This set of Computer Networks Multiple Choice Questions & Answers (MCQs) focuses on “Error Detection and Correction Techniques”.
1. What is the hamming distance between the words 1010101, 1100110?
a) 4
b) 5
c) 8
d) 3
View Answer
Explanation: Hamming distance can be obtained by performing X-OR operation on two words of same length, then the number of 1s in the resulting word will be the hamming distance. It is used to find and correct the errors.
2. What is the disadvantage of the burst errors over single bit errors in error detection and correction?
a) Difficult to correct
b) Difficult to detect
c) High error rate
d) Frequent occurring
View Answer
Explanation: The burst errors can be detected easily but it is difficult to correct them, as they happen to appear as blocks. When the error rate is low, burst errors are convenient to detect compared to single bit errors.
3. What is the minimum hamming distance between the words 0001, 0100, 0110, 1111?
a) 1
b) 2
c) 3
d) 4
View Answer
Explanation: In order to find the minimum hamming distance between the set of words, the hamming distance between all pairs of the set should be calculated. The least number in the resulted hamming distances will be the minimum hamming distance.
4. Which of the following is an error correcting code?
a) Checksum
b) Parity
c) Cyclic redundancy check
d) Hamming codes
View Answer
Explanation: Hamming code assigns consecutive numbers for code word; the code word will have check bits in place of the numbers that are powers of 2 and the remaining numbers contain bits of data word.
5. An error is burst error, if two or more consecutive bits of the data unit are corrupted.
a) True
b) False
View Answer
Explanation: An error is considered as a burst error, if two or more bits get corrupted, but they don’t need to be consecutive bits. Length of the burst independent of sequence, that is length between first burst bit to last burst bit is considered as length of burst.
6. How many bits will be added to a data word to become a code word in simple parity check code?
a) 1 bit
b) 2 bits
c) 0 bits
d) 4 bits
View Answer
Explanation: Code word will have total of k+1 bits where k is the number of bits in data word. This extra parity bit helps the code word to have even number of 1s and it can be 0 or 1. Parity code is an error detecting code but doesn’t correct the errors.
7. Which of the following error correcting code is not a linear bock code?
a) Low density parity check
b) Hamming codes
c) Binary convolution codes
d) Reed-Solomon codes
View Answer
Explanation: Unlike block codes, convolution codes don’t have encoding boundary. The encoder in this convolution codes deals with sequence of bits and outputs sequence of bits using constraint length of the code.
8. Which error correcting code works well for burst errors among the following?
a) Cyclic redundancy check
b) Hamming code
c) Checksum
d) Reed Solomon codes
View Answer
Explanation: Hamming codes and convolution codes works well for single bit errors, whereas Reed Solomon codes works well for burst errors. Because it uses m bit symbols, which are used to treat single bit and m bit burst errors equally as a single symbol error.
9. What is the maximum number of errors that can be detected using two-dimensional parity check code?
a) 4
b) 5
c) 3
d) 2
View Answer
Explanation: Two-dimensional parity check was introduced to overcome the problem in simple parity check code that it is able to find only odd numbers of errors. Two-dimensional parity arranges the code into columns and rows and add parity bits in each column and row to find errors up to 3.
10. Simple parity-check is used to detect the even number of errors also.
a) True
b) False
View Answer
Explanation: Simple parity check uses only one bit for parity and the bit that is added to a data word can detect odd number of errors only. Because, the even parity will be same after even number of errors occur and it shows no error.
11. Which of the following is not an error detecting code?
a) Hamming code
b) Checksum
c) Parity check
d) Cyclic redundancy check
View Answer
Explanation: Given all codes are error detecting codes other than hamming code. Error detecting codes are used for noisy and wireless links whereas error correcting codes are used for channels like optical fiber.
12. How cyclic redundancy check code can also be called?
a) Fletcher’s checksum
b) Polynomial code
c) Generator polynomial code
d) Checksum
View Answer
Explanation: Polynomial codes has the polynomial representation for bit strings and these polynomials have 0 or 1 as their coefficients. It makes use of generator polynomial to do further calculation in finding errors.
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