CITS3002

Week 1

4 min read

Introduction to Computer Networks

1. Basic Terminology

  • Computer Network: An interconnected collection of autonomous computers (meaning they operate independently without a permanent master-slave relationship).
  • Interconnection: The capability of exchanging information over various mediums (copper wire, optical fibre, radio frequencies, etc.).
  • Physical Network: The actual hardware layout (cables, switches, routers).
  • Logical Network: The virtual, software-based structure showing how devices communicate.

2. Network Protocols

Definition: An agreed format for messages, expressed by a packet header, an optional message component, and a set of rules for the exchange of messages between computers.

  • Both endpoints must understand the protocol for communication to occur.
  • Protocols must be formally defined and unambiguous.
  • Examples:
    • HTTP: Accessing web pages.
    • IPv6: Enabling devices to connect to the Internet.
    • SSH: Secure remote access.

3. Network Models (OSI vs. TCP/IP)

To manage the immense complexity of network communication, functions are separated into different protocol layers.

  • Why Layer?
    • Provides a well-defined, independent function at a different level of abstraction.
    • Minimizes traffic between layers/interfaces.
    • Makes updating or changing one layer possible without breaking the whole system.

The OSI Reference Model (7 Layers)

Mnemonic (Bottom-Up): Please Do Not Throw Sausage Pizza Away

  1. Physical Layer (Layer 1): Transmits raw bit streams (0s and 1s) over the physical communication medium (handles electrical & mechanical interfaces).
  2. Data Link Layer (Layer 2): Takes raw bits and constructs logical chunks called frames. Performs limited error detection/recovery. MAC addresses operate here.
  3. Network Layer (Layer 3): Routes data between systems across networks using logical addresses (like IP addresses). IP protocol operates here.
  4. Transport Layer (Layer 4): Provides reliable end-to-end service independent of network topology (Flow control, congestion control, connection management). TCP operates here.
  5. Session Layer (Layer 5): Manages the dialogue/conversation between end systems (synchronization & negotiation).
  6. Presentation Layer (Layer 6): Provides standard formats for transferred info (data encoding, display technologies).
  7. Application Layer (Layer 7): Interface to the user. Allows apps (browsers, email) to access network services. HTTP operates here.

The TCP/IP Model (4 Layers)

Based specifically on the Internet (rather than a generic standard like OSI).

  1. Network Access Layer
  2. Internet Layer
  3. Transport Layer
  4. Application Layer

4. Network Topologies

Topology refers to the physical and logical network layout.

  • Bus: All devices connect to a single shared cable (backbone).
    • Pros: Simple, inexpensive.
    • Cons: Backbone is a single point of failure. Data is visible to all devices.
  • Star: All devices connect to a central hub/switch via single cables.
    • Pros: Easy to manage, scalable, point-to-point connections. (Most commonly used today, e.g., home Wi-Fi).
    • Cons: Hub is a single point of failure.
  • Mesh: Every computer connects to every other computer.
    • Pros: High reliability and redundancy.
    • Cons: Very complicated wiring, high cost, tricky to troubleshoot.
  • Ring: Devices connected in a closed loop. Data travels in one direction using a “token” (only one sender at a time).
  • Tree (Star Bus): A root node branching out to children. Easy to expand.

5. Network Standards

  • IEEE (Institute of Electrical and Electronics Engineers): Focuses on hardware and lower-layer technologies (e.g., 802.3 Ethernet, 802.11 Wi-Fi). Ensures interoperability between manufacturers.
  • IETF (Internet Engineering Task Force): Focuses on Internet architecture and protocols. Publishes standards as RFCs (Request for Comments), e.g., RFC 793 for TCP.

6. The Internet & Network Types

  • The Internet: A “network of networks”. Evolved from the ARPANET (1960s). TCP/IP developed by Robert Kahn and Vint Cerf.
    • Note: The Internet is the infrastructure; the WWW (World Wide Web) is just a system that runs on top of the Internet.
  • LAN (Local Area Network): Physically close devices (home, business). High speed, multi-access (e.g., Ethernet).
  • WAN (Wide Area Network): Connects computers physically far apart (cities, continents). Point-to-point, typically slower than LANs (e.g., Fiber optics, ATM).

Lab 1 Key Takeaways

  • IP Address (Logical Address): Operates at the Network Layer (L3). Represents a device’s location in a network structure rather than the physical hardware. It can change when you connect to a different network.
  • MAC Address (Physical Address): Operates at the Data Link Layer (L2). Uniquely identifies the device’s Network Interface Card (NIC). It is permanently assigned by the manufacturer and never changes.