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

 

🔴 UNIT 2 – DATA LINK LAYER & MEDIUM ACCESS CONTROL

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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.

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✅ Main Functions

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

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2️⃣ Framing

🔹 Definition

Framing divides the data stream into manageable units called frames.

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🔹 Methods of Framing

1. Character Count

• Frame size specified in header
  ❌ Error-prone

---

2. Byte Stuffing

• Special character added to data
  ✔ Used in HDLC

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3. Bit Stuffing

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

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3️⃣ Error Detection Techniques

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🔹 1. Parity Check

• Adds 1 parity bit

• Detects single-bit error

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🔹 2. Checksum

• Sum of data bits

• Used in TCP/IP

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🔹 3. Cyclic Redundancy Check (CRC)

• Most powerful method

• Uses polynomial division

✔ Highly reliable
✔ Widely used

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4️⃣ Error Control

Error control ensures reliable data delivery.

Methods:

✔ Error detection
✔ Error correction
✔ Retransmission

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5️⃣ Flow Control

Definition:

Controls the amount of data sent to avoid receiver overflow.

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Methods:

1. Stop-and-Wait

2. Sliding Window

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6️⃣ Sliding Window Protocol

Types:

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🔹 1. Stop-and-Wait ARQ

• Send one frame

• Wait for ACK

• Simple but slow

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🔹 2. Go-Back-N ARQ

• Sends multiple frames

• Retransmits all frames after error

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🔹 3. Selective Repeat ARQ

• Retransmits only erroneous frame

• Efficient but complex

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7️⃣ Medium Access Control (MAC)

Definition:

MAC controls who can transmit data on shared medium.

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8️⃣ Channel Allocation Methods

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🔹 1. Static Channel Allocation

• Fixed bandwidth

• Inefficient for bursty traffic

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🔹 2. Dynamic Channel Allocation

• Bandwidth allocated as needed

• Used in LANs

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9️⃣ Multiple Access Protocols

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🔹 ALOHA Protocol

Types:

1. Pure ALOHA

2. Slotted ALOHA

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Pure ALOHA

• Send anytime

• High collision

• Low efficiency (~18%)

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Slotted ALOHA

• Time divided into slots

• Higher efficiency (~36%)

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🔹 CSMA (Carrier Sense Multiple Access)

Types:

1️⃣ CSMA/CD (Collision Detection)

• Used in Ethernet

• Detects collision and stops transmission

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2️⃣ CSMA/CA (Collision Avoidance)

• Used in Wi-Fi

• Avoids collision before transmitting

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1️⃣0️⃣ IEEE Standards

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

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1️⃣1️⃣ Data Link Layer Protocols

✔ HDLC
✔ PPP
✔ Ethernet

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📌 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

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1️⃣ Introduction to Data Communication

✅ Definition

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

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Components of Data Communication

1. Sender

2. Receiver

3. Message

4. Transmission Medium

5. Protocol

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Characteristics of Data Communication

✔ Delivery
✔ Accuracy
✔ Timeliness
✔ Jitter-free communication

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2️⃣ Goals of Computer Networks

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

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3️⃣ Network Structure & Architecture

Network Architecture:

Defines how network is designed and how devices communicate.

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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

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👉 Exam Tip:

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

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5️⃣ Services Provided by OSI Model

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

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6️⃣ Network Topologies

🔹 1. Bus Topology

• Single cable

• Low cost

• Difficult troubleshooting

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🔹 2. Star Topology

• Central hub

• Easy maintenance

• High reliability

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🔹 3. Ring Topology

• Circular connection

• Data flows in one direction

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🔹 4. Mesh Topology

• Each node connected to every other

• High cost

• Highly reliable

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7️⃣ Network Design

Network Design Factors:

✔ Delay
✔ Bandwidth
✔ Cost
✔ Reliability

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8️⃣ Physical Layer

Functions:

✔ Bit transmission
✔ Electrical specifications
✔ Data rate control

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9️⃣ Transmission Media

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🔹 Guided Media

1. Twisted Pair Cable

• UTP

• STP
  ✔ Cheap
  ❌ Noise sensitive

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2. Coaxial Cable

✔ Better shielding
✔ Used in cable TV

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3. Optical Fiber

✔ High speed
✔ Long distance
✔ Secure

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🔹 Unguided Media

✔ Radio waves
✔ Microwave
✔ Infrared

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🔟 Switching Techniques

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🔹 Circuit Switching

• Dedicated path

• Used in telephone network

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🔹 Packet Switching

• Data divided into packets

• Used in Internet

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🔹 Message Switching

• Store and forward technique

• Not used now

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📌 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

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🟢 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

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🔹 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.

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🟢 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

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🟢 3. Structure of Shell Script

#!/bin/bash
echo "Hello World"

Explanation:

• #!/bin/bash → Shebang line

• echo → Print output

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🔹 Running a Shell Script

chmod +x file.sh
./file.sh

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🟢 4. Variables in Shell

User Defined Variable

name="Amit"
echo $name

---

System Variables

$HOME
$USER
$PATH

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🟢 5. Read Command

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

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🟢 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

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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.

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🔹 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

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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

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📄 File Commands

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

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📘 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:

1. Command mode

2. Insert mode

3. 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

📘 PAPER 5 – OPERATING SYSTEM (UNIT 3 – FILE SYSTEM & I/O MANAGEMENT) university of allahabad

 

🔴 UNIT 3 – FILE SYSTEM & I/O MANAGEMENT

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1️⃣ File System – Introduction

✅ What is a File?

A file is a collection of related information stored on secondary storage.

✅ File System

The file system is a method used by the OS to:

• Store files

• Retrieve files

• Organize data efficiently

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2️⃣ File Attributes

Each file has the following attributes:

Attribute	Description
Name	File name
Type	File extension
Size	File size
Location	Disk location
Protection	Access rights
Time & Date	Creation/modification time

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3️⃣ File Operations

Common file operations:

✔ Create
✔ Open
✔ Read
✔ Write
✔ Close
✔ Delete

---

4️⃣ File Access Methods

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🔹 1. Sequential Access

• Data accessed sequentially

• Example: Text file

✔ Simple
❌ Slow for random access

---

🔹 2. Direct Access

• Access any record directly

• Example: Database files

✔ Fast
✔ Efficient

---

🔹 3. Indexed Access

• Uses index to locate data

• Combination of sequential & direct

---

5️⃣ Directory Structure

Types of Directory Structures:

---

🔹 1. Single Level Directory

• All files in one directory
  ❌ Name conflict

---

🔹 2. Two-Level Directory

• Separate directories for each user

---

🔹 3. Tree Structure Directory

• Hierarchical structure

• Most commonly used

---

🔹 4. Acyclic Graph Directory

• Shared files

• No cycles

---

6️⃣ File Allocation Methods

---

🔹 1. Contiguous Allocation

• Files stored in continuous memory

✔ Fast access
❌ External fragmentation

---

🔹 2. Linked Allocation

• File blocks linked using pointers

✔ No fragmentation
❌ Slow access

---

🔹 3. Indexed Allocation

• Index block stores pointers

✔ Fast access
✔ No fragmentation

---

7️⃣ Free Space Management

Methods used to manage free disk space:

✔ Bit vector
✔ Linked list
✔ Grouping
✔ Counting

---

8️⃣ Disk Scheduling

Purpose:

Reduce disk access time

---

🔹 Disk Scheduling Algorithms

---

1️⃣ FCFS (First Come First Serve)

✔ Simple
❌ High seek time

---

2️⃣ SSTF (Shortest Seek Time First)

✔ Better than FCFS
❌ Starvation possible

---

3️⃣ SCAN (Elevator Algorithm)

✔ Moves in one direction
✔ Reduces starvation

---

4️⃣ C-SCAN

✔ Uniform wait time
✔ Used in modern OS

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9️⃣ I/O Management

I/O System Responsibilities:

✔ Device communication
✔ Error handling
✔ Buffering
✔ Spooling

---

🔹 I/O Techniques

1. Programmed I/O

CPU controls I/O
❌ Slow

---

2. Interrupt Driven I/O

CPU works during I/O
✔ Better performance

---

3. Direct Memory Access (DMA)

• Data transferred directly to memory

• CPU not involved

✔ Fastest
✔ Efficient

---

🔟 Protection & Security

---

🔹 Protection

Ensures controlled access to resources.

🔹 Security

Prevents unauthorized access.

---

Protection Mechanisms:

✔ Passwords
✔ Access control lists
✔ Encryption

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📌 EXAM IMPORTANT QUESTIONS (UNIT 3)

✔ Explain file allocation methods
✔ Disk scheduling algorithms
✔ Explain DMA
✔ File access methods
✔ Directory structure
✔ Difference between FCFS and SSTF

📘 PAPER 5 – OPERATING SYSTEM ( UNIT 2 – MEMORY MANAGEMENT) university of allahabad

 

🔴 UNIT 2 – MEMORY MANAGEMENT

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1️⃣ Memory Management – Introduction

✅ Definition

Memory management is the function of the OS that:

• Allocates memory to processes

• Deallocates memory after use

• Manages primary and secondary memory efficiently

---

2️⃣ Memory Management Requirements

✔ Relocation
✔ Protection
✔ Sharing
✔ Logical organization
✔ Physical organization

---

3️⃣ Memory Management Techniques

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🔹 1. Multiprogramming with Fixed Partitions

Concept:

• Memory is divided into fixed-size partitions

• One process per partition

Advantages:

✔ Simple
✔ Easy implementation

Disadvantages:

❌ Internal fragmentation
❌ Wastage of memory

---

🔹 2. Multiprogramming with Variable Partitions

Concept:

• Memory allocated as per process size

• No fixed partitions

Advantages:

✔ Less internal fragmentation

Disadvantages:

❌ External fragmentation

---

4️⃣ Memory Allocation Strategies

---

🔹 First Fit

Allocates first available memory block

✔ Fast
❌ Fragmentation

---

🔹 Best Fit

Allocates smallest possible block

✔ Less wastage
❌ Slow

---

🔹 Worst Fit

Allocates largest block

❌ Poor utilization

---

5️⃣ Swapping

Definition:

Process is temporarily moved from main memory to disk.

Purpose:

✔ Free memory
✔ Improve multiprogramming

---

6️⃣ Virtual Memory

Definition:

Virtual memory allows execution of processes even if:

• Entire program is not in memory

Uses:
✔ Secondary memory
✔ Demand paging

---

7️⃣ Paging

Concept:

• Memory divided into pages

• Physical memory divided into frames

Address Format:

Page number | Offset

---

Advantages:

✔ No external fragmentation
✔ Easy memory management

---

Disadvantages:

❌ Internal fragmentation

---

8️⃣ Segmentation

Concept:

Memory divided based on logical units:

• Code

• Data

• Stack

Address Format:

Segment number | Offset

---

Advantages:

✔ Logical view
✔ Better protection

Disadvantages:

❌ External fragmentation

---

9️⃣ Paging vs Segmentation

Paging	Segmentation
Fixed size	Variable size
No external fragmentation	External fragmentation
Hardware based	Logical view
Fast	Flexible

---

🔟 Virtual Memory Techniques

---

🔹 Demand Paging

Pages loaded only when needed.

---

🔹 Page Fault

Occurs when required page is not in memory.

---

🔹 Page Replacement Algorithms

1. FIFO

Oldest page removed

2. LRU

Least recently used page removed

3. Optimal

Replaces page not used for longest time (best but impractical)

---

1️⃣1️⃣ Thrashing

Definition:

System spends more time paging than executing.

---

Causes:

• High degree of multiprogramming

• Insufficient memory

---

Prevention:

✔ Reduce multiprogramming
✔ Increase RAM
✔ Use good page replacement

---

📌 EXAM IMPORTANT QUESTIONS (UNIT 2)

✔ Explain paging and segmentation
✔ Difference between paging & segmentation
✔ Explain virtual memory
✔ Explain page replacement algorithms
✔ What is thrashing?
✔ Explain memory allocation strategies