1. Fundamentals of Computer Networks
A Computer Network is a collection of interconnected autonomous computing devices that exchange information and share resources. The primary purpose of networking is Resource Sharing (printers, storage) and Communication (email, chat).
To understand networking, one must first categorize networks based on their geographical span. This classification is fundamental to the PGT syllabus.
1.1 Types of Networks (Geographical Classification)
- LAN (Local Area Network):
- Scope: Confined to a small localized area such as a single room, a building, or a campus (approx. 1 km range).
- Characteristics: High data transfer rates, low error rates, and privately owned.
- Example: An ethernet network inside a computer lab or Wi-Fi in a home.
- MAN (Metropolitan Area Network):
- Scope: Covers a larger geographical area like a city. It interconnects multiple LANs.
- Characteristics: Moderate speed and ownership can be private or public.
- Example: Cable TV network in a city or a city-wide Wi-Fi network provided by the municipality.
- WAN (Wide Area Network):
- Scope: Spans across countries or continents.
- Characteristics: Uses satellites and telecommunication links. Lower data rates compared to LANs and higher error rates due to distance.
- Example: The Internet is the largest WAN. The railway reservation system (IRCTC) is another example.
2. Network Topologies: The Architectural Layout
Network Topology refers to the physical or logical arrangement of nodes (computers) and links (cables) in a network. The choice of topology impacts cost, reliability, and scalability.
2.1 Bus Topology (Linear Bus)
- Architecture: A single central cable (Backbone) to which all nodes are attached via drop lines.
- Pros: Requires the least amount of cabling; easy to install.
- Cons: Single Point of Failure—if the backbone breaks, the entire network goes down. Diagnosis of faults is difficult.
2.2 Star Topology (The Standard)
- Architecture: All nodes connect directly to a central device (Hub or Switch).
- Pros: Easy to troubleshoot. If one cable fails, only that node is affected.
- Cons: Requires more cabling than Bus. If the central device fails, the whole network collapses.
2.3 Tree Topology (Hierarchical)
- Architecture: A variation of Star topology where “Star” networks are connected to a central bus backbone. It mimics a parent-child hierarchy.
- Pros: Scalable and easy to manage large networks (e.g., dividing a school network by departments).
2.4 Mesh Topology (The Reliable)
- Architecture: Every node is connected to every other node (Fully Connected).
- Pros: Most reliable (Fault tolerant) and secure. No traffic congestion.
- Cons: Most expensive due to massive cabling requirements.
- Formula: Number of cables needed = \(\frac{n(n-1)}{2} \).
3. Network Devices: The Hardware Backbone
This section is critical for the exam. Questions often ask to identify the correct device for a specific scenario (e.g., “Which device connects two dissimilar networks?”).
3.1 Modem (Modulator-Demodulator)
- Function: Converts Digital signals (from computer) to Analog signals (for telephone lines) and vice versa.
- Use Case: Connecting to the internet via telephone lines or cable networks.
3.2 Repeater
- Function: An electronic device that receives a signal and regenerates it. It does not filter or interpret data.
- Use Case: Used to extend the range of a network when the signal becomes weak (attenuated) over long distances.
3.3 Hub ( The “Dumb” Device)
- Function: A central connecting point for a LAN.
- Operation: When it receives data, it broadcasts it to all connected ports, regardless of the destination.
- Drawback: High traffic collision and security risks.
3.4 Switch (The “Smart” Device)
- Function: Connects devices in a LAN but handles data intelligently.
- Operation: It learns the MAC Address of connected devices. When data arrives, it sends it only to the specific destination port.
- Advantage: Reduces network traffic and increases security compared to a Hub.
3.5 Router
- Function: Connects two distinct networks (e.g., your Home LAN to the ISP’s WAN).
- Operation: It works on IP Addresses (Layer 3) to determine the best path for data packets to travel across the internet.
3.6 Gateway
- Function: A device that connects networks using different protocols (e.g., TCP/IP network to a Novell network).
- Operation: It acts as a “translator” between dissimilar systems.
4. Internet and Web Technologies
The syllabus distinguishes between the infrastructure (Internet) and the service (Web).
4.1 Internet vs. WWW
- Internet: The physical network of networks (Hardware/Infrastructure).
- WWW (World Wide Web): The collection of information (Webpages/Documents) accessed via the Internet using HTTP.
4.2 The URL (Uniform Resource Locator)
A URL is the address of a resource on the web.
- Structure:
https://www.larasacademy.com/notes.html- Protocol:
https(HyperText Transfer Protocol Secure) - Domain:
www.larasacademy.com - Path:
/notes.html
- Protocol:
4.3 Web Hosting & Servers
- Web Server: A powerful computer running software (like Apache or Nginx) that stores website files and serves them to users upon request.
- Web Hosting: The service of providing storage space on a server for a website.
4.4 Static vs. Dynamic Web Pages
- Static Web Page: Displays the same content for every user. Written purely in HTML/CSS. Content only changes if the developer edits the code.
- Dynamic Web Page: Content is generated on the fly based on user interaction, time, or database queries (e.g., Facebook Feed, Amazon Dashboard). Uses server-side languages like Python (Django), PHP, or Node.js.
5. Web Browsers & Security
5.1 Cookies
Cookies are small text files stored on the user’s computer by a website.
- Purpose: To remember user preferences (login status, shopping cart contents, language settings).
- Privacy Concern: Third-party cookies can track browsing history across different sites for ad targeting.
5.2 Add-ons and Plug-ins
- Add-ons (Extensions): Small software modules that add new features to the browser (e.g., AdBlocker, Grammarly).
- Plug-ins: External software components that allow the browser to display specific content (e.g., Flash Player—now obsolete, PDF Readers).
6. Societal Impacts & Ethics
6.1 Intellectual Property Rights (IPR)
Rights given to creators over their creations (code, art, music).
- Copyright: Protects the expression of an idea (e.g., source code).
- Patent: Protects the invention/functional idea itself.
- FOSS (Free and Open Source Software): Software that comes with the freedom to run, study, change, and distribute (e.g., Linux, Python, Mozilla Firefox).
6.2 Cyber Crime & Safety
- Phishing: Fraudulent attempt to obtain sensitive info (passwords) by disguising as a trustworthy entity (fake bank email).
- Hacking: Unauthorized intrusion into a computer system.
- Cyber Bullying: Harassment using digital devices.
- Digital Footprint: The trail of data you leave behind while using the internet (search history, social media posts).
Exam Corner: Critical Distinctions
- Hub vs. Switch: A Hub broadcasts to all; a Switch sends only to the destination.
- Router vs. Gateway: A Router connects networks with the same protocol (IP routing); a Gateway connects networks with different protocols (Protocol Translation).
- MAC vs. IP Address: MAC is physical/permanent (burned into hardware); IP is logical/temporary (assigned by the network).
- CC vs. BCC in Email:
- CC (Carbon Copy): All recipients see who else received the mail.
- BCC (Blind Carbon Copy): Recipients are hidden from each other.
