🧠 Computer Networks: Connecting the World!

Hello future Computer Scientists! Welcome to the exciting world of Computer Networks. This is the chapter where we learn how devices communicate, how data travels across the globe, and the rules (protocols) that make it all possible.
Let's break everything down clearly to prepare for your OxfordAQA exams!

1. What is a Network and Why Do We Use Them?

Definition and Purpose

A computer network is two or more computer systems or devices connected together to communicate and share resources.

  • Communication: Sending emails, instant messages, and video calls.
  • Resource Sharing: Sharing hardware (like a single printer for an entire office) or software/data files. This saves money and time.
  • Data Management: Centralising backups and security management across systems.

Quick Takeaway: Networks allow devices to communicate, share hardware/software resources, and manage data centrally.

2. Types of Networks: LAN vs. WAN

Networks are categorised mainly by their geographical coverage.

A. Local Area Network (LAN)
  • Definition: A network connecting devices over a small, limited geographical area.
  • Area Covered: A single room, building, office, or school campus.
  • Ownership: Usually privately owned and managed by the organisation or individual that uses it.
B. Wide Area Network (WAN)
  • Definition: A network connecting LANs across a large geographical area.
  • Area Covered: Multiple cities, countries, or continents.
  • Ownership: Often uses shared or rented third-party telecommunication infrastructure. The most famous WAN is the Internet.

3. Network Topologies

A topology describes the layout or structure in which devices (nodes) are arranged and connected on a network.

A. Star Topology
  • Structure: Every device is connected via its own dedicated cable directly to a central node (such as a switch or hub).
  • Advantages: If a single cable or device fails, no other device is affected; easy to add new devices without disrupting the network.
  • Disadvantages: If the central switch/hub fails, the entire network goes down; can require more cabling and hardware, making it costlier.
B. Bus Topology
  • Structure: All devices are connected to a single central backbone cable with terminators at each end.
  • Advantages: Cheap and easy to install as it requires minimal cabling.
  • Disadvantages: If the main backbone cable breaks, the whole network fails; data collisions increase as more devices are added, slowing down transmission.

4. Essential Network Hardware and Media

A. Network Hardware
  • Router: Connects different networks together (e.g., connecting a LAN to the Internet/WAN). It directs data packets using their destination IP addresses.
  • Switch: Connects devices together within a LAN. It forwards data packets directly to the destination device using its MAC address.
  • Network Interface Card (NIC): Internal hardware component (wired or wireless) enabling a device to connect to a network.
B. Transmission Media
  • Copper Cable (e.g., Twisted Pair / Ethernet): Transmits data as electrical signals. Cheap and common, but limited over long distances.
  • Fibre Optic Cable: Transmits data as pulses of light. Extremely fast, supports high bandwidth over long distances, and immune to electrical interference.
  • Wireless (Wi-Fi): Transmits data using radio waves. Highly convenient for mobile devices. Wi-Fi is a trademark for wireless local area networks (WLANs).

5. Network Protocols and the 4-Layer TCP/IP Model

A protocol is a set of rules that govern how data is transmitted and received across a network.

A. The 4-Layer TCP/IP Stack

Network communications are organised into four conceptual layers where each layer has a distinct role:

  • 1. Application Layer: Provides network services directly to end-user applications and programs (e.g. web browsers, email clients).
    Protocols: HTTP, HTTPS, FTP, SMTP, IMAP.
  • 2. Transport Layer: Sets up communication between endpoints, splits data into packets, and ensures end-to-end transport.
    Protocols: TCP, UDP.
  • 3. Internet / Network Layer: Handles the logical addressing (IP addresses) and routes packets across different networks.
    Protocol: IP (Internet Protocol).
  • 4. Link / Data Link Layer: Handles physical transmission of raw data across hardware media and local network routing using MAC addresses.
    Protocols: Ethernet (family of wired protocols), Wi-Fi (family of wireless protocols).
B. Key Protocols Explained
  • TCP (Transmission Control Protocol): A connection-oriented protocol that ensures reliable delivery. It establishes a connection, checks that all packets arrive, requests retransmission of lost packets, and reassembles them in order.
  • UDP (User Datagram Protocol): A connectionless, lightweight protocol. It sends packets without establishing a dedicated connection or checking for packet arrival, making it faster and ideal for real-time video/voice streaming or gaming.
  • IP (Internet Protocol): Assigns logical addresses to packets and routes them to their destination across networks.
  • HTTP / HTTPS: Used by web browsers to request and view web pages. HTTPS adds encryption to secure communications.
  • FTP (File Transfer Protocol): Used for transferring files between a client and a server.
  • SMTP (Simple Mail Transfer Protocol): Used for sending emails from clients to servers and between mail servers.
  • IMAP (Internet Message Access Protocol): Used for retrieving and managing emails stored centrally on a mail server across multiple devices.

6. Addressing: IP vs. MAC Addresses

  • IP Address: A logical network address assigned to a device dynamically or statically when connected to a network. It changes when moving across networks.
  • MAC Address (Media Access Control): A unique physical address permanently assigned to the Network Interface Card (NIC) at manufacture. It does not change.

7. Network Security Methods

  • Firewall: Hardware or software that inspects incoming and outgoing network traffic, allowing or blocking data packets based on predefined security rules.
  • Encryption: Scrambling plaintext into unreadable ciphertext using an encryption key so unauthorized parties cannot read intercepted data.
  • Authentication: Verifying user identity before granting access to network resources (e.g., passwords, biometric scans, two-factor authentication).
  • MAC Address Filtering: A security mechanism on a router/switch that only permits devices with approved, whitelisted MAC addresses to access the network.

Exam Summary: Remember to distinguish between connection-oriented (TCP) and connectionless (UDP) protocols, identify which protocol belongs to each of the 4 TCP/IP layers, and understand how Star vs. Bus topologies function!