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ARQ strategies: Error Detection and correction, Parity Bits, Hamming Codes and CRC.
DAY 15 – ARQ STRATEGIES, ERROR DETECTION AND CORRECTION

1) ARQ – AUTOMATIC REPEAT REQUEST
Definition:
ARQ is an error-control technique used in data communication where the receiver detects errors and requests retransmission of corrupted data from the sender.

Purpose:
Ensure reliable data transmission
Detect and recover from transmission errors

Basic Working:
1. Sender transmits frame
2. Receiver checks for errors
3. If correct → ACK (Acknowledgment) sent
4. If error → NAK or timeout → retransmission

Common ARQ Strategies:
Stop-and-Wait ARQ
Go-Back-N ARQ
Selective Repeat ARQ

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2) STOP-AND-WAIT ARQ

Definition:
Sender sends one frame at a time and waits for acknowledgment before sending the next frame.

Steps:
1. Send frame
2. Wait for ACK
3. If ACK received → send next frame
4. If timeout → resend frame

Advantages:
Simple implementation

Disadvantages:
Low efficiency in high-speed networks

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3) GO-BACK-N ARQ

Definition:
Sender can send multiple frames before receiving acknowledgment, but if an error occurs, it retransmits that frame and all subsequent frames.

Characteristics:
Sliding window protocol
Receiver accepts only correct order frames

Advantage:
Better efficiency than Stop-and-Wait

Disadvantage:
More retransmissions if error occurs

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4) SELECTIVE REPEAT ARQ

Definition:
Only the corrupted or lost frames are retransmitted instead of the entire sequence.

Characteristics:
Receiver stores out-of-order frames
More complex but efficient

Advantage:
Minimum retransmissions

Disadvantage:
Higher complexity

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5) ERROR DETECTION AND CORRECTION

Error Detection:
Technique used to identify whether data has been corrupted during transmission.

Error Correction:
Technique used to detect and fix errors without retransmission.

Examples:
Detection → Parity, CRC
Correction → Hamming Code

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6) PARITY BITS

Definition:
A parity bit is an extra bit added to data to detect errors.

Types:
Even Parity:
Total number of 1s becomes even.

Odd Parity:
Total number of 1s becomes odd.

Example:
Data: 1011001
Number of 1s = 4 (even)
Parity bit (even parity) = 0

Advantage:
Simple and fast

Disadvantage:
Detects only single-bit errors

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7) HAMMING CODE

Definition:
Hamming code is an error-correcting code that can detect and correct single-bit errors.

Key Idea:
Extra parity bits are placed at positions that are powers of 2.

Positions:
1, 2, 4, 8, 16 …

Working:
1. Insert parity bits
2. Calculate parity values
3. Receiver checks parity
4. If error detected → identifies exact bit position

Advantage:
Can correct single-bit errors

Disadvantage:
More redundant bits required

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8) CRC – CYCLIC REDUNDANCY CHECK

Definition:
CRC is an error detection technique that uses polynomial division to detect errors in data.

Working:
1. Sender treats data as binary polynomial
2. Divides by generator polynomial
3. Remainder becomes CRC bits
4. Receiver performs same division
5. If remainder = 0 → no error

Advantages:
Detects burst errors
Highly reliable

Disadvantages:
Only detects errors (does not correct)

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9) SUMMARY TABLE

Technique Purpose
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Parity Bit Error detection
Hamming Code Error correction
CRC Error detection
ARQ Error recovery (retransmission)