Friday, January 2, 2026

📘 PAPER 6 – DATA COMMUNICATION & NETWORKS (UNIT 4 – TRANSPORT, SESSION & PRESENTATION LAYERS)

🔴 UNIT 4 – TRANSPORT, SESSION & PRESENTATION LAYERS

🟦 PART A: TRANSPORT LAYER

1️⃣ Transport Layer – Introduction

✅ Definition

The Transport Layer is responsible for:
✔ End-to-end communication
✔ Reliable data transfer
✔ Flow control
✔ Error control

It works between:
➡ Application Layer and Network Layer

2️⃣ Functions of Transport Layer

✔ Segmentation & Reassembly
✔ Service point addressing (Port numbers)
✔ Flow control
✔ Error control
✔ Connection control

3️⃣ Transport Layer Protocols

🔹 1. TCP (Transmission Control Protocol)

Features:

✔ Connection-oriented
✔ Reliable
✔ Error-free transmission
✔ Flow control
✔ Congestion control

TCP Services:

TCP Header Contains:

4️⃣ TCP Connection Management

🔹 Three-Way Handshake

1️⃣ SYN →
2️⃣ SYN + ACK →
3️⃣ ACK →

✔ Connection established

🔹 Connection Termination

Uses:

5️⃣ UDP (User Datagram Protocol)

Features:

✔ Connectionless
✔ Fast
✔ No error checking
✔ No flow control

Used In:

6️⃣ TCP vs UDP

TCP	UDP
Reliable	Unreliable
Connection-oriented	Connectionless
Slower	Faster
Error control	No error control
Used for emails	Used for video

🟦 PART B: SESSION LAYER

7️⃣ Session Layer – Introduction

Definition:

Session layer manages:
✔ Session establishment
✔ Session maintenance
✔ Session termination

Functions:

Dialog control
Synchronization
Session checkpointing

Example:

✔ Video conferencing
✔ Online login sessions

🟦 PART C: PRESENTATION LAYER

8️⃣ Presentation Layer – Introduction

Definition:

Presentation layer is responsible for:
✔ Data formatting
✔ Encryption
✔ Compression

Functions:

🔹 1. Data Translation

Converts data format (ASCII, EBCDIC)

🔹 2. Encryption

Provides security

🔹 3. Compression

Reduces data size

9️⃣ Encryption Techniques

🔹 Symmetric Encryption

Same key for encryption & decryption
Example: AES

🔹 Asymmetric Encryption

Public key + Private key
Example: RSA

🔟 Comparison of Layers

Layer	Main Function
Transport	End-to-end delivery
Session	Session control
Presentation	Data formatting & security

📌 EXAM IMPORTANT QUESTIONS (UNIT 4)

✔ Explain TCP/IP
✔ TCP vs UDP
✔ Three-way handshake
✔ Functions of session layer
✔ Functions of presentation layer
✔ Encryption techniques

📘 PAPER 6 – DATA COMMUNICATION & NETWORKS (UNIT 3 – NETWORK LAYER) university of allahabad

🔴 UNIT 3 – NETWORK LAYER

1️⃣ Network Layer – Introduction

✅ Definition

The Network Layer is responsible for:

Routing packets
Logical addressing
Path selection
Congestion control

👉 It ensures data reaches the correct destination network.

2️⃣ Functions of Network Layer

✔ Logical addressing
✔ Routing
✔ Packet forwarding
✔ Congestion control
✔ Internetworking

3️⃣ Network Layer Services

🔹 Connection-Oriented Service

Establishes connection before transmission
Reliable
Example: Virtual circuits

🔹 Connectionless Service

No prior connection
Fast but unreliable
Example: Internet (IP)

4️⃣ Routing

✅ Definition

Routing is the process of selecting the best path to transfer data from source to destination.

5️⃣ Types of Routing

🔹 1. Static Routing

Fixed routes
Manually configured
Used in small networks

🔹 2. Dynamic Routing

Routes change automatically
Uses routing algorithms

6️⃣ Routing Algorithms

🔹 1. Distance Vector Routing

Features:

Based on distance and direction
Uses Bellman-Ford algorithm

Example:

RIP (Routing Information Protocol)

Problems:

❌ Count to infinity
❌ Slow convergence

🔹 2. Link State Routing

Features:

Each node knows full topology
Uses Dijkstra’s algorithm

Example:

OSPF (Open Shortest Path First)

Advantages:

✔ Fast
✔ Accurate

7️⃣ Congestion Control

✅ Definition

Congestion occurs when:

Network traffic exceeds capacity.

Causes:

Too many packets
Limited bandwidth
Slow routers

Congestion Control Techniques:

🔹 Open Loop

Prevent congestion
No feedback

Examples:
✔ Traffic shaping
✔ Admission control

🔹 Closed Loop

Detect & correct congestion
Uses feedback

Examples:
✔ Backpressure
✔ Choke packet

8️⃣ Internetworking

Definition:

Connecting multiple networks together to form the Internet.

Devices Used:

Router
Gateway
Bridge

9️⃣ Internet Protocol (IP)

✅ IP

Responsible for:

Logical addressing
Packet delivery

🔹 IPv4 Address

IPv4 Classes

Class	Range
A	1–126
B	128–191
C	192–223
D	Multicast
E	Experimental

🔹 IPv6

Features:

✔ 128-bit address
✔ Larger address space
✔ Better security
✔ No NAT needed

Example:


2001:0db8:85a3::8a2e:0370:7334

🔟 Difference Between IPv4 and IPv6

IPv4	IPv6
32-bit	128-bit
Limited addresses	Large address space
NAT required	No NAT
Less secure	More secure

📌 EXAM IMPORTANT QUESTIONS (UNIT 3)

✔ Explain Network Layer
✔ Routing algorithms
✔ Difference between distance vector & link state
✔ Congestion control
✔ IPv4 vs IPv6
✔ IP addressing

📘 PAPER 6 – DATA COMMUNICATION & NETWORKS (UNIT 2 – DATA LINK LAYER & MEDIUM ACCESS CONTROL) university of allahabad

🔴 UNIT 2 – DATA LINK LAYER & MEDIUM ACCESS CONTROL

1️⃣ Data Link Layer – Introduction

✅ Definition

The Data Link Layer is the second layer of the OSI model.
It is responsible for reliable data transfer between two directly connected nodes.

✅ Main Functions

✔ Framing
✔ Error control
✔ Flow control
✔ Access control
✔ Physical addressing

2️⃣ Framing

🔹 Definition

Framing divides the data stream into manageable units called frames.

🔹 Methods of Framing

1. Character Count

Frame size specified in header
❌ Error-prone

2. Byte Stuffing

Special character added to data
✔ Used in HDLC

3. Bit Stuffing

Extra bit inserted after 5 consecutive 1s
✔ Used in modern networks

3️⃣ Error Detection Techniques

🔹 1. Parity Check

Adds 1 parity bit
Detects single-bit error

🔹 2. Checksum

Sum of data bits
Used in TCP/IP

🔹 3. Cyclic Redundancy Check (CRC)

Most powerful method
Uses polynomial division

✔ Highly reliable
✔ Widely used

4️⃣ Error Control

Error control ensures reliable data delivery.

Methods:

✔ Error detection
✔ Error correction
✔ Retransmission

5️⃣ Flow Control

Definition:

Controls the amount of data sent to avoid receiver overflow.

Methods:

Stop-and-Wait
Sliding Window

6️⃣ Sliding Window Protocol

Types:

🔹 1. Stop-and-Wait ARQ

Send one frame
Wait for ACK
Simple but slow

🔹 2. Go-Back-N ARQ

Sends multiple frames
Retransmits all frames after error

🔹 3. Selective Repeat ARQ

Retransmits only erroneous frame
Efficient but complex

7️⃣ Medium Access Control (MAC)

Definition:

MAC controls who can transmit data on shared medium.

8️⃣ Channel Allocation Methods

🔹 1. Static Channel Allocation

Fixed bandwidth
Inefficient for bursty traffic

🔹 2. Dynamic Channel Allocation

Bandwidth allocated as needed
Used in LANs

9️⃣ Multiple Access Protocols

🔹 ALOHA Protocol

Types:

Pure ALOHA
Slotted ALOHA

Pure ALOHA

Send anytime
High collision
Low efficiency (~18%)

Slotted ALOHA

Time divided into slots
Higher efficiency (~36%)

🔹 CSMA (Carrier Sense Multiple Access)

Types:

1️⃣ CSMA/CD (Collision Detection)

Used in Ethernet
Detects collision and stops transmission

2️⃣ CSMA/CA (Collision Avoidance)

Used in Wi-Fi
Avoids collision before transmitting

1️⃣0️⃣ IEEE Standards

Standard	Description
IEEE 802.3	Ethernet
IEEE 802.11	Wi-Fi
IEEE 802.5	Token Ring
IEEE 802.15	Bluetooth

1️⃣1️⃣ Data Link Layer Protocols

✔ HDLC
✔ PPP
✔ Ethernet

📌 EXAM IMPORTANT QUESTIONS (UNIT 2)

✔ Explain Data Link Layer
✔ Framing techniques
✔ Sliding Window Protocol
✔ ALOHA vs CSMA
✔ Error detection methods
✔ CRC explanation

📘 PAPER 6 – DATA COMMUNICATION & NETWORKS (UNIT 1 – INTRODUCTION & PHYSICAL LAYER) university of allahabad

📘 PAPER 6 – DATA COMMUNICATION & NETWORKS

🔴 UNIT 1 – INTRODUCTION & PHYSICAL LAYER

1️⃣ Introduction to Data Communication

✅ Definition

Data Communication is the process of transferring data from one device to another using a transmission medium.

Components of Data Communication

Characteristics of Data Communication

✔ Delivery
✔ Accuracy
✔ Timeliness
✔ Jitter-free communication

2️⃣ Goals of Computer Networks

✔ Resource sharing
✔ Communication
✔ Reliability
✔ Cost reduction
✔ Scalability

3️⃣ Network Structure & Architecture

Network Architecture:

Defines how network is designed and how devices communicate.

4️⃣ OSI Reference Model

OSI has 7 Layers

Layer	Name	Function
7	Application	User interface
6	Presentation	Encryption
5	Session	Session control
4	Transport	Reliable delivery
3	Network	Routing
2	Data Link	Framing
1	Physical	Transmission

👉 Exam Tip:

Remember as:
A-P-S-T-N-D-P

5️⃣ Services Provided by OSI Model

✔ Data transmission
✔ Error control
✔ Flow control
✔ Connection management

6️⃣ Network Topologies

🔹 1. Bus Topology

Single cable
Low cost
Difficult troubleshooting

🔹 2. Star Topology

Central hub
Easy maintenance
High reliability

🔹 3. Ring Topology

Circular connection
Data flows in one direction

🔹 4. Mesh Topology

Each node connected to every other
High cost
Highly reliable

7️⃣ Network Design

Network Design Factors:

✔ Delay
✔ Bandwidth
✔ Cost
✔ Reliability

8️⃣ Physical Layer

Functions:

✔ Bit transmission
✔ Electrical specifications
✔ Data rate control

9️⃣ Transmission Media

🔹 Guided Media

1. Twisted Pair Cable

UTP
STP
✔ Cheap
❌ Noise sensitive

2. Coaxial Cable

✔ Better shielding
✔ Used in cable TV

3. Optical Fiber

✔ High speed
✔ Long distance
✔ Secure

🔹 Unguided Media

✔ Radio waves
✔ Microwave
✔ Infrared

🔟 Switching Techniques

🔹 Circuit Switching

Dedicated path
Used in telephone network

🔹 Packet Switching

Data divided into packets
Used in Internet

🔹 Message Switching

Store and forward technique
Not used now

📌 EXAM IMPORTANT QUESTIONS (UNIT 1)

✔ OSI model with diagram
✔ Transmission media
✔ Network topology
✔ Switching techniques
✔ Physical layer functions

📘 PAPER 5 – OPERATING SYSTEM (UNIT 5 – SHELL PROGRAMMING & FILTER COMMANDS) university of allahabad

🔴 UNIT 5 – SHELL PROGRAMMING & FILTER COMMANDS

🟢 1. Introduction to Shell

✅ What is a Shell?

A Shell is a command-line interpreter that:

Accepts user commands
Executes them
Acts as an interface between user and OS

🔹 Types of Shells in Linux

Shell	Description
sh	Bourne shell
bash	Bourne Again Shell (most used)
csh	C shell
ksh	Korn shell
zsh	Advanced shell

👉 Bash is most commonly used.

🟢 2. Shell Programming

✅ Definition

Shell programming is writing a script to automate tasks using shell commands.

🔹 Advantages

✔ Saves time
✔ Automates tasks
✔ Easy to write
✔ Used in system administration

🟢 3. Structure of Shell Script


#!/bin/bash
echo "Hello World"

Explanation:

#!/bin/bash → Shebang line
echo → Print output

🔹 Running a Shell Script


chmod +x file.sh
./file.sh

🟢 4. Variables in Shell

User Defined Variable


name="Amit"
echo $name

System Variables


$HOME
$USER
$PATH

🟢 5. Read Command


echo "Enter name:"
read name
echo "Hello $name"

🟢 6. Conditional Statements

🔹 if Statement


if [ $a -gt $b ]
then
  echo "A is greater"
fi

🔹 if-else


if [ $a -eq $b ]
then
  echo "Equal"
else
  echo "Not equal"
fi

🔹 Case Statement


case $choice in
1) echo "One";;
2) echo "Two";;
*) echo "Invalid";;
esac

🟢 7. Looping Statements

🔹 for Loop


for i in 1 2 3
do
  echo $i
done

🔹 while Loop


i=1
while [ $i -le 5 ]
do
  echo $i
  i=$((i+1))
done

🟢 8. Command Line Arguments


echo "File name: $0"
echo "First arg: $1"
echo "Second arg: $2"

🟢 9. Filter Commands

Filter commands process text input and give output.

🔹 1. `grep`

Search text in file


grep "hello" file.txt

🔹 2. `sed`

Stream editor (search & replace)


sed 's/old/new/' file.txt

🔹 3. `awk`

Used for report generation


awk '{print $1}' file.txt

🔹 4. `cut`

Extract columns


cut -d ":" -f1 file.txt

🔹 5. `sort`

Sort data


sort file.txt

🔹 6. `uniq`

Remove duplicate lines


uniq file.txt

🟢 10. Pipes

Used to connect commands.


ls | grep ".txt"

🟢 11. Shell Keywords

Keyword	Use
if	Condition
then	Execution
fi	End if
for	Loop
do	Loop start
done	Loop end

🟢 12. Shell Script Example

Program: Check Even or Odd


echo "Enter number:"
read n

if [ $((n % 2)) -eq 0 ]
then
  echo "Even"
else
  echo "Odd"
fi

📌 IMPORTANT EXAM QUESTIONS (UNIT 5)

✔ Explain shell scripting
✔ Write shell program for even/odd
✔ Explain grep, sed, awk
✔ Explain pipes & filters
✔ Write notes on shell variables
✔ Explain case statement

📘 PAPER 5 – OPERATING SYSTEM (UNIT 4 – UNIX & LINUX COMMANDS) university of allahabad

🔴 UNIT 4 – UNIX & LINUX COMMANDS

1️⃣ Introduction to UNIX / Linux

✅ UNIX

UNIX is a multi-user, multitasking operating system developed at AT&T Bell Labs.

✅ Linux

Linux is an open-source UNIX-based operating system.

🔹 Features of Linux

✔ Open source
✔ Secure
✔ Portable
✔ Multi-user
✔ Multitasking

2️⃣ Linux File System Structure

Directory	Purpose
/	Root directory
/bin	System commands
/etc	Configuration files
/home	User files
/usr	User programs
/var	Variable files
/tmp	Temporary files

3️⃣ Basic Linux Commands

📁 Directory Commands

Command	Description
`pwd`	Show current directory
`ls`	List files
`cd`	Change directory
`mkdir`	Create directory
`rmdir`	Remove empty directory

📄 File Commands

Command	Description
`touch`	Create file
`cat`	View file
`cp`	Copy file
`mv`	Move/rename file
`rm`	Delete file
`file`	File type

📘 Viewing Files

Command	Use
`more`	View page by page
`less`	Scrollable view
`head`	First 10 lines
`tail`	Last 10 lines

4️⃣ File Permission Commands

Permission Types:

Symbol	Meaning
r	Read
w	Write
x	Execute

Example:


-rwxr-xr--

Change Permission:


chmod 755 file.txt

5️⃣ Disk Related Commands

Command	Description
`df`	Disk free space
`du`	Disk usage
`mount`	Mount disk
`umount`	Unmount disk

6️⃣ Process Management Commands

Command	Function
`ps`	Show running processes
`top`	Real-time process
`kill`	Kill process
`nice`	Set priority
`sleep`	Delay execution

7️⃣ Input / Output Redirection

Symbols:

Symbol	Use
`>`	Output redirect
`>>`	Append
`<`	Input redirect
`	`

Example:


ls > file.txt
cat file.txt | grep abc

8️⃣ Background Processing


command &

9️⃣ Process Scheduling Commands

Command	Purpose
`at`	Schedule job
`cron`	Repeated jobs
`crontab`	Cron table

🔹 Example Cron Job:


* * * * * echo "Hello"

🔟 File Editing Commands

Editors:

✔ vi
✔ vim
✔ nano

vi Editor Modes:

Command mode
Insert mode
Exit mode

Important vi Commands:

Command	Function
`i`	Insert
`:w`	Save
`:q`	Quit
`:wq`	Save & quit
`dd`	Delete line

📌 EXAM IMPORTANT QUESTIONS (UNIT 4)

✔ Explain Linux file system
✔ Explain Linux commands
✔ File permission in Linux
✔ Explain I/O redirection
✔ Explain process management commands
✔ Explain cron & at