Computer Networks - Notes - Free Download

Introduction to Computer Networks

Definition:

A computer network is a system of interconnected devices that communicate with each other to share resources, data, and applications.

Applications of Computer Networks:

• Communication: Emails, messaging, video conferencing (e.g., Zoom, WhatsApp)
• Resource Sharing: Shared printers, cloud storage (e.g., Google Drive, OneDrive)
• Data Sharing: File transfer via FTP, peer-to-peer sharing
• Entertainment: Online gaming, streaming services (e.g., Netflix, YouTube)
• E-commerce: Online shopping platforms (e.g., Amazon, Flipkart)

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Network Types

1. Local Area Network (LAN):

• Covers a small area (e.g., home, office, school)
• High-speed data transfer (e.g., Ethernet, Wi-Fi)
• Example: Office network connecting multiple computers

2. Metropolitan Area Network (MAN):

• Covers a city or large campus
• Faster than WAN but slower than LAN
• Example: City-wide broadband services

3. Wide Area Network (WAN):

• Covers large geographical areas (e.g., country, worldwide)
• Slower due to long-distance communication
• Example: The Internet

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Network Topologies

1. Bus Topology:

• Single central cable (backbone) connects all devices
• Advantage: Cost-effective, easy to expand
• Disadvantage: If the main cable fails, the network fails
• Example: Small office networks

2. Star Topology:

• Central hub/switch connects all devices
• Advantage: Easy troubleshooting, reliable
• Disadvantage: If the central hub fails, the network goes down
• Example: Home or office Wi-Fi network

3. Ring Topology:

• Devices are connected in a closed loop
• Advantage: Equal access to data, no data collision
• Disadvantage: Failure in one device affects the entire network
• Example: Token Ring network

4. Mesh Topology:

• Every device is connected to every other device
• Advantage: High redundancy, reliable
• Disadvantage: Expensive and complex
• Example: High-security networks

5. Hybrid Topology:

• Combination of two or more topologies
• Advantage: Flexible and scalable
• Disadvantage: Complex structure
• Example: Enterprise networks

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OSI & TCP/IP Models

OSI Model (7 Layers):

1. Physical Layer - Transmission of raw data (e.g., cables, Wi-Fi signals)
2. Data Link Layer - Error detection and MAC addressing (e.g., Ethernet)
3. Network Layer - Routing and IP addressing (e.g., Routers, IP protocol)
4. Transport Layer - Ensures reliable data transfer (e.g., TCP, UDP)
5. Session Layer - Manages sessions between applications
6. Presentation Layer - Data encryption and formatting (e.g., JPEG, MP3)
7. Application Layer - User interface (e.g., HTTP, FTP, SMTP)

TCP/IP Model (4 Layers):

1. Network Interface - Physical and data link layers combined
2. Internet Layer - Handles IP addressing and routing
3. Transport Layer - TCP/UDP for reliable/unreliable communication
4. Application Layer - User applications like web browsing, email

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Network Devices

1. Router:

• Connects different networks (e.g., LAN to the Internet)
• Uses IP addresses for data transmission
• Example: Home Wi-Fi router

2. Switch:

• Connects multiple devices within a LAN
• Uses MAC addresses for data transmission
• Example: Office network switch

3. Hub:

• Broadcasts data to all devices in a network
• No intelligence, slower than a switch
• Example: Outdated networking setups

4. Bridge:

• Connects two LANs together
• Works at the Data Link Layer
• Example: Connecting two office buildings' networks

5. Gateway:

• Connects different network architectures
• Converts protocols (e.g., IPv4 to IPv6)
• Example: Email gateway

6. Modem:

• Converts digital data to analog for transmission over telephone lines
• Example: DSL modems for broadband internet

7. Repeater:

• Amplifies and extends network signals
• Used in long-distance communication
• Example: Wi-Fi range extenders

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Practice Questions

1. What is the main purpose of a computer network?
2. Differentiate between LAN, MAN, and WAN.
3. Explain the advantages and disadvantages of different network topologies.
4. What is the difference between a router and a switch?
5. Compare OSI and TCP/IP models.
6. Why is a repeater used in networking?
7. What are the functions of a gateway in networking?

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OSI Layers

Physical Layer

Transmission Media (Wired & Wireless)

• Wired Media:
  • Twisted Pair Cable: Used in LANs, reduces interference.
  • Coaxial Cable: Used in cable TV networks, provides better shielding.
  • Fiber Optic Cable: High-speed data transfer, immune to electromagnetic interference.
• Wireless Media:
  • Radio Waves: Used in Wi-Fi, Bluetooth.
  • Microwaves: Used in satellite communication.
  • Infrared: Used in remote controls.

Data Encoding Techniques

• NRZ (Non-Return to Zero): No transition for 1s, transition for 0s.
• Manchester Encoding: XOR operation between the clock and data.
• 4B/5B Encoding: Maps 4-bit data into 5-bit symbols for synchronization.

Signal Transmission (Analog vs Digital)

• Analog Signals: Continuous, used in radio and telephone systems.
• Digital Signals: Discrete, used in computers and digital communication.
• Example: Telephone voice transmission (analog) vs Ethernet (digital).

Multiplexing (FDM, TDM, WDM, CDMA)

• FDM (Frequency Division Multiplexing): Different frequencies for different signals (e.g., radio channels).
• TDM (Time Division Multiplexing): Time slots assigned to different signals (e.g., telephony systems).
• WDM (Wavelength Division Multiplexing): Used in fiber optics to transmit multiple signals.
• CDMA (Code Division Multiple Access): Each user has a unique code (used in mobile networks).

Switching Techniques (Circuit, Packet, Message Switching)

• Circuit Switching: Dedicated path, used in telephone networks.
• Packet Switching: Data broken into packets, used in the internet.
• Message Switching: Entire message transmitted at once, used in email systems.

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Data Link Layer

Framing Techniques

• Character Count: Frame length specified in the header.
• Flag Bytes with Byte Stuffing: Special flag bytes indicate frame boundaries.
• Bit Stuffing: Extra bits inserted to prevent false frame detection.

Error Detection & Correction

• Parity Check: Adds a parity bit (even/odd) for error detection.
• Checksum: Sum of data bits checked at receiver.
• CRC (Cyclic Redundancy Check): Uses polynomial division.
• Hamming Code: Used for error correction by adding redundant bits.

Flow Control Protocols

• Stop-and-Wait: Sender waits for acknowledgment before sending next frame.
• Sliding Window: Allows multiple frames to be sent before acknowledgment.

MAC Addressing & ARP

• MAC Address: Unique identifier for network devices.
• ARP (Address Resolution Protocol): Maps IP addresses to MAC addresses.

Medium Access Control (ALOHA, CSMA/CD, CSMA/CA)

• ALOHA: Random transmission, used in early wireless networks.
• CSMA/CD (Carrier Sense Multiple Access with Collision Detection): Used in Ethernet.
• CSMA/CA (Collision Avoidance): Used in Wi-Fi to avoid collisions.

Ethernet & IEEE 802.3 Standards

• Ethernet: Wired LAN standard.
• IEEE 802.3: Defines Ethernet specifications, including frame structure and speeds.

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Network Layer

IP Addressing (IPv4 & IPv6)

• IPv4: 32-bit address (e.g., 192.168.1.1).
• IPv6: 128-bit address, supports more devices.

Subnetting & Supernetting

• Subnetting: Divides a network into smaller sub-networks.
• Supernetting: Combines multiple subnets into a larger one.
• Example:
  • Subnet Mask: 255.255.255.0 divides a network into 256 addresses.

Routing Algorithms

• Distance Vector: Uses hop count (e.g., RIP - Routing Information Protocol).
• Link State: Uses complete network topology (e.g., OSPF - Open Shortest Path First).
• RIP (Routing Information Protocol): Distance vector-based, slow convergence.
• OSPF (Open Shortest Path First): Link-state-based, fast convergence.
• BGP (Border Gateway Protocol): Used for inter-domain routing.

Network Address Translation (NAT)

• Purpose: Converts private IPs to public IPs.
• Example: Home router assigns private IPs (192.168.x.x) and uses a public IP for internet access.

Internet Control Message Protocol (ICMP)

• Purpose: Used for error reporting and network diagnostics.
• Example: Ping command uses ICMP.

Virtual LAN (VLAN)

• Definition: Logically segments a network into different broadcast domains.
• Example: Separating HR and IT departments on the same physical switch.

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Sample Questions

1. What is the difference between wired and wireless transmission media?
2. Explain Manchester encoding with an example.
3. How does FDM differ from TDM in multiplexing?
4. Compare circuit switching and packet switching.
5. What is the role of ARP in networking?
6. How does CRC detect errors in data transmission?
7. Explain the differences between IPv4 and IPv6.
8. What are the advantages of using VLAN in a network?
9. Describe the working of CSMA/CD in Ethernet.
10. What is the purpose of NAT in networking?

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Transport Layer

TCP vs UDP

• TCP (Transmission Control Protocol): Connection-oriented, reliable, ensures data delivery in order.
  • Example: Web browsing (HTTP/HTTPS), Email (SMTP, IMAP, POP3)
• UDP (User Datagram Protocol): Connectionless, faster but unreliable, data may be lost or out of order.
  • Example: Online gaming, VoIP, DNS

Connection Establishment & Termination (3-way Handshake)

• 3-Way Handshake (TCP Connection Establishment):
  1. SYN: Client sends a synchronization request.
  2. SYN-ACK: Server acknowledges with SYN-ACK.
  3. ACK: Client sends acknowledgment; connection established.
• Connection Termination (4-way Handshake):
  1. FIN (from client)
  2. ACK (from server)
  3. FIN (from server)
  4. ACK (from client)

Flow & Congestion Control

• Leaky Bucket: Packets are stored and processed at a constant rate to prevent bursts.
• Token Bucket: Tokens are generated at a constant rate; packets are sent only if tokens are available.

Sockets & Ports

• Sockets: Combination of IP address & port number, used for communication.
• Ports:
  • Well-known ports (0-1023): HTTP (80), HTTPS (443), FTP (21)
  • Registered ports (1024-49151): Custom applications
  • Dynamic ports (49152-65535): Temporary communication

Quality of Service (QoS)

• Definition: Prioritizing network traffic to ensure performance.
• QoS Parameters:
  • Bandwidth: Data transfer rate
  • Latency: Delay in data transmission
  • Jitter: Variations in delay
  • Packet Loss: Dropped packets due to congestion

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Application Layer

Common Protocols

• DNS (Domain Name System): Resolves domain names to IP addresses.
• DHCP (Dynamic Host Configuration Protocol): Assigns IP addresses dynamically.
• FTP (File Transfer Protocol): Transfers files (Port 21).
• SMTP (Simple Mail Transfer Protocol): Sends emails (Port 25).
• POP3 (Post Office Protocol 3): Retrieves emails (Port 110).
• IMAP (Internet Message Access Protocol): Accesses emails (Port 143).
• HTTP (Hypertext Transfer Protocol): Web browsing (Port 80).
• HTTPS (Secure HTTP): Secure web browsing using SSL/TLS (Port 443).

Web Services & APIs

• Web Services: Enable communication between applications over the internet.
• APIs (Application Programming Interfaces):
  • REST (Representational State Transfer)
  • SOAP (Simple Object Access Protocol)

Remote Access Protocols

• Telnet: Remote login without encryption (Port 23).
• SSH (Secure Shell): Secure remote login (Port 22).

Email Services & Protocols

• SMTP: Sending emails.
• POP3: Downloads emails to local storage.
• IMAP: Accesses emails from multiple devices.

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Network Security

Basics of Cryptography

• Symmetric Encryption: Uses a single key for encryption & decryption (e.g., AES, DES).
• Asymmetric Encryption: Uses a public and a private key (e.g., RSA, ECC).

Network Attacks

• DoS (Denial of Service): Overloading a server to make it unavailable.
• DDoS (Distributed Denial of Service): Multiple devices attacking a server.
• Phishing: Deceptive attempts to steal sensitive information.
• MITM (Man-in-the-Middle): Intercepting communication between two parties.

Firewalls & VPNs

• Firewall: Filters traffic based on rules (hardware/software-based).
• VPN (Virtual Private Network): Encrypts data over public networks for secure communication.

Intrusion Detection & Prevention Systems

• IDS (Intrusion Detection System): Monitors network traffic for suspicious activities.
• IPS (Intrusion Prevention System): Detects and actively blocks threats.

Public Key Infrastructure (PKI)

• Definition: Framework for managing digital keys & certificates.
• Components:
  • Certificate Authority (CA): Issues digital certificates.
  • Public/Private Key Pair: Ensures secure communication.
  • Digital Signatures: Validates sender authenticity.

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Example Questions

1. Explain the difference between TCP and UDP with examples.
2. Describe the steps involved in a TCP 3-way handshake.
3. How does a token bucket algorithm help in congestion control?
4. What are sockets and why are they important in networking?
5. What is Quality of Service (QoS) and how does it affect network performance?
6. How does DNS resolve a domain name to an IP address?
7. Explain the difference between POP3 and IMAP.
8. What is the role of a firewall in network security?
9. Differentiate between IDS and IPS.
10. What is a Man-in-the-Middle (MITM) attack and how can it be prevented?

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Wireless and Mobile Networks

Wi-Fi (802.11 Standards)

Wi-Fi is a wireless networking technology based on IEEE 802.11 standards that allows devices to communicate over a local area network (WLAN) without physical cables.

Common 802.11 Standards:

• 802.11a: 5 GHz, 54 Mbps
• 802.11b: 2.4 GHz, 11 Mbps
• 802.11g: 2.4 GHz, 54 Mbps
• 802.11n: 2.4/5 GHz, up to 600 Mbps
• 802.11ac: 5 GHz, up to 1 Gbps
• 802.11ax (Wi-Fi 6): 2.4/5 GHz, improved efficiency and speed

Example: A home router providing wireless internet to multiple devices.

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Bluetooth & ZigBee

Bluetooth

• Short-range wireless technology (10-100 meters)
• Used for file sharing, audio streaming, and IoT devices
• Low power consumption
• Operates in the 2.4 GHz band

ZigBee

• Designed for low-power, low-data-rate applications (IoT)
• Uses the 2.4 GHz band
• Supports mesh networking

Example: Bluetooth headphones, ZigBee-based smart home devices.

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Mobile IP & Cellular Networks (2G, 3G, 4G, 5G)

Mobile IP

• Protocol allowing mobile devices to maintain the same IP address while moving across networks.

Cellular Networks:

• 2G: GSM, CDMA; used for calls and SMS
• 3G: UMTS, EV-DO; supports internet access
• 4G (LTE): High-speed internet, VoLTE
• 5G: Ultra-low latency, high bandwidth, supports IoT & smart cities

Example: Using mobile data on a smartphone while traveling.

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Ad-hoc Networks & MANET

• Ad-hoc Networks: No centralized infrastructure, devices communicate directly.
• Mobile Ad-hoc Networks (MANETs): Self-configuring networks where mobile nodes form a temporary network.

Example: Disaster recovery networks after an earthquake.

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Network Programming & Simulation

Socket Programming (Python & C)

• Allows communication between devices using TCP/UDP protocols.

Python Example:

import socket
s = socket.socket()
s.bind(('localhost', 8080))
s.listen(1)
print("Waiting for connection...")
conn, addr = s.accept()
print("Connected to", addr)
conn.close()

C Example:

#include <stdio.h>
#include <sys/socket.h>
#include <netinet/in.h>
int main() {
    int sockfd = socket(AF_INET, SOCK_STREAM, 0);
    printf("Socket created\n");
    return 0;
}

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Network Simulation Tools

• NS2 & NS3: Simulate network performance.
• Cisco Packet Tracer: Visualize network setups.
• Wireshark: Packet analysis tool.

Example: Using Packet Tracer to simulate a router network.

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Advanced Topics & Emerging Trends

Software-Defined Networking (SDN)

• Separates control and data planes in networking.
• Centralized network management via controllers.

Example: OpenFlow protocol for SDN implementation.

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Internet of Things (IoT) Networks

• Connects physical devices to the internet.
• Uses protocols like MQTT, CoAP.

Example: Smart home automation systems.

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Cloud Networking

• Delivers network services via cloud infrastructure.
• Includes Virtual Private Networks (VPNs) and Software-Defined WAN (SD-WAN).

Example: AWS Virtual Private Cloud (VPC).

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Edge & Fog Computing

• Edge Computing: Processing data near the source.
• Fog Computing: Extends cloud computing closer to the user.

Example: Self-driving cars using edge computing.

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Blockchain in Networking

• Decentralized network security and data integrity.
• Used in secure transactions and identity management.

Example: Blockchain-based DNS systems.

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Practice Questions

1. What are the differences between 802.11a, 802.11b, and 802.11g?
2. Explain the role of Mobile IP in cellular networks.
3. Describe the working of ZigBee in IoT applications.
4. What is the advantage of SDN over traditional networking?
5. How does blockchain improve network security?

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