Error detection (parity, CRC, checksum) in Computer Networks - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to calculate the parity bit for a given byte.

Computer Networks

def calculate_parity(byte):
    count = 0
    for bit in byte:
        if bit == '1':
            count += [1]
    return '1' if count % 2 != 0 else '0'

Drag options to blanks, or click blank then click option'

A1

B2

C0

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 instead of 1 to count bits.

Adding 2 or more per bit instead of 1.

The parity bit counts the number of '1's. Each '1' adds 1 to the count.

2fill in blank

medium

Complete the code to compute the checksum by summing all bytes and taking modulo 256.

Computer Networks

def checksum(data):
    total = 0
    for byte in data:
        total += [1]
    return total % 256

Drag options to blanks, or click blank then click option'

Atotal

Blen(byte)

C1

Dbyte

Attempts:

3 left

💡 Hint

Common Mistakes

Adding the length of the byte instead of its value.

Adding a constant instead of the byte.

The checksum sums the value of each byte in the data.

3fill in blank

hard

Fix the error in the CRC calculation loop by completing the missing operation.

Computer Networks

def crc(data, poly):
    crc = 0
    for bit in data:
        crc = (crc << 1) | int(bit)
        if crc & (1 << (len(poly) - 1)):
            crc = crc ^ [1]
    return crc

Drag options to blanks, or click blank then click option'

Adata

Bpoly

Cbit

Dcrc

Attempts:

3 left

💡 Hint

Common Mistakes

XORing with data or bit instead of the polynomial.

Not XORing at all.

The CRC is XORed with the polynomial when the highest bit is set.

4fill in blank

hard

Fill both blanks to complete the parity check function that returns True if parity matches.

Computer Networks

def check_parity(data, parity_bit):
    count = sum(1 for bit in data if bit == '1')
    return (count [1] 2) == [2]

Drag options to blanks, or click blank then click option'

A%

B==

C!=

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' instead of '==' for comparison.

Using '>' instead of modulo operator.

The function checks if the count of '1's modulo 2 equals the parity bit.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps each byte to its checksum if checksum is greater than zero.

Computer Networks

checksums = {{'{'}}[1]: [2] for [1], [2] in data.items() if [2] [3] 0{{'}}'}}

Drag options to blanks, or click blank then click option'

Abyte

Bchecksum

C>

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'data' as a variable name inside the comprehension.

Using '<' instead of '>' in the condition.

The comprehension creates a dictionary of bytes and their checksums where the checksum is positive.

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