Computer Science | Networks

Welcome to Topic 3: Networks!

Hello future computer scientists! Get ready to explore one of the most vital concepts in modern technology: Networks. Everything you do online—from checking grades to streaming videos—relies on complex systems of interconnected devices.

In this chapter, we will break down how computers talk to each other, the rules they follow, and how we keep all that important information secure. Don't worry if terms like "protocol" or "IP address" seem tricky; we'll use simple analogies to make sure every concept sticks!

Why are Networks Important?

• They enable communication (email, chat).
• They allow for resource sharing (printers, files).
• They support collaboration across global distances.

1. Fundamentals of Networking

What is a Network?

At its most basic, a Computer Network is simply two or more computer systems linked together to share resources, exchange files, or communicate electronically.

Key Network Terminology

• Bandwidth: This is the maximum rate of data transfer across a given path. Think of it as the width of a pipe—a wider pipe (higher bandwidth) can handle more water (data) at once. It is usually measured in megabits per second (Mbps) or gigabits per second (Gbps).
• Data Compression: The process of reducing the number of bits needed to represent data. This is crucial for faster network transfers. (Example: Zipping a large file before emailing it.)
• Latency: The time delay before a transfer of data begins following an instruction for its transfer. Low latency is essential for real-time activities like video conferencing or gaming.

Common Networking Hardware

These are the physical devices that keep the data flowing:

1. Router: A device that connects different networks together and directs data packets between them.

   Analogy: A postal worker who determines the quickest path for a letter traveling from one city (network) to another.

2. Switch: A device that connects devices *within* a single network (like a LAN). It intelligently forwards data only to the specific device that needs it, using the device's MAC address.
3. Hub: An older, less efficient device than a switch. A hub sends all incoming data to *every* connected device, wasting bandwidth. (Switches replaced hubs because they are smart!)
4. Server: A powerful computer or program that provides services (like file storage or web pages) to other computers (clients) in the network.
5. Client: A device (like your laptop or phone) that requests services from a server.
6. Firewall: A network security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules.

2. Network Types and Transmission Media

Types of Networks (Scope)

Networks are typically categorized by the geographic area they cover:

• LAN (Local Area Network): Covers a small, limited area, like a home, school, or office building. (Example: All the computers in your school library.)
• WAN (Wide Area Network): Connects devices over a large geographical area, often across cities, countries, or continents. WANs are formed by connecting multiple LANs.

  The best example of a WAN is the Internet itself.

• PAN (Personal Area Network): Very small network, typically centered around one person. (Example: Your phone communicating with your wireless headphones via Bluetooth.)

Memory Aid: LAN is Local, WAN is Worldwide.

Transmission Media (The Physical Path)

Data needs a medium to travel. This can be wired or wireless.

Wired Transmission (Physical Cables)

Wired connections generally offer higher speed, better security, and lower latency.

• Copper Cable (Twisted Pair/Ethernet): Standard cables used in LANs. They are cheap but can suffer from interference over long distances.
• Fibre Optic Cable: Uses light signals transmitted through glass fibres.
  • Pros: Extremely high bandwidth, very fast, immune to electromagnetic interference, and can cover huge distances.
  • Cons: Very expensive and difficult to install/repair.

Wireless Transmission

Uses electromagnetic waves (radio, microwaves) to transmit data. This offers flexibility but introduces security risks and interference issues.

• Wi-Fi: Uses radio waves to link devices to a LAN.
• Bluetooth: Used for very short-range connections (PANs).
• Microwave/Satellite: Used for very long-distance WAN connections, often where laying physical cable is impractical.

3. Protocols: The Rules of the Road

Imagine trying to speak to someone who only understands French when you only speak Spanish. Communication fails because you don't share a common set of rules. In networking, protocols are these common rules and formats that computers must follow to exchange information successfully.

Key Internet Protocols

1. TCP/IP (Transmission Control Protocol/Internet Protocol)

   This is the fundamental suite of protocols that powers the Internet. It is actually two separate protocols working together:

   • IP (Internet Protocol): Deals with addressing and routing. It assigns unique IP addresses to every device and ensures the data packet finds its destination. (Like the postal address on a letter.)
   • TCP (Transmission Control Protocol): Deals with reliability. It breaks the data into smaller, manageable chunks called packets, ensures all packets arrive, and reassembles them in the correct order at the destination.
2. HTTP (HyperText Transfer Protocol) & HTTPS (Secure)

   This protocol is used to access and transfer web pages (HTML files) on the World Wide Web. HTTPS is the secure version, which uses SSL/TLS encryption.

3. FTP (File Transfer Protocol)

   Specifically designed for the transfer of computer files from a server to a client or vice versa. (Used when uploading files to a website host.)

4. POP3 / IMAP / SMTP

   These are protocols used for email:

   • SMTP (Simple Mail Transfer Protocol) sends emails.
   • POP3 (Post Office Protocol 3) or IMAP (Internet Message Access Protocol) retrieves emails.

Addressing and Naming

IP Address

A numerical label assigned to every device participating in a computer network that uses the Internet Protocol. IP addresses serve two main functions: host or network interface identification and location addressing.

DNS (Domain Name System)

Humans prefer names (like www.google.com), but computers need numbers (IP addresses). The DNS acts as the Internet's directory service, translating human-readable domain names into the numerical IP addresses required for locating computer services and devices worldwide.

Analogy: When you type a website name, your device asks the DNS server, "What is the IP address for this name?" The DNS replies with the number, and your device can then connect.

4. The Concept of Layering (The TCP/IP Stack)

Don't worry if this seems tricky at first—layering is just a way to manage complexity!

Instead of having one massive, complex protocol that handles everything (addressing, security, physical transmission, and web formatting), network communication is broken down into smaller, self-contained stages called layers.

The most widely used conceptual model is the TCP/IP Protocol Suite.

Why use a Layered Model?

1. Standardization: Ensures that different technologies (e.g., Apple hardware and Windows software) can communicate.
2. Modularity: If one layer needs updating (e.g., a faster physical cable), the other layers (like the application layer) do not need to be completely rewritten.
3. Troubleshooting: Makes it much easier to isolate where a problem is occurring (e.g., "It's not the application, it's a problem at the transport layer.").

Simplified TCP/IP Layers (From Top to Bottom)

1. Application Layer: (The software you use). This is where protocols like HTTP, FTP, and SMTP live. It interacts directly with the user's application. (What you see: the web browser interface.)
2. Transport Layer: (The organizer). This is where the TCP protocol lives. It manages reliable data transfer, handles error checking, and ensures data is broken into packets and reassembled correctly.
3. Internet Layer: (The addresser/router). This is where the IP protocol lives. It handles logical addressing (IP addresses) and routing the packets across networks.
4. Network Interface (Link) Layer: (The physical connection). This layer handles the physical transmission of data over the cable or wireless medium (Wi-Fi, Ethernet).

5. Network Security and Threats

With vast amounts of sensitive data flowing through networks, security is paramount. We must protect data from unauthorized access, modification, or destruction.

Common Network Security Threats

• Malware: Short for malicious software (viruses, worms, ransomware) designed to damage, disable, or gain unauthorized access to a computer system.
• Phishing: A social engineering attempt to trick users into revealing sensitive information (like passwords or credit card numbers) by impersonating a trustworthy entity in an electronic communication (e.g., a fake bank email).
• Denial of Service (DoS) / Distributed DoS (DDoS): An attack designed to overwhelm a network resource (like a server) with excessive requests, causing it to slow down or crash, thus denying service to legitimate users.
• Brute Force Attack: Repeatedly attempting to guess a password or key, often by automated software that tries every combination.

Security Countermeasures

1. Authentication and User IDs

This confirms the identity of the user before allowing access to network resources.

• User ID and Password: The most common form of authentication.
• Biometric Data: Using physical characteristics (fingerprints, retina scan).
• Two-Factor Authentication (2FA): Requires two different types of verification (e.g., a password AND a temporary code sent to a mobile phone). This drastically improves security.

2. Firewalls

A firewall acts as a security gatekeeper between two networks (e.g., your LAN and the Internet). It filters network traffic based on predefined rules, blocking unauthorized access attempts.

3. Encryption

Encryption is the process of scrambling data so that it can only be read by authorized parties who possess the correct decryption key. Even if an attacker intercepts the data, it appears as useless nonsense (ciphertext).

• Public Key Encryption (Asymmetric): Uses two different keys—a public key (used for encryption) and a private key (used for decryption). This is fundamental to secure web browsing (HTTPS).
• Symmetric Encryption: Uses the exact same key for both encryption and decryption. Faster, but requires a secure way to share the key initially.

Specific Security Concerns for Wireless Networks

Wireless networks are inherently less secure than wired networks because the transmission medium (airwaves) is accessible to anyone within range.

• Range and Eavesdropping: Signals can travel outside the intended area, making it easy for an attacker to "eavesdrop" on the data if it is not encrypted.
• Interference: Wireless signals are susceptible to interference from other devices and objects.

To combat this, Wi-Fi networks use protocols like WPA2 and WPA3, which enforce strong encryption (like AES) between the access point and the client device.

Key Takeaways from the Networks Chapter

You have successfully learned the language and structure of the digital world!

• Networks are groups of devices sharing resources and following common protocols.
• The core protocol suite is TCP/IP, which handles reliability (TCP) and addressing (IP).
• The DNS translates human-readable domain names into computer-readable IP addresses.
• Network communication is organized into a layered model to manage complexity.
• Security relies on tools like Firewalls, Authentication methods, and strong Encryption to protect data integrity and privacy.

Keep these foundational concepts clear, and you'll be well-prepared for any network-related questions!