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

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

flowchart LR

    L1["<div style='text-align:center;'><span style='color:#000; font-size:14px;'>L1 · Please</span><br/><strong style='font-size:18px;'>Physical</strong></div>"]
    
    L2["<div style='text-align:center;'><span style='color:#000; font-size:14px;'>L2 · Do</span><br/><strong style='font-size:18px;'>Data Link</strong></div>"]
    
    L3["<div style='text-align:center;'><span style='color:#000; font-size:14px;'>L3 · Not</span><br/><strong style='font-size:18px;'>Network</strong></div>"]
    
    L4["<div style='text-align:center;'><span style='color:#000; font-size:14px;'>L4 · Throw</span><br/><strong style='font-size:18px;'>Transport</strong></div>"]
    
    L5["<div style='text-align:center;'><span style='color:#000; font-size:14px;'>L5 · Sausage</span><br/><strong style='font-size:18px;'>Session</strong></div>"]
    
    L6["<div style='text-align:center;'><span style='color:#000; font-size:14px;'>L6 · Pizza</span><br/><strong style='font-size:18px;'>Presentation</strong></div>"]
    
    L7["<div style='text-align:center;'><span style='color:#000; font-size:14px;'>L7 · Away</span><br/><strong style='font-size:18px;'>Application</strong></div>"]

    L1 --> L2
    L2 --> L3
    L3 --> L4
    L4 --> L5
    L5 --> L6
    L6 --> L7

The TCP/IP Model (4 Layers)

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

Network Access Layer
Internet Layer
Transport Layer
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.

CITS3002

Explorer

Week 1