Error detection (parity, CRC, checksum) in Computer Networks - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of error detection in data communication?

Error detection helps find mistakes in data sent over a network so the receiver knows if the data is correct or needs to be resent.

beginner

What is parity in error detection?

Parity adds an extra bit to data to make the number of 1s either even (even parity) or odd (odd parity), helping detect single-bit errors.

beginner

How does a checksum help detect errors?

A checksum adds up all data parts into a small number. The receiver adds the data again and compares it to the checksum to check for errors.

intermediate

What is CRC (Cyclic Redundancy Check) used for?

CRC uses a special math formula to create a code from data. The receiver uses the same formula to check if data changed during transmission.

intermediate

Which error detection method is best for detecting burst errors?

CRC is best for detecting burst errors because it can find multiple bits of errors in a block of data.

What does a parity bit do?

A Uses a polynomial to check data

B Adds all data values together

C Encrypts the data for security

D Adds a bit to make the number of 1s even or odd

A parity bit is added to make the total number of 1s either even or odd to detect single-bit errors.

Which error detection method uses a polynomial division?

A CRC

B Parity

C Checksum

D None of the above

CRC uses polynomial division to generate a code that helps detect errors.

What is a checksum mainly used for?

A Detecting single-bit errors only

B Summing data parts to detect errors

C Encrypting data

D Adding parity bits

A checksum sums parts of data and sends the result to help detect errors during transmission.

Which error detection method is simplest but least reliable?

A Parity

B Checksum

C CRC

D All are equally reliable

Parity is simple and detects only single-bit errors, making it less reliable than checksum or CRC.

Why is CRC preferred in networks for error detection?

A It uses less data

B It is faster than parity

C It detects burst errors effectively

D It encrypts data

CRC is preferred because it can detect burst errors, which are multiple errors occurring close together.

Explain how parity, checksum, and CRC differ in detecting errors.

Think about how each method checks data and what types of errors they catch.

Describe a real-life example where error detection is important and which method might be used.

Consider how your computer checks if a downloaded file is correct.

Practice

1. What is the main purpose of error detection methods like parity, CRC, and checksum in data communication?

easy

A. To find mistakes in data transmission

B. To increase the speed of data transfer

C. To compress data for storage

D. To encrypt data for security

Solution

1. Step 1: Understand the role of error detection

   Error detection methods are designed to identify errors that occur during data transmission.

2. Step 2: Compare options with the purpose

   Only To find mistakes in data transmission correctly states that these methods find mistakes in data transmission, while others describe unrelated functions.

3. Final Answer:

   To find mistakes in data transmission -> Option A

4. Quick Check:

   Error detection = Find mistakes [OK]

Hint: Error detection finds transmission mistakes fast [OK]

Common Mistakes:

• Confusing error detection with data encryption
• Thinking error detection speeds up data transfer
• Mixing error detection with data compression

2. Which of the following correctly describes a parity bit?

easy

A. A bit added to make the total number of 1s even or odd

B. A bit used to encrypt data

C. A bit that compresses the data size

D. A bit that stores the checksum value

Solution

1. Step 1: Define parity bit function

   A parity bit is added to data to ensure the total number of 1s is either even or odd, helping detect errors.

2. Step 2: Match options with definition

   A bit added to make the total number of 1s even or odd matches this definition exactly; other options describe unrelated functions.

3. Final Answer:

   A bit added to make the total number of 1s even or odd -> Option A

4. Quick Check:

   Parity bit = Even/Odd 1s check [OK]

Hint: Parity bit counts 1s to check even or odd [OK]

Common Mistakes:

• Thinking parity bit encrypts data
• Confusing parity bit with checksum
• Assuming parity bit compresses data

3. Consider a data block with bits: 1101011. If even parity is used, what should be the parity bit added?

medium

A. 0

B. Depends on checksum

C. 1

D. Cannot determine without CRC

Solution

1. Step 1: Count the number of 1s in the data block

   The bits are 1 1 0 1 0 1 1. Counting 1s: 1+1+0+1+0+1+1 = 5 ones.

2. Step 2: Determine parity bit for even parity

   Since 5 is odd, to make total number of 1s even, parity bit must be 1.

3. Final Answer:

   1 -> Option C

4. Quick Check:

   Odd 1s + parity 1 = even total [OK]

Hint: Count 1s; parity bit makes total even [OK]

Common Mistakes:

• Counting zeros instead of ones
• Choosing parity bit 0 when 1 is needed
• Confusing parity with checksum or CRC

4. A checksum is calculated by summing data bytes and sending the result. If the receiver's checksum does not match, what is the likely problem?

medium

A. Data was compressed incorrectly

B. Data was corrupted during transmission

C. Data was encrypted with wrong key

D. Data was sent too fast

Solution

1. Step 1: Understand checksum purpose

   Checksum verifies data integrity by comparing sums at sender and receiver.

2. Step 2: Interpret checksum mismatch

   If checksums differ, data was likely corrupted or changed during transmission.

3. Final Answer:

   Data was corrupted during transmission -> Option B

4. Quick Check:

   Checksum mismatch = Data corruption [OK]

Hint: Checksum mismatch means data error [OK]

Common Mistakes:

• Assuming compression or encryption caused checksum error
• Ignoring data corruption possibility
• Thinking transmission speed affects checksum

5. A data packet uses CRC with a generator polynomial of x^3 + x + 1. If the data bits are 1011, which CRC remainder bits should be appended to detect errors?

hard

A. '000' after dividing data by polynomial

B. '000' because CRC always appends zeros

C. '111' after adding parity bit

D. '100' after dividing data by polynomial

Solution

1. Step 1: Understand CRC calculation

   CRC divides data appended with zeros by the generator polynomial and uses the remainder as CRC bits.

2. Step 2: Perform division for data 1011 and polynomial x^3+x+1

   Dividing 1011 (with 3 zeros appended) by polynomial gives remainder 100.

3. Final Answer:

   '100' after dividing data by polynomial -> Option D

4. Quick Check:

   CRC remainder = '100' [OK]

Hint: CRC remainder from polynomial division is appended [OK]

Common Mistakes:

• Confusing CRC remainder with parity bits
• Assuming CRC always appends zeros
• Skipping polynomial division step