Computer Networks & Internet: Protocols, Topologies, and Real-World Systems

1. Introduction to Computer Networks

A computer network is a system connecting multiple devices (computers, servers, phones) to share resources (data, printers, internet) via communication channels. Networks enable collaboration, remote access, and distributed computing.

Why Networks Exist

• Resource sharing: Printers, databases, and storage (e.g., NTC’s shared internet backbone).
• Communication: Email, messaging (WhatsApp), and VoIP (Pathao’s ride-hailing app).
• Cost efficiency: Centralized servers reduce hardware costs (e.g., Daraz’s cloud-based inventory).
• Reliability: Redundancy prevents single-point failures (e.g., Ncell’s 4G/5G network).

Types of Networks

Classified by scope and geometry:

Network Topologies

The physical or logical arrangement of devices affects performance, cost, and scalability.

1. Bus Topology

• Structure: All devices share a single communication line (backbone).
• Advantages:
  • Simple and cheap to install.
  • Easy to add new devices.
• Disadvantages:
  • Single point of failure: If the backbone fails, the entire network crashes.
  • Traffic congestion: All devices compete for the same channel (collisions).
• Example: Old Ethernet networks in schools.

flowchart TD
    A["Device 1"] -->|"Shared Bus"| B["Backbone"]
    C["Device 2"] -->|"Shared Bus"| B
    D["Device 3"] -->|"Shared Bus"| B
    E["Device 4"] -->|"Shared Bus"| B
    B -->|"Terminator"| F["End"]

2. Star Topology

• Structure: All devices connect to a central hub/switch.
• Advantages:
  • Fault isolation: Failure of one device doesn’t affect others.
  • Easy troubleshooting: Isolate issues by checking the hub.
  • Scalable: Add devices without redesigning the network.
• Disadvantages:
  • Centralized dependency: Hub failure = network failure.
  • Higher cost: Requires a central device (switch/router).
• Example: Modern home Wi-Fi routers, NTC’s fiber-optic network.

flowchart TD
    A["Central Switch"] --> B["Device 1"]
    A --> C["Device 2"]
    A --> D["Device 3"]
    A --> E["Device 4"]

3. Ring Topology

• Structure: Devices form a closed loop; data travels in one direction.
• Advantages:
  • Predictable traffic: No collisions (unlike bus).
  • High performance: Suitable for real-time systems (e.g., industrial control).
• Disadvantages:
  • Single point of failure: Break in the ring = network down.
  • Complex management: Requires token-passing protocols.
• Example: Token Ring networks (legacy IBM systems), some railway signaling systems.

flowchart TD
    A["Device 1"] --> B["Device 2"]
    B --> C["Device 3"]
    C --> D["Device 4"]
    D --> A

Hybrid Topologies

Combine two or more topologies for optimized performance. Example:

• Star-Bus: Multiple star networks connected via a bus (used in large offices).
• Mesh Topology: Every device connects to multiple others (redundant paths; used in IoT sensors).

2. The Internet: A Global Network

The Internet is a WAN of networks connecting billions of devices worldwide using standardized protocols.

How the Internet Works

1. Packets: Data is split into small chunks (packets) with source/destination addresses.
2. Routers: Devices that forward packets toward their destination using routing tables.
3. Protocols: Rules governing data transmission (e.g., TCP/IP).

A simplified path of a packet from your phone to a server in the US, showing routers, ISPs (NTC/Ncell), and the global backbone. (Image: Pluke, CC0, via Wikimedia Commons)

Key Protocols: TCP/IP

The Transmission Control Protocol/Internet Protocol (TCP/IP) is the foundation of the Internet. It consists of four layers:

Function of TCP/IP:

• TCP (Transmission Control Protocol): Ensures reliable, ordered delivery of data (used for emails, file transfers).
  • Uses acknowledgments and retries if packets are lost.
• IP (Internet Protocol): Handles addressing (e.g., 192.168.1.1) and routing (finding the shortest path to a destination).
  • IPv4: 32-bit addresses (limited to ~4.3 billion devices).
  • IPv6: 128-bit addresses (supports trillions of devices; being deployed globally).

Real-World Example: eSewa’s Payment Network

When you pay via eSewa:

1. Your phone sends a TCP packet to eSewa’s server with transaction details.
2. The server routes the request via NTC’s internet backbone (IP).
3. The bank verifies funds and sends back a TCP-acknowledged response.
4. Your phone displays "Payment successful" (HTTP response).

3. World Wide Web (WWW)

The WWW is a service built on top of the Internet, enabling hypertext documents (webpages) via URLs and HTTP/HTTPS.

Key Components

• URL (Uniform Resource Locator): Address of a webpage (e.g., https://www.daraz.com).
  • https:// = secure (HTTPS), http:// = unsecure.
• HTTP/HTTPS: Protocols for transferring webpages.
  • HTTP: Unencrypted (vulnerable to eavesdropping).
  • HTTPS: Encrypted (secure; used by eSewa, banks).
• HTML/CSS/JavaScript: Languages that define webpage structure, styling, and interactivity.

4. Network Security Basics

Security threats include hacking, malware, and data breaches. Mitigation tools:

• Firewalls: Filter traffic between networks (e.g., NTC’s firewall blocking malicious IP addresses).
• VPNs: Encrypt data for secure remote access (used by banks for employee logins).
• Encryption: Scrambles data (e.g., HTTPS uses TLS to protect eSewa transactions).

Functions of a Firewall:

1. Packet filtering: Blocks/unblocks packets based on rules (e.g., block port 21 for FTP attacks).
2. Stateful inspection: Tracks active connections (e.g., allows return traffic for a legitimate request).
3. Proxy services: Acts as an intermediary (e.g., school firewalls cache websites to reduce bandwidth).

5. Case Study: Daraz’s Order Processing Network

Scenario: A customer orders a laptop from Daraz.

1. Client-Server Model:
   • User’s phone (client) sends an HTTP POST request to Daraz’s server with order details.
2. Database Query:
   • Server checks inventory (SQL query) and updates stock in real-time.
3. Payment Gateway:
   • Redirects to eSewa/Khalti (UDP for fast payment processing).
4. Order Fulfillment:
   • Daraz’s warehouse system (IoT sensors + WAN) tracks the laptop’s location via GPS.
5. Delivery Confirmation:
   • Pathao’s app sends an SMS/email (SMTP protocol) to the customer.

Why TCP/IP?

• Reliability: TCP ensures the order details aren’t lost in transit.
• Scalability: Daraz’s servers use load balancers (distribute traffic across multiple servers).

## In the Real World

1. eSewa/Khalti: Cryptography and Protocols

   • Idea: Uses HTTPS (TLS encryption) and TCP for secure transactions.
   • How: When you pay ₹1,000, your phone encrypts the data and sends it via TCP to eSewa’s server. The server decrypts it, verifies funds, and sends back a signed receipt (digital signature).

2. NTC/Ncell: Packet Switching and Routers

   • Idea: Uses IP routing and packet switching to deliver your WhatsApp message.
   • How: Your message is split into packets, routed through NTC’s backbone, and reassembled at the recipient’s phone. Routers use OSPF (Open Shortest Path First) to find the fastest path.

3. Pathao: IoT and Real-Time Data

   • Idea: Uses MAN (Metropolitan Area Network) to connect driver apps, GPS, and payment gateways.
   • How: When you book a ride, Pathao’s backend (running on a cloud server) checks driver availability via a real-time database query, updates the driver’s app, and processes payment via Khalti (UDP for speed).

## Exam Tips

1. Definitions:

   • Always define terms briefly but clearly. For example:
     • Internet: A global WAN connecting billions of devices via TCP/IP.
     • Protocol: A set of rules for data exchange (e.g., TCP for reliability, UDP for speed).
     • WWW: A service on the Internet using HTTP/HTTPS to deliver webpages.
   • Common mistake: Don’t confuse "Internet" with "WWW." The Internet is the infrastructure; the WWW is an application.

2. TCP/IP Layer Comparison:

   • Memorize the four layers and their protocols. Draw a table like above in exams.
   • Example question: "Explain the role of TCP in data transmission." → Answer: TCP ensures reliable, ordered delivery with acknowledgments and retries.

3. Topology Questions:

   • For bus/star/ring, always include:
     • Structure (diagram or description).
     • Advantages/disadvantages.
     • Real-world example (e.g., star topology in NTC’s network).
   • Example question: "Why is star topology preferred in modern networks?" → Answer: Fault isolation, scalability, and easy troubleshooting.

4. Security Focus:

   • Firewalls are highly examinable. Mention:
     • Packet filtering, stateful inspection, and proxy services.
     • Example: "How does a firewall protect NTC’s network from DDoS attacks?" → Answer: Blocks excessive traffic from a single source.

5. Case Study Practice:

   • Expect application-based questions (e.g., "How does Daraz use TCP/IP for order processing?").
   • Break it down:
     1. Client-server model (HTTP request).
     2. Database interaction (SQL queries).
     3. Payment gateway (UDP for speed).
     4. IoT integration (real-time tracking).

6. Worked Example: Binary Addition (Bonus)

   • Even though this is from Unit 1, TCP/IP uses binary/hexadecimal for addressing (e.g., IP 192.168.1.1 is binary 11000000.10101000.00000001.00000001).
   • Question: "Add 23 (binary 10111) and 12 (binary 1100) and verify in decimal."
     • Step 1: Binary addition:

         10111 (23)
       +  1100 (12)
       --------
         11011 (35)
       

     • Step 2: Verify in decimal: 23 + 12 = 35 ✓.

## Summary Table for Quick Revision

Final Tip: For short-answer questions, use bullet points and diagrams (even if hand-drawn). For long answers, structure like this note—define, explain, compare, and apply to real-world examples. Good luck!