complement (Negative Binary)

Two's complement is how computers represent negative integers. The most significant bit (MSB) acts as the sign bit: 0 = positive, 1 = negative.

Binary Subtraction Using Two's Complement

1.1.7 Character Sets — ASCII & Unicode

ASCII — The First Universal Standard

ASCII (American Standard Code for Information Interchange) was developed in the 1960s. It uses 7 bits to represent 128 characters (0–127).

Limitations of ASCII — Why Unicode Was Born

Unicode in Practice — Sample Code Points

Unicode organises characters into blocks (ranges). Each character has a unique code point written as U+XXXX (hexadecimal).

📋 Chapter 1.1 Summary Checklist

🎨 1.2 Multimedia — Graphics

1.2.1 Bitmap Image Encoding

A bitmap image is made up of a grid of tiny squares called pixels. Each pixel has one colour, and that colour is stored as a binary number.

Key Bitmap Terms — Visual Memory Guide

1.2.2 Calculating Bitmap File Size

1.2.3 Effects of Changing Resolution & Colour Depth

1.2.4 Vector Graphics

Unlike bitmaps, vector graphics store images as a series of mathematical drawing objects (shapes, lines, curves) rather than individual pixels.

Vector Graphics Terms — Visual Memory Guide

1.2.5 Bitmap vs Vector — Choosing the Right Format

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🔊 1.2 Multimedia — Sound

1.2.6 How Sound is Represented & Encoded

Sound is analogue — a continuous wave of changing air pressure. Computers are digital — they can only store discrete (separate) values. Sampling bridges this gap.

Sound Terms — Visual Memory Guide

1.2.7 Impact of Changing Sampling Rate & Resolution

📋 Section 1.2 Summary Checklist

📦 1.3 Compression

1.3.1 The Need for Compression

1.3.2 Lossy vs Lossless Compression

1.3.3 Run-Length Encoding (RLE)

RLE is a lossless compression method. It identifies consecutive repeated values (runs) and replaces them with: the count + the value.

RLE on Text

RLE on Bitmap Images

1.3.4 Compression by File Type

Summary Decision Flowchart

📋 Section 1.3 Summary Checklist

2.1 Networks including the Internet

Throughout this chapter, we will follow Alex and Jamie as they build a network from scratch — starting with just two computers, and growing all the way to connecting to the global Internet. At each step, you'll face the same problems they face. Try to solve each problem yourself before reading the answer.

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2.1.1 Why Do We Need Networks? (The Problem Before the Solution)

Step 1: Two Computers, One Cable

Alex and Jamie start with the simplest possible setup: two laptops, one cable connecting them directly.

This works! Alex can send a file to Jamie directly. But now a third student, Sam, joins the project. Problem: With only one cable port on each laptop, how do you connect three computers?

🤔 What would YOU do? (Try before reading)

Three possible solutions:

1. Daisy chain: Alex → Jamie → Sam. But if Jamie's computer is off, Sam and Alex can't communicate.
2. Add more cables: Alex connects to both Jamie and Sam. But now Alex needs two cable ports.
3. Use a central device: Everyone connects to a switch. Data goes to the switch, which forwards it to the right person.

Solution 3 is what real networks use. You've just invented the need for a network switch!

Step 2: The Sharing Problem Grows

Now there are 30 students in the class, each with a computer. They also have one printer. Before networking:

• ❌ To print, you save your file to a USB stick, walk to the printer's computer, and print from there
• ❌ To share a file with everyone, you email it 30 times or pass a USB stick around
• ❌ To update software, you visit each computer individually
• ❌ To back up files, you need 30 separate backup drives

What networking solves: A single network connects all 30 computers and the printer. Now anyone can print from anywhere. Files are shared instantly. Software is updated once from a central server.

🔍 Practice 1 — Why Network?

Question: A primary school has 20 computers in one lab and 1 printer. There is no network. List three problems this causes, and explain how networking solves each one.

Sample Answer:
1. Problem: Students must save work to a USB stick to print it. Solution: Networking allows all computers to access the printer directly.
2. Problem: The teacher must manually install software on each of 20 computers. Solution: Software can be installed once on a central server and distributed to all computers.
3. Problem: If a student's computer crashes, their unsaved work may be lost. Solution: Files can be saved centrally on a server with automatic backup.

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2.1.2 How Should We Arrange the Connections? (Network Topologies)

⏱️ Era: Late 1970s — Early 1980s

🔹 1. Bus Topology — "The Party Line"

⏱️ Era: 1990s — Today (The Modern Standard)

🔹 2. Star Topology — "The Hub-and-Spoke"

⏱️ Era: 1960s (military/ARPANET) — Modern (wireless mesh & critical infrastructure)

🔹 3. Mesh Topology — "The Spider's Web"

⏱️ Era: 2000s — Today (Real-world networks)

🔹 4. Hybrid Topology — "The Best of Both Worlds"

📊 Historical Timeline Summary

✏️ Practice 2 — Which Topology?

1. A primary school has 5 computers and a very limited budget. They want to connect them to share one printer. Which topology would you recommend? State one advantage and one disadvantage.
2. A stock exchange needs a network where no single failure can stop trading. Which topology is most suitable? Explain why.
3. A company has 50 computers across 3 floors. Each floor is a star network with its own switch. All switches are connected to a central router. If the switch on Floor 2 fails:
   a) Which floor(s) lose network access?
   b) What topology does this overall design represent?
4. In a bus topology, what happens when two computers try to send data at the same time? What is this problem called?
5. Exam-style question: A hospital needs a network connecting 4 departments: A&E (critical), Surgery (critical), Pharmacy, and Admin. The hospital requires:
   • Maximum fault tolerance for A&E and Surgery
   • Cost-effectiveness for Pharmacy and Admin
   Design a suitable network topology arrangement for the hospital. Justify your choice.
6. Extension question: In a star topology, examine the animation above. Why does the data packet go to the switch before being forwarded to the printer? What would happen if every device could send directly to every other device (like in a mesh)? Compare the cost vs reliability trade-off between star and mesh.

🔍 Check your answers

1. Bus topology. Advantage: Lowest cost (one cable, no switch). Disadvantage: If the backbone cable breaks, the entire network goes down / performance degrades with more devices.
2. Mesh topology. Explanation: Multiple redundant paths mean no single point of failure. If one connection fails, data automatically reroutes through alternative paths.
3. a) Only Floor 2 loses access. Floors 1 and 3 continue working because each floor has its own switch. b) Hybrid topology.
4. A data collision occurs — the signals crash into each other and both messages are destroyed. Each computer must wait a random time and try again. This is called CSMA/CD (Carrier Sense Multiple Access / Collision Detection).
5. Sample answer: Use a hybrid topology. A&E and Surgery should use a partial mesh for redundancy. Pharmacy and Admin can use star to save costs. Connect all via a central router. This balances fault tolerance with cost efficiency.
6. The switch acts as a traffic manager — it receives the packet, reads the destination address, and forwards it only to the correct device. This prevents collisions and ensures only the intended recipient gets the data. In a full mesh, every device connects to every other — this gives maximum reliability (no single point of failure) but is very expensive (N(N-1)/2 cables). Star is a cost-effective trade-off: cheaper than mesh, but the switch is a single point of failure.

2.1.3 How Big Should the Network Be? (LAN and WAN)

Networks are classified by their geographical scope. The two most important categories are LAN (Local Area Network) and WAN (Wide Area Network). The difference isn't just about distance — it affects who owns the infrastructure, how fast data travels, and what hardware you need.

🔍 Practice 3 — LAN vs WAN

Question: A multinational company has offices in London, Tokyo, and New York. Computers within the London office are connected. The three offices are also connected to each other. Explain whether each connection is a LAN or WAN and why.

Sample Answer:
London office network: LAN — all computers are in the same building (small area), using privately owned cables.
Connection between offices: WAN — offices are on different continents (large area), requiring leased infrastructure like undersea fibre-optic cables.

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2.1.4 What Hardware Makes a Network Work?

Every device on a network needs specific hardware to send and receive data. Each piece of hardware solves a specific problem:

🔹 Step 1: The NIC — How Does a Computer Even Connect?

WNIC (Wireless NIC): What if you don't want a cable? A WNIC does the same job but wirelessly:

• Acts as an antenna — receives analogue radio waves and converts them to digital/binary
• Takes digital input and converts it to analogue radio waves for transmission
• Checks incoming transmissions for the correct MAC/IP address
• Encrypts and decrypts data for secure wireless communication

🔹 Step 2: The Switch — How Do Many Devices Talk to Each Other?

🔹 Step 3: The Server — Where Do We Keep Shared Files?

🔹 Step 4: The WAP — What About Phones and Tablets?

🔹 Step 5: Extending the Network — Bridge and Repeater

🔹 The Router — Connecting Your Network to the World

Router Functions in Detail

• Receives packets from devices or the internet
• Reads the destination IP address in each packet
• Consults its routing table to find the most efficient path
• Forwards packets to the next hop on that path
• Assigns private IP addresses to devices on the LAN (using DHCP)
• Maintains a table of MAC and IP addresses for local devices

🔍 Practice 4 — LAN Hardware

Question 1: Distinguish between the role of a switch and the role of a router in a network.

Sample Answer:
A switch connects devices within a single LAN and forwards data to the specific device it is intended for using MAC addresses. A router connects two or more networks (e.g., a LAN to the Internet) and forwards data packets between them using IP addresses and a routing table.

Question 2: Explain why a repeater might be needed in a large office network.

Sample Answer:
Electrical signals in a cable weaken over distance (signal degradation). If the office is large and cables run over 100 metres, the data signal becomes too weak to read. A repeater restores and amplifies the signal so it can continue travelling to its destination.

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2.1.5 How Should We Organise the Computers? (Client-Server vs P2P)

🔹 Client-Server — "The Library"

Example — Downloading a file:

1. Teacher saves a homework file on the school's file server
2. Alex opens his browser (client software) and requests the file
3. The server checks: "Is Alex authorised to access this file?" ✓
4. The server sends the file to Alex's computer

Benefits: Centralised security, centralised backup, easier management, less powerful (cheaper) client computers needed.

🔹 Peer-to-Peer (P2P) — "The Potluck Dinner"

Benefits: No expensive server hardware (save money), easy to set up (just connect and share), resilient (if one peer goes offline, others still work).

Drawbacks: Reduced security (each computer manages its own, weakest link vulnerability), no central backup (data lost if computer fails), performance issues (peers slow down when accessed by others).

Thin-Client vs Thick-Client

In a client-server network, clients come in two flavours. The difference is how much work the client does vs how much the server does:

🔍 Practice 5 — Network Models

Question 1: A school needs a network where the IT department can control which websites students visit and ensure all files are backed up centrally. Should they use client-server or P2P? Justify.

Sample Answer:
Client-server. The server provides centralised management: IT can set internet filtering policies once on the server, all student files are stored on the server (enabling central backup), and students require usernames/passwords to access resources, improving security.

Question 2: A hospital installs thin-client computers at nurse stations. Give two characteristics of a thin-client that make this suitable.

Sample Answer:
1. Data is not stored on the client — patient records remain on the central server, improving security and privacy.
2. The client performs minimal processing — nurse stations can use cheap, low-power devices that only display the server's output.

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2.1.6 To Cable or Not to Cable? (Wireless vs Wired)

The choice between wired and wireless isn't about which is "better" — it's about which trade-offs matter for your situation.

Wired Transmission Media

Copper vs Fibre-Optic Comparison (Exam Focus):

• ✅ Fibre-optic: greater bandwidth (more data per second), longer distance without boosting, more secure (harder to tap), no electromagnetic interference, lighter weight
• ✅ Copper: cheaper, easier to install (more flexible), technology is more established/widespread, easier to make terminations

Wireless Transmission Media

Wired vs Wireless — The Key Trade-Offs

🔍 Practice 6 — Wired vs Wireless

Question: A hospital wants doctors to access patient records from portable tablets as they move between wards. However, patient data is highly sensitive. Advise whether the hospital should use wired or wireless networking, explaining the trade-offs.

Sample Answer:
The hospital should use wireless networking with strong encryption because the doctors need mobility to access records while moving between wards — wired connections would not allow this. The trade-off is reduced security (wireless signals can be intercepted), so the hospital must implement robust encryption (e.g., WPA3) and access controls to protect patient data. If absolute security were the only concern, wired would be better, but mobility is essential for patient care.

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2.1.7 How Do We Avoid Traffic Jams? (Ethernet & CSMA/CD)

How CSMA/CD Works — Step by Step

Drawbacks of CSMA/CD

• ❌ Random waiting time increases each time — can lead to infinite waiting on a busy network
• ❌ Not scalable — more devices means exponentially more collisions
• ❌ Cannot prioritise certain devices (all are equal)
• ❌ Only suitable for short-distance networks

🔍 Practice 7 — CSMA/CD

Question: Describe what happens when two devices on an Ethernet network try to transmit data at exactly the same time.

Sample Answer:
1. Both devices sense the channel is free and begin transmitting simultaneously.
2. Their signals collide on the shared medium.
3. Both devices detect the collision and send a jamming signal to notify all devices.
4. Both devices wait a random amount of time before attempting to retransmit.
5. Because the wait times are random, one device will likely transmit before the other, avoiding another collision.

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2.1.8 Connecting to the World (The Internet and WWW)

Hardware That Supports the Internet

🔍 Practice 8 — Internet vs WWW

Question: A student says "I use the Internet to check my email and browse the WWW." Explain why this statement is both correct and slightly inaccurate.

Sample Answer:
The statement is correct in everyday language, but inaccurate in technical terms. The Internet is the global network infrastructure (TCP/IP) that carries data. Email and the WWW are both services that run on the Internet. The WWW is specifically web pages accessed via HTTP, while email uses different protocols (SMTP, POP3, IMAP). Both use the Internet as their transport medium.

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2.1.9 How Do We Find Anything on This Giant Network? (IP Addresses)

IP Addresses — The Internet's Addressing System

Every device on a network needs a unique identifier. Just as your home has a unique postal address, every device on the Internet has a unique IP address (Internet Protocol address).

IPv4 vs IPv6

Public vs Private IP Addresses

Key insight: Your home router has one public IP address, but assigns private IPs (like 192.168.0.x) to all your devices. When you visit a website, the router uses NAT (Network Address Translation) to map your private IP to its public IP and back. This means 100 devices in your home only need 1 public IP address — conserving the limited IPv4 space.

Static vs Dynamic IP Addresses

Subnetting

Subnetting divides a large network into smaller, manageable subnetworks. Each subnetwork shares the same network ID but gives each device a unique host ID. Benefits include: reduced traffic (data stays within its subnet), improved security (compromised device doesn't expose the whole network), easier maintenance, and simpler expansion.

🔍 Practice 9 — IP Addresses

Question: A company uses private IP addresses for its internal network. Explain two reasons for this choice and how employees still access the Internet.

Sample Answer:
Security: Private IP addresses are not visible from the Internet, so external attackers cannot directly target internal devices. Address conservation: Only the router needs one public IP address for all employees, reducing the number of public IP addresses needed. Employees access the Internet through NAT (Network Address Translation) on the router, which translates private IPs to the router's public IP for outgoing traffic.

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2.1.10 How Do We Remember All Those Numbers? (URLs and DNS)

URL Structure

A URL (Uniform Resource Locator) is the full address of a resource on the web:

https://www.cscompass.cn/courses/python

Protocol   Subdomain   Domain name       Path
  |           |            |               |
https://   www.    cscompass.cn    /courses/python

• Protocol: How to communicate (HTTP, HTTPS, FTP)
• Subdomain: A specific section of the domain (www, mail, etc.)
• Domain name: The human-readable name of the website
• Path: The specific page or resource within the website

How DNS Works — The Complete Journey

🔍 Practice 10 — DNS

Question: Describe the steps from a user typing www.example.com into a browser to the web page being displayed. Include the role of DNS.

Sample Answer:
1. The browser sends the URL to the nearest DNS server.
2. The DNS server looks up the domain name in its database and finds the matching IP address.
3. If not found, the request is forwarded to a higher-level DNS server until the IP is located.
4. The IP address is returned to the browser (and cached).
5. The browser sends an HTTP request to the web server using the IP address.
6. The web server retrieves the requested page and sends it back.
7. The browser interprets the HTML code and displays the web page.

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2.1.11 Watching Video Without Downloading (Bit Streaming)

Bit streaming is the continuous transmission of data over a network so the recipient can process it as it arrives — without waiting for the entire file to download.

How Video Bit Streaming Works

1. Video is compressed before transmission (reduces file size)
2. The video is hosted on a media server
3. The server sends the data continuously as a series of bits
4. The client computer receives the data into a buffer (temporary storage)
5. The user's media player reads from the buffer while the next data arrives

Real-Time vs On-Demand Streaming

Importance of Broadband Speed

Bit streaming requires a minimum broadband speed to work smoothly. If the connection is too slow, the buffer empties faster than data arrives, causing the video to stop and "buffer" (a spinning wheel). Higher bit rate = better quality, but also requires faster internet.

🔍 Practice 11 — Bit Streaming

Question: Distinguish between real-time bit streaming and on-demand bit streaming.

Sample Answer:
Real-time streaming broadcasts a live event as it happens (e.g., a football match), and the user cannot pause or rewind. On-demand streaming delivers pre-recorded content when the user requests it (e.g., Netflix), and the user can pause, rewind, or fast-forward at any time.

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2.1.12 Storing Data Elsewhere (Cloud Computing)

Benefits: Access from anywhere, no hardware to maintain, automatic backups, easy collaboration, scalable storage.

Drawbacks: Requires internet access, reliant on provider's security, potential privacy concerns, ongoing subscription costs.

Public vs Private Cloud

🔍 Practice 12 — Cloud Computing

Question: A hospital is deciding whether to use a public cloud or private cloud for storing patient records. Advise which option is more suitable and justify your answer.

Sample Answer:
A private cloud is more suitable. Patient records are highly sensitive and subject to strict privacy regulations. A private cloud gives the hospital greater control over security and privacy — they can decide exactly who has access, where data is stored, and how backups are managed. A public cloud would expose patient data to third-party infrastructure, increasing the risk of a data breach.

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📚 Key Terms — Chapter 2

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📋 Chapter 2 Summary Checklist

⏱️ Era: Late 1970s — Early 1980s

🔹 1. Bus Topology — "The Party Line"

⏱️ Era: 1990s — Today (The Modern Standard)

🔹 2. Star Topology — "The Hub-and-Spoke"

⏱️ Era: 1960s (military/ARPANET) — Modern (wireless mesh & critical infrastructure)

🔹 3. Mesh Topology — "The Spider's Web"

⏱️ Era: 2000s — Today (Real-world networks)

🔹 4. Hybrid Topology — "The Best of Both Worlds"

📊 Historical Timeline Summary

✏️ Practice 2 — Which Topology?

1. A primary school has 5 computers and a very limited budget. They want to connect them to share one printer. Which topology would you recommend? State one advantage and one disadvantage.
2. A stock exchange needs a network where no single failure can stop trading. Which topology is most suitable? Explain why.
3. A company has 50 computers across 3 floors. Each floor is a star network with its own switch. All switches are connected to a central router. If the switch on Floor 2 fails:
   a) Which floor(s) lose network access?
   b) What topology does this overall design represent?
4. In a bus topology, what happens when two computers try to send data at the same time? What is this problem called?
5. Exam-style question: A hospital needs a network connecting 4 departments: A&E (critical), Surgery (critical), Pharmacy, and Admin. The hospital requires:
   • Maximum fault tolerance for A&E and Surgery
   • Cost-effectiveness for Pharmacy and Admin
   Design a suitable network topology arrangement for the hospital. Justify your choice.
6. Extension question: In a star topology, examine the animation above. Why does the data packet go to the switch before being forwarded to the printer? What would happen if every device could send directly to every other device (like in a mesh)? Compare the cost vs reliability trade-off between star and mesh.

🔍 Check your answers

1. Bus topology. Advantage: Lowest cost (one cable, no switch). Disadvantage: If the backbone cable breaks, the entire network goes down / performance degrades with more devices.
2. Mesh topology. Explanation: Multiple redundant paths mean no single point of failure. If one connection fails, data automatically reroutes through alternative paths.
3. a) Only Floor 2 loses access. Floors 1 and 3 continue working because each floor has its own switch. b) Hybrid topology.
4. A data collision occurs — the signals crash into each other and both messages are destroyed. Each computer must wait a random time and try again. This is called CSMA/CD (Carrier Sense Multiple Access / Collision Detection).
5. Sample answer: Use a hybrid topology. A&E and Surgery should use a partial mesh for redundancy. Pharmacy and Admin can use star to save costs. Connect all via a central router. This balances fault tolerance with cost efficiency.
6. The switch acts as a traffic manager — it receives the packet, reads the destination address, and forwards it only to the correct device. This prevents collisions and ensures only the intended recipient gets the data. In a full mesh, every device connects to every other — this gives maximum reliability (no single point of failure) but is very expensive (N(N-1)/2 cables). Star is a cost-effective trade-off: cheaper than mesh, but the switch is a single point of failure.

Content for 3.1 Computers and their Components coming soon.

Content for 3.2 Logic Gates and Logic Circuits coming soon.

Content for 4.1 Central Processing Unit (CPU) Architecture coming soon.

Content for 4.2 Assembly Language coming soon.

Content for 4.3 Bit Manipulation coming soon.

Content for 5.1 Operating Systems coming soon.

Content for 5.2 Language Translators coming soon.

Content for 6.1 Data Security coming soon.

Content for 6.2 Data Integrity coming soon.

Content for 7.1 Ethics and Ownership coming soon.

Content for 8.1 Database Concepts coming soon.

Content for 8.2 Database Management Systems (DBMS) coming soon.

Content for 8.3 Data Definition Language (DDL) and Data Manipulation Language (DML) coming soon.

Content for 9.1 Computational Thinking Skills coming soon.

Content for 9.2 Algorithms coming soon.

Content for 10.1 Data Types and Records coming soon.

Content for 10.2 Arrays coming soon.

Content for 10.3 Files coming soon.

Content for 10.4 Introduction to Abstract Data Types (ADT) coming soon.

Content for 11.1 Programming Basics coming soon.

Content for 11.2 Constructs coming soon.

Content for 11.3 Structured Programming coming soon.

Content for 12.1 Program Development Life Cycle coming soon.

Content for 12.2 Program Design coming soon.

Content for 12.3 Program Testing and Maintenance coming soon.