Computer Networks

1. Evolution of Networking

Computer networks have evolved significantly over time. The journey from early experimental networks to today's global Internet represents one of the most important technological developments.

1.1 Introduction to Computer Networks

A computer network is a collection of interconnected computers and devices that can communicate and share resources with each other. Networks enable sharing of data, resources (printers, storage), and services among multiple users.

Need for Computer Networks:

• Resource sharing: Share hardware and software resources
• Data sharing: Exchange information between computers
• Communication: Send messages and files quickly
• Cost reduction: Share expensive resources like printers
• Reliability: Backup and redundancy of data
• Scalability: Easy to expand the network

1.2 Evolution of Networking - History

a) ARPANET (1969)

• Full form: Advanced Research Projects Agency Network
• Created by: U.S. Department of Defense ARPA (now DARPA)
• Purpose: To connect computers in universities and research centers for resource sharing
• Technology: Packet-switching technology
• Year: First connection on September 2, 1969
• First nodes: UCLA, Stanford Research Institute, UC Santa Barbara, University of Utah
• Significance: Foundation of modern Internet, introduced packet switching
• Growth: By 1971, connected 15 computers; by 1981, over 200 computers

b) NSFNET (1985)

• Full form: National Science Foundation Network
• Created by: National Science Foundation (NSF)
• Purpose: Provide high-speed network connectivity to universities and research institutions
• Connection to ARPANET: NSFNET connected to ARPANET, helping the Internet grow beyond military use
• Impact: Made Internet accessible to more universities and research centers
• Commercial growth: Eventually led to commercial Internet access
• Decommissioned: 1995 (as other networks became more efficient)

c) INTERNET (1990s onwards)

• Background: ARPANET was decommissioned in 1990; Internet emerged as the successor
• World Wide Web (WWW): Invented by Tim Berners-Lee at CERN (1989), made Internet user-friendly
• Commercial phase: 1995 - Commercial Internet Service Providers (ISPs) began offering public access
• Global connectivity: Connected computers worldwide, transcending national boundaries
• Growth: Exponential growth from millions to billions of devices connected
• Modern Internet: Supports World Wide Web, email, social media, cloud computing, IoT, and more

2. Data Communication Terminologies

2.1 Concept of Communication

• Communication means sharing information from one place to another
• Data communication: Exchange of data between devices through a communication medium
• Key: Exchange of data + sender + receiver + medium

2.2 Components of Data Communication

• Sender: Device that sends data (Computer, Mobile)
• Receiver: Device that receives data (Laptop, Server)
• Message: The actual data being sent (Text, Image, Video)
• Communication Medium: Path through which data travels (Cable, WiFi)
• Protocol: Rules that control communication (HTTP, TCP/IP)

2.3 Measuring Capacity of Communication Media

Bandwidth:

• Definition: Maximum amount of data that can be transmitted through a communication channel in a given time
• Unit: Bits per second (bps)
• Formula: Bandwidth = Data size / Time
• Example: If a file of 100 MB can be downloaded in 10 seconds, bandwidth = 100 MB / 10 s = 10 MB/s

Data Transfer Rate:

• Definition: Actual speed at which data is transferred per second
• Unit: bps, Kbps (Kilobits/s), Mbps (Megabits/s), Gbps (Gigabits/s)
• Difference: Bandwidth is capacity; Data Transfer Rate is actual speed
• Comparison table:

2.4 IP Address

• Unique numerical address assigned to every device connected to a network
• Example: 192.168.1.1
• IPv4: 32-bit address (most common) - Format: xxx.xxx.xxx.xxx (each octet 0-255)
• IPv6: 128-bit address (new version) - Supports more devices
• No two devices on the same network can have the same IP address

2.5 Switching Techniques

a) Circuit Switching

• Dedicated path created between sender and receiver before communication
• Fixed path, continuous connection
• Example: Traditional telephone call (once connected, dedicated line)
• Steps: Establish connection → Communicate → Terminate connection
• Disadvantage: Wastes bandwidth if no data is sent (connection holds resources)
• Suitable for: Voice communication, real-time data

b) Packet Switching

• Data is broken into small packets and sent separately
• Each packet may take a different path to reach destination
• Packets are reassembled at destination in correct order
• Example: Internet and email communication, file transfer
• Advantages: Efficient use of network, no dedicated path needed, better bandwidth utilization
• Disadvantage: Slight delay due to packet reassembly
• Suitable for: Data communication, file transfer, email

3. Transmission Media

Transmission media refers to the path through which data travels from sender to receiver.

3.1 Wired (Guided) Communication Media

Data is transmitted through physical cables.

a) Twisted Pair Cable

• Consists of two insulated copper wires twisted together
• Types: UTP (Unshielded) and STP (Shielded)
• Low cost, easy to install
• Used in: Telephone lines, Ethernet networks
• Limitation: Short distance, lower speed than fiber

b) Co-axial Cable

• Central copper conductor surrounded by insulation and shielding
• Better protection from noise than twisted pair
• Higher bandwidth
• Used in: Cable TV, broadband internet
• Limitation: Bulky and costlier than twisted pair

c) Fiber-Optic Cable

• Transmits data using light signals
• Made of glass or plastic fibers
• Very high speed and long-distance transmission
• Immune to electromagnetic interference
• Used in: Internet backbone, submarine cables
• Limitation: Expensive and difficult to install

3.2 Wireless (Unguided) Communication Media

Data is transmitted through air (no physical cable).

a) Radio Waves

• Long-distance transmission
• Can pass through walls
• Used in: Radio, television, mobile phones
• Limitation: Prone to interference

b) Micro Waves

• Travel in straight lines
• Require line-of-sight communication
• High frequency and speed
• Used in: Satellite communication, mobile towers, Wi-Fi
• Limitation: Obstacles can block signals

c) Infrared Waves

• Used for short-range communication
• Cannot pass through walls
• Used in: TV remotes, short-range data transfer
• Limitation: Limited range

4. Network Devices

4.1 Modem

• Full form: Modulator–Demodulator
• Converts digital signals ↔ analog signals
• Used to connect a computer/network to the Internet
• Layer: Physical

4.2 Ethernet Card (NIC)

• Connects a computer to a LAN
• Each NIC has a unique MAC address
• Can be wired or wireless
• Layer: Data Link

4.3 RJ45 Connector

• Used to connect Ethernet cable to devices
• Commonly used in LAN connections
• Layer: Physical

4.4 Repeater

• Boosts or regenerates weak signals
• Used to increase network distance
• Layer: Physical

4.5 Hub

• Connects multiple devices in a network
• Sends data to all connected devices
• Causes more data collisions
• Layer: Physical

4.6 Switch

• Smarter version of hub
• Sends data only to the intended device
• Uses MAC address
• Faster and more secure than hub
• Layer: Data Link

4.7 Router

• Connects two or more networks
• Routes data using IP address
• Commonly used to connect LAN to Internet
• Layer: Network

4.8 Gateway

• Connects different types of networks
• Converts protocols
• Layer: All layers

4.9 Wi-Fi Card

• Allows devices to connect wirelessly
• Uses radio signals
• Found in laptops, mobiles
• Layer: Data Link

5. Network Topologies and Network Types

5.1 Types of Networks (Network Scale)

PAN (Personal Area Network)

• Covers a very small area (around a person)
• Range: Up to 10 meters
• Examples: Bluetooth, connecting mobile to earphones, smartwatch
• Usually private ownership
• Low speed, very low cost
• Devices Used: Bluetooth, USB, IR
• Transmission Media: Wireless
• Security: More secure

LAN (Local Area Network)

• Covers a small area like a room, office, school, or lab
• Distance: Up to 1 km (room/building)
• High speed and low cost
• Examples: School computer lab network, office network
• Usually private ownership
• Devices Used: Switch, Ethernet cable
• Transmission Media: Wired
• Security: More secure

MAN (Metropolitan Area Network)

• Covers a city or town
• Connects multiple LANs
• Distance: 5–50 km (city)
• Examples: Cable TV network, city-wide broadband
• Can be government or service-provider owned
• Medium speed, moderate cost
• Devices Used: Fiber optics, routers
• Transmission Media: Wired
• Security: Less secure, needs encryption & firewalls

WAN (Wide Area Network)

• Covers a large geographical area (country or world)
• Uses public or leased communication links
• Distance: Above 50 km (country/world)
• Example: Internet (largest WAN)
• Can be government or service-provider owned
• Comparatively slower due to long distance
• Devices Used: Satellite links, leased lines
• High cost
• Security: Less secure, needs encryption & firewalls

5.2 Network Topologies (Physical Arrangement)

A network topology is the physical or logical arrangement of devices in a network.

Bus Topology

• All devices are connected to a single backbone cable
• Data travels in both directions
• Advantages: Low cost, easy to install
• Disadvantages: Backbone failure stops entire network, data collision
• Example: Small temporary networks

Star Topology

• All devices are connected to a central device (hub/switch)
• Easy to install and troubleshoot
• Advantages: Easy fault detection, high performance
• Disadvantage: Central device dependency
• Example: School computer lab

Tree Topology

• Combination of Star + Bus topology
• Devices arranged in hierarchical structure
• Advantages: Easy expansion, scalable
• Disadvantages: Backbone failure affects many nodes
• Example: Large organizations, colleges

6. Network Protocols

A network protocol is a set of rules that defines how data is transmitted over a network.

6.1 HTTP (HyperText Transfer Protocol)

• Used to access web pages
• Data is not encrypted
• Example: Opening a normal website

6.2 HTTPS (HyperText Transfer Protocol Secure)

• Secure version of HTTP
• Uses encryption for safe data transfer
• Example: Online banking websites

6.3 FTP (File Transfer Protocol)

• Used to upload and download files
• Not secure by default
• Example: Uploading files to a web server

6.4 SMTP (Simple Mail Transfer Protocol)

• Used to send emails
• Cannot receive emails
• Example: Sending an email via Gmail

6.5 POP3 (Post Office Protocol – Version 3)

• Used to receive emails
• Downloads emails from server to client
• Example: Receiving mail in Outlook

6.6 TELNET (TELecommunication NETwork)

• Used for remote login
• Not secure (no encryption)
• Example: Remote server access

6.7 TCP/IP (Transmission Control Protocol / Internet Protocol)

• Backbone of the Internet
• TCP ensures reliable data delivery
• IP handles addressing and routing

6.8 PPP (Point-to-Point Protocol)

• Used for direct communication between two devices
• Commonly used by ISPs
• Example: Dial-up Internet connection

6.9 VoIP (Voice over Internet Protocol)

• Used for voice and video calls over Internet
• Example: WhatsApp call, Skype

7. Introduction to Web Services

7.1 World Wide Web (WWW)

• System of interlinked web pages accessed through the Internet
• Invented by Tim Berners-Lee
• Uses HTTP/HTTPS protocol
• Note: WWW is a service on the Internet, not the Internet itself

7.2 Website

• Collection of related web pages stored on a web server
• Has a unique domain name
• Example: www.wikipedia.org

7.3 Web Page

• Single document on the web written using HTML
• Has a unique URL
• Displays text, images, videos

7.4 Web Browser

• Software used to access and display web pages
• Examples: Google Chrome, Mozilla Firefox, Microsoft Edge
• Browser = Client side application

7.5 Web Server

• Computer that stores websites and delivers web pages to users
• Examples: Apache, Microsoft IIS
• Stores and delivers HTML pages via HTTP

7.6 Web Hosting

• Service that allows websites to be stored on a web server
• Websites must be hosted to be visible online
• Examples: GoDaddy, Hostinger, Bluehost

7.7 HTML (HyperText Markup Language)

• Standard language used to create web pages
• Uses tags like <html>, <body>, <p>
• Defines structure of a web page
• Used for displaying data, not storing data

7.8 XML (Extensible Markup Language)

• Markup language used to store and transport data
• User-defined tags
• Self-descriptive
• Focus on data storage

7.9 Domain Name

• Human-readable address of a website
• Examples: google.com, amazon.in, cbse.gov.in
• Domain name = easy name instead of IP address

7.10 URL (Uniform Resource Locator)

• Complete address used to locate a web page on the Internet
• Example: https://www.google.com/search
• Parts: protocol (https), domain (google.com), path (/search)
• URL = address of a specific web page